Question95 New Yorker, 3.5L, auto,has rough idle. Power balance test indicates that no.4 cylinder is weak. Driving is ok, no missfire under heavy accelertation. Compresion are good .Wires , spark plugs and injector of No.4 cy have been changed.I suspect vancuum leak , but I can't confirm it .the problemm still remains .
thanks!
AnswerHi Derek,
I am not familiar with the power balance test but in any case it appears that you have covered many of the possible reasons for a rough idle. I have a couple of ideas:
Have you checked for any fault codes that might have been recognized by the engine controller? If not here is a brief instruction:
The most useful thing to do would be to try to get the fault codes that are likely stored in the engine controller memory to readout. Try using the ignition key: turn it "on-off-on-off-on" and leave it "on" (doing this quickly, no longer than 5 seconds). Then watch the 'check engine' light to begin flashing, then pause, flashing, pause, etc. Count the number of flashes before each pause and keep track of the numbers. Repeat the readout and verify the counts are correct. Then group them in pairs in the order that they came out, thus forming two digit numbers. You may notice that the pause is shorter between the digits of a given number, and longer between the numbers themselves. Then send me a 'follow-up' question telling me the results of your readout. By the way, 55 will be the last number (two groups of 5 flashes each) and that is the code for "end of readout".
There is an essay on fault codes at the site:
http://www.allpar.com/fix/codes.html
which gives the meaning of the code numbers. But then you need to get specific info for what exactly might be the diagnostic tests or parts to replace to complete the repair. So write back if you get a code that you need some help dealing with.
The other idea is that you may have an egr valve that is not fully closed at idle, which is a good candidate for causing rough idle. Have you tried taking if off and checking it for excessive build up of gunk that is preventing it from closing fully?
The vacuum leak is a little more doubtful with this feedback O2 sensor type of engine controller. Usually a leak will cause an increase in idle rpm rather than a rough idle which was certainly the case in the "good old days" of carburetors and injector systems without the O2 sensor capability. It would have to be a local leak in the manifold right around the #4 cylinder to cause a rough idle so you might check to see if the nuts are tight in that area, and maybe try spraying a bit of starter fluid/carb cleaner in that area to see if the idle smoothes out when you do so which would be an indication of a localized leak. But don't spray very much to avoid risk of a fire.
Let me know if any of these ideas are helpful.
Roland
