Help w/ Sudden Idle Problem
Thread Starter
Racer
Joined: Jun 2003
Posts: 308
Likes: 0
From: San Antonio TX
Help w/ Sudden Idle Problem
I need help with an idle problem that developed out of the blue yesterday. I drove the car all day and it performed perfectly. I got home from work and parked the car in the garage around 5 pm. Around 9 pm I tried to move it out of the garage and it would start, sputter at very low RPM's, then die. Same thing this morning. Once it starts, if I give it some gas and raise the RPM's it runs fine. It even drives fine, but when I stop I have to shift into nuetral and raise the RPM's to keep it from dying.
I'm in San Antonio, TX, and we've had about 12 inches of rain over the last couple of days so my first thought was something got wet or I got water in my gas somehow, but wouldn't this affect it across the entire RPM band and not just at idle?
The first thing I did was look for a vacuum leak but couldn't find any. As soon as my wife gets home from work I'm going to go get a new fuel filter and see if that helps. If not, can anyone offer any advice as to where to look next?
The car's an '89 L98 coupe, 79k miles, and stock.
Thanks in advance for help.
I'm in San Antonio, TX, and we've had about 12 inches of rain over the last couple of days so my first thought was something got wet or I got water in my gas somehow, but wouldn't this affect it across the entire RPM band and not just at idle?
The first thing I did was look for a vacuum leak but couldn't find any. As soon as my wife gets home from work I'm going to go get a new fuel filter and see if that helps. If not, can anyone offer any advice as to where to look next?
The car's an '89 L98 coupe, 79k miles, and stock.
Thanks in advance for help.
Team Owner
Joined: Aug 2000
Posts: 40,156
Likes: 45
From: San Pedro CA
there is a thingy on the throttle body called the "IAC" - Idle Air Control.
Any codes???
It might be dirty or in need of adjustment.
First, make sure it's still connected!!! That happened to me once! oopsie.
It's on the right side of the throttle body, at the bottom of the TB. It has a square connector with 4 wires.
If thats ok, remove the intake tube and look inside the TB. I'll bet it's nasty! At the bottom of the inside of the TB, near the throttle plates, is a smallish hole. That is the IAC port. Get some carb/injection cleaner and soak it all over. Inside that hole, mostly! I have seen chunks of crap flow out....
If it runs better after this GREAT!
It might not be a bad idea to pull the TB and clean it completely. It is not hard to remove, will require a new gasket, tho...
if you remove the IAC, it will REQUIRE a special reinstall process. Move the pintle out 28mm from the edge of the IAC body (this includes threads!).
DO NOT TURN ON THE ENGINE WITH THE IAC ATTACHED AND IN INSTALLED!!! IT WILL EXPLODE!!!
Any codes???
It might be dirty or in need of adjustment.
First, make sure it's still connected!!! That happened to me once! oopsie.
It's on the right side of the throttle body, at the bottom of the TB. It has a square connector with 4 wires.If thats ok, remove the intake tube and look inside the TB. I'll bet it's nasty! At the bottom of the inside of the TB, near the throttle plates, is a smallish hole. That is the IAC port. Get some carb/injection cleaner and soak it all over. Inside that hole, mostly! I have seen chunks of crap flow out....
If it runs better after this GREAT!
It might not be a bad idea to pull the TB and clean it completely. It is not hard to remove, will require a new gasket, tho...
if you remove the IAC, it will REQUIRE a special reinstall process. Move the pintle out 28mm from the edge of the IAC body (this includes threads!).
DO NOT TURN ON THE ENGINE WITH THE IAC ATTACHED AND IN INSTALLED!!! IT WILL EXPLODE!!!
Racer
Joined: Sep 2004
Posts: 323
Likes: 1
From: Muncie Indiana
Moisture
Remove the distributor cap and check for moisture. You may have a crack in cap or it may be condensing moisture from the high humidity in the air with all the rain. Dry it out and see how car runs. My moneys on the cap being bad. A long reach would be the wires have absorbed moisture in them. 
