Is my ECM Toast?

[gbmidyear66]

I seem to be in over my head here. I've been chasing a SES light that comes on intermittent (no codes set) AND an erratic high idle problem for over a week. After checking many possibilities, I am coming to the unhappy conclusion that I may have an ECM problem. Here is what I have done/eliminated:

- vacuum pump does not cycle
- no apparent vacuum leaks, all hoses on tight
- engine stalls (as it should) if I block the TB air intake
- TPS sensor reads correct voltage at idle, and voltage values cycle up/down smoothly with throttle position
- MAP vacuum hose is clear and connected, and voltage values from sensor (from scanner) look good
- replaced my custom PROM with the factory AUAH PROM (just to eliminate the custom PROM) - and strangely - the CAGS function no longer kicks in like it used to (the CAGS light flickers once in a while), and the idle and SES behavior is the same as with the custom PROM (it doesn't hang high then drop down after you come to a stop - like it did before with the AUAH PROM)
- when I can kick the idle down, it will hunt around 900 to 1100, won't come down to 750 like it used to
- disconnected and cleaned the ECM connectors at the harness with spray contact cleaner - no effect
- throttle body is clean, throttle cables move freely

- IAC is clean, and I know it is not "dead" - because if I disconnect it, the idle gets much worse (stuck hard at about 2,500), can an IAC go "bad" but not "dead"?

- Should I swap the IAC? ( just in case), or does everything point to the ECM?

Glenn

[tomtom72]

Glenn, just so you know someone is thinking on this issue I'm posting up. I'm at work and when i get time I'll get the FSM and do some reading & be back with something useful ( hopefully ) to post up.

In the mean time I do have a spare ECM so we got the swap & test covered if it comes to that.

:cheers:
Tom

[QB93Z]

Glenn

I worked on a ZR-1 with an idle problem for quite a while that turned out (thanks Pete) to be an improperly cut throttle body gasket. There was no hole for the Idle Air to pass through.

I would approach the trouble shooting with the assumption that the intermittent SES light and the idle problem are separate issues.

Do you have access to a Tech 1? Have you verified that the Secondaries function properly when the SES light is not on?

What work has been done to the engine?

Jim

[XfireZ51]

So you've checked PCV connection at the F connector?

[GOLDCYLON]

IAC is clean, and I know it is not "dead" - because if I disconnect it, the idle gets much worse (stuck hard at about 2,500), can an IAC go "bad" but not "dead"? - Should I swap the IAC? ( just in case), or does everything point to the ECM? Glenn


Glenn its sounding like the TB. the last failure I had on an IAC was the unit physically sperated (Spring/Plunger) etc into the side of the TB. I had to pull it out of the TB.

[XfireZ51]

Does the SES light blink once and then come on steady when ignition is
"ON"? If it does, that's the ECM self-check indicating system
is OK. If it does not blink, that's an issue with prom or ECM. Is it possible that
the Min Air adjustment needs to be re-done? TPS position being at 0% means nothing because TPS% will auto-zero at idle.
The hunting idle could be happening because at 900-1100 rpm,
the calibration is not set for idle.

[gbmidyear66]

Tom, thanks for your effort.

Jim - secondaries operate just fine. I have an AutoXray 6000 scanner. 40K mile car, engine is bone stock (currently ;)) - I replaced the injectors (multec look alikes from FIC) and plugs last year. I cleaned and secured everthing under the plenum at that time. Previous owner had wires and coils replaced some years back by Marc Haibeck.

Dom, PCV's were replaced last year and are strapped firmly to the "F" hose. Thats great info on the ECM start sequence - I'll check behaviour when I get home tomorrow (on the road - didn't want to risk taking the ZR1 given its curent state). On Min air adjustment, are you talking about adjusting the factory set TPS position ? I had the battery disconnected when I cleaned the ECM contacts - should that not have triggered a full re-learn sequence?

Nothing has been touched to have triggered the issue - haven't touched anything on the motor in the last year / 3,000 miles.

[gbrtng]

Try a known good ECM next. Weird gremlins when you've done nothing to it in 3000 miles points to a flakey ECM to me. Especially since you don't see any error codes.

[XfireZ51]

Dom, PCV's were replaced last year and are strapped firmly to the "F" hose. Thats great info on the ECM start sequence - I'll check behaviour when I get home tomorrow (on the road - didn't want to risk taking the ZR1 given its curent state). On Min air adjustment, are you talking about adjusting the factory set TPS position ? I had the battery disconnected when I cleaned the ECM contacts - should that not have triggered a full re-learn sequence?

Nothing has been touched to have triggered the issue - haven't touched anything on the motor in the last year / 3,000 miles.

Since you've put a scanner on it, what are the IAC steps at idle? If they're 0, I would suspect a vacuum leak of some sort or the Throttle Blades may need to be closed up a bit.
Its not adjusting the TPS position, although you will need to do that after you adjust the throttle blade stop. Its the set screw on the drivers side of the throttle body. PITA to do unless you have replaced it with a hex head. It doesn't take much to make a diff. Could even be wear.
I'm assuming you have driven the car since disconnecting the battery and that the IAC has reset.

