Wassup TGO?

Here's a good one for y'all experts

Camaro IROC-Z, bone stock, LB9, M5, 3.08, low milage, top shape maintenance.
My own custom tune, runs very smooth, no issue.
ECM is '165 from factory, but it says remanufactured by Delco on it. Don't understand that one!
EPROM is stock APYS
Moates G1 memory adapter
Moates 27SF512 chip

When cruising, ECM will enter Limp Home Mode (LHM) on/off rapidly making the car jerk around (like fuel being cut off). SES will flash on/off in synch with the jerk around. It'll stay in LHM for a while. The car is actually drivable. Then, SES light will turn off and car runs smooth as ever. Like nothing ever happened.

I know it's LHM because of following:

• SES light comes on solid and flicker like a fake Xmas candle light
• Idle goes to 1500rpm
• Runs rich and rough
• Fan is on steady
• Lack power

I did the following test: I removed the 27SF512 chip and left the EPROM with the memory adapter. Basically running the ECM without the chip. Same results, car goes into LHM. It will start and it's drivable. Runs with same symptoms except the SES will stay solid, no flickering.

No code stored
When it runs smooth, all data is normal on datastream

Soooo.... I am thinking there is something wrong with the ECM/EPROM/Memory Adapter/27SF512

I have inspected the ECM circuitry with magnifier. Nothing appears loose or burnt. While the engine was running, I have tapped & shaken the ECM, wiggle the wires and even rocked and tapped on the EPROM. Nothing would trigger the LHM.

I don't have another '165 ecm in stock, but I will get one soon. It's good to have as a back-up I guess.

I can't get my hands on a replacement APYS EPROM. Quite rare.

I have changed the 27SF512 chip and Memory Adapter for other ones I had in stock. I am waiting for good weather for testing.

I guess my question: Is there anything else that can make the ECM go into LHM (real LHM, not Fail Soft Mode)? https://www.thirdgen.org/forums/diy-...ode-final.html

I'm reading about bad ground, but I would think that the ECM would not work at all with bad ground. Not sure. Plus the car is almost Concours condition. Never been hacked or tempered with. No rust or any kind of wire corrosion.

Any input would be appreciated as I am just starting the trouble shooting process.

Thanks

 

@Tuned Performance

 

Can you datalog to see if you catch a code ?
im thinking a piggyback connection issue or bad ecm .

 

Thanks!

That's what I am thinking also.
I could try datalogging. Thing is, It could take some time for it to happen. And sometimes I lose my connection if engine stalls.

 

I wish you were closer to loan an ecm, I’m thinking that’s the issue. Sounds odd but if you wrap on the ecm with a screw driver handle while running it flexes the circuit board. If it acts up I’d say bad ecm.

 

Ummm, I know you mentioned that in different threads. I did: "While the engine was running, I have tapped & shaken the ECM, wiggle the wires and even rocked and tapped on the EPROM. Nothing would trigger the LHM". Maybe I did not hit hard enough lol. I'm a bit careful not to brake something that is not. I'm thinking it could be related to heat. It seems to happen more with cold weather and running the heater. Again all speculation.

I'll get an ecm and do more testing.

Thanks for input

 

Probably not this. When I got my 88 it was doing some weird things, not exactly this though. Took about a year for me to actually find that one of the main grounding connections at the back of the head was loose and burnt/corroded. Tough area to get into but I got all the connections cleaned and together tightly. Major difference in how it ran after that one fix. Worth a look at that and all other grounding points. Those intermittent deals are tough.

 

Yep the famous ground gremlins! I will go over all of them again but I doubt. Like I said before: "So what ever you read or someone suggests checking, check it, recheck it and triple check it"
So I'll practice what I preach lol
Peace

 

Some kind of intermittent electrical noise getting into the system? Did you change/add some new piece of electronic hardware elsewhere in the car? Failing alternator A/C suppression diodes perhaps?

Also I thought certain errors had to linger long enough to actually store a code, otherwise the SEL may come on w/o storing a code? RBob or one of the other gurus may know for sure...

 

Thanks for input

Electrical noise I doubt
Bone stock no add on
Voltage is steady on gauge

 

I had the exact same problem 15 years ago with a 7730 ECM. After a month of testing, head scratching, bunch of new parts and a lot of curses to the damn car I figured it was a bad ECM. Not the memcal, it was the ECM itself that caused all the trouble.

 

Thanks for input

 

Update:

I was thinking of doing multitude of testings by changing one thing at the time. Well, I don't have the time and I want to enjoy the car and not get stuck "limping home" with a rich engine.

So I changed the following:

New 27SF512 chip
New Memory Adapter
Reman ECM from Cardone 77-6571
Disabled VATS in tune
New battery (it was planned anyway, I changed them every 5 years)
Checked all grounds, all good

Most likely cause was: 27SF512 chip or Memory Adapter or ECM

So far so good. No more in and out of Limp mode. But I will wait before calling it a win.

 

Occasionally when I'm doing some real time tuning, and I do an upload to the Ostrich, something will malfunction and it'll go into limp mode... and yeah, it'll start running at around 11:1 AFR. I'll have to shut the car off, do another upload and then restart the car.

 

Yep, common issue in real time. I don't use it anymore because of it.

Not the case here though. No Ostrich involved.

Thanks