ok a friend of mine has an '88 C1500 which had come with a 305 tbi stock.. he put in a 350 keeping all the stock sensors etc.. he had problems with the old engine and is still having same issues with the new one.. it's been almost 2 years now and no one can figure out the problem. he's had it at about 6 different shops.. here's whats going on..

on cold start it has no bog & lots of power.. as it warms up has major bog, exhaust smells very rich, and when you drop it in gear it almost stalls & kicks back up(probably IAC?).. now here' swhat's been changed..

ignition coil, cap,rotor, plugs, wires, knock sensor, tried another tps, tried another ecm, tried my superchip which is programmed for 350, checked fuel pressure(good - 12psi).. the only thing I could think of is maybe the temp sensor is out of whack and it thinks it's colder than it actually is.. I made the cable to use the aldl software.. could using this software help diagnose?

thanks

 

This may seem obvious/simple, but has the fuel filter ever been changed? This would cause a similar symptom, and may be an intermittant problem....

I would also suspect a restricted exhaust, does the cat. converter ever rattle? Maybe check that out as well.

Good luck.

 

possible o2 sensor?

 

It sound like the engine is going into open/closed? loop and having problems. Id have to say it could be a computer/electrical problem. Especially since its happened with both enginesl.

 

my bro has a88 C2500 that ran like crap. it turned out to be the computer.i think he said it revervts to a fuel chip to run the truck. kind of a limp home mode. he replaced the computer[$70 at autozone] and hes got a whole new truck. his check engine light was on and it went out after. i know thats kind of vague but adealership found it on their diagnostic tool and he replaced it hisself.

 

yeah.. he's already tried another computer..

 

check the e.g.r. valve and control solenoid. pull off the vacuum line that hooks to the egr valve and then drop it into gear and see what the engine behaves like. i have seen where the egr valve will open up very early, and it causes a decrease in manifiold vacuum which the map sensor interprets as additional load on the motor, and adds more fuel-when it isnt required- ignition coils seem to take a poop at around 150k and if you have a known good coil to test(swap) ck.that out too. im assuming you have done your homework and have timing set correctly,no intake leaks,good fuel pressure,etc-ya know, the basics. hope this helps

 

I just posted about something similar in another thread which may be related. My car does 2 things, which I think both relate to the tem sensor. 1 is like you said here, Gobs of power on startup. My T/A with the mods listed, upon startup will go from 650 RPM idle, to the full 5500 RPM in gear in what I'd guess to be less than a second....it also burns up tires. As it warms up, power decreases. However, mie gets worse/better (depending on how you look at it) My SES light will come on, and I'll regain some of that power. The code was O2 Lean, and it would go back into open loop (verified by a scan tool) My guess is that at first the motor correctly compensates for the temp, then as the motor gets warmer, it keeps actlng like its cold....very cold, and continues to dump extra fuel in, until that code is set, then it backs off, but no longer corrects for the temp, so I don't get all the power back. Also, on cold days, the problem is less pronounced. The other thing it does, is if the car is allowed to warm up, then be shut off, for 10 mins to 1/2 an hour, you'll come back and it won't fire. But it doesn't seem to do this really on cold days, leading me to suspect the CTS
Good luck
Corry

 

Nothing beats a begged/borrowed or stolen scan tool for figuring this stuff out. Since you have thrown a bunch of money into parts I would suggest you STOP doing it that way and diagnose it correctly. It could be almost anything causing the problem. You might hit it dead-bang first time out or you might be chasing it still a year from now.

 

scan tool sure wont diagnose a bad (intermittent) coil or, with that ecm, a faulty egr valve.

 

ok well.. tonight I decided to go and look at his truck for him.. first thing I notice is the vac line from the tb to the egr solonoid is all craked and barely on.. the other thing I noticed is there's no ground from the batt to the fender(probably not that big of a deal) the ground from the batt to the engine and chassis to engine looked ok but I told him to remove those and clean them and also do the ones for the ecm that ground to the intake and use dielectric grease on them.. He's suppose to try that tomorrow.. I guess will see.. but that egr line sounds like a big time problem in itself.. vac leak + faulty vac to egr solonoid..