What causes shaky/erratic idle?
10-05-2006, 11:12 AM
#1
What causes shaky/erratic idle? I'm asking in a more general sense than necessarily what causes it on our cars. I'm trying to track down an erratic idle on a different car and am not sure where to start. I know a vacuum leak could possibly cause it, but what else?
10-05-2006, 11:42 AM
#2
Is this car fuel injected or carburated? If carb'd, make sure it's in good working order, then check your ignition system (cap, rotor, plugs & wires). If injected, check plugs & wires, airflow (idle air controller mechanism, clean maf sensor), could be dirty/dying fuel injector(s). That'll give you a starting point.
10-05-2006, 11:50 AM
#3
It's FI. It's a 95 LeBaron. The plugs were just replaced and the old ones looked like crap. The plug wires are pretty new as well. So it's certainly running better than it did. It used to have a slight hesitation before the new plugs and I'm not sure if it had the erratic idle then but I'm sure it did. What can I use to clean an IAC or MAF sensor? I did buy some STP complete fuel system cleaner that I'm about to put in. I'm just waiting for the tank to get low first.
10-05-2006, 12:36 PM
#4
TECH Addict
Plugs the right kind and gapped correctly? (Don't trust them to come properly pre-gapped) The old ones were just worn and not caked with oil or anything, right?
Motor/tranny mounts?
Try Sea-foam, run 1/4 to 1/2 a bottle through the brake booster (or FPR) vacuum lines into the manifold. (Just noticed you got the STP stuff, that should be better than nothing)
Do the RPMs rise by themselves and/or not fall easily? That's commonly a vacuum leak. Pinch each line at the manifold and see if the behavior changes.
Clean IAT, TB, etc. Can use brake cleaner or throttle body cleaner. Make sure not to get any of that stuff on paint, it will eat it. Might as well clean EGR too while you're at it.
You might have a bad sensor that just happens to be still giving voltages in a range close enough to let the car run. Sensors just wear out over time, particularly O2 sensors and coolant temperature sensors.
Motor/tranny mounts?
Try Sea-foam, run 1/4 to 1/2 a bottle through the brake booster (or FPR) vacuum lines into the manifold. (Just noticed you got the STP stuff, that should be better than nothing)
Do the RPMs rise by themselves and/or not fall easily? That's commonly a vacuum leak. Pinch each line at the manifold and see if the behavior changes.
Clean IAT, TB, etc. Can use brake cleaner or throttle body cleaner. Make sure not to get any of that stuff on paint, it will eat it. Might as well clean EGR too while you're at it.
You might have a bad sensor that just happens to be still giving voltages in a range close enough to let the car run. Sensors just wear out over time, particularly O2 sensors and coolant temperature sensors.
10-05-2006, 01:12 PM
#5
Originally Posted by dragonrage
Plugs the right kind and gapped correctly? (Don't trust them to come properly pre-gapped) The old ones were just worn and not caked with oil or anything, right?
Motor/tranny mounts?
Try Sea-foam, run 1/4 to 1/2 a bottle through the brake booster (or FPR) vacuum lines into the manifold. (Just noticed you got the STP stuff, that should be better than nothing)
Do the RPMs rise by themselves and/or not fall easily? That's commonly a vacuum leak. Pinch each line at the manifold and see if the behavior changes.
Clean IAT, TB, etc. Can use brake cleaner or throttle body cleaner. Make sure not to get any of that stuff on paint, it will eat it. Might as well clean EGR too while you're at it.
You might have a bad sensor that just happens to be still giving voltages in a range close enough to let the car run. Sensors just wear out over time, particularly O2 sensors and coolant temperature sensors.
Motor/tranny mounts?
Try Sea-foam, run 1/4 to 1/2 a bottle through the brake booster (or FPR) vacuum lines into the manifold. (Just noticed you got the STP stuff, that should be better than nothing)
Do the RPMs rise by themselves and/or not fall easily? That's commonly a vacuum leak. Pinch each line at the manifold and see if the behavior changes.
Clean IAT, TB, etc. Can use brake cleaner or throttle body cleaner. Make sure not to get any of that stuff on paint, it will eat it. Might as well clean EGR too while you're at it.
You might have a bad sensor that just happens to be still giving voltages in a range close enough to let the car run. Sensors just wear out over time, particularly O2 sensors and coolant temperature sensors.
10-05-2006, 01:30 PM
#6
Originally Posted by ArcticZ28
I gapped each one of the new plugs myself so they are gapped correctly. The car gets great gas mileage for having 160k on it. The numbers new were 21 city and 27 highway and it gets pretty damn near that now. The rpm's don't rise and they return properly, it's just that the needle kinda shakes at idle and varies about 50 rpm up and down erratically. The car doesn't sound to be idling rough, it's just the erratic needle behavior. It has no more hesitation or anything, I was just wondering what might be goin on.
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10-07-2006, 12:12 AM
#8
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Good news is, the problem can only be related to either one or a combo of three things; air, fuel, and/or spark. Bad news is, there are TONS of things on a modern engine that can negatively effect any or all of those three catigories.
PS. Try a new fuel filter if you haven't already.
PS. Try a new fuel filter if you haven't already.
10-08-2006, 10:54 AM
#12
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Mine did too. For some reason it would find its groove and stay. I only noticed it when it idled sometimes. It didnt do it everytime it idled. Then a couple weeks later during my cam swap we pulled out the pushrods and sure enough one was bent like hell. I would check them because its very easy to pull valve covers and rocker arms.
