D0UG

1991 C1500
350 engine (non CA)
Auto Transmission
160k miles

My truck just started backfiring (mostly back through throttle body) when I entered the Interstate Highway. It runs fine when cold but starts the backfiring after the truck has been driven at least 20 minutes. Last night, I was able to make it back home by just barely opening the throttle. If the throttle is opened more than 1/4 or if it is openned suddenly, the engine backfires and stumbles like crazy.

The Service Engine code indicated that the probable cause was either the engine temperature sensor or Throttle Postiion Sensor (TPS). I replaced both but that did not fix it. I also read that this can be caused by a poor electrical ground at the thermostat housing. I cleaned that ground terminal and the main battery/engine ground then added a 12 GA strap between the two but that did not fix it. The batytery is about 4 months old.


I am unemployed and cannot afford to keep replacing parts just to see which one is faulty, and I sure cannot afford the dealer service center. I am suspecting it is an ignition advance issue rather than fuel but at this point I am just guessing. I hope someone here can offer some good guidance.

Thanks in advance.

go-part

Have you checked your spark plugs, wires and distributor?
The next possibility could be your timing is off, see if it jumped a few teeth.

D0UG

I have not checked the timing yet; however, it would not run so well cold if it jumped a few teeth. I have never heard of a bad spark plug causing a V8 to backfire and stumble like this. Going down hwy at 60 MPH, then open the throttle half open to pass, the truck will backfire and stumble down to less than 30 MPH.

SImpleManFore

Defiantly Check Timing And Wires For Hole Were It Might Be Arching Not To Make You Sound Dumb But Make Sure Firing Right Order If You Replaced Plugs Ive been There Done That Also Check Harmonic Balancer

kevinkpk

How old are the wires (plug)? Could be insulation breaking down when hot. When it starts acting up, I'd suggest with a broom handle or something wood, pull the plug wires away from the engine. If the insulation is breaking down that will tell you if the spark is going to ground. Seen this issue before. Pull up the plug wires with it running, and the engine will settle down.

D0UG

The spark plugs, wires, distributor cap, and rotor were replaced about 6 years ago. The truck has been driven less than 3 thousand miles per year.

I do not have a dynometer so no way I can open the throttle 1/2 way and move the wires whiel the truck is parked. The engine would over-rev and float the valves.

kevinkpk

You don't need a dyno, I'd suspect still plug wires. Drive it around till it starts to miss, take it home, then check for wires. Six years old on what, a cheap set of plug wires, good luck.

D0UG

Well if it does turn out to be the spark plug wires, I will just buy a cheap set this time. The Accel SuperStock Spiral 8mm Wire Set cost me over $80 several years ago. Cheap sets last longer than 5 years.

crotchety old man

What you are describing is a lean pop. For some reason when an engine has a lean condition and you try to accelerate it, it will pop back through the intake.The fact that it runs fine cold and starts causing problems hot would indicate that for some reason the system is going into a lean condition warm.

A couple of things to try that don't cost money!! Drive the truck until it reaches the engine temp where it starts to pop. Remove the air cleaner and shine a flashlight across the bottom of the injectors, this will highlight the fuel spray. Sharply accelerate the engine and observe the fuel spray, careful because if it is a lean issue this could cause the lean pop which will probably blow a flame out of the throttle body so don't have any body parts in the way. When accelerating the engine you should see a noticible increase in the amount of the fuel spray, if not you have a problem but what is the question. While you are in there check the connectors on the injectors as they were quite problematic on the TBI. You did say that you replaced the engine temp sensor, not to question your abilities but are you certain that you replaced the temp sensor for the computer and not the one for the temp gauge or idiot light? They are not generally the same sensor.

Second thing to try and I do not know if this works on a chevy. I found out by accident when helping a friend with a turbocharged 4 cyl mustang that by disconnecting the O2 sensor it put the computer into the open loop mode which puts all the parameters into preset factory settings. The Mustang would run fine cold but when the O2 sensor would warm up and put the system into closed loop it would instantly start blowing black smoke and running terrible. As soon as we disconnected the O2 sensor putting the computer back into open loop mode it ran fine.

The bad news was that the problem was in the computer. We switched it out with a known good unit and fixed it.

Good news is if disconnecting the O2 sensor malkes it run it you have now probably narrowed the problem down to fuel as opposed to any ignition problems. I say probably because the injectors do trigger off of the distributor module, but the fuel flow is controlled by the computer.

Just out of curiosity how much fuel is in the tank? I once had an '89 with a bad tank. The baffle that was supposed to keep the fuel from sloshing away from the fuel pickup was broken loose and with a low tank sharp acceleration would kill the engine. I do not think it is the problem as yours seems to be temp related and mine was not but threw it out anyway, you just never know.

Hopefully something here can get you pointed in the right direction.

Best of luck to you