redzr1

I have a 92 coupe with ~52k miles that I picked up a couple months ago. I was aware when I purchased it that it was setting a code 36 and expected it was being set due to a bad Optispark. After purchasing the vehicle, I also noticed that the water pump was leaking.

Tonight after having completed replacing the Optispark with a unit from Chandler Motorsports and putting a new replacement water pump, I went out and drove the car to verify if the problem with setting a code 36 was solved. I would start out from a dead stop and the engine would run up to between 3000-3200 RPM and then just stay at that engine speed unless I let up on the throttle. After attempting the same thing a couple more times the check engine light came on (later verified that it is a code 36). Once the check engine light was on, I could then start from a dead stop and run up to around 5200 RPM or so where the transmission would shift multiple times.

Any insight as to why I would get this behavior priort to a check engine light being set for a code 36, but not after? I have looked over the diagnostic flow chart in the factory shop manual and it gives no insight to resolving the issue. Is my replacement Optispark also faulty?

TWISTERUP

Since you had a code 36 before opti replacement and if you had EXACTLY the same performance scenario--ie no ck eng. light won't rev past 3k rpms--then code sets/ck eng. light is on and then revs normal-I wouldn't suspect the replacement opti is bad. It's posible the old opti wasn't bad either. Did you remove the cap on the old opti to check for signs of coolant inside?

Check VERY closely the harness connector at the opti--be sure the terminals are clean/tight w/no signs of corrosion from water intrusion. This opti harness is approx. 2 ft. in length and plugs into the main eng. harness above and to the front of the right valve cover. Check both connectors at each end very closely. Check the wiring in the harness for any rubbed areas or poor insulation. This a very low voltage circuit and it doesn't take much to loose a signal--set a code and run like crap. Could be a problem in the main harness/connector at the ECM or the ECM itself. The short section of opti harnesses are known to give problems and as far as I know are still available so ck. it carefully

The low resolution circuit which is used for crank/cam position is usually what fails and you have a "no start".
The high resolution circuit (your code 36) is used for precise ign. timing adjustments. What is likely happening in your case is that once code 36 is set--the ECM uses a default/limp program with predetermined ign. timing and allows the engine to rev in a fairly normal way

redzr1

Twister,

Prior to your posting I did look an initial look at the opti harness per the diagnostic flow chart in the shop manual, as I had seen some others indicate having problems with corrosion in the connectors. I did not see any problems with the harness and am getting the same reference voltage on both ends of the opti harness on the high res line. I did clean the connectors with a wire brush before reinstalling and still see the "rev limiting". Just before looking at the harness and after, I have not been able to get the code 32 to set so it never defaults to the limp home mode. I plan to take a much closer look at the harness tonight along with also checking the actual resistance of the high res line all the way back to the ECM connector. I know that the voltage I am measuring is in the correct range, but that is into a high impedance digital multimeter and when current actually flows the voltage drop could be too much in the harness to allow the high res pulses to be valid.

I did send a note to Chandler Motorsports and they have indicated once in the past a customer had a very similar issue and it was not resolved until they replaced the ECM. The ECM is GM part#16159278 and I have not been able to find anywhere online or from a local parts supplier to obtain and put in the car. A couple parts stores in the area can send mine in to be rebuilt, but the turnaround time is 10-14 days. I have a few other areas that I am going to check to see if I can get one on shorter order to try out. I am going to check with our service department here at Delphi where I work to see if they can recommend one of their reman shops to check with to purchase a replacement ECM as they will have put in any known production fixes, which I am much more comfortable with than buying a used one off of Ebay that could fail down the road if common repairs/upgrades are not made.

redzr1

I have had the ECM checked by Service Engineers at Delphi with the same tester and program used at the reman center for GM's ECMs (versus aftermarket like A1 Cardone). The ECM passed with no issues. I also swapped in a new AC Delco ICM and it showed the problem right away without it even having to heat up. Also swapped out the coil with no improvement in driveability. Was able to get the ECM to register another Code 36 by going WOT while in 2nd gear and forcing a downshift, which bumped the RPMs up to around 4500. With code 36 present, I am able to run back up to ~5500 RPM. Code 36 did go away on the next ignition cycle and am having trouble reproducing again.

I have borrowed a Tech 2 controller . I have seen on it that when running at around 1250 RPM, the low resolution signal pulse is registering 15+/- ms. At the same time the high resolution pulse reads out as an increasing value from 1ms up to ~255ms, at which point it drops back down to 1ms. This is indicating to me that the ECM is not seeing the high resolution pulse at all. I would expect that the high resolution pulse width on the Tech 2 should be roughly 1/90th of the low resolution pulse. I just have not been able to find what the expected high res pulse width should be versus the low res. When I checked the harness a couple days ago with ignition on, I was seeing the high resolution signal biased to ~4.75 volts at both the ignition test connector and also at the end of the connector that plugs into the optispark. This was in the 4-6 volt range that the FSM indicates is correct.

I also checked all four lines in the wiring harness back to the connectors for the ECM and all four run 0.3 to 0.4 ohms. I visually checked the contacts on all the connectors and saw no corrosion present on any of them. About the only thing I could think of to further confirm the replacement optispark has a bad high resolution signal is to borrow an oscilliscope and look at both the low and high resolution signals right at the ECM.

redzr1

Wanted to make an update for anyone that looks at this thread while searching to repair their car. After replacing the optispark for the second time and not seeing the issue go away, I then put in new AC Delco Spark Plugs and wires. This did not solve the issue, so I borrowed another ECM (GM#16159278) and put it into the car with the PROM out of my original ECM. The rev limiting at 3500-4000 RPM went away with the ECM change and then came back when I put the original ECM back in.

I am now looking for a good replacement ECM and also will be following up with our service engineers to figure out why the test for remanufactured units does not catch the problem seen with the one in my car. Thanks to all for their input. Unfortunately I still have to pull the water pump and reinstall as it is leaking a little bit after putting the last optispark into the car.

jaa1992

See my reply to your other post. We should get the powers that be to add it to the sticky post. That 278 ECM is a pain. Its really too bad we can't go to a later ECM.

DaveD2

This is for redz1 I have the same problems you described but I bought an opti off of eBay and installed it and it ran great. Perfect to be exact, rev great idled perfect and it is a 383 stroker now. So after installing everything because it checked out to run great as soon as I fired it up again the high resolution code came up again code 36. I have a brand new harness to the opti, I have two good ECM's and was wondering why you never finished the thread or did you ever fix your problem? I am about to go get a new opti somewhere else instead of eBay. This is a brand new motor and installing that new opti was the first time it ran great for the couple minutes that it did run anyway.
So My question is did you fix that code 36 problem???
My engine is running like the timing is way off, but for the first run with the new opti it was perfect so that leaves me to believe that the opti is the culprit here.
Thanks ahead of time
Daved2

DaveD2

It is my understanding that the new ECM when installed has to learn itself by running for a while before picking up any codes like the old ECM. IT took 5 miles of driving for code 36 to set on my ECM the first time.
I had someone tell me about the learning process the ECM has to go through not sure about it myself but I would like to know from other experienced Vette mechanics what other problem could it be except the ECM or Opti?
When code 36 is setting.
With a brand new harness and a cheap Opti off of eBay, two of them to be exact, from the same place!