It happened to me about 4 times this summer, each time leaving me with more questions than answers.
While accelerating (not hard) in first or second gear, the car would briefly stammer, then shut off. After checking with the usual suspects (spark and fuel) and having both, the car would restart after about 5 or 10 minutes and run without further problem. Each instance happened about a month apart. Each time, the car restarted when I took the air cleaner off, and kept running after I put it back on (eliminating the air filters from the line-up). The motor wasn't saying nuthin'.
The victim was an '86 GT5-S with (as far as I know) a stock 351C of about 300-350 hp, I'm figuring around a 9:1 compression ratio, with a Holley 750 vac sec calling the shots. Hanging out in the shadows is a low pressure (5.5 - 9 psi) electric fuel pump mounted on top of the gas tank.
The first thing I did after stalling was to verify the fuel pump's alibi. It was working. To do this I listened to it and heard it humming away, and even put my hand on it and felt it vibrating. Then I roughed up a spark plug wire a bit until it let go of it's spark plug. Then I stuck a screwdriver into it and laid it on the engine cover. I got a strong and consistent spark. Yeah, it was singing all right! I didn't smell any gas while cranking, though. Something was 'amiss'. After taking off the B&M 'dogbone' style air cleaner with it's dual K&N air filters, I would observe fuel squirting into the carb. The engine would then fire up. I'd put the air cleaner back on and be on my way, looking over my shoulder and wondering when our paths would cross again...
The stalling always happened on hot days, always when the engine was thoroughly warmed up, and always while accelerating in 1st or 2nd gear. This perp had it's act together! I thought either the accelerator pump in the carb was bad, or it was vapor lock. (For those who don't know, vapor lock is when the fuel turns from liquid to a gaseous state somewhere along the fuel system. As a result, you lose fuel pressure, and the engine starves for fuel. It is caused by excessive heat.)
I closed in on it the 5th time it struck. I was about 20 miles from home when I started my car up ice cold. It was about 80 degrees outside (certainly not 'excessively' hot). I warmed the car up for a minute or so and took off nice and easy to warm the car up slowly, so no one would get hurt. Before I got to the corner the car shut off. Vapor lock was clearly being framed, so it was released on it's own recognizance. I didn't even get out of the car this time. After 2-3 minutes, the car started back up, but shut off again after a few blocks. I jumped on the highway and quickly accelerated to about 60 mph where the car would shut off. I would leave the key forward (fuel pump on), and coast to about 30 mph. Then I would crank it and as soon as it came to life, I'd release the clutch in 3rd gear, charge up to about 60 mph where it would stall again. I stayed on locked it's tail, and it was a back and forth struggle all the way home! Not exactly fun, and a little hard on the clutch. It was time for the 'takedown'!
By this time I'm figuring there's a blockage somewhere in the fuel system. I was hoping the fuel filter was clogged, and that it wasn't a blockage in the gas tank. No one wants to tangle with 'The Tank'!
I started the hunt for the culprit by removing the fuel line at the outlet side of the fuel filter. I turned the key forward: nothing. The pump was going, but not a drop of gas. I removed the fuel filter to separate the elements of the 'crew' to get individual statements. It didn't feel heavy, and made no sound when shook. I blew through it and got plenty of air flow on the other side, and a bad taste on my lips. The fuel filter was innocent by a preponderance of the evidence. Next, I disconnected the fuel line from the pressure regulator. Key forward, and BAM! Nothing. Isolating the fuel regulator further cleared it of guilt. Next, I disconnected the fuel line on the outlet side of the fuel pump. Key forward, pump humming, nothing. Not a drop, not even any air flow.
Now I'm getting ready to go 'undercover' with the gas tank. I'm thinking the tank is going to have to come out and go away for a while. I figured even if I could blow the tank line out, 'The Tank' probably has a thousand tiny 'minions' in it ready to 'move up' in rank, and put a strangle hold on fuel flow. Not cool by a long shot.
I disconnected the fuel line at the inlet side of the fuel pump. I was going to get my suction gun, connect it to the line going into 'The Tank', and fire away. But just for sh*ts and giggles, I turned the key forward to check the inlet side of the fuel pump first. What... no suction? Apparantly, I caught it totally exposed. No blowing on the output side, no suction on the inlet side, but the pump was humming away! It was telling me what I wanted to hear, but the story just didn't add up. Book 'em Danno, for fraud!
I got a new fuel pump and filter, reconnected all the hoses, and checked flow again at the fuel filter output. Fuel was now flowing like a water fountain. I connected the fuel filter, and the car fired right up. It actually ran much better. I haven't cut the old fuel pump open yet to investigate what caused it to go 'bad' (it's currently in solitary confinement in a box in my garage). I'm guessing something tore the diaphragm inside it, and it was a downward spiral from there. The new fuel pump is identical with the exception of a pre-filter screwed directly into the inlet side, probably to help prevent this type of crippling failure.
'The Tank' and I have a silent agreement: you don't mess with me, and I won't mess with you!
Vapor lock skipped town (probably headed down south), but I have a hunch it may want to return next summer.
The cold hard lesson here is just because an operation looks and sounds legit, doesn't always mean that it is. Even though the fuel pump sounded like it was doing it's part, it actually wasn't. An electric fuel pump usually fails when the motor quits, making it easy to diagnose. This one just turned out to be a liar.
I'm just passing along this info so that maybe it'll help someone who encounters similar symptoms one day to quickly 'close the case'.
Mike 'Spade'
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