Need ECM help for idle problem.
Thread Starter
Racer
Joined: Jun 2000
Posts: 428
Likes: 1
From: Friendswood Texas
Need ECM help for idle problem.
I'm having an idle problem (stock 86L coupe/auto) and maybe you ECM guru's can give me a clue - please.
The problem: idle, in neutral, system is closed loop. Idle starts at approx 600 rpm, then goes to 800 rpm, then goes to 400 rpm and almost stalls, then goes back to 600 rpm and repeats cycle over and over again. Complete cycle takes only a couple of seconds. Turn the A/C on and idle steadies out to 600 rpm, then turn A/C off and idle remains at 600 rpm, steady.
What is closed loop logic using to control the idle? What would cause this "ringing" of the control loop.
What I've eliminated so far: new IAC, new O2 sensor, new MAT sensor, new TPS, new (rebuilt MAF) - check for vacuum leaks - found 2 small ones and corrected them. At ldle fuel pressure is 36psi - engine vacuum is 20 in.
Oh - this started happening after replacing stock fuel injectors with FMS 24#.
I like a good puzzle, but this is starting to drive me crazy.
:confused:
The problem: idle, in neutral, system is closed loop. Idle starts at approx 600 rpm, then goes to 800 rpm, then goes to 400 rpm and almost stalls, then goes back to 600 rpm and repeats cycle over and over again. Complete cycle takes only a couple of seconds. Turn the A/C on and idle steadies out to 600 rpm, then turn A/C off and idle remains at 600 rpm, steady.
What is closed loop logic using to control the idle? What would cause this "ringing" of the control loop.
What I've eliminated so far: new IAC, new O2 sensor, new MAT sensor, new TPS, new (rebuilt MAF) - check for vacuum leaks - found 2 small ones and corrected them. At ldle fuel pressure is 36psi - engine vacuum is 20 in.
Oh - this started happening after replacing stock fuel injectors with FMS 24#.
I like a good puzzle, but this is starting to drive me crazy.
:confused:
Thread Starter
Racer
Joined: Jun 2000
Posts: 428
Likes: 1
From: Friendswood Texas
Re: Need ECM help for idle problem. (DOCTOR J)
Thanks, DOCTOR J. Very good hypothesis - I'll check into it. The other hypotesis I'm looking into is a vacuum leak induced by me during the injector R&R - o-rings or gaskets.
Funny thing is it idles steady and smooth when cold - open loop. Someimes it idles steady closed loop also. It's not a consistant symptom.
Funny thing is it idles steady and smooth when cold - open loop. Someimes it idles steady closed loop also. It's not a consistant symptom.
Thread Starter
Racer
Joined: Jun 2000
Posts: 428
Likes: 1
From: Friendswood Texas
Re: Need ECM help for idle problem. (Mike263) - followup
I've reinstalled the injectors, including runner and plenum gaskets, and have eliminated any vacuum leaks. It still has the erratic idle problem - sometimes.
While I was setting the base idle (IAC was unplugged) the erratic idle was there (800rpm - 400rpm - 800rpm). It eventually steadied out after I pulled a vacuum line from the plenum (induced a vacuum leak!). I plugged the vacuum line back in and the idle stayed steady. I then set the base idle.
It's almost like it's going way lean - way rich -lean -rich. Does the O2 sensor come into play during idle?
Since I'm the sixth owner and don't have the complete history of the car, I'm going to drop the ECM and see if it has a stock chip.
:confused: :confused: :confused:
While I was setting the base idle (IAC was unplugged) the erratic idle was there (800rpm - 400rpm - 800rpm). It eventually steadied out after I pulled a vacuum line from the plenum (induced a vacuum leak!). I plugged the vacuum line back in and the idle stayed steady. I then set the base idle.
It's almost like it's going way lean - way rich -lean -rich. Does the O2 sensor come into play during idle?
Since I'm the sixth owner and don't have the complete history of the car, I'm going to drop the ECM and see if it has a stock chip.
:confused: :confused: :confused:
Thread Starter
Racer
Joined: Jun 2000
Posts: 428
Likes: 1
From: Friendswood Texas
Re: Need ECM help for idle problem. (dgoodhue)
Since you replace the IAC, did you reset the Minimum Air properly?
What I did:
jump ADL to put ECM in diagnostic mode
turn key on (don't start car), wait 30 sec for IAC to fully close
disconnect IAC
pull ADL jumper
start car
adjust throttle body idle screw to set idle to 450 rpm - used a DMM to read rpm (tapped into distributor tach line)
stop car
reconnect IAC
done.
Thread Starter
Racer
Joined: Jun 2000
Posts: 428
Likes: 1
From: Friendswood Texas
Re: Need ECM help for idle problem. (Mike263)
Just a followup. A new MAF and MAF relays solved the problem. Old MAF was apparently sending erratic data to the ECM, but no codes were thrown.
Idle is now steady and smooth.
Thanks for all the replies.
Idle is now steady and smooth.
Thanks for all the replies.
Melting Slicks
Joined: Apr 2002
Posts: 2,118
Likes: 0
From: St. Charles, MO Route 66 Corvette Club
Re: Need ECM help for idle problem. (Mike263)
Mike: I just put in a new MAF in my '89 L98 due to a rough idle. The new sensor has helped but the idle is still not smooth. Why did you replace the MAF relays and how did this affect the idle? You comments would be appreciated as the MAF sensor was not cheap!!! :confused:
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Thread Starter
Racer
Joined: Jun 2000
Posts: 428
Likes: 1
From: Friendswood Texas
Re: Need ECM help for idle problem. (Sam Lam)
Sam,
No specific reason for replacing the relays. The relays were cheap and I read somewhere to go ahead and replace them with the MAF. I quess if the burnoff relay failed the MAF would get dirty and not work properly after a while. If the power relay failed I think you get a code immediately.
I don't believe the realys affected the idle in my case.
BTW
I wrestled with a rough idle for almost 9 months last year. I'm not sure which change I made did the most good. The last thing I did was a new ignition module and AC Rapid Fire plugs.
Prior to that I did stock plugs, plug wires, distributor cap, rotor, coil, and fuel filter.
Melting Slicks
Joined: Apr 2002
Posts: 2,118
Likes: 0
From: St. Charles, MO Route 66 Corvette Club
Re: Need ECM help for idle problem. (Mike263)
Mike: It looks like we have a similiar idle problem except you have solved yours and mine is still "buggin me". I tried the Rapid Fire plugs and ended up replacing them with standard plugs. I am in the process of replacing plug wires. My MAF sensor was definitely bad and made some improvement. The base minimum idle adjustment also made a slight improvement. I will replace the fuel filter and then the ignition module. Beyond this, I think I may have to visit Mr. Goodwrench!!! If you have any "words of wiscom", I am open to any suggestions for my garage queen! Regards, Sam. :confused:
Thread Starter
Racer
Joined: Jun 2000
Posts: 428
Likes: 1
From: Friendswood Texas
Re: Need ECM help for idle problem. (Sam Lam)
Sam,
A vacuum leak or EGR valve problem may cause a rough idle also. I checked/replaced every vacuum line on the car. I also checked the EGR valve.
I know how you feel.
[Modified by Mike263, 6:58 PM 5/13/2002]
A vacuum leak or EGR valve problem may cause a rough idle also. I checked/replaced every vacuum line on the car. I also checked the EGR valve.
I know how you feel.
[Modified by Mike263, 6:58 PM 5/13/2002]
