Rough Idle, Runs Fine at higher RPM

This problem has been getting worse over the last 2 years. When the car starts it idles fine at about 1000 RPM. As soon as it warms up, it starts to idle rough, between 500 and 750 RPM. It almost quits running. As soon as you give the car some gas it runs just fine. About 3 weeks ago I got a “Engine Lean” check engine light error. I reset the system and have not gotten the error again. Since then I have:

changed the MAF

cleaned the EGR (made sure it closes completely and ohm out the coils in the actuator)

changed the O2 meter on the exhaust manifold

cleaned out the throttle body

Ohmed out the air idle controller.

Any idea on what I should check next. I was planning to check the fuel pressure in case it is a fuel system problem. Any help would be greatly appreciated.

It is a 1999

You didn’t tell us if it’s the 4 cylinder or v6 so I’m gonna assume the 4. Most common cause for a P0171 which I bet is your lean code is bad PCV hose or leaking intake.

Checking the fuel pressure is never a bad idea.

The first thing I’d be looking for is a vacuum leak - anywhere in the air intake, intake manifold or vacuum lines themselves. Its probably somewhat small b/c it apparently doesn’t disturb the cold idle too much (richer mixture at that point). A smoke test at a shop might be the fastest/easiest way to go, but you have to pay for those. Some people spray something like carb cleaner around vacuum connections & manifold looking for changes in idle. I like to feed propane/butane around from an unlit torch (less messy).

Yeah a lean codition is almost always caused by vacume leaks…however on most cars a vac leak would lead to high idle…but it is possible your leak doesnt affect your system te same as it does on Honda and Toyotas. Id search for a vacume leak…the old way is to spray around the intake man and assoc hoses with wd40 or similar…when the idle changes with the spray, you found the leak.


first thanks for the advice,

My mazda 626 is indeed a 4 cylinder. I measured the vacuum and it was 15 inHg steady. Is that low? Does that indicate a vacuum leak? I tried the propane trick and couldn’t find it.

any help would be greatly appreciated

What is your altitude? At something like zero to a couple thousand feet its probably low. If you’re up at 5/6,000 feet or higher 15in could be normal.

Do you have another car that’s running really well that you could compare to?