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Ugh, been getting a bit of hesitation & rough running & some backfire on my 427 Fontana Fuel Injected car. I see in 2011 I replaced both fuel filters due to hesitation under load, but I didn't take note of the reading on the fuel pressure gauge reading before (rookie!). Today I had the wife stand in the back by the fumes while I ran it and she sees it is reading about 40 on idle and maintains at 40 with steady increase of the gas pedal which I see from some google searching is pretty much normal. Seems to have started back in Dec so I cleaned the plugs, but the past two runs seems to have started acting up again. I suppose I could pull a plug and take a look at that too and also be sure all is still connected, but is 40 pretty much normal for fuel pressure, or anything else I could look at? I guess I'm trying to talk myself out of a messy fuel filter change if not needed, but if that's all it is I suppose that's not bad from a financial perspective at about $100/filter.IMG_4028PSI40


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If you have a steady 40 psi fuel pressure reading, then I would say that you don't have a plugged filter or a fuel pressure issue. Whose ECU are you using to run your injection system? Back firing could indicate an ignition problem. You need to get into the program and check the fuel air ratio settings along with your ignition settings. I am running a FAST XFI 2.05 system that is port injection and in sequential mode and my normal fuel pressure is about 39 psi. I also have a 9.2 deck Fontana block, but am using CHI 3V heads and displacing 388 cubic inches.

39 psi should be fine.  Jffr, are you running eight stacks? If so, are you using a remote IAC valve? Are you running the FAST or the Holley dual sync distributor? I'm running the FAST XFI 2.05 with individual throttle bodies without the IAC valve and need to adjust the cold idle rpm tune. I also have a Fontana 9.2'' deck hight displacing 422 c.i. running in bank to bank mode right now, will be switching to sequential mode eventually with the Holly low profile dual sync distributor which I already have. Any input would be appreciated! Thanks




I have the MSD 6AL ignition system.  12 years since I owned the car and its the first time it got a bit funky (besides the time I had to change the two filters and in Dec when I cleaned the plugs). Now I'm thinking maybe it wasn't the plugs needing cleaning but instead something else. Not sure what's up with it, way beyond my scope of abilities if its something up with the ECU/ignition system. Will probably just need to drive it in to my fuel injection engine guy. He programed it when I first got the car. Maybe they occasionally need readjustment? Its not real bad but I notice its not up to par like it usually is. I thought maybe it was a warm up issue, but I ran it a good while and didn't improve.

Last edited by does200

Good thought bossman. I was actually thinking the same, I have one 5 gal can to take out the existing gas but just need to grab another 5 gal can to put in new. I'd like to take out as much as possible and I think I have about 5 in the car now and could run it in the driveway a bit to squeeze out some more or use other containers. I really thought it was the gas filters acting up again, but that 40 psi looks right. I always use the same place for all my cars but who knows bad gas may be a longshot but worth checking anyway.

Well I syphoned out the existing gas from the car (after tasting some) and put in 5 gal of new gas. But in doing so I noticed the gas from the car (Mobile 91 octane) looked quite a bit darker than the new gas (Chevron 91 octane). I only assume a different brand gas could have different color, but who knows I'm no gas man (the wife could debate that!). Anyway in the photo Mobile is on the left and Chevron on the right. Didn't take the car out yet though. If the car still runs odd I may try and rig my gopro near the fuel pressure gauge in the engine bay to see what the PSI is under load.IMG_E4322


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Jan, as gas ages it starts to tarnish like that, it has lost some of the volatiles.

An ECU map shouldn't need updating, what is likely occurring is one of the inputs is reacting differently, a sensor or maybe an injector clogged, especially is the vehicle is sitting for long periods. What are you running for a MAP/MAF sensor?

Yeah it seems something is a bit off. One time it even stalled just after start up and idling.  I usually get it out for a good run at least every couple of weeks, but back in January had to let it sit for close to a month. I wonder if I just run it more or just start it up more if it may work itself out on its own such as if gums & varnish happened from sitting. Anyway, will test it out next week with the new Chevron gas in it. I am not sure of the specs on MAP/MAF but looking at build photos it seems it has a Speed Pro ECU (that is if it didn't get changed out from when I bought it.



