How-To: Troubleshooting Idle Problems

i'll get straight to the point:

after either a cold or warm start, when in park, the car idles at about 1100 to 1200. if i wait a while sometimes it will go down to a nice 800ish, and sometimes it will just stay high (under the same conditions). if i don't wait and shift into drive, it shifts roughly. either way, when i shift into drive it'll be at 700 to 800. great. when i come to my first stop, and every stop thereafter, the rpms hover at 500 and the car kinda feels like it wants to stall, but it never does. it's subtle but i can definitely notice it. when i leave the light or stop sign, if i press on the gas very lightly the feeling is no longer subtle and you can tell the car REALLY wants to stall. but it never does! (the car has actually stalled ONCE when starting it. almost immediately after i stopped turning the key. not sure what that was about, but that might not be related)

so,

when in park it idles high and shifts roughly into drive.
when in drive and stopped it idles very low and acts like it wants to stall.

p.s. the car passed inspection yesterday with my mechanic only telling me i will need rear brakes soon. plugs and wires were changed a month ago, along with the fuel filter and cap/rotor. transmission fluid is clean and leveled out. this is the ONLY non-cosmetic issue my s12 has at the moment and i would really like to get this fixed.

please help me out! ask me any questions and i will give you an answer. i think i can describe my problem(s) pretty well.

If your car only runs like that when the brake pressed in I would say to check vacuum lines. Your brake booster uses the vacuum lines from your motor so that is my guess. Hope this might help.

I agree about the vacuum lines, also might want to try high octane gas..

SprockCapnIf your car only runs like that when the brake pressed in I would say to check vacuum lines. Your brake booster uses the vacuum lines from your motor so that is my guess. Hope this might help.

soapdudeI agree about the vacuum lines, also might want to try high octane gas..

I had a real rough idle, similar to yours, maybe worse, but again never stalled.

I finally go around to replacing the spark plugs and the spark plug wires...and that brought the idle up significantly.

Those plugs anhd wires were next to cooked though...unless you've replaced those in the past few years that might be a good option.

Gap your plugs properly as well.

I had a disgustingly rough idle, make sure the plugs and order of the wires on the distributor are all proper as mis-firing in the wrong order is never a good thing.

Another thing I'd check for is the condition of the MAF sensor.

Also, any idea how old your ignition coils are?

i'm sure the order the wires are in is correct, but i guess you never really know if you don't do it yourself. i'll look. and i'll check out the MAF sensor as well. also, i'll bet the ignition coils are about ready to go. i briefly considered doing the coil-mod when i first found this website.

i'll check those things. thanks, lupich.

keep 'em coming, guys.

No problem, I plan on doing the dual-coil mod when I have some more knowledge of the S12 heh.

noticed when i left for work this morning: if i let the car sit for a few minutes after starting it, leaving it in park, the RPMs will drop to 500ish just like they sometimes will when i'm stopped. and the car doesn't seem to idle too high lately. at least not for long. which is normal as far as i know. so this should narrow it to vacuum, right? i wish more people would reply to this thread.

Unfortunately there is many things that could cause a rough idle. If possible I'd suggest just replacing vacuum tubes anyways as you certainly wouldn't regret it later.

a tin pan alleythat's two for vacuum.

4 for vacum

I agree with everyone also try some better fuel in her and see if a few miles she might clean herself out.

a tin pan alleyalso, my manual (original that came with the car!) says nothing lower than 91. gas stations here only sell 87, 89, and 93 (i rarely see 91, and SOMETIMES i see 92), so it always gets 93.

a rough idle can be caused by a multitude of things. A low idle speed can also be caused by alot of things, but make sure you do not confuse the two!

What i see is an erratic idle. the hard shifting? well, thats easy. your transmission is applying the bands (or whatever..) at a higher speed than normal (750 +/- 100 rpms). its a quicker, harsher engagement, so you get a lot of feedback.

The low idle, high idle thing. The VG has a picky ass IACV (Idle air control Valve). Its bolted to the passenger front area of the upper intake manifold. There are two adjustments that can be made. Loaded idle (AC, headlights, blower motor, etc). and unloaded idle (idling in park, minimal loads on).

So lets figure out wtf is happening. If you start the car, let it run for about one minute. it should idle a bit higher (1000ish is ok), but then creep down. WITH NO LOADS on, what is your idle at? it should be around 750. if its too high, or to low, you need to adjust it. loaded idle can be checked the same way, but make sure your headlamps are on, and that you have your unloaded idle correct first.

before you go adjusting your idle, check over some things..

