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Unread 10-17-2015, 03:57 PM
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Default Re: Vorshlag Budget TT Build: Project DANGER ZONE

continued from above

The Optispark previously used in this C4 was an aftermarket $500 MSD unit that had less than 3 hours of use on it, so I doubted it could be bad. Maybe a ground wire was missed, or a wire cracked during the install (from age) on the factory harness? Ryan and Brad broke out the factory 1992 Corvette manuals and went through the Ignition Troubleshooting Flow Chart, step by step. Everything they checked worked, we just didn't have spark. Voltages were checked in the harness and into the Opti at several locations. The coil was swapped out for an MSD LT1 unit we had in the shop. No help.

Finally we broke down and ordered a stock replacement $150 Optispark unit from O'Reilys, which showed up 10 minutes later. It was 3 pm the day before our scheduled dyno day and track test, so it was worth the peace of mind.


This small victory was short lived - the problem never really was in the Optispark

We plugged the new unit into the engine harness, put the coil lead near the cylinder head, and spun the drive on the Opti... pop-pop-pop. We had spark, so it was proven to work. So out came the nearly brand new MSD Optispark unit, which meant the water pump we had just installed had to come back off. Order more gaskets while that's being swapped in. While the MSD Opti it was off the car they did the same "off the car" test and... damn it, the MSD unit worked like this, too. So I had to make the call... put the problematic MSD unit back on or have them install the brand new OEM replacement Opti? I told them go with the brand new unit.

By this point we had called and spoken to several LT1 experts and got suggestions from "bad ground", "bad ignition module", "bad ECM" and of course "Its the Opti!" It is almost always the Opti, but I doubted that more and more as we tested the old MSD unit off the car and it came back as good.


Another short lived victory - it started with the new Opti, but ran poorly

After a long day of troubleshooting the engine and with the new Opti installed, the engine fired up and ran briefly with the water pump off, so that was good. Then the water pump and coolant hoses had to go back on, the coolant get filled (with distilled water + Redline water wetter) and burped again, and the front of the engine went back together - drive belt, intake tube, etc.



What should have taken an hour to wrap up that Thursday morning ended up taking all damned day. The engine was finally started, it ran (poorly), the coolant system burped, temps checked out and loaded into the trailer for a dyno test the next morning. It was 7 pm and everyone was glad to go home.



Still, it just wasn't running that smoothly. The engine was idling way too high, way up at 2000 rpms, and it had a throttle tip in stumble. Our LT1-expert Ed stopped by and got the set-screw on the throttle blade set to turn ever so slightly and now it would idle at 1600 rpms, but it wouldn't go lower. Hmm. Also, after the engine warmed up, it wouldn't start again easily. It would crank, but wouldn't fire. Something still wasn't right. After 10-15 minutes of cooling down (it barely got up to operating temps) it would fire up, but once it was warm again and we shut it off, then tried to restart it - nothing. Just cranked, no fire. What is going on here? We scoured the car looking for other issues - grounds, broken wires, etc.

The guys at True Street had been ready for us to come by mid-day Friday for a dyno check and re-tune, but with the Optispark + other issues, we were delayed by several hours and missed their deadline. I called them at 4 pm and told them we were going to miss that window. Luckily they were hosting a big "dyno day" setup the next morning, so I planned to get there early and be first in line.... then head to ECR for a member day track test right after that.

Dyno Check at True Street, Saturday October 10, 2015

Loading the car into the trailer with a triple disc clutch is a PITA, but we hadn't stopped long enough to make front (or rear) tow hooks for this car. So when we got to True Street Motorsports at 8 am Saturday morning, an hour before they opened, Amy and I just rolled the car out of the trailer. Then I tried to start it...



NOPE. It cranked like a champ, but once again the coil wasn't firing. It won't start now, even cold. We fought with it for a solid hour, clearing the battery and trying again and again. It got to where the battery was losing voltage, and it still wasn't starting.



I thought about asking the guys at True Street for a hand, but they were all busy getting their shop cleaned up and ready for 200+ people, who had started arriving in droves. They had a food truck setting up, and cars starting to get dyno'd when we decided to abort the dyno test and load up.

Missing this open track day at ECR was a huge disappointment, and it put me in a foul mood for the rest of the day. I knew we would have to spend more (unbillable) hours the next week to get the C4 running "right" and starting consistently before going to NASA @ TWS the following weekend, without any test time on track. With a new motor, new radiator, and new crankcase breather. Testing this many new things during a competition weekend sounded like a bad idea, and it is.

