Hi everyone, I have been lurking for a while here and this is a great forum!

Okay, so I have a '95 EX Coupe, 1.6l, 5spd., and I have the dreaded fluctuating idle problem. I have, one at a time, disconnected the O2 sensor on the exhaust manifold, brake booster (plugged the port), fuel pressure regulator (plugged the port), and idle air control valve, but still get the fluctuation. It fluctuates from ~1,000 to ~1,250rpm, up and down over the course of a few seconds for each interval. I have no vacuum leaks as far as I can tell. I changed the PCV valve and this helped greatly (it used to rev from ~1,100 to ~1,500rpm before this), but did not cure the problem. Incidentally, when driving at a high steady speed (like driving on the freeway) with the engine cold ONLY, the CEL light comes on and gives a 'fuel pressure' code. I don't know if this is a related problem or not. Here is the weird part: if the idle is fluctuating, sometimes I can tap the gas pedal, bringing the rpm up to 1500-2000rpm and a few seconds after coming back down the idle will return to a steady ~700rpm or so and not fluctuate anymore. I can drive the car for a few blocks and then the fluctuation will return. Tapping the gas pedal doesn't always stop the fluctuating idle. I'd say it works about 65% of the time. Would this be the throttle position sensor? I haven't had time to test it yet. Any ideas? Thanks for any help!

 

Don't rule out vacuum leaks yet, because they can be very pesky and tough to pinpoint. But if the idle does return to a normal steady speed occasionally, you may not have a vacuum leak. Usually low fuel pressure won't increase idle speed, but lower it and cause a rough idle. Before I make any assumptions, the best thing you can do is find the correct OBD scanner that can read operational data from the sensors and other engine components. Check everything, or have someone who has a trained eye try and spot something unusual. This may sound stupid, but try looking at the throttle plate while the engine is running and the idle is fluctuating to make sure it actually closes completely. Maybe there is something binding in the accelerator pedal linkage or cable that won't allow it to close all the may and that's why tapping the pedal works sometimes. Open and close the throttle by hand with the engine both off and on...maybe you'll find something.