Rough idle...

Guy Montaquila

Registered User
Hi everyone,
My '90 SC normally idles at 700 to 800 RPM, but when I turn on the AC, it drops to about 600 RPM and I get a bit of vibration in the shifter and the steering wheel. Does anyone have any ideas? Thanks for any input.
 
Hi, Guy.

My first thought to share is, ..are you ready?..., motor mounts. My 35th idled really bad soon after I bought it and was worse when I turned on the A/C. Long story short, I read about motor mounts sagging and causing all sorts of problems. I had them changed, to solids by the way, and the car was 300% smoother. I had to check the tach driving home to make sure the car was still running. Vacuum leaks at the SC tubes can be a source as well but if you have a lot of miles on the car, mounts would be a good idea to replace anyway. Just my .02. Good luck to you.
 
Same problemo here. A/C used to cause the car to die.. no problem after new engine mounts installed. Makes no sense so far, but there must be a reason and we'll find it eventually.
:)
 
Ditto. When my motor mount went, it felt like a solid mounted 4 cyl when you put it into reverse. I thought it was major!
It idled poorly and shook. After that was fixed it was fine.
 
Hey everyone, I had the motor mounts, tranny mount and rear diff bushings replaced yesterday. The mounts were in sad shape. They litterally fell apart. My bird rides much better now, but I still have an little idle problem. This morning she was idling at 1000 RPM after it warmed up and then it fluctuated from 1000 to 650 RPM. I still get a vibration when I turn the AC on. Could it be the bypass valve? Any advice.
 
Disconnect your negative battery cable and leave it for a few minutes. Then reconnect it, turn off all accesories, such as the radio, heater etc.
Start the car and let it idle for one minute, so the computer can relearn the idle procedure.
The EEC computer will gradually learn and collect data as you start/stop the engine and drive. Give it a couple of days for it to work the bugs out of itself. It may have been confused because the broken mounts would send virbration to the knock sensor located in the block and retard the ignition timing.
Now that your mounts are fixed and the fake "knock" is gone, the EEC should have the idle re learned.
Hope this helps.
 
I agree with Katoom. When the motor mounts go bad, there is metal to metal contact in the mount, the mount travel limiter, oil pan, and other places. This fakes the knock sensor which causes the ECM to retard the ignition timing, resulting in low idle and stalling especially with the A/C on and in reverse. You should feel additional vibration at idle in gear with your foot on the brake.

Check the mounts limiters -- the metal tang going thru the square hole -- to see if there is contact. If there is, change mounts.
 
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rough idle

check your plugs, if indications are its running rich, check for vac leaks on i/c tubes. also check all vac lines and tubes just to make sure. had to replace all mine and the i/c tube seals to get it to idle ok. just have to put motorcraft plugs back in. It hates bosch!!!!
 
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