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carguyjosh, ASE Certified Technician
Category: Car
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Experience:  16 yrs. in professional shop,,ASE auto technician,service on all makes and models
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My 1998..#4 cylinder not firing. i changes spark plugs..coils

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My 1998 Z24 was running horrible... 180k miles. #4 cylinder not firing. i changes spark plugs (one was completely fowled), caps and coils. still ran rough. had a friend change out entire aluminum cowling that included coils and ignition computer from a running z24 but didn't change plugs. compression check good; removed catalytic converter (worried clogged cause exhaust was "sucking". still no luck. in fact ran worse. Checked computer. got code P0300. changed spark plugs again and fuel filter in case clogged. Started up but now idles at 3800 RPM. Haven't checked spark plugs to see if #4 cylinder is firing and can't tell if running better cause of fast idle. So to sum up as of now.....1. new spark plugs, 2. new plug caps 3. used coils 4. used coil housing 5. used ignition computer (thin computer under the aluminum cowling) 4. removed catalytic converter 5. new fuel filter. 6. now idles at 3800 RPM and can't tell if misfiring.
i'm about to put a .40 caliber slug in it and call it a day. car is worthless so not willing to spend tons of money on it....just wanted to delay the inevitable (it's my beater). Could this be a vacuum problem? What should i do next? is this an easy fix if i go to the right mechanic?

Good evening.


It sounds as though your right on the money with a vacuum leak causing your high idle condition and possibly the rough idle.

I just had your identical car in the shop with the same types of problems and found that the brake booster had failed internally and was causing the problem.

Thats not to say that a fuel injector, wiring or computer couldn't be the cause of your problem.


The first thing that I would do is pinch off the vacuum hoses that are coming off of the intake and restart the engine.

If the idle speed settles down, you know that it is indeed a vacuum leak and can then start to trace down the source...

If you are still having a high idle problem, we will need to go deeper...


As far as the last part of your question, the diagnostics should be fairly simple for the right mechanic. We are able to plug in our scan tools and watch the engine control computer and the commands that are given to help determine the source of the malfunction.


Please let me know what you find when you pinch off the vacuum hoses and we'll take it from there.

Customer: replied 6 years ago.
Wow. Sometimes it takes an expert advice to fix stupidity. While i was digging around looking for the vacuum hose, i came across a small electrical plug on the intake that had become disconnected...probably when i switched the spark plugs this last time. i forgot to mention the injectors were also changed previously. botXXXXX XXXXXne, after changing spark plugs and fuel filter tonight, it seems that it at least has power back. Seems like all 4 cylinders are firing. it's idling right at 1000 now that the connector was plugged in. however, it is still idling rough. I will drive it tomorrow. i estimate about twice the power i had prior to the fuel filter and plug change and seems almost back to "normal", any idea what the rough idle is from? i also changed O2 sensor on the exhaust 6 months ago when this all started. any other cheap, quick sensors/parts i should consider next to get idle normal vs. rough besides a vacuum check?.

Without looking at the computer with a scanner I don't want to tell you to throw parts at it. As you already know, it costs way to much.


I'm glad that we atleast now have a reasonable idle speed. You may just need a good run to clean all of the carbon out of the engine that was caused by the high uncontrollable idle speed..


If your rough idle continues, it may be worth having a shop take a look with a scanner to verify all of the computer perimeters...


Please let me know how she does.,.,Thanks, Josh

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