1989 Bad ECM/PROM??
#1
Racer
Thread Starter
1989 Bad ECM/PROM??
Y'all...I'm sure you've seen my monster thread I posted a couple weeks ago about a bad TPS, etc. Well, it morphed into some complex stuff and I need some guidance. I'm a little defeated and need some C4 forum love right now
As the story goes, i've been restoring my 89 and have done immense amount of work (mechanical/cosmetic) to get her cleaned up and back on the road after a tough life of abuse from the PO. Back in Feb is when I officially got her back on the road. At the time she ran a little rough because the PO put a LT4 hot cam in it and never had the car re-tuned. I did notice the car sometimes would like to idle at 1000 then settle to 8 when coasting in neutral or at a light. Fast forward. PCM built a tune for me and it completely changed the car for the better, ran like a champ except I noticed some sluggish throttle response. Checked the TPS volts and they were super low and WOT was only giving me like 1.75v. Went through 4, that's right, 4 new TPSs that were failures out of the box. I eventually had to purchase an original from a forum member here that had a nice original. That TPS was perfect and read proper volts. However, during all of this process, the car started to develop this odd idle issue where it simply would not idle under 13-1500. No matter how many times I reset the IAC and TPS, it would bounce back to 13-1500. Took IAC out and it was pretty gross and I cleaned it up; installed with no change. I had some rewards at the parts store, so I threw a new IAC at it for like $20, no change.
I then turned to the MAF. Unplugging it would drop the revs right down to 800 and idle perfect. Rich, but perfect. The MAF was a new Bosch unit I put on the car when I first got it because it literally shut down on me twice while driving and the new MAF cured that. The unit was still in warranty so I dropped it off at the store and they supplied me with a new Spectra unit. I installed that tonight with an exact result of a 1500 idle. Oddly, tonight when disconnecting MAF, it only dropped revs to 1000. Trust me when I say i've investigated vacuum leaks. There are 0 vacuum leaks in that car.
The only real variable that has changed with this car is the temps we're in right now. I'm in NC and we've had some 90s/high 90s for like 2-3 weeks now. I know ECMs when failing tend to be more temp sensitive. I once read a quick test is to stick the ECM in the fridge for like an hour and see if that makes a difference. At this point i'm a bit lost and would love to have some feedback. Always, thank you all greatly
As the story goes, i've been restoring my 89 and have done immense amount of work (mechanical/cosmetic) to get her cleaned up and back on the road after a tough life of abuse from the PO. Back in Feb is when I officially got her back on the road. At the time she ran a little rough because the PO put a LT4 hot cam in it and never had the car re-tuned. I did notice the car sometimes would like to idle at 1000 then settle to 8 when coasting in neutral or at a light. Fast forward. PCM built a tune for me and it completely changed the car for the better, ran like a champ except I noticed some sluggish throttle response. Checked the TPS volts and they were super low and WOT was only giving me like 1.75v. Went through 4, that's right, 4 new TPSs that were failures out of the box. I eventually had to purchase an original from a forum member here that had a nice original. That TPS was perfect and read proper volts. However, during all of this process, the car started to develop this odd idle issue where it simply would not idle under 13-1500. No matter how many times I reset the IAC and TPS, it would bounce back to 13-1500. Took IAC out and it was pretty gross and I cleaned it up; installed with no change. I had some rewards at the parts store, so I threw a new IAC at it for like $20, no change.
I then turned to the MAF. Unplugging it would drop the revs right down to 800 and idle perfect. Rich, but perfect. The MAF was a new Bosch unit I put on the car when I first got it because it literally shut down on me twice while driving and the new MAF cured that. The unit was still in warranty so I dropped it off at the store and they supplied me with a new Spectra unit. I installed that tonight with an exact result of a 1500 idle. Oddly, tonight when disconnecting MAF, it only dropped revs to 1000. Trust me when I say i've investigated vacuum leaks. There are 0 vacuum leaks in that car.
The only real variable that has changed with this car is the temps we're in right now. I'm in NC and we've had some 90s/high 90s for like 2-3 weeks now. I know ECMs when failing tend to be more temp sensitive. I once read a quick test is to stick the ECM in the fridge for like an hour and see if that makes a difference. At this point i'm a bit lost and would love to have some feedback. Always, thank you all greatly
#3
Le Mans Master
….. Start with basics … set the timing and the minimum idle speed per the instructions in the Factory Service Manual … you might want to set the min idle a little higher (600-700rpm) because of the cam … Lubricate the throttle body shaft where it passes through the TB with some silicone or Teflon based lube … unplug/disconnect then re-plug/re-connect EVERY electrical connector from the ECM out to the engine … this cleans the contacts … as mentioned above , get a scanner and check IAC counts and idle air flow … IAC should be somewhere around 10 to 30 at a normal idle at operating temperature … less than 10 means the throttle blades are open too far , taking idle control away from the IAC … more than 30 and the IAC is trying hard to control the idle because the throttle blades aren't open enough … idle air flow should be approx. 8-11 grams per second at normal idle speed at operating temps …..
