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Q: Car misfires after new spark plugs and multiple replacements

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My car has a problem with misfiring. There is a new o2 sensor, as well as new spark plugs, spark plug wires, MAP sensor, ignition coils, and air filter. I have determined that the problem is not an exhaust leak and the misfire is too big to be a vacuum leak. It also has a direct ignition system, so the problem is not timing. I performed a compression test and all cylinders are normal. The spark plugs are gapped correctly. I am considering that perhaps it is a sticky valve. What is your assessment and diagnosis?

A: Hey there, thanks for writing in about your...

Hey there, thanks for writing in about your 1987 Pontiac Fiero. I have the 2.8L engine in my '87 Cavalier Z24 and I know it well. I'm guessing your Fiero has the same engine. One of the best ways to narrow down potential misfire causes is to monitor the block learn multiplier and integrator parameters with an OEM scan tool.

These are close to the same thing as modern long term fuel trim and short term fuel trim, respectively. These values can help determine if the vehicle is running rich or lean, which will help narrow down the cause of the misfire.

The stoichiometric efficiency is 128, and anything less than that would mean the vehicle is running rich. Anything more would mean that it's running lean. The integrator should vary somewhat once the vehicle is in closed loop, but should not get too far away from 128.

As far as the sticking valve goes, it may or may not be revealed during a compression test. I say this because, if the valve is closed during the test, but sticks intermittently, the compression readings will be OK. The next step would be to perform a leakdown test, which is more successful in pinpointing problems like a sticking valve.

There are too many things to list on a old car like this that could cause a misfire. My best advice is to have a professional, such as one from YourMechanic, diagnose and repair the misfire issue for you.

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