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C/K1500 · 10-29-2008, 07:58 PM

1. There is nothing wrong with your cat. The motor has no idea if there even is one, since it is mounted behind the O2 sensor. If it is plugged, you will lose pwer on the highway. At idle there would be no issue.

2. You have not mentioned if your injectors are spraying nicely or dribbling fuel. If they are dribbling, that could be your problem.

3. You could have an air leak at the throttle shaft. Grab it and see if there is play. If there is, see if the play is the same at different throttle openings. Most wear occurs (and the most impact to drivability) at the throttle tip-in point.

4. Make sure you ignition wires, etc. are good. SOmeone pasted an Autozone link and that shows you good ways to rule out an ignition problem.

5. Your vacuum is too low at idle. 1978 Chevys idled at 21 inches of mercury. There is no problem with your cam (unless of course it has flat lobes). Since you have no problems at higher rpm or under load, I would rule out the cam as a problem. You might still have a vacuum leak due to improperly installed manifold gaskets or a warped manifold. Since you feel that the mechanic who reinstalled the manifold gaskets was a gibroni, perhaps he did not do this job well.

6. A bad EGR valve will cause drivability problems, such as excessive pinging at speed. I have seen brand new, faulty EGR valves right out of the box. These engines need exhaust recirculation to run well.

7. A bad knock sensor will cause - your engine to knock (ping) like crazy. You have not reported that as a problem, so look elsewhere.

8. You should have a 195 degree thermostat. That is what your ECU is calibrated for. The warmer the better for driveability and economy. For drag strip performance, yes, go cooler. But not on a street car or with today's gas prices.

9. To set the timing with a timing light, you must disconnect that wire as described in a previous posting. Set the timing as far ahead as you like. It doesn't matter that much, as long as the engine does not ping once you reconnect everything and drive it. You will notice that retarding the timing tends to give a smoother idle, but the engine will have no balls. Advancing slightly, the engine idles with a bit of a 'roll' or occasional hiccup, but has more power. In general, run the most timing for the grade of gasoline that you use that does not cause the engine to ping under the type of driving you normally do.

10. A bad MAP sensor could cause problems. Did you already change yours? I believe that you did. But a MAP sensor is not the first place I would be looking as they tend to fazool things farther up the power band.

Lastly, remember the first law of auto repair shops: the problem you have can only be fixed by whatever high-margin parts that shop sells. So, if you take the truck to Meineke, it will need a new cat for whatever problem you showed up with. As so on and so forth.

Good luck, and let us know.

10-29-2008, 07:58 PM	#100
C/K1500 Registered User Join Date: Oct 2008 Location: USA Posts: 8	Re: 1991 chevy 1500 5.7 tbi erratic idle 1. There is nothing wrong with your cat. The motor has no idea if there even is one, since it is mounted behind the O2 sensor. If it is plugged, you will lose pwer on the highway. At idle there would be no issue. 2. You have not mentioned if your injectors are spraying nicely or dribbling fuel. If they are dribbling, that could be your problem. 3. You could have an air leak at the throttle shaft. Grab it and see if there is play. If there is, see if the play is the same at different throttle openings. Most wear occurs (and the most impact to drivability) at the throttle tip-in point. 4. Make sure you ignition wires, etc. are good. SOmeone pasted an Autozone link and that shows you good ways to rule out an ignition problem. 5. Your vacuum is too low at idle. 1978 Chevys idled at 21 inches of mercury. There is no problem with your cam (unless of course it has flat lobes). Since you have no problems at higher rpm or under load, I would rule out the cam as a problem. You might still have a vacuum leak due to improperly installed manifold gaskets or a warped manifold. Since you feel that the mechanic who reinstalled the manifold gaskets was a gibroni, perhaps he did not do this job well. 6. A bad EGR valve will cause drivability problems, such as excessive pinging at speed. I have seen brand new, faulty EGR valves right out of the box. These engines need exhaust recirculation to run well. 7. A bad knock sensor will cause - your engine to knock (ping) like crazy. You have not reported that as a problem, so look elsewhere. 8. You should have a 195 degree thermostat. That is what your ECU is calibrated for. The warmer the better for driveability and economy. For drag strip performance, yes, go cooler. But not on a street car or with today's gas prices. 9. To set the timing with a timing light, you must disconnect that wire as described in a previous posting. Set the timing as far ahead as you like. It doesn't matter that much, as long as the engine does not ping once you reconnect everything and drive it. You will notice that retarding the timing tends to give a smoother idle, but the engine will have no balls. Advancing slightly, the engine idles with a bit of a 'roll' or occasional hiccup, but has more power. In general, run the most timing for the grade of gasoline that you use that does not cause the engine to ping under the type of driving you normally do. 10. A bad MAP sensor could cause problems. Did you already change yours? I believe that you did. But a MAP sensor is not the first place I would be looking as they tend to fazool things farther up the power band. Lastly, remember the first law of auto repair shops: the problem you have can only be fixed by whatever high-margin parts that shop sells. So, if you take the truck to Meineke, it will need a new cat for whatever problem you showed up with. As so on and so forth. Good luck, and let us know.