1994 GMC Med Duty - 5.7L Motor Surge

[alphaauto]

I have a 94 medim duty truck; it says GMC on cab & gm 5.7 liter motor, but states on tag Manufacturer chassis is isuzu. The truck surges & wants to die out when cold. When t-stat opens it will idle but still have no power & back fire.

The customer has been throwing parts at it for 2 weeks. TPS, IAC, EGR, EGR solenoid, fuel pump, fuel filter, major tune up..even injectors & O2 sensors. Nothing worked.

I changed the coolant sensor on intake & with sensor out of intake & not touching anything. Truck idles fine cold...but as soon as i screwed it into intake & replugged it and started truck, back to the same surge & die out.

Any ideas.?????????

[fortknoxx]

if it is a new aftermarket sensor, it looks like the old one (you will find that pn covers many models and makes)it might be out of range. find a delco/gm one it will prob work.

fill what are the first 8 and the 10th digs of the vin?

[steven kiser]

don't let the gmc isuzu issue throw you. this model is the npr and we've been working on them for years. treat it as a 5.7 3500. with the sensor out and it running fine was the hole in the intake plugged or was it sucking air? if it was sucking air and running fine the engine is probably running rich. it's been a while for me but look at the map sensor and with a scanner check voltage at the front 02's. check fuel pressure and see what the reading is. another little trick is see how fast the pressure drops when key is shut off. carefully close off the return line and if the pressure falls off you most likely have an injector leaking. it's just a 5.7, treat it like that. load it into the scanner as a 3500 or use global. i'm thinking the 02's are reading lean forcing rich or the fuel pump is weak. a vac leak will also cause this. without looking it up if this is tbi see if the pcv tube has vac or if it sounds like a vac cleaner in the tbi. the tbi's had a issue where the gasket split under the body as well as the port for the pcv plugging. another issue i've seen with these trucks and still on occasionally do is some times drivers put dls fuel in them. if a driver did and even if he caught himself after a few gallons the injectors are probably junk and 02's are useless. i service these trucks on a regular basis and see wrong fuel issues on somewhat a regular basis. last year i heated my shop a 80 degrees burning mixed fuels. check for soot in the tail pipe.

[alphaauto]

Lets see where do i begin.Besides pulling my hair out. The owner of the truck put in new o2 sensor,iac & tps sensor.Then he stated he put a new fuel pump & washed out the tank due to grime in strainer....Plus he stated he put in reman injectors......We found the injector gasket old & leaking fuel.Replaced gasket & cleaned tbi unit..Also replaced sticking egr valve & egr solenoid due to no vacuum at idle cold start.....Then one night i remembered a cts could cause a lean or rich idle condition cold.Thats when we changed the cts & left the old one screwed in the intake & justed pluged in the new one for sh&ts & grins & walla car ran fine.Figure ureeka got it changed them out,plugged it in & crap back to square one. What type of fuel psi reading should we get on this motor???? It sound good from tbi unit lots of suction.Spray in injector looks odd.....I firsts was suspecting bad fuel pump because when cold & trying to run,was able to spray small amounts of carb cleaner in tbi & vehicle ran fine.....Am i wrong in the assumption pump could be bad????????Also with my solus hooked up it states that tps sensor closed should read 0.55 volts i got 0.48 cold closed??????But it is a new tps sensor.by customer!!!!!!
Thanks
for any more help!!!!!

[steven kiser]

sounds like customer may have monkeyed with the throttle body along with everything else. i had more than a few issues with the temp sensor for the pcm. what's your scanner reading? you mention that the pump was replaced. i don't see a mention of pressure. the spray pattern may be a clue. if there was debris in the tank there is a chance that micro fibers got into the strainer at the injector. it appears what we have is a train wreck. there are probably 10 viable issues here that are borderline and could cause the issue you have, not to mention the owner compounding the problem. i remember dealing with a fleet of cut vans and the pcm's were under the drivers seat and the drivers were tossing things under the seat causing huge issues with the wiring.

[alphaauto]

Customer took truck after a shouting match that fuel pump needs to be replaced. He installed 3 pumps but fuel filter was installed backwards. Told me to * off & he is mechanically inclined & i was a crook.

[steven kiser]

happens to the best of us. maybe one day you'll get the satisfaction of seeing him on the side of the road broken down. throw him the wheel.

[Tim Martin]

Yea, sounds like you got in for the short end of the stick, as we say around here. As my Dad would have said, if ya never get treated bad, it means ya have't done much yet. When I get into those kind of situations I try to examine the whole thing and make a determination of what I can learn from it. So, it ain't all bad.

[steven kiser]

Told me to * off & he is mechanically inclined & i was a crook.

made me think of "just because i know that if i slam my head against the wall it will hurt doesn't mean i'm a psychic." be thankful that you shook this guy as soon as you did and smile when you see the truck at some other shop, i have.