TJ993

Just read entire thread. Wow! What an ordeal. Hoping the Spring Runs puts that 993 smile back on your face!
Cheers
Thank You for sharing.
Lots for me to consider before I enter the S $$ Slope for sure!
TJ

SwayBar

Have you done a compression and/or leakdown on the new engine? Perhaps the faulty ECU advanced the timing too much causing detonation and cracking a ring - I hope not.

JPS

I haven't yet, and I may, just because it was new and ran at high revs suboptimally. But neither the diagnostic shop nor the build shop felt it was demonstrating any indication of a core engine issue, and thus didn't feel the need to do either.

JPS

Should have said hello!
I'm terrible at connecting RL folks to their real life side.

JPS

So, it's still not working.

Even after what I thought was an ECU repair, it was better, but after a long drive (which could have been better or could have been lower ambient temps in Nov/Dec) I gave up and got a rebuilt ECU. Completely different from mine. Rebuilt. Still has the problem (severe timing retardation after it gets hot and stomping on it).

I've replaced:
2 coils/wires
Ignition module (several times)
Fuel pump
Distributor/rotors/caps, etc.
Plugs
Pulled and cleaned the fuel injectors.
Wires were replaced just a few years / 30k miles ago

Consulting Steve W next week, but open to suggestions.

AOW162435

^ wow. Sorry you’re still dealing with this.



Andreas

CT944

Assuming all the big stuff was replaced or repaired, any thought on any of the temperature or position sensors being bad, could they be telling the ECU to do something they shouldn’t when the motor gets hot? I’ve had the same types of issues on a 944 and a 500e due to either bad temp sensors, position sensors and cracking wiring harnesses. Not sure if the 993 has those old plastic bodied fuses, but I changed all those on a 944 as well as the ECU relay and a lot of gremlins went away. Also cleaning the ground straps helps with electrical issues. Also, unplugging and cleaning relays and reinstalling can help ensure connections are good and tight.

JasonAndreas

Ignition timing is only pulled in response to a sensors input. What does the "Actual Values" menu in a Bosch Hammer or the like, tell you the sensor values are when this is happening?

JPS

ODB1 doesn't give a ton of info...

JasonAndreas

The sensor values are exactly the same with OBD1 and OBD2. When the timing gets pulled: what are the intake and cylinder head temperature values? knock count? Do the MAF values correspond with engine load?

CT944

Exactly.

JPS

So kinda getting back to this. Seems a few others have had problems with ignition modules failing, so want to share to avoid the pain, expense, and missed track days I've had.

Rebuilt due to the above cracked valve. Decided to go "mild" 3.8 as above.

Originally slower at the Glen than pre-rebuild. Looked like the ignition module had a bad channel, so running on 6 plugs. Replaced.

Back to the track. Now when hot and consistently above 4k RPM, would sputter or stop revving when punching it from 4k to 5k, sometimes even hesitating at 5k.

Unplugged coils sequentially. Car died on one. Replaced coil (I thought we replaced both, but turns out later I was wrong).

Same problem.

Checked so much stuff I forgot, same problem.

Sent my ECU out, one problem found, and vendor upped the signal to the ignitor. Same problem. Replaced ignition module again, same problem.

Sent injectors out for cleaning. Reduced flow and poor dispersion. Figured that was it. Nope.

Swapped the ECU, still a problem. Replaced plugs and distributor. Seemed to be better. Swapped the ECU back. Problem came back (or just resurfaced, never sure).

Gave up and bought rebuilt ECU. Problem still there. Livid.

Replaced ignition unit again, seemed to be better for awhile, then got so hot the mechanic burned his hand on it.

Replaced ignition unit and replaced what I found out was the 1, not 2, coils replaced with an aftermarket coil (grrr.) Replaced with 2 OEM units.

So far haven't been able to replicate, but haven't been on the track. Summer, vacation, work, etc. Hoping to get out in Oct/Nov and flog it. But hopefully that fixed it.

Points to the need as per above to have a mechanic who is not afraid of diagnostics. Living in NYC the car was always in a shop that was too far to do anything myself, and not sure I would have figured out the oscilliscope myself. Plus, since it only happened after 25+ minutes at above 4k (well, for awhile it was only 10 min, then over 50), it was also pretty hard to replicate. The signal may have been fine cold, who knows. But one really needs not just a repairing kind of mechanic on these older cars, but a diagnosing sort of mechanic.

In any case, fingers crossed.

Chuck W.

I just read all five pages..... wow. Fingers are crossed.

MarinS4

Without diving into the specifics I can offer you some good guidance for troubleshooting. Between my time as a mechanic and color copier technician I’ve become quite good at problem solving.

The most difficult problems to solve have been technician induced from what I’ve seen. I suspect this will be the case here as well. I don’t believe you have a component problem but rather an electrical connection problem.

Because of the nature of the problem it’ll be tough to rely on a mechanic to solve it. You (or someone competent) will need to take it on and own the problem. Mechanics just can’t camp out on your car the same way you can. It’s not financially or logistically feasible.

Arm yourself with a good meter and scope if possible. Before attempting to replicate the problem I’d start with ohming out the ground connections. In all likelyhood you’re loosing ground when things get hot. BTDT! Start pulling on wires in the engine bay looking for loose connection issues. Retrace all the steps the guys would have taken to R&R the engine. The factory engine harness is a known problem. All it takes is for some logic circuit to have a slight short to ground and that in turn causes the ECU to get confused. Seen this many times on copiers. One circuit does not have to be related to the other, it’s how they travel internally in the ECU and how that voltage feedback, loss of ground and arcing connection scrambles the ECU brain. You may never truly understand the why but I am certain you’ll find something suspect and like magic all your problems will disappear.

Good Luck!!!!

JPS

Well I'm hoping it's fixed now so we shall see.