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Still a bit rough with the new gas, so I put in some Techron fuel system cleaner anyway and over time see if that has any improvement. Not sure what's up with it. Trying to locate the pressure regulator next. I see some references that say the fuel pressure gauge should maintain pressure when you turn the car off however mine shoots right from 40 to 0. But maybe that's normal for my car with the set up I have. Talked with the fuel injection manufacture and he suspects the throttle body. It seems there's 4 with my car but probably beyond my capabilities to get into that. In the end may need to just drive it to Harv in Whittier.

The fuel pressure drops off after a few minutes or less after you shut your engine off. You really should get you car hooked up to a lap top and see what is going on. Have you tried checking your exhaust header tube temperatures with an infrared temperature gun? That can at least tell you if you are getting good and even combustion on all cylinders.

Thanks for the gun suggestion. Funny I was just thinking of that too, since I am unsure of the real problem. Will need to buy one, any suggestion for heat range? Amazon seems to have several weeks back order on all of theirs, but a parts store here has one that goes up to 840 degrees. I also have a code reader as well that I use on my other cars on occasion, so I may look to see if I can find the input (if there is one). Never saw one under the dash, but could be near the ECU.

Last edited by does200

Get a thermal gun that goes to 1500 degrees "F". That should be all that you need to check header tube temperatures. I doubt that a modern day automotive code reader will work on your after market ECU. As far as I know all of the after market companies have their own program and you need their software program to get into the ECU.

 I tried this for analyzing fuel mixture. Don't be surprised by considerable variation in temp. from these guns. I found the average idle temp on header pipes not far from the heads was around 980F on a Costco temp gun. Shooting the exact same spot on multiple round tubes is difficult, and minute changes in vertical distance away, angle or position can change the indicated temp by 100F or more. You can't tell if its exhaust gas temp variation, or whether its because the spot you checked is not identical on all 8 tubes. Or if its just a cheap gun.

The lower the temp, the less all this seems to affect the gun reading. So its much more accurate on radiator header tanks and useless for checking exhaust mixtures. My 2¢....

Thanks all. I still may get that heat gun, as I probably should have one anyway. Willing to give it a shot just to see if there is any drastic temp differences. I may even take off a stack to take a look at how the throttle body looks inside. If its easy to do, I suppose I could look into each throttle body because the manufacturer said sometimes those valves can bend with a backfire. It looks like I just unscrew these couple of screws on each side and the stack should come off.IMG_E4431


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Last edited by does200

I have been bothered from the first post , but  rather reluctant to chime in.

The fuel pressure caught my eye.  In a FACTORY EFI system with a fuel pressure regulator and return line to the fuel tank , the pressure varies.   At idle and low loads the fuel pressure would be more like 28 psi and as engine load increases the pressure also rises by about 8 to  10 psi.  That is done so the pressure across the injector remains constant. (delta is the term)   The fuel pressure rises to match the increasing manifold absolute pressure.  (decreasing vacuum)

The picture you posted shows a fuel pressure regulator with a small hose on the right side of the regulator. (passenger side)  That should be your reference to the manifold pressure.  To test it , you would pull the hose off with the engine running and fuel pressure would go up that 8-10 psi.  The hose should be dry , any fuel and you have a regulator problem.  There should also be some vacuum at the now removed hose.

  I have zero experience with a engine with IR manifold and efi.  I understand the "vacuum " signal can be erratic.  (bounce around)  Most manifolds have all cylinders drawing from a common plenum and one large throttle body to meter air flow.  Manifold pressure (vacuum) is more stable .

   I would look to see the small hose is connected to the manifold and in good condition (not rotten).  It is possible the MAP sensor has lost it's connection as well if not bolted to the manifold directly.

   I use the Harbor Freight contactless infrared thermometer for $25 .  I use it for "looking" for heat in brake rotors and bearings.   A industrial quality instrument has a correction factor that has to be entered for the material you are scanning. Copper vs steel vs skin vs what ever.

   Another thought is a scan tool does NOT pick up all faults, only sensors that are monitored.  It can not tell a clogged air filter or tell a bad plug wire from a fouled plug.  They only point you in a direction.

   Boss wrench has good advice , start with the logical and simple things and work your way throgh the problem rather than go full "Mike the Snake" mode.

   I hope that is food for thought and wish you luck in your repair.

   Please keep us posted.