#4
Racer
Thread Starter
….. Start with basics … set the timing and the minimum idle speed per the instructions in the Factory Service Manual … you might want to set the min idle a little higher (600-700rpm) because of the cam … Lubricate the throttle body shaft where it passes through the TB with some silicone or Teflon based lube … unplug/disconnect then re-plug/re-connect EVERY electrical connector from the ECM out to the engine … this cleans the contacts … as mentioned above , get a scanner and check IAC counts and idle air flow … IAC should be somewhere around 10 to 30 at a normal idle at operating temperature … less than 10 means the throttle blades are open too far , taking idle control away from the IAC … more than 30 and the IAC is trying hard to control the idle because the throttle blades aren't open enough … idle air flow should be approx. 8-11 grams per second at normal idle speed at operating temps …..
#6
Racer
Thread Starter
Have i I missed an obvious one I need to investigate? Even crimping off PCV/CCV doesn’t change it. I’ve wondered if there was the random possibility of an intake to block leak with how bad this was. But thought that was silly considering 3.5 weeks ago there wasn’t this huge issue
#7
Le Mans Master
..... Have you looked at the throttle blades with the idle screw backed out to see if there is any light coming around them ? Is the throttle shaft tight where it passes thru the TB ? … TB off engine of course … what intake manifold ? TPI has numerous opportunities for air leaks … smoke test ? … any air leaks between the MAF and TB ? … brake booster itself ? …..
Last edited by C409; 06-21-2018 at 08:32 AM.
#8
Racer
Thread Starter
..... Have you looked at the throttle blades with the idle screw backed out to see if there is any light coming around them ? Is the throttle shaft tight where it passes thru the TB ? … TB off engine of course … what intake manifold ? TPI has numerous opportunities for air leaks … smoke test ? … any air leaks between the MAF and TB ? … brake booster itself ? …..
Once the cable gets here Monday and I scan accessory and run data and if it leads me to vac leak, I might go ahead and pick up a new gasket set for the TPI and swap it out. It’s like $10. That way during the disassembly process I can examine each piece. Hell, at this point I wouldn’t mind just getting a mini ram and swapping out the old TPI
#9
Racer
Thread Starter
So I picked up some of those awful Incense stick things that smell like garbage but smoke up a storm. Went through every inch of that intake and not even one trace of smoke being disturbed.
I backed the set screw out and looked at the blades and they are a nice fit, barely any visible gap around the edges. There was no play or rattle in the spline running through.
I did notice something interesting tonight. I did the coolant bypass a long long time ago with no issues. And I could always remember after a drive I could come back and put my hand on the plenum and it would feel ambient temp. Just having the car idle for that short period of time tonight made the plenum like fire hot. EGR isn’t suppose to start pulsing until actual vehicle movement correct? At idle there should be absolutely no EGR. With there now being no overt plenum leak and feeling how hot them plenum has been getting. I wonder if there is an issue with the the EGR valve or the ECM commanding pulses when it shouldn’t. Is this a stupid thought? Feel free to say so, I’m just tossing ideas at this point
I backed the set screw out and looked at the blades and they are a nice fit, barely any visible gap around the edges. There was no play or rattle in the spline running through.
I did notice something interesting tonight. I did the coolant bypass a long long time ago with no issues. And I could always remember after a drive I could come back and put my hand on the plenum and it would feel ambient temp. Just having the car idle for that short period of time tonight made the plenum like fire hot. EGR isn’t suppose to start pulsing until actual vehicle movement correct? At idle there should be absolutely no EGR. With there now being no overt plenum leak and feeling how hot them plenum has been getting. I wonder if there is an issue with the the EGR valve or the ECM commanding pulses when it shouldn’t. Is this a stupid thought? Feel free to say so, I’m just tossing ideas at this point
#11
Racer
Thread Starter
Y'all, she's fixed. Long story short, I got the ALDL cable and got TunerPro RT going and I found that my multimeter for whatever reason was not giving me an accurate reading when I was back-probing the TPS. With the set screw out completely, I was still showing 1.3v. I was running out of daylight so I eyeballed the set crew at 5 turns from blade contact and set the TPS at 0.54 per the ECM output on screen. Idled right at 750. I still need to go through the min idle reset process, but what a lesson learned. I verified the multimeter worked just fine, shows proper volts on other equipment. This leads me to believe I was either getting poor contact from on back probe or was picking up some resistance somehow in the readout. So from this point out, no more multimeter FSM procedures, I'm going straight to scan data...
Thank you all for all of your help, it's seriously appreciated
Thank you all for all of your help, it's seriously appreciated