Rough idle, shaking at driving speeds?

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Rough idle, shaking at driving speeds?

Postby Tazman » Mon Jun 04, 2018 5:24 am

I have a '95Z26 3100, it idles a lil rough. When I drive it around 60mph the engine shakes. If I get on the gas it stops shaking. When I try to hold a constant speed it starts to shake again. Any help or advice on this issue is greatly appreciated.

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Re: Rough idle, shaking at driving speeds?

Postby Rettax3 » Tue Jun 05, 2018 12:38 am

This could be almost anything from a bad ECU to a dirty MAF. Any check engine light? Do you have any way of reading sensor data through the OBD connector?

Since it is happening at idle, you should be able to run some tests and see the problem. A vacuum leak is the first place to look, or more accurately to listen, so poke around under the hood with your ears open. Carefully spray carburetor-cleaner around at the engine, intake boot, and any vacuum-lines (like the brake booster). I would be looking at the FPR (Fuel Pressure Regulator) too -pull the vacuum line off of it while the engine is idling rough, plug the vacuum line with your finger, and give the FPR a few moments -watch for fuel coming out of the vacuum port indicating a broken FPR. While you are looking into the fuel side, attach a test gauge to the pressure port, look for the pressure to be about 35-42 while idling, and it should rise a few PSI when you disconnect the vacuum line from the FPR. Another test while you have the tester on the engine is shut down the engine and watch the gauge. The pressure SHOULD NOT bleed off. If it does, you likely have a sticking fuel injector, bad FPR, or bad fuel pump. You could still have a bad or dirty MAF, or an injector problem. Clean the MAF and retest, sometimes even a bad MAF will work okay for a little while after a good cleaning. Injector problems can be a little harder to diagnose, but since your engine is an SFI system, you can Ohm-test each one individually -look for one or two that differ significantly from the others. The connector is towards the back of the engine, get a wiring diagram to find your color-codes for each injector, the connector is a 10 or 12 pin black plug.

You could also have a problem caused by the EGR system, like a leaking valve, a leaking pipe, or the valve opening due to a bad ECU signal. A bad catalytic converter could be getting plugged and can cause similar issues too, but you will usually have more issues while accelerating. Worn or damaged ignition wires, a bad coil, or defective ICM (Ignition Control Module) could also be the problem... I picked up a newer Malibu that had similar problems, only issue was a cracked spark plug -previous owner should have taken it to a shop... :wink:

It really just keeps going on and on. Pulling your spark-plugs and reading them to find out if the problem is one cylinder, or engine-wide could be another good direction to take. Without trouble codes or any way to read sensor data, it is really hard to tell what else it is likely to be. Good luck, keep us posted!

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