My GMC k2500 with 350 tbi will barely run after warmed up. It idles good, but misses under load and backfires. I have replaced the tbi and distributor with known good ones. All new tuneup parts. All the sensors under the hood have been replaced. I have temporarily fixed it by plugging the coolant sensor wire into a sensor that is out exposed to the air. The computer thinks the engine is cold and runs great, but I don’t want to leave it that way. Any ideas?

The reason it runs OK cold is it's in "open loop", which is to say the computer isn't running things, it's running off of per-programed parameters. What kind of trouble codes are you getting?

I forgot to mention that the check engine light has never come on and all I get is the normal cobe 12. I have tried 3 different chips. The stock one and 2 aftermarket.

I would change the oxygen sensor.

If that doesn't do it, check the fuel pressure cold and hot.

I had two s10s that did that and turned out to be coolant temp sensor. It wasn’t telling the computer the motor was warm.

Cam lobes gone flat?

quote:

I had two s10s that did that and turned out to be coolant temp sensor. It wasn’t telling the computer the motor was warm.

I'd do this.

Good indicator the sensor went south. When plugged into a reader. You will see a value on an average of -20 to -30 below. Going into closed loop it may not. Open loop is up to 600 F on exhaust temp pending all 5 Volt references are true. One other thing. On the older ones. make sure the grounds on the back of the heads are clean and tight. just a little help.... Very common issue with the coolant sensors going bad.

You said all sensors had been replaced. Does that include the one you're swapping out to the one laying on the intake? Failure of those sensors causes it to act just like you described and the one laying on the intake registers to the computer a lot warmer than a failed one, so it stops dumping furl to try to warm itself up.

The new temp sensor is installed in the intake where it belongs. Since the new one made no difference, I figured the old one is probably good, so I use it as the one exposed to the air. I am thinking the sensors are doing their job and telling the computer to go into closed loop when the engine warms up. Then for some reason the computer is causing the tbi to go too lean. Could it possibly be something in the computer other than the chip?

If it runs fine cold, IE: Open Loop, then most likely you have fuel pressure, the coolant temp sensor is working since it starts out fine, then as it warms up the ECM goes into Closed Loop and runs like crap, but if you plug the external coolant temp sensor in and lay it on the intake it goes Open Loop and runs fine again.....

Have you changed the oxygen sensor?

Since you don't have a MAF sensor you don't need to worry about that, but it can cause the same issues for the cars with TPI.

You TBI system is very simple as you see, very few sensors.

I also had issues with my 96 Chevy 4x4 with 350 right when I was ready for new truck which was probably fuel pressure regulator or sensor which ever it is that’s located in the spider but I replaced the whole injection spider and it ran great. I didn’t want to take top half of I take off twice.

Mine sounded like it was hitting rev limiter if I remember correctly when I stomped on it.

The O2 sensor is new. It seems strange that the engine can run lean enough to miss, yet not turn on the check engine light. One of the reasons I have kept this truck is because it’s simple to fix. This has been a stumper.

quote:

Originally posted by green1:
I also had issues with my 96 Chevy 4x4 with 350 right when I was ready for new truck which was probably fuel pressure regulator or sensor which ever it is that’s located in the spider but I replaced the whole injection spider and it ran great. I didn’t want to take top half of I take off twice.

That design was such a POS!

Pretty spectacular explosions at times though!

I know the S-10's and S-15's have a bundle of ground wires what are on one of the thermostat housing bolts and get corroded very easily. Not sure about the V-8's in the full size trucks.

I had an issue years ago, same scenario, when cold running rich enough to compensate, but the fuel filter was clogged, I know it's one of those things you might not believe but easy enough to check

Something is screwey for sure if your old coolant temp sensor sitting on the intake makes it run right. It makes the engine think it's warm, not cold - if it thought the engine was cold, it would dump more fuel in to richen the mixture and warm things up just like it does when that sensor is AFU.