The only eight stack EFI system that I worked on had a vacuum chamber that was being fed by lines that came off of each throttle body. It allowed for a constant and stable vacuum reading. I agree that the photo of the fuel pressure regulator does seem to have a small line on the right side which looks more like a vacuum sensing line than a fuel return line. The aftermarket fuel injection system that I am using in my Pantera is a FAST XFI 2.05 and it has a rather large fuel return line that comes off of the pressure regulator and goes back to the gas tank. The fuel pump pressure stays the same at all  times. I have watched it from the ECU on the dash monitor and my in dash fuel pressure gauge. There is also a mechanical fuel pressure gauge that is on my fuel pressure regulator. It sounds like over kill on my system, but I already had an in dash fuel pressure gauge in my car before I switched it over to fuel injection, so I just replaced the gauge with one that worked with the higher fuel pressure that EFI has. Many newer factory EFI systems use a variable pressure fuel pump which does change fuel pressure according to load. I also agree with just starting to look at simple things on this system, but the best approach is to start with getting any fault codes that are coming up in the ECU. That of course means hooking it up to a laptop with the correct software program. This entire problem could still just be a batch of old gas that needs to purged out of the system. The simplest way to do that is just get the car out and drive it like you stole it until it clears itself out!

Yeah thanks for putting up with my dumbness. I could just take the car in, but I like to learn as well. If my engine guy was more local I could just drive it in and may do so in the end. Yes I have been trying to find the fuel pressure regulator, so maybe this is it as shown in the photo (the piece top of the gauge?). I did inspect that small tube on the right and that hose is connected at each end. Maybe that's the vacuum hose? I didn't go pulling on it because I thought maybe it was heat wrap secured and didn't initially want go pulling on it. I do have a vacuum test tool, so if that is the vacuum hose I could try pulling it off and testing. I still think its a bit odd though that my PSI drops so fast when turning the car off, because from google searches it seems to indicate that PSI should remain at least for a few minutes. I forgot all about Harbor Freight for tools, so I'm on my way to pick up the heat gun because I figure its good to have one anyway.QJCCE1449


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Last edited by does200

Jffr you are right, you don't have to vary the fuel pressure.  Jeep and Dodge systems put the fuel pump , filter , and pressure regulator in the basket in the tank.   There is no reference to the manifold pressure and runs around 39-40 psi all the time.

    With a reference line showing in the photo I had assumed it is not a dummy. A 1/8 inch pipe plug or a rubber cap from the auto parts store would blank it off if not in use.  They do make regulators without reference as well.  They also make regulators that are not 1 to 1 but don't know how or when they are used. 

 I have seen a equalizing chamber with spider legs to each throttle body used on IR systems.  I would expect with out one the engine would be running alpha n  fueling.    MAP sensors can be directly mounted to a manifold or remotely mounted with a rubber hose.  Another thing to check.  A rotten or split hose would mess things up here.  

 I would expect a back fire to be tough on a MAP sensor. 

 A digital dash on a tablet or phone would be a help here.

  I think the Speed Pro system is now part of F.A.S.T.

  Looking for bent throttle blades might be jumping the gun.

 You are NOT dumb!!  Auto mechanics might not be your specality but every day is a oppertunity to learn something new.  I have very little hands on experience with EFI but have bought a pesonal library of text books off Amazon.  It may be a morbid sense of curiosity.

  Trouble shooting auto mechanics is a art.  Think it through , start with the simplist or cheapest possible fault and work it forward.  Some systems have known weakness and it doesn't hurt to know your enemy.  Youtube is not a bad resource , you just have to recognize BS when you hear it.

   The injector cleaner and a Italian tune up is not a bad idea.  ( drive like it is stolen)   I doubt any Pantera owners drive like old ladies , the Pantera has long legs.


IMG_E4455Your dash monitor looks great. Had trouble getting the Fuel Pressure Regulator hose off, but it does move around so I know it isn't fixed. But for a quick test I wrapped the connection itself real tight with some electrical tape and tried the car and still bad. Idols around 50 then is still rough when I give it some gas. Then by the time I turn the car off take the key out and run to the back the PSI is 0. I thought there should be at least some PSI remaining with immediate turn off.  Tomorrow will try and get that hose off and look at my vacuum tool instructions to see how to test at the FPR and at the hose too. But for my sake in understanding my car can anyone tell me what items 1, 2 and 3 are for? I see the eight hoses labeled 2 go into the manifold but can't see where the eight #1 hoses go. Maybe item 3 screw is for some type of test gauge or just secures it to the car straight through the FPR?



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Last edited by does200

  Hope you have a fire extingisher in the garage and is handy.  Those are vacuum lines and no fuel should be present.  # 2, should be 8 of them and they appear to all be going to a common chamber so you should have a solid vacuum signal.  That is what jffr refered to in a previous reply.

