Low idle problem on my '89.
Thread Starter
Pro
Joined: Jun 1999
Posts: 525
Likes: 0
From: San Jose, Ca
Low idle problem on my '89.
Hey guys,
I've been noticing my idle dropping of intermitantly. Most of the time it's at 600rpm solid. Every once and a while it will drop tp 500 RMP and stay there for the duration of the trip. It started about 4 months ago
So if it starts in the "low idle" mode, it will keep doing it usually until I stop the car and let it sit for a long period of time. Then when I go back and fire it up it's fine again.
There is no surging or missing from what I can tell. It just drops off and stays there for a while. This is what I've done in the last year.
Plugs, wires, cap rotor....last month
TPS adjustment ....last month
New injectors...............1 year ago
Cleaned throttle boby.......1 year ago
Replaced IAC, TPS, ECM...1 year ago (I was having a surging idle last year ended up being the injectors)
Anyone got any ideas?
Thanks,
Christ
I've been noticing my idle dropping of intermitantly. Most of the time it's at 600rpm solid. Every once and a while it will drop tp 500 RMP and stay there for the duration of the trip. It started about 4 months ago
So if it starts in the "low idle" mode, it will keep doing it usually until I stop the car and let it sit for a long period of time. Then when I go back and fire it up it's fine again.
There is no surging or missing from what I can tell. It just drops off and stays there for a while. This is what I've done in the last year.
Plugs, wires, cap rotor....last month
TPS adjustment ....last month
New injectors...............1 year ago
Cleaned throttle boby.......1 year ago
Replaced IAC, TPS, ECM...1 year ago (I was having a surging idle last year ended up being the injectors)
Anyone got any ideas?
Thanks,
Christ
Burning Brakes
Joined: Nov 2000
Posts: 855
Likes: 0
just a thought but try scanning the car if you can and check to make sure the coolent temps are being accurately measured. A faulty temp sensor can cause what you described. My '86 is doing this same thing, but runs fine once the enging is warmed up. It's just another thing I added to list of many needed repairs.
Thread Starter
Pro
Joined: Jun 1999
Posts: 525
Likes: 0
From: San Jose, Ca
Thanks Rick, I checked the temp sensor, seems to be right on the money.
Could it be a voltage problem? If the voltage in the system drops, would that in turn drop the voltage to the TPS causing the idle to drop?
I have noticed that the voltage seems to swing at times dropping to 12.5 then it slowly makes it's way back up to 13.6-14. ( drop occurs typically when the car comes to a stop)
Could it be a voltage problem? If the voltage in the system drops, would that in turn drop the voltage to the TPS causing the idle to drop?
I have noticed that the voltage seems to swing at times dropping to 12.5 then it slowly makes it's way back up to 13.6-14. ( drop occurs typically when the car comes to a stop)
Last edited by Black'69; Dec 29, 2005 at 12:17 PM. Reason: Additional info
Le Mans Master
Joined: May 2000
Posts: 7,033
Likes: 12
Your TPS only sees 5v max. If you are dropping down to 12.5v that is showing your battery to be in a state of discharge. Perhaps your alternator is getting ready to take a dump. I don't think it would be the first (millionth) to do so. 
Try turning your air on once it drops and see what happens to the idle.
Try turning your air on once it drops and see what happens to the idle.
Le Mans Master
Joined: Dec 2000
Posts: 7,839
Likes: 22
From: San Diego, Ca
If I had to guess, I'd say your fan is coming on as you roll to a stop and zapping the life out of it. If the aux fan is coming on too, that's another probability (with a clean cooling system, the aux fan should rarely if ever come on). A scanner will show you ECM voltage and injector pulse width, though an ordinary Auto Xray is probably not going to be fast enough to pick it up. Essentially, the voltage drops as the fan turns on and the ECM fattens up the injector on time to compensate. You get an overly rich mixture dragging down your idle. I would want to scan it, monitor voltage, pulse width before and after it drops, and Block Learn. That's not to say that your alternator isn't the culprit, but I'd want to look at the fan first. You might even be able to duplicate it in your garage by turning on the a/c and monitoring voltage as the fan cycles on and off with pressure (or with the a/c off and with the engine temp below 226 disconnect and reconnect the fan switch on the high side line a couple of times and see where the voltage goes - watch for a spike too).
