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I am working on a Pantera with a freshly rebuilt engine.  It starts, runs, and sounds great.  It also idles nicely.  But on shutdown it diesels badly.  It has a new Holley 750 with vacuum secondaries, electronic ignition, Autolite 25 spark plugs, closed chamber 351C heads with stainless valves, stroked to 408 cubic inches, dished forged pistons, compression ratio 9.5 or 10, hydraulic roller cam, and adjustable roller rockers.

As a temporary workaround I made a steel sleeve that slips over the idle stop and I can pull it off to kill the engine.  I just finished it and I'm not sure how well it will work to keep it from dieseling.  Its initial test seemed okay.

I tried a different carburetor on the engine and it dieseled the same way on shutdown.

I am at a loss on how to proceed with this and I am looking for suggestions on how to proceed.  Thanks in advance.


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It diesels with idle set low, even 800 RPM.

Compression ratio is estimated because of the dished pistons.  The heads are 1970 4V 351C D0AE closed chamber 62.8cc.  I might try Autolite 24 spark plugs.

A vacuum leak is a possible root cause.  The engine idles with 14 inches vacuum.  I worked on another similar engine and it had 17 inches.  I will start looking for a vacuum leak.

Thank you all.

Low octane fuel and hot engine?

If ignition is off there is no power to make a spark (excepting the rad fan possibility), timing adjustments would be moot, no?

I put 5 gallons 87 into the car once in a pinch, it dieseled so badly when I got home I had to put it in gear and release the clutch to stall it.

5 gallons of 110 race gas fixed it. Never ran 87 again.

This car is a new build and I am trying to get it up on its feet.  I have not yet produced enough heat to get the radiator temperature sensor to engage the fans.  Tomorrow I will focus on vacuum leaks and ignition timing.

For testing I even bypassed the entire wiring harness and hot wired the coil to run the car.  Here is a photo of the temporary ignition switch.  The black wires run from the battery to the coil through the temporary switch.  The factory ignition switch on steering column is still fully functional and usable.


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The engine is dieseling because the combustion chamber temperature is too hot, possibles roots causes : bad spark plug timing, too low octane fuel, bad timing (generally too much advance), too hot sparkplug or vacuum leaks.

The another possibility is that the ignition coil is still powered when you turn off the ignition, possibles roots causes: radiator fans running or bad electrical connection (perhaps try to disconnect the orange wire from the starter relay).

Look to the vacuum setup to the power brake booster. The booster itself can have an internal leak or the plumbing  from the carb to it may have developed a leak.

The tube itself is steel and can develop a pinhole from rust where you can't see it or the booster diaphragm itself can be leaking.

As far as the timing causing this, it can be but if the timing light is showing correctly be aware that with the 4v balancers it is VERY COMMON for the balancing ring to have slipped. That can account for the actual timing being retarded while the balancer actually showing that it is ok.

It only needs to be off a few degrees to do this. A 351c set at 0 to 2° can cause this.

The only way you can find this without pulling off the balancer and having that ring position checked is to set the timing up, i.e., advance it and see if the dieseling stops. With an original untouched balancer you have a high likeliness that it has slipped.

You can have them rebuild so no need to panic but it will just be "down" while it is out for a rebuild.

For your information, the 4v balancer that was supplied new on the 351cj engines is not the best balancer to use. The D1ZZ Boss or the D2ZZ HO balancers would be.

The difference between the two would be the factory balance on them. The D1 is matched to the factory domed pistons while the D2 is for the dished pistons. You can run either together since the difference in balance is not something you normally will see until high rpm's.

The D1's are expensive and rare but the D2's occasionally come up used for sale. There are more 351 HO's out there then you would expect.

Last edited by panteradoug

The balancer is aftermarket and brand new.  We checked that the TDC marking on the balancer matched to cylinder #1 at the top of its stroke.

The car is being driven but only for a short distance while sorting things out.  The distance is not long enough to get the fans going.  When the car is stationary I can run it with my hot wire setup and the ignition switch off.  This helps to eliminate electrical issues as the source of the dieseling.  

Vacuum advance is plumbed to the full vacuum port on the Holley carb.

