'93 1500 Runnin a little rough

[Charlie LT]

When I start out in the morning, my '93 1500 5.7 5-spd manual runs perfect... nice and smooth. Couldn't be better!

After it warms up a little (about 5 minutes), there is a detectable miss that comes and goes in pulses. It's rough for a few seconds, then smooth for a few seconds, then rough again. It's usually detectable between 1100 to 1900 rpms... and even more detectable in 1st, 2nd, and 3rd gear due to torque... being a standard and in a lower gear, the roughness causes the truck to get kinda jerky. So far, I have not noticed this problem if I take it over 2000 rpms. The problem does not seem to be detectable if I'm just sitting there in the driveway... only really notice it while driving.

So far, I've changed the usual items... plugs, wires, rotor, cap.

My shop looked it over and told me they were unable to localize the problem... and they did not want to "guess" what "might" be failing, and start changing other parts at random... which would run up a bill and possibly not fix the issue. As far as they can tell, all systems are operating as they should.

The dash readings all seem fine... and the check engine light goes off right after the engine starts. Remember... it starts out nice and smooth first thing in the morning and the issue shows up after about 5 minutes of running.

Any ideas?

[2000CAYukon]

Originally Posted by Charlie LT

When I start out in the morning, my '93 1500 5.7 5-spd manual runs perfect... nice and smooth. Couldn't be better!

After it warms up a little (about 5 minutes), there is a detectable miss that comes and goes in pulses. It's rough for a few seconds, then smooth for a few seconds, then rough again. It's usually detectable between 1100 to 1900 rpms... and even more detectable in 1st, 2nd, and 3rd gear due to torque... being a standard and in a lower gear, the roughness causes the truck to get kinda jerky. So far, I have not noticed this problem if I take it over 2000 rpms. The problem does not seem to be detectable if I'm just sitting there in the driveway... only really notice it while driving.

So far, I've changed the usual items... plugs, wires, rotor, cap.

My shop looked it over and told me they were unable to localize the problem... and they did not want to "guess" what "might" be failing, and start changing other parts at random... which would run up a bill and possibly not fix the issue. As far as they can tell, all systems are operating as they should.

The dash readings all seem fine... and the check engine light goes off right after the engine starts. Remember... it starts out nice and smooth first thing in the morning and the issue shows up after about 5 minutes of running.

Any ideas?

I have a 90 GMC K1500 5.7 700R4 that I have owned since new. Mine developed the surging idle a while ago and I was determined to find the cause. I also noticed that with a timing light on it, the timing would jump around at idle when warm. At one point, I thought my timing light was broken and bought a new one. For me the problem would get worse the longer I drove it. If you read this forum (and others) as much as I do, you will see that this is a fairly common issue with TBI trucks now that they are getting up there in age. I replaced some parts just due to age and others I guessed were the problem. For me, the issue turned out to be vacuum leaks at the PCV gromet and the hose to the MAP sensor.

Since your issue starts after it warms up, the problem shows up when the engine goes into closed loop mode (i.e. it looks at O2, MAP and TPS to determine the amount of fuel to deliver to the engine). Looking at a scanner when the idle problem occurs will really help pinpoint your issue. It could be IAC, EGR, EGR Solenoid, pickup coil (inside dist), MAP sensor, 02 sensor, timing chain, dist drive gear, leaking injector, leaking fuel pressure regulator, vacuum leak, engine coolant temp sensor, exhaust backpressure, intake gaskets, etc

Again, I highly recommend getting a scanner hooked up to it so you can see if it thinks the engine is running lean or rich, what temp the engine running at, and other sensor values.

The MAP and EGR Solenoid (TBI side) should hold vacuum. Check for loose rubber ends on EGR and MAP vacuum lines.

Good luck.

//2000CAYukon