  # 3 is the adjustment for fuel pressure and does NOT need to be adjusted. The hoses appear to be hard plastic and may damage the end trying to get it off the regulator.  You are not apt to find hose like that easily.   The fitting on the regulator is a barbed adapter.  It is slightly tapered on has sharp edges around its diameter to grab the hose.  You may not want to take it off and damage any thing.  Pretty obvious you have a vacuum signal to the regulator.   You could try to ID the brand and part number off the regulator and go read about that regulator and see how it works.   With a vacuum line reference I think the pressure should vary with load.  There is no other reason to supply a vacuum reference.  The specs may say "vacuum/boost rise ratio 1:1" , that means on a boosted engine 1 pound boost raises fuel pressure 1 pound.  On vacuum it is around 2 inches HG (mercury) to equal 1 pound less fuel pressure.

   The sudden lose of fuel pressure would lead to hard HOT restarts.   Without pressure the hot fuel can boil in the fuel rails and when you try to start, the injectors and fuel lines have vapor rather than liquid fuel.    At this point we don't know if that is related to the problem or has always done that but no one noticed.   Again in theroy it should hold some pressure for hours. 

    You should try a phone call to the guy that tuned the car and hope he can / will answer a couple of questions.

    A fuel pressure regulator is about $150 for a house brand (summit racing) to $350 for aeromotive brand , so determine that really is your problem rather than throw money at it.

   I said before no hands on expereince with a system like this but have read about so I am not selling myself as a expert.  


#3 in the photo is the fuel pressure regulator adjustment screw and lock down nut. #2 in the photo appears to be vacuum lines that are coming off of the throttle bodies and going to the vacuum supply pod. I don't see the #1 photo, but it looks like there is another line coming from the fuel pressure regulator body that also goes to the vacuum pod. That type of set up looks like what my old factory 1988 Ford Ranger 2.9 V6 engine used on its fuel injection system and it was tied to the fuel return that went back to the gas tank. On that particular system, when the fuel pressure regulator began to  fail it caused major flooding problems with the engine. Without actually being there to look at your engine's fuel injection set up,  it  would be hard to pin point exactly what is going on. It would help if you could find out who built the original system for that car and what tuning mode is being used. From what I understand about those type of IR throttle body systems, they either use Alpha N or Speed Density tuning. I think Lance Nist  from southern California built at least one system like that in the late 1980's or early 1990's. I will bet that someone on this forum knows where he is these days, but Lance certainly knows how these systems work and was a pioneer with fuel injection and other cool stuff for Panteras.

why not try that go pro camera you have.  aim it at the fuel pressure gauge and go for a short test drive.  include idle  , economy cruise and a very brief high load.

no need to go on the express way or any high speed driving just show a range of loads.  I maintain the fuel pressure should "mirror" engine load.  low load low fuel pressure , high load higher fuel pressure.  did the fuel pressure vary?

If   NO  , loosen the lock nut on the regulator and back the adjustment screw out until fuel pressure drops to around 28-30 psi.  Re tighten the lock nut.  now a short test drive keeping speeds and loads low.  did drivability inprove?  Remember that leaned the fueling so no hot rodding.  now restore the fuel pressure setting and decide what to do.  careful around the opening in the bellhousing and the rotating flywheel / clutch  and don't let the car roll away on you. ( wheel chocks)  Not trying to be insulting , just safe.

    If you have a oxegen sensor in the exhaust it will try correct the mixture.  no action needed just an observation.

    Again see if you can get any information on your regulator.

   Now you should know enough to talk to your tuner , or some one that knows your system.


I've been interested in EFI conversions for the Cleveland (or Fontana) but I really know very little about the systems. In one of the books written by a company called "TPS" decades ago (1988), they mentioned fine-tuning their GM system by varying the fuel pressure. On a dyno, they ran from 40 psi on the low end to 55 psi and the throttle response & bhp varied considerably.  And that was with stock GM-style injectors. Do 8-hole throttle bodies change the rules on varying fuel pressure? 40 psi seems low based on what little I think I know.

That threaded stud (#3)  with locknut is how one changes fuel pressure on a regulator. If you remove the 4 small screws, that part of the housing comes off and there should be a rubber diaphragm underneath with a fairly weak coil spring. Possibly the old diaphragm has a small puncture maybe from using CA gas and that's why the thing doesn't hold fuel pressure when switching off.