Fuel filter - is it good? when did you replace it last?
Air filter - ditto.
throttle body - is it CLEAN? some carb cleaner & a toothbrush will work wonders.
MAF - is it clean & is the screen free of holes/obstructions? use ONLY MAF cleaner. otherwise you'll jack up the maf.
Ignition system - ok, you've replaced plugs and wires. cap n rotor as well i hope? what about you timing? iirc you need to be at 20* bdtc for stock, but closer to 25* works well.
egr valve is it stuck open a bit? yeah, that'll mess up alot.
TPS is it set properly? the TPS on an automatic needs to have continuity at WOT, and NO throttle. manual is just no throttle. (..and yes, its adjustable).
Note: there are other things that can be bad, but these are the likely culprits.

Now, if all the above is correct, and in good working order, you can adjust the idle. Im going off memory here, but what you do, is let it idle in park (or neutral for 5spd). slowly turn the large flat screw in or out, until you get the desired effect. If nothing is happening at all when turning it, i told you the wrong screw. try the other one. but BE PATIENT. it may take a few seconds for the idle to respond to your adjustments. now, lets assume you've set the idle. and its humming right along in park. turn your lights on, where is the idle? if its lower than 1150 (check the FSM for this value, im working off memory), you need to raise it so it is above said number. You adjust it by using the OTHER screw on the IACV. DO NOT turn the screw you adjusted the first time. you'll F it all up, and have to start over. turning EITHER screw for the loaded adjustment will make the idle change. you need to be certain you are turning the right one!!

Lemme know if this helps.

one last tidbit. Premium fuel does nothing to help a poor running vehicle! the SE calls for 87 octance. use nothing else. THe octane rating is essentially how difficult it is for a fuel to burn (ie, how stable a fuel is). So, the higher the rating, the harder it is to burn. So, assume you burn 95% of your 87 octane in the combustion chamber. well, lets try it with 94 octane. you may only burn 85%. guess what happens to the unburnt portions? some heads to the EGR valve system, some heads to the convertor, and some heads BACK into the intake in the form of a black soot (carbon). too much of that carbon, and your intake wont flow like it should, and efficiency drops.

my percentages are made up, but you get the idea. If you're running boost, hell yea you want premium, because you do NOT want pre-ignition. ever. but the VG isnt boosted (here) and as such, RUN 87 OCTANE ONLY. (ok, if you live in Denver, 91 may work a bit better due to elevation issues.)

i didnt really read this, but....

to check for vacuum leaks, spray around with carb cleaner. listen for dips/rises in the idle. thats yo leak sucka.

to check for simple ignition problems, involving arcing, like bad cap/wires etc, fill an old windex bottel with some water, and mist it over the ignition system with the engine running, and look for sparks. you shouldnt see any on a properly working system.

and after scanning, something i dont see mentioned is pcv valve. if its stuck open or closed it can cause idle problems.

did anyone mention coolant temp sensor? they should be changed as perventative maintainence, not just when they go bad.

they accuracy as they get corroded inside the cooling system.

if you ecu thinks the car is hotter or colder then it is it can run funky.

again, these are all just random facts popping out of my head, if they dont apply, or have been said already, oh well.

• Car has a rough idle.
• Car will start then the idle will die quickly.
• When its cold the car will start but die shortly after.
• Car will have idle problems after sitting around a while (8+ hours)

• A loose vacuum line somewhere in the engine bay is known to cause a rough idle or an idle that dies quickly. Trace your vacuum lines and replace old crusty ones. This is NEVER a bad idea.
• Using gas with an octane rating lower than 91 has been known to cause some idle (and general) issues in cars, specifically those with higher performance engines. Use some higher octane fuel or purchase some octane booster. (Check your cars manual for the proper octane level to use.
• Fuel Injectors are old and worn, and possibly need to be replaced. Chances are if you haven't changed the injectors on your 20 year old car, nows a good time. Or if you're too cheap then try using some 'Fuel Injector Cleaner' which can be bought at any auto store.
• Check your spark plugs and spark plug wires, as well as making sure your distributor is not dirty or wet. Replacing these with new delicious spark plugs and wires is a good idea every once in a while.
• Ignition Coils are a possibility if they are old and failing, replacing these is a good idea if yours are very old. Also check out the dual coil mod.
• Your MAF (Mass Air Flow Sensor) is not plugged in right or your Air Filter is dirty as hell, you're using the wrong one, you're missing one, or your current one is broken. The MAF helps regulate the amount of fuel the car gets based on the air it gets so this is pretty important to check. A MAF sensor can be expensive in a store so eBay is a good place to look for a replacement.
• TPS (Throttle Position Sensor) is incorrectly set causing a low idle that dies out, depending on the engine in your car, there will be a small screw you can play with to increase or lower your idle right under and behind the TPS.
• Your battery is not supplying proper power. it is possible your battery is past its prime or incorrectly grounded, additionally your alternator could be sucking up the juice.