Is my ECM Toast? - ZR-1 Net Registry Forums

gbmidyear66 · 09-13-2011

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 · 09-13-2011

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.

Tom

QB93Z · 09-13-2011

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 · 09-13-2011

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

GOLDCYLON · 09-13-2011

Quote:

Originally Posted by gbmidyear66

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 · 09-13-2011

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 · 09-13-2011

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 · 09-13-2011

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 · 09-13-2011

Quote:

Originally Posted by gbmidyear66

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 · 09-13-2011

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.

09-13-2011	#1
gbmidyear66 Join Date: Feb 2008 Location: Vancouver, BC Posts: 252	Is my ECM Toast? 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 __________________ [SIGPIC][/SIGPIC] __________________ 1990 Corvette ZR-1 #2824 4.10's, Custom PROM, Flowmaster, Top End Port NCRS BC Chapter

09-13-2011	#2
tomtom72 Join Date: Dec 2004 Location: Jacksonville, FL USA Posts: 4,645	Re: Is my ECM Toast? 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. Tom __________________ 1990 ZR-1, Black/grey, #2233, stock. ZR-1 Net Reg Founding Member #316 & NCM member

09-13-2011	#3
QB93Z Join Date: May 2007 Location: Westminster, Maryland Posts: 3,689	Re: Is my ECM Toast? 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

09-13-2011	#4
XfireZ51 Join Date: May 2007 Location: Chicagoland, IL Posts: 9,708	Re: Is my ECM Toast? So you've checked PCV connection at the F connector?

09-13-2011	#6
XfireZ51 Join Date: May 2007 Location: Chicagoland, IL Posts: 9,708	Re: Is my ECM Toast? 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.

09-13-2011	#7
gbmidyear66 Join Date: Feb 2008 Location: Vancouver, BC Posts: 252	Re: Is my ECM Toast? 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. __________________ [SIGPIC][/SIGPIC] __________________ 1990 Corvette ZR-1 #2824 4.10's, Custom PROM, Flowmaster, Top End Port NCRS BC Chapter

09-13-2011	#8
gbrtng Join Date: Feb 2006 Location: San Antonio area Posts: 1,179	Re: Is my ECM Toast? 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. __________________ “Buy the best example of what you want, and pay whatever it takes. That way, you cry only once.” ~ Bruce Meyer

09-13-2011	#10
gbmidyear66 Join Date: Feb 2008 Location: Vancouver, BC Posts: 252	Re: Is my ECM Toast? 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. __________________ [SIGPIC][/SIGPIC] __________________ 1990 Corvette ZR-1 #2824 4.10's, Custom PROM, Flowmaster, Top End Port NCRS BC Chapter