Question about Chevrolet S 10

91 S10 rough idle, stalling ALDL problems


My 91 S10 2.5L 5 speed 2wd pickup isn't running correctly. Idles erratically, eventually dies, engine light comes and goes. Loses power on the road off and on. I've replaced the tps, water temp sensor for ecm, plugs, cap,rotor, air filter, oil and oil filter and fuel pump. O2 sensor, air temp sensor and ignition module test good. Jumping the A-B terminals on the ALDL fails to flash any codes. The engine light will only flash once then stay on permanent when jumping the ALDL. I connected a timing light to check the timing and noticed that it seems like it skips firing every now and then, corresponding to the rough idling. I can't get computer codes so I'm hunting and pecking. Could my ecm be bad? magnetic pickup? wiring?

  • lboy1971 I forgot to mention that I did remove the EGR and cleaned all the soot from it. It wasn't too bad. I replaced it and could feel the diaphram going up and down as I reved the engine up and down. I did spray some carb cleaner around the intake manifold, the egr valve, base of the TBI and along the vac lines to the brake cylinder and any other vacuum lines I could see and didn't notice any change in the engine, other than the normal problem it already has. When I pulled a vac line from on the intake manifold below the TBI or from the brake booster the engine would rev up.

    Dec 17, 2009

2 Answers


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  • on Dec 17, 2009
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Answer # 1

Make sure there is no vacuum leaks.

Vacuum leak at intake manifold and vacuum hose.

Bad Idle Air Control Motor (IAC)

Low compression (170 psi + is OK)

Start cleaning and repair these trouble spots and see is there any improvement.


For more advance DIY'er then you may try this.

Use a can of Berryman carburetor clean to locate the vacuum leak.


This is a 20-30 minutes job.

Vacuum leaks can occur in following locations.

Intake manifold AND/OR vacuum lines.

Intake manifold is not accessible unless the engine head if off.

You can still check the potential leak by spraying some Berryman near the intake manifold.

Listen for rpm increase after you introduce the Berryman (FUEL) at idle.

Do the same starting from the brake booster unit (the round unit) on the drive side fire all.

Carefully listen for increase of RPM right after to spray the Berryman to the potential vacuum spot.

Get a friend to HEAR the RPM increase or monitor for RPM on the tech.

DO NOT AIM the Berryman into any heat source.

PLEASE RATE my answer if it is useful to you.

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Answer # 2

The only thing I didn`t hear you say was the egr valve.If this is stuck open it will run just like you are explaining.You could also have a bad ecm .If possible go to the salvage yard and get a used one.Just remember to swap the prom.Hope this helps.Good luck.

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  • Salvador Quintanilla If the engine is idling and then you rev it up the egr should not be moving .The only time the egr should move is when driving and you give it gas.I believe there is also an egr solenoid on these that can go bad.The vacuum lines will go from the tbi to the solenoid to the egr.It will also have an electrical connector with 2 wires.

    Dec 17, 2009

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