a little rough at idle

[Guest Butch91]

hi guys;

i am having a slightly rough idle on my 1989 8-valve TC. it is idling at around 600rpm on the tach inside the car at stop sign. and at stop it just seems to be idling rough, i wouldn't say it is missing but just rough idle. any foot on the gas and it takes off fine, no roughness at all and runs smooth and just fine.

I have replaced plugs, cleaned all the injectors, new air filter, but still it seems to be idling rough.

if i have it in park and just put foot slightly on gas to get rpm to 800-900rpm then it is very smooth again.

no hesitation at all when driving away from stop sign.

Any suggestions as to what to look at next?

thanks for any suggestions and help.

[Guest LIT MUSE]

I would continue with the small replacements: fuel filter, distributor cap, PCV, plugs cables.

[ghostymosty]

Sounds like a dirty throttle body, idle air control.

[Guest Butch91]

thanks for the input; already did the fuel filter, distributor cap, pvc etc. i will take a look at the throttle body and clean out. i am not familiar with the idle air control?

can you please describe where to find it and how to clean.

much thanks for all help

[tjackmil]

Is it possible that your idle speed is too low? Manual gives allowable range of 650 to 900 RPM (measured with Diagnostic Tool, not dashboard tach). The fact that you say it smooths out when speed increases to 800-900 RPM would lead me to try to get an accurate reading on your real idle speed.

[Guest Butch91]

Thanks, tjackmil. i will get out the real tach and get a more accurate reading.

Do you know if there is any way of adjusting the idle speed RPM if in fact i am even below the 600 RPM when i get a more accurate reading?

[ghostymosty]

The IAC, idle air control valve, is attached to the throttle body and controls how much air gets through the throttle when the engine is idling. If its small opening is dirty then you limit the amount of air that gets through and your idle speed goes down.

[Guest george w]

Your idle speed is in the right ballpark. I suspect that your right side motor mount has deteriorated and that's why you're feeling the inherent idle roughness in these engines. Lift the hood and take a look at the right mount. You'll see the squareish mount core surrounded ( bolted to the inner fender structure ) by the metal support structure that connects to the engine. You should be able to just about fit your small finger in the space between the mount and the inverted "U" portion of the mount structure above it.

As far as the idle speed itself, it's not adjustable. It's maintained by the engine computer. The AIS motor would compensate of a dirty throttle body so in and of itself a midly dirty throttle body bore and balde wouldn't affect the idle speed. The AIS would simply be "opened up" by the computer to maintain speed. Cleaning the throttle body bore and blade with some TB cleaner and an old toothbrush is never a bad idea.

An easy check to see if the AIS is functioning is t simply start the car on a cold start. The engine should start and idle up to about 1200-1500 rpm for a few seconds and then slow down to below 100rpm in a couple of seconds. If it doesn;t then I would supect a sticky AIS pintle or clogged AIS passage. If you do get the "fast idle" for a few seconds and the engine then slows down then the AIS and the computer are doing their jobs correctly.

If the TB is really dirty I'd suggest taking it off the car and cleaning it on the workbench. It's easy to R & R.

These sag over time and as they do more of the engine vibration at idles passes through to the body. Getting the rev's up a bit just smooths out the engine naturally.

[Guest Butch91]

thank you very much guys for the recommendations, i will look at all options.

Yes, during cold start the engine does indeed start out at the 1200-1500rpm and then come down to the typieca idle rpm, in my case around 600 rpn on the dash tach.

I will take a good look at the motor mount, i did not think of that possibility at all.

I will be replacing the 4 sway bar bushings this weekend so it will be a good time to look at the motor mounts and clean the throttle body etc.

thanks again guys i really appreciate your help and recommendations.

[Guest LIT MUSE]

I was wondering if one of you guys can add a picture to show the idle air control valve, where and how to clean the throttle body. Thank you!

[Guest LIT MUSE]

I invested today in cleaning my throttle body, bought the spray disassembled the connecting hose and clean it real good. The rough start went away, the idle is stady. No engine cuts so far. Feels good.

[Guest Butch91]

guys;

i finally got the part to replace the motor mount on the right, passenger side, of the 1989 8-valve. George; you were right, the old one was shot, the engine slumping down about half to 3/4 of an inch compared to the new one.

2 more questions

1 -- is there? and if so, where, is it located a motor mount on the drivers side of the car? it was getting too dark and i could not see much last night, if there is one i might as well replace that one as well?

2 -- the motor mount on the passenger side also has a small 'shock' or stabalizer or damper type mechanism which i had to take off of the original motor mount and reposition on the new motor mount.

does anyone know where to get a replacement 'shock' or damper that goes onto the motor mount for this application? anyone know the part number for this small 'shock'?? And/or where to order one??

thanks again to all; for all your help and suggestions