Team Owner
Joined: Jan 2003
Posts: 20,335
Likes: 264
From: Summerland B.C. Canada
Mine did that once. I had to turn up the min idle just so I could use ther car. When I got out of the parts store it returned to normal and I had to return my min idle to original. Don't know what caused it, but it did return to normal 
Melting Slicks
Joined: Jun 2004
Posts: 3,032
Likes: 4
What RPM's are you surging to/from?
In mine, once the car was warmed up (closed loop I think it is) I would get a continuous Idle surge fromm 600-900 RPMs while Idling.
I replaced the IAC
it now idles steady at 600 RPMs when warmed up...
You did say this only happened after it got really wet...
The distributer getting wet would be my second guess then.
Luckily you have an L98, and once it dries off you should be fine if that's the case...
In mine, once the car was warmed up (closed loop I think it is) I would get a continuous Idle surge fromm 600-900 RPMs while Idling.
I replaced the IAC
it now idles steady at 600 RPMs when warmed up...
You did say this only happened after it got really wet...
The distributer getting wet would be my second guess then.
Luckily you have an L98, and once it dries off you should be fine if that's the case...
Last edited by mikey whipreck; Nov 24, 2004 at 04:22 PM.
Pro
Joined: Oct 2001
Posts: 554
Likes: 1
From: CA
Do any work on the car lately? Reason I ask is because I've changed out tons of parts in the past to hoping to solve various problems and it all came back to something loose...loose battery connection, loose distributor wire connection, loose vacuum connection, etc. Save yourself some money by first making sure everything is properly connected. And if the idle problem still exists, I'd go the IAC route.
Good Luck and hope it doesn't cost you much.
Good Luck and hope it doesn't cost you much.
Advanced
Joined: Jul 2004
Posts: 66
Likes: 0
don't wanna hijack this post, but i was wondering if there is something that could make a IAC go bad? I've put in two in the last year, and the they seem to run fine, for a few weeks, then the surging and dying when you let off of throttle returns.
Melting Slicks
Joined: Jun 2004
Posts: 3,032
Likes: 4
Originally Posted by Ronstrap01
don't wanna hijack this post, but i was wondering if there is something that could make a IAC go bad? I've put in two in the last year, and the they seem to run fine, for a few weeks, then the surging and dying when you let off of throttle returns.
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Pro
Joined: Oct 2001
Posts: 554
Likes: 1
From: CA
Originally Posted by Ronstrap01
I've reset things several times, and I've cleand the TB within the last year. I suppose I could try to clean it again.
Race Director
Joined: Dec 1999
Posts: 11,182
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From: Almost all Skyline Cruises Vettes at Waterside 1-5
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Originally Posted by gpagaduan
Join the club Ron, I've used GM, Autozone...I think they just go bad...although my original one last a good ten years. There is a post that mentioned a bad ECM could adversly affect the way your IAC works. As I mentioned above, check all the basis stuff first. I bought the IAC valve, fuel filter, ECM, did the TB cleanin, etc and it all came down to a loose wire!!!
These cars are very susceptible to not just loose connections but also to "dirty" connections.By dirty I don't mean that it has to be corroded "green"!
Both the positive and negative leads servicing the sensors are "stacked" one atop another and secured to common distribution blocks.
According to the Guru, Gorden killebrew, GM experienced a lot of problems in Texas, Virginia and North Carolina with high humidity "corroding" these connections.
Again, don't mean obvious "green", but barely visible signs! These sensors are operating on minivolts/amps and slight changes in resistance can affect performance.
Last fall and spring I was chasing numerous electrical gremlins, including some that were transient. While I had some bad relays (MAF) and the IAC, it wasn't till I undid all the positive and negative leads and cleaned them that my problems not only went away, but the car is running better than ever
All the leads "looked clean", but apparently an invisible layer of "corrosion" had set in.
On my 88 all the positive leads attach to a block behind the battery and the negative ends bolt to the transmission case.
Jack
