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I have a 95 Nissan Pathfinder with 83K miles that is idling rough and seems to be missing. When the car is running above 2000 RPM everything is fine. However, the car misses at low RPMs and lower speeds. It will not pass an emissions test at this time. The hydrocarbon reading is 900 at idle speed and the max allowed is 220. The car is not throwing any codes through the ECU. I have replaced the rotor, distributor cap, spark plugs, wires and PCV valve. In addition I have run every type of engine cleaner through it. I also had a Nissan dealership look at it and they told me that the Oxygen Sensor and Air Flow Meter had the incorrect voltage but they weren't convinced that was the problem. They said that a compression test was needed to further troubleshoot. The cylinders ranged from 140 for a high to 100 as a low. Most cylinders were between 110 and 125. Some mechanic I have talked with seem to think that it is a valve problem while the Nissan dealership says an engine will run fine with a compression of 90. I have ordered an Oxygen Sensor and an Air Flow Meter and I hope that will fix the problem. However, I occasionally have no brake pressure when I first start the car. I have read that a vacuum leak causes the car to run lean but mine is running rich. Is there a possibility that I have a vacuum leak somewhere? What is the best way to detect a vacuum leak?

If it is a vacuum leak then you need to isolate the cylinder that is missing at idle! You can do this by removing one spark plug wire at a time to see which one does not affect the idle speed. Once you find a cylinder that does not make any difference when you remove the plug wire. You need to pull the spark plug and look at it very closely. Is the tip of the spark plug look brand new and clean? Or does it look black and sooty? If it looks "brand new looking" then you MAY have vacuum leak or maybe a burnt valve. If it black and sooty then you are running too rich and may have a leaky injector or just a bad spark plug. To see if the intake gasket is leaking carefully spray WD 40 around the intake gasket area to see if the seems to “smooth out”. If it does seem to make a difference then a vacuum hose or intake gasket may be at fault. This may be a job for you to seek professional help. Paying someone that will guarantee their work will save you time and more money down the road.

 

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