Within a few hours I should be able to try a new carb gasket and/or find a vacuum leak.  

The engine is pulling a steady vacuum reading.  There is very little fluctuation.

I replaced the blue plastic plug in the manifold with a brass plug.  The blue plug was not tight.  I also replaced the thick carb gasket with a normal gasket.  The dieseling did not change.  Normal advance on this engine 16 degrees.  I tried decreasing that to 10 degrees and no help.

The best way I have to stop the engine at this time is to turn down the idle screw until it stops running.

After putting in the normal carb gasket the throttle cable and linkage are dragging on the manifold.  Hence the reason for the thick gasket.  It looks like I will go back to it or use a spacer and two gaskets.

Finally some improvement. I set the idle timing much higher (simulating vacuum advance) and adjusted idle mixture screws to maximize vacuum. The dieseling is almost gone. Here are the settings for the Progression Ignitions distributor:

Thus at idle there is a lot of advance and as soon the throttle is blipped the advance drops to around 16 degrees and gradually increases with RPMs.


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The Y axis KPa.  I think it is vacuum in reverse sequence (or something like that).  101 is butterflies closed and 20 is WOT (wide open throttle).  At startup timing is reduced to 10 degrees making for easy starts.  This distributor is somewhat like having 1/2 of EFI.  It has a MAP sensor.  It is now my default distributor and I really like it.

I was baffled by the dieseling and have not seen anything like it before.  My hunch is cam profile is involved.  I increased the advance for low RPMs from 23 to 25 (and adjusted idle mixture on the lean side) and the dieseling stopped entirely.  Next week I will get Tommy Hodges and Wilkinson involved and maybe get some insight.  This is Wilkinson's car.

I am even more impressed as the distributor uses vacuum as a timing input.

Seems to me as if it gives you a very flexible, and predictable programming ability, for something that once only had a few variables (and they were dependent variables, like the timing curve) that you could control!

Interesting stuff!  Thanks for posting that screen capture!


It is important to note on this posting that all Panteras came with vacuum advance and that ignition timing is set with the vacuum canister disabled. As John Taphorn mentioned, timing advance is normally set at 16 degrees. If you check the timing again after reconnecting the vacuum canister you will see that advance is much higher, probably in the low to mid twenties at idle. I spoke with Progression Ignition about this a while back and they said their software does not automatically emulate this characteristic. They said if you want such just go back and add eight degrees to the first two columns in the table except for the ones that support startup (those remain at 10). It was not until yesterday afternoon that I made this adjustment and the dieseling stopped. This particular engine requires the extra the advance normally supplied by a stock distributor to avoid dieseling. I have worked on other engines that did not diesel with the default table. It is now apparent to me that eight degrees should be added to the default table idle columns when using this distributor. I think Progression Ignition software should have a check box labeled “Include vacuum advance in idle columns” and it should default to yes.

my 2 cents if you have not done it already is to check timing. Find out if you did not assemble the engine, if the cam was degreed. If it was then your pointer should be correct if not you could be off and making a pointer is easy so you are not chasing your tail in the middle of a forest which sound like you are or were there. Start at the basics.

In my experience, Cleveland head engines like a lot of initial advance. Boss 302's use 16°. If you are running iron heads you just want to limit the total to less then 36°.

Aluminum heads only need about 32°total but will tolerate 36°.

Most aftermarket cams I've used will only give you around 14 inches of vacuum at idle. Having only 14 is not an indication that something is wrong.

"my 2 cents if you have not done it already is to check timing."

Using the timing light on the balancer, the readings from the timing tape match the electronic distributor display.  I think the timing (rotation of the distributor) is correct.  Next week I will check with Tommy Hodges about the cam degree.  He assembled the motor.   Perhaps that is a factor in this engine needing a lot of vacuum advance at idle.  

The engine is running well and the dieseling is gone.  I am happy with it at this point.

Last edited by stevebuchanan

The dieseling problem is completely fixed and the engine runs well.  The fix was to increase timing below 950 RPM's.  It is easy to do with this electronic distributor by making table changes (I wish I knew that earlier).  Once idle timing is increased then the butterfly openings can be reduced and idle air mixture can be leaned.  These changes were enough to stop the dieseling.

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