91 Chevy 5.7 chugs, stalls


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Old 01-30-12, 02:41 PM
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91 Chevy 5.7 chugs, stalls

1991 Chevy truck, 5.7 engine. Starts and runs fantastic, then sometimes with no warning it starts dieseling and chugging badly before stalling. If I can get it into neutral quickly enough and floor it, it MIGHT gain RPMs and within half a minute or so it will suddenly start running PERFECT - the change is so sudden it's as if someone threw a "OK let's run normally now" switch. A few miles later it may do this again, or it might run fine all day.
When it starts that dieseling crud it's most likely going to stall, and I can only restart it with full throttle and some difficulty, then when it starts it sounds like it's running on two cylinders. In that case it will only stay running at full throttle, then if I keep it running it will eventually, and very suddenly, start running perfectly fine.
Years back it was doing this and after changing the fuel filter and putting in a throttle body rebuild kit it was OK. But, here's what I've done so far this time:
New IAC
New Delco EGR valve and EGR selenoid
Throttle body rebuild kit (the last time i did this it was full of gunk, but this time everything looked OK in there)
What next?
Oh, one more thing - I once put in a new fuel pump but I used the cheap brand, not the Delco. For my next step I'm thinking about putting a T into the fuel line and intalling a fuel pressure gauge for a while.
 
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Old 01-30-12, 04:03 PM
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Definitely get a fuel pressure gauge on it. I'm running the same engine (and probably fuel pump) in my '94 wrecker. Can't say I've had those exact symptoms, but I've chased plenty of gremlins over the years. The one that had me flummoxed for a while was intermittent stalling that turned out to be a worn distributor - the shaft had a pronounced wobble. All of your other routine stuff is up to date, i.e. plugs, wires, cap, rotor? You didn't mention codes; have you checked to see if anything is stored?

If it ends up being the pump definitely use a Delco; I haven't had much luck using the cheap ones that I tried.
 
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Old 01-30-12, 08:12 PM
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It does sound more ignition related. Failing coil comes to mind.
 
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Old 01-31-12, 03:21 AM
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so kind of you to help!

Worn distributor?! Good point! I didn't think to check it since I have such a nice starting and running truck otherwise.

Ignition: I keep vehicles well tuned but the coil was original. I should have mentioned that I put a new one in just for kicks at the same time I did the throttle body overhaul.

Finally: Should also have mentioned that I did have a code 32 but I cleared it and it hasn't come back after I changed the EGR valve and the EGR solenoid. No other codes.

AGAIN, THANKS SO MUCH FOR REPLIES!!! and... I'll get back here if I learn anything else with the fuel pressure gauge, or my distributor inspection.
 
 

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