Rough and misfire goes away after driving

[mark460]

98 Suburban, Vortec, 200K miles. Plugs, cap and rotor about 6months old. Just replaced plug wires thinking it was the problem. It starts up fine and has light misfire while driving. It doesn't turn on check engine light and it typically goes away after about 30 min of driving and then runs smooth for the rest of the day. Any ideas what could be wrong? Coil maybe?

[speedygonzales]

While a coil is considered a tune up item and should be replaced, I don't think it's your problem since they usually get worse when they get hot. If it's age is unknown or it has white spots on it, I would change it. Be sure to remove the ICM and clean and re-heat sink compound it.

The only things I can think of that would make it get better is fuel(ing) related. When you start the vehicle cold, it starts out in open loop and runs slightly rich until a prearranged temperature. Then it kicks over to closed loop and begins monitoring and adjusting fuel amounts. BUT, here's the rub. It does NOT take 30 minutes to get there.

Some times the ECT sensor is bad and the PCM never knows when the engine is warm enough to go into closed loop. But those usually stay crappy. Perhaps your ECT sensor is lazy and takes long to get there.

Or you have a slightly leaky injector and it has to cut fuel back enough to make it run correctly.

What I would do is first change the ECT sensor then monitor fuel pressure after I changed the fuel filter. Check the critical fuel pressure with KOEO. Which should be 60psi min. Then check to see how fast the pressure drops. The speed at which it drops is critical. They all drop pressure slowly. Fast is an indication of a problem.

[mark460]

I replaced the ECT sensor since it was quick and easy to do. It still has the problem. Its crazy, it seriously will run crappy for 20-30 min. of driving and then it clears up and runs great for the rest of the day, even after shutting it down and driving it later. Next morning its back to stumbling and running erratic. I'll go and check the fuel press, etc like you recommended, next. Fuel mileage seems like it has dropped at times, so maybe your right about a failing injector.

[mark460]

I still haven't figured out whats wrong yet, but I have noticed when it is running erratic in the morning, it is at light throttle. If I punch the gas, it instantly clears up and runs strong. Would it be possible its an O2 sensor problem?

[Seabolt]

Originally Posted by mark460

I still haven't figured out whats wrong yet, but I have noticed when it is running erratic in the morning, it is at light throttle. If I punch the gas, it instantly clears up and runs strong. Would it be possible its an O2 sensor problem?

I'm guessing but have you verified your check eng light is working because everything that has been mentioned should throw a code in the ECM.

[Seabolt]

Originally Posted by mark460

I replaced the ECT sensor since it was quick and easy to do. It still has the problem. Its crazy, it seriously will run crappy for 20-30 min. of driving and then it clears up and runs great for the rest of the day, even after shutting it down and driving it later. Next morning its back to stumbling and running erratic. I'll go and check the fuel press, etc like you recommended, next. Fuel mileage seems like it has dropped at times, so maybe your right about a failing injector.

if it was fuel pressure I don't think it would perform well when you punch it? I'd check for a vacuum leak. You might try to take a look at the fuel injectors spray pattern first thing in the morning? I'm trying to think of things that could cause the problem without throwing a code on the ECM.

[Seabolt]

Originally Posted by mark460

I replaced the ECT sensor since it was quick and easy to do. It still has the problem. Its crazy, it seriously will run crappy for 20-30 min. of driving and then it clears up and runs great for the rest of the day, even after shutting it down and driving it later. Next morning its back to stumbling and running erratic. I'll go and check the fuel press, etc like you recommended, next. Fuel mileage seems like it has dropped at times, so maybe your right about a failing injector.

replacing the ECT or CTS (coolant temp sensor) is a good thing to do since they are pretty cheap and have a lot to do with the tbi. Had one drive me nuts for a while and found out the black wire going to the sensor was low on ohms. Electrical stuff is complicated and has been for me, but I'm getting better at using a meter with tbi problems.

[Seabolt]

You might try pulling AIC (air inlet sensor) on the tbi and see if it's gummed up? A little bit of brake kleen or carb cleaner will take the black stuff away.

[badbadleroybrown]

Are you losing any coolant?

[mark460]

Thanks for all the replies. I had a guy put it on a high dollar scanner and he noticed the cam offset for the distributor was about 14, so we set it to specs. The coil was also shorting out and I put on a new coil, so the combination of these fixes got it running good now.