Rough idle

Good morning,

The first thing I would do is a compression test to be sure it is not an internal issue with the engine.

https://www.2carpros.com/articles/how-to-test-engine-compression

A burned valve could cause this.

https://www.2carpros.com/articles/engine-misfires-or-runs-rough

Can you eliminate t down to the one cylinder that is missing? You can do this by disconnecting an injector one by one. When the engine RPM does not change, that is the cylinder that is the problem.

Roy

Hello my name is Dave.

The first place to start would be the spark plugs as they tell a lot about the engine. If they are black then first thing I would check would be the fuel pressure regulator, it is on the fuel rail and has a red plastic vacuum line going to it (assuming it hasn't been replaced), if you hook your vacuum pump up to the regulator it should hold vacuum but before you do that check and make sure there isn't gas coming out of the port on the regulator.

I would recommend running the engine a few minutes before performing the tests, then pull the vacuum line off and see if it's wet and smells of gas. If there is any gas smell the diaphragm has a hole in it and would cause the engine to run rough, especially once warmed up. If it passes these 2 tests I would move on the Idle control valve, it is located just behind the throttle body on the side of the intake and are notorious for causing idle issues with these engines.

Another issue can be the ignition module, on your truck it is located on the driver's fender close to the master cylinder. I would recommend checking with your local parts store and see if they can test it for you, if you can make sure they test it at least 10 times since a lot of the time they only have an issue after they warm up. If they can test it I would go ahead and have it tested to eliminate it as the cause since the testing should be free.

Finally it is possible that the catalytic converter could be getting restricted, this is usually cause by an engine running rich so be sure to check the other issues I have outlined to prevent a recurrence of the problem if that proves to be (part of) the problem.

Please keep us up to date on your progress and thank you for using 2CarPros!

Good morning. Thank you for replying my post. I have done compression test, fuel pressure test, manifold vacuum test and it’s all in normal range. I have also checked spark plug but it was nice brown color. I’m not sure if my cat is clogged up or not. I do smell a lot of exhaust fumes from my tailpipe. My truck came with TFI-IV system so it doesn’t have any ignition module mounted on fender, but I have replaced entire distributor and coil including TFI module.

Did you eliminate it down to a particular cylinder and do the test I suggested?

Roy

I have not done the test yet, but the injector is right under the intake manifold, so there is no way that I can disconnect while it’s running.

Okay, then remove the spark plug wires one by one. You will need a pair of insulated pliers to keep you from getting a shock.

Roy

Will I be able to do the test by removing it with engine off and start it?

With the engine running. Remove one plug at a time until the engine pitch does not change. When that happens, that is the cylinder that is missing.

Roy

Does using in-line spark tester works as well for the test?

That is for testing spark and is not intended for disabling a cylinder while doing this cylinder balance test.

Roy