1994 5.7 350 Twitchy RPMs when stopped

[94Bobby]

Hello Everyone,

I have been battling this problem for quite some time now, I have a 1994 Sierra C1500 with a new motor (less than 15,000 miles). I’m having this issue where the RPMs will rev randomly, most of the time it’s quick and not noticeable like in the video, but other times the truck will jump forward. When I start the truck as well it will have a rhythmic up and down movement as it starts to move down to regular idle. I don’t have an high idle, it usual ranges from 700-800. I have replaced all the know sensors, spark plug, wires, distributor rotor, and distributor cap. I have not replaced the MAP or O2 sensor. I have also rebuilt the TBI with a kit from Oreilys. The engine also doesn’t feel like it’s at its full power, sometimes it will be, but other times it feels lazy almost. Any advice would be appreciated and I would like to solve this problem to help others who may have something similar.

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[movietvet]

More Information for DELPHI CV10033

Check this idle air control valve. When you remove it, there may be a bunch of carbon build up where it mounts and also in the passage it works in. Was that addressed during the TBI rebuild?

When you say all known sensors, did you replace the coolant temp sensor? Not the sender for the gauge but the sensor for the computer.

When you say new motor, are you also saying that when this problem happens, there is no CEL? I would do a compression test, with all plugs out when you do it. You could have a valve seat problem. You can clean the MAP with cleaner from the parts store and see what happens. Also, IMO, new engine means new O2 sensors.

Hell, I almost forgot, have you done a fuel pressure test from a cold start to warm and while driving? Can get a long hose and run out back of hood and tape gauge to w/s.

 

[GoToGuy]

Truck moves up and down or the idle surges up and down. At a stop in drive the idle jumps up causing the truck to lurch forward ?
It's a little hard to understand.

 

[DerekTheGreat]

Hook a scan tool up to it and see what the Idle Air Control motor and throttle position sensor are doing. My '89's idle would surge randomly at stop lights (manual transmission). Turned out the TPS was sending erratic signals to the ECM, which was opening and closing the IAC as a result. I'd start there and then check timing. Also, flip your phone over before hitting the record button, makes for better videos. Portrait mode should be saved for portraits.

 

[94Bobby]

More Information for DELPHI CV10033

Check this idle air control valve. When you remove it, there may be a bunch of carbon build up where it mounts and also in the passage it works in. Was that addressed during the TBI rebuild?

When you say all known sensors, did you replace the coolant temp sensor? Not the sender for the gauge but the sensor for the computer.

When you say new motor, are you also saying that when this problem happens, there is no CEL? I would do a compression test, with all plugs out when you do it. You could have a valve seat problem. You can clean the MAP with cleaner from the parts store and see what happens. Also, IMO, new engine means new O2 sensors.

Hell, I almost forgot, have you done a fuel pressure test from a cold start to warm and while driving? Can get a long hose and run out back of hood and tape gauge to w/s.

Taking out the IAC to clean/look at it is something I have not done yet. I replaced it when I got the new motor since the mechanic said my old one was trashed, which it was. I have also replaced the CTS that on the top of the motor which goes to the computer, that fixed a starting issue I had. I have not had any CEL during this whole process of figuring out why the idle would twitch/rev randomly. I thought about the fuel pressure since I know that can be an issue but will update this thread when all is done.

Hook a scan tool up to it and see what the Idle Air Control motor and throttle position sensor are doing. My '89's idle would surge randomly at stop lights (manual transmission). Turned out the TPS was sending erratic signals to the ECM, which was opening and closing the IAC as a result. I'd start there and then check timing. Also, flip your phone over before hitting the record button, makes for better videos. Portrait mode should be saved for portraits.

So what fixed the issue? Also what tool can I use to monitor this, I know the older trucks can be a little weird, I use a paper clip to read engine codes, so if I knew what scanner that would be great help. Did replacing the TPS fix the issue or fixing the IAC, please let me know. Thank you for the camera tip will make sure to do next time.

Truck moves up and down or the idle surges up and down. At a stop in drive the idle jumps up causing the truck to lurch forward ?
It's a little hard to understand.

As seen in the video its kind of just twitchy, I do not know what technical term to use to describe the issue. Please everyone continue to give suggestion or thoughts on the issues, I will do an update once I have tried different suggestions.

 

[TreeGeared]

IAC would be my first guess as well. You mentioned a new engine. 15000 miles would have been some time ago. I would check if you have any air leaks where the TB bolts to the manifold or anywhere else on the manifold for that matter. A small leak to the atmosphere would be enough to cause the erratic idle. Also check the plug wires. May be a signal issue or if they are old they could be arcing. Since we checked air and spark let's do fuel. Hook up a fuel pressure gauge and make sure you have good constant pressure at idle.

 

[94Bobby]

Small little update

I cleaned both the MAP sensor and IAC valve. The IAC valve did have some carbon deposits as did the passage it goes into. I cleaned it out throughly, but the problem still occurs. Next thing on my list is doing all check of fuel and a vacuum leak. Will continue to update.

 

[Schurkey]

I have replaced all the know sensors, spark plug, wires, distributor rotor, and distributor cap.

Replacing sensors means NOTHING if the problem is in the wire harness between the sensor and the computer.

Connect a scan tool, VERIFY EVERY SENSOR and the computer outputs--IAC, EGR, Electronic ignition advance, AIR diverter, (if equipped) etc. There are multiple threads right here in the "Engines" section of the forum providing suggestions on scan tools.

Replacing the tune-up items--cap, rotor, plug wires, spark plugs--is a good idea. Be sure the PCV system is working properly, along with the heated air intake (if equipped.)

You can clean the MAP with cleaner from the parts store and see what happens.

I've seen aerosol cleaners labeled for carburetors, for throttle bodies, and for MAF sensors.

I've never cleaned a MAP sensor.

I almost forgot, have you done a fuel pressure test from a cold start to warm and while driving? Can get a long hose and run out back of hood and tape gauge to w/s.

Fuel pressure test is a good plan.

 

[movietvet]

I guess I would have to say I agree with you. I don't even know if it could be cleaned.

 

[DerekTheGreat]

...So what fixed the issue? Also what tool can I use to monitor this, I know the older trucks can be a little weird, I use a paper clip to read engine codes, so if I knew what scanner that would be great help. Did replacing the TPS fix the issue or fixing the IAC, please let me know. Thank you for the camera tip will make sure to do next time.

As seen in the video its kind of just twitchy, I do not know what technical term to use to describe the issue. Please everyone continue to give suggestion or thoughts on the issues, I will do an update once I have tried different suggestions.

A "new" TPS from the treasure yard fixed me right up. With the engine simply idling, I saw the TPS voltage jumping about a tenth of a volt. When that happened, the IAC counts would rise. So I swapped out the existing TPS for one in my stash and it's been going fine ever since. I haven't watched the video, but I'd say my idle was twitchy.