[gbmidyear66]

Dom, regarding the IAC values - yes something is not 100% right here.

When I can get the idle to behave, car runs at about 775 RPM with IAC=0, if I turn on the A/C, it runs at about 725RPM with IAC about 13 or so(from memory). I agree - with A/C off I should not see IAC = 0 and 775 RPM.

The above implies a "leak ", but I don't think it is the typical situ with air leaking in from various hoses/tubes, etc given that I can stall the engine by blocking the throttle body.

I started out thinking I had an issue with a sticking throttle plate(s) - but the SES indicator and CAGS no longer working led me to suspect the ECM.

[XfireZ51]

Dom, regarding the IAC values - yes something is not 100% right here.

When I can get the idle to behave, car runs at about 775 RPM with IAC=0, if I turn on the A/C, it runs at about 725RPM with IAC about 13 or so(from memory). I agree - with A/C off I should not see IAC = 0 and 775 RPM.

The above implies a "leak ", but I don't think it is the typical situ with air leaking in from various hoses/tubes, etc given that I can stall the engine by blocking the throttle body.

I started out thinking I had an issue with a sticking throttle plate(s) - but the SES indicator and CAGS no longer working led me to suspect the ECM.

The IAC counts should be somewhere at 10-20 at idle. I think mine are at about 5-10. A/C should require more IAC steps to deal with drag of compressor. Again, it could imply a leak but if the IAC is opening up when AC compressor is on, it could be the Min Air adjustment. That would be consistent with the stalling motor when blocking TB. As for the SES light, are you saying it does not come on with ignition ON only, no engine run?
If you close up the throttle blades and see the IAC counts increasing, then there's no vacuum leak.

[gbmidyear66]

I spent some quality time with my FSM last night. Regarding the ECM....

You put the ECM into diagnostic mode if you connect ALDL pins A and B (on a 90 - Ign on, engine off). If the DIC flashes 12, 12, 12, xx, xx, xx, 12, 12, 12 (xx indicates a stored fault code, there may be none, or multiple) - then the ECM has passed all its internal self tests and is presumably good.

Mine indicates good. I'm going back to looking at the throttle body for my issues.

[GOLDCYLON]

I'm going back to looking at the throttle body for my issues.


:thumbsup:

[gbmidyear66]

Well, I eliminated one suspect leak tonight (vacuum pipe connection to PCV header (Breather Mold).

I then started the car and let it warm up, everything was beautiful - idle drops to 675 RPM, IAC at 6 steps. Almost ready to hit the couch and crack a beer, but I thought I would blip the throttle a few times to see what happens. Well, it won't come back down below 750RPM, and IAC is at 0.

I think this is telling me that the butterflies in my TB are not sealing reliably?

I thought I read somewhere that you can grease the circumference of them as an interim solution? Anyone done this?

Glenn

[XfireZ51]

Glenn,

I clean my throttle blades, the bores and the shafts.

[tomtom72]

Glenn,

I clean my throttle blades, the bores and the shafts.

Just make sure you use Throttle Body Cleaner and not carb cleaner. Also, be careful when cleaning the DAG, gently is the key or you might start to remove it and loose the seal.

just a thought :o you're sure that the cables and their cams are not gummed up & binding a bit?

:cheers:
Tom

[XfireZ51]

Just make sure you use Throttle Body Cleaner and not carb cleaner. Also, be careful when cleaning the DAG, gently is the key or you might start to remove it and loose the seal.

just a thought :o you're sure that the cables and their cams are not gummed up & binding a bit?

:cheers:
Tom

Tom,

I've heard and read about the DAG but my TB is bored out to 63mm. My idle w the cams is now in the 775-825 range.

[ScottZ95ZR1]

Well, I eliminated one suspect leak tonight (vacuum pipe connection to PCV header (Breather Mold).

I then started the car and let it warm up, everything was beautiful - idle drops to 675 RPM, IAC at 6 steps. Almost ready to hit the couch and crack a beer, but I thought I would blip the throttle a few times to see what happens. Well, it won't come back down below 750RPM, and IAC is at 0.

I think this is telling me that the butterflies in my TB are not sealing reliably?

I thought I read somewhere that you can grease the circumference of them as an interim solution? Anyone done this?

Glenn

Not sure the post linked below is where you read about greasing the TB plates, but I remembered Scott stated he does.

http://www.zr1.net/forum/showpost.php?p=89151&postcount=12

[gbmidyear66]

Thanks for digging up the link to Scott's post.

Well, I tried greasing the circumference of the plates today, but this just made the idle hang situ much worse. So I get out my trustry can of TB cleaner to remove some of the grease on the plates and I skillfully manage to shoot the plastic extension straw off of the can and down into the Plenum. Managed to fish it out with some double sided tape on my telescoping magnet, and get back to where I started at the beginning of the day.

If I apply finger pressure to the primary plate - my idle drops down nicely, I think my next step will be to send it out to Marc Haibeck to have it serviced this winter.

Glenn