Thanks for all your help. Sorry, I got caught in all the Huntington Beach protest mess about beach closures but am home now. Yes you are right the car does have an Aeromotive FPR. I am not sure if this one in the link below is the one I have (or may need) but for illustration it does show what the small vacuum connector looks like. So it looks like I should be able to slide the hose off without damage and test at the FPR metal line and/or at the hose/engine line. I have a hand vacuum pump as shown but can't find my instructions or even youtube or google for use on the FPR. But I am thinking if I attach it to the FPR with the engine off and pump the vacuum pump to lets say 10 or 20, then shouldn't the pump hold that same pressure? If it would drop maybe the diaphragm may have gone bad? I like the suggestions to drive the car hard too, but if the FPR is bad I am thinking it may be putting small amounts of gas in the engine which may not be good. Thus trying to test the FPR first.






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  Boss Wrench is right could be a hole in the diaphram.   Maybe areomotive might sell just that part as a repair kit.

   A hole in the diaphram would explain why you don't hold pressure when the engine stops.  It would explain why the fuel pressure does not move from 40 psi.  It would explain why the car is not running well.

   You are right the hand pump should pull a vacuum and hold when you stop pumping.  

    If you do get the small hose off you will have fuel present IF the diaphram is ruptured. 

   You might take a electric heat gun or a hair dryer to heat the plastic line to soften it.  to aid in removal.  NO FLAMES might encounter raw fuel and that extingisher!!!

  If the diaphram is ruptured , it has been adding  fuel like a 9th injector.

  Might sniff the oil on the dip stick to see if there is raw fuel in the pan.  That would be extreme but doesn't cost a dime.




Great idea with the heat gun on that plastic line, maybe it will help it come off. Yes, another excuse for Harbor Freight tomorrow, $10 heat gun on sale. I forgot how great that store is. Main thing is to test that FPR at this point. I had already sniffed the dip stick and there was slight engine smell, but not really gassy smell that I could detect. Already had planned oil & filter, but now waiting till this is solved. When that day comes I will save an oil sample too. Would also like to know how to do a vacuum test at the engine hose. Maybe its the same process or does the engine need to be running?

that heat gun would have no trouble melting the hose so easy does it.  I have 2 of those heat guns.  softened sheet flooring for removal in the house.  I hate sending any more money to China but buy the heat gun else where and it may have rolled off the same assembly line. 

    here is a link to a youtube video that is decent on fuel pressure regulators.


If its running rich / poorly maybe even smoking looking closely at the FPR is a good place to go based upon my recent experience with the 348.  The FPR diaphragm developed a small hole in it.  Fuel would push out the diaphragm and into the vacuum line.  This caused the engine to progressively run richer and richer.  When the problem just began I noticed it just wasn't running quite right.  Finally the CEL came on and we began hunting for the problem.  By the time we found the problem it was smoking really bad it was so rich.  It eventually dumped so much raw fuel into the intake manifold that I had to change the oil / plugs and run the car quite a while before all the un-burnt fuel cleared up.  Two months later it began running slightly off again.  This time I changed the other FPR on the right side and problem went away.  I did dissect / destroy both old FPR's and found both had holes in the diaphragm.

I don't know if that's your problem but this is what happened to my 348.

That's right, back in December I suspected the plugs as a possibility so I cleaned them up even though they didn't look too bad. It did run nice after that but on the next run it started acting up a little. That's when I began hunting around on the web and with you guys for suggestions. I got a heat gun which did help in getting the plastic vacuum line off, then hooked up my vacuum pump to the FPR. Took it up to 20 and it dropped pretty fast (see video). I even tried some other connections to be sure my connections were as tight as possible & thumbed at the end too. Did the plastic hose sniff and it seemed to have some fumes there too. So it seems like that is most likely the issue. Now to find the actual Areomotive unit I need, and as suggested maybe I could just replace the diaphragm from Aeromotive rather than the entire unit. My thought is that I would not prefer to mess with any of the existing fuel line connections, especially if the top just opens and I could replace the bad diaphragm that way. Thanks for all your help with this. I guess in the end the tell tail sign was that pressure drop when turning the car off. 


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Jan, If the FPR is your problem I would just replace it instead of trying to replace the diaphragm.  I just went to the Summit Racing web site and looked them up.  They range in cost from low $100's to low $200's.  Look for the correct fitting size and inlet / outlet ports etc. and replace it.  Not a difficult job at all.

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