More Testing At The Shop + True Street

Monday morning we unloaded the Corvette at Vorshlag again and Ryan and Brad spent most of the day tracing circuits and testing everything, wiggling the harness, and looking up issues with these cars. More and more evidence started to point to a bad ECM - the 25 year old engine computer might be at fault. My ops manager Steve started chasing down replacement 92 Corvette ECMs and that search went well into Tuesday. He called dozens of wrecking yards and LT1 re-programmers, parts suppliers and GM dealerships across the country. Nothing, nada, zip - no 1992 Corvette ECMs anywhere.

Just to eliminate any last "part problems" we ordered a new ignition module and replaced it. This was mounted it to the coil/module bracket with proper Heat Sink Compound. Then we spaced the module + coil bracket away from the cylinder head for heat protection from the engine. No change.

Sometime around mid-day Ryan had done some sleuthing online and found a number of 92 Corvette owners with eerily similar problems. One suggestion was to remove the PROM chip inside the ECM housing and re-seat it. Another said to "whack the ECM a few times, hard!" After doing those two things the car started up again, but still ran a bit rough. More tests were done, but it at least ran. We called True Street, who were backed up with tuning work for 3 weeks... I begged, and they agreed to call me back if they had an opening. As luck would have it they had a last minute cancellation and were able to sneak us in for a dyno check-up. I towed the C4 up there Tuesday after lunch and they got to work.

After I left, it wouldn't start again. Sean played with things, checked some codes, and late that day they had found a bad TPS sensor. With the TPS unhooked it would start, but you can't drive the car like this. We had replaced the TPS with a new sensor back in February, after I noticed a weird "throttle dead spot" at the January NASA event. Hmm... red flags were going up.



I spoke with Paul Costas later that night and told him the symptoms we were having, and he related a similar issue he had on his 92 Camaro (which is a GenI TPI V8, but used a similar vintage computer and EFI tech). About a year ago 92 started running poorly and burning up TPS sensors. He talked to an EFI expert familiar with this vintage GM EFI hardware, who said when these ECMs go bad they often lose their reference voltage outputs for 5V circuits like the TPS (Throttle Position Sensor) and IAC (Idle Air Control). He told Paul to test voltage signal at the TPS while driving... and sure enough it spiked way above 5 volts several times, which was what was burning up the sensors.

And we've just burned up another TPS sensor, and the car had a weird throttle response issue when it would run. And the idle was super high (IAC circuit) at some points. I was more convinced than ever that the ECM was dying and emitting some weird reference voltages. But with no replacement ECM for this car to be found nationwide, the chances of making TWS looked slimmer by the hour.



I was stewing over this for days and mid-day Wednesday I decided to scrub the TWS event. We had ECM problems and there was no way to fix it in 24 hours we had left. Crap, crap, crap!

What's Next?

Long term plans had always been to convert the car to a different ECM and harness from a later 1993-96 Base Trim Level Corvette. There were some EFI changes through the 1992-96 LT1 Corvette era which included:
  • 1992 was the first year of the TL1, speed density air metering, and had the weakest computer with the least number programmable inputs
  • 1993 was still speed density but had a more powerful computer with more inputs to alter
  • 1994-95 was another step up in inputs and computing power with the added change of a move to Mass Air Metering, but still OBD-I. Yet these can be reprogrammed via the OBD-I port, no more E-PROMS.
  • 1996 was another big change, such as the move to OBD-II standards, the ECM had a significantly more powerful processor, a LOT more data channels and programmable inputs, and also no longer relied on an E-PROM to change programming.

Checking with all of the parts suppliers we use it appears that new AC Delco ECMs for the 1996 Corvette are readily available, and around $200 (with the core charge). The other years are "iffy" or just downright impossible to come up with reliable units. So it looks like I am going to be chasing down a good 1996 LT1 Corvette engine harness, ECM, Mass Air Sensor, and intake tube next. We could not hope to pull this off in the one day we had remaining before TWS, so that event had to be scrubbed.



We had ordered new R7 Hoosiers, procured a 2nd set of wheels (for the scrub set of tires), event fees were paid, Amy had taken off work, arrangements for the weekend were set, and lots of money thrown at ignition parts that weren't bad - but hey, "that's racing."

Some of you might question scrubbing this 3 day race weekend - since the engine "technically starts" now. Sure, you can unhook the TPS and it might start, but it cannot be driven like this in anger. It is not reliable, and will almost certain break and/or run poorly once we get this thing on track. If we plug in the TPS it will burn that sensor up in short order.

I'm not giving up on this car, and it will likely be run in 2016 up until our "Shop Car" LSx BMW is complete. So look for us at NASA events in January and more likely some TEST EVENTS before then, to verify the new engine and ECM changes we have in store. My next update will be after we make the wiring harness and ECM changes.

Cheers,
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