91 Comanche 4.0 dies when warm (Fixed!)

[mfendley]

2 hours ago, HOrnbrod said:

 

Yes - same connector.

Great, just picked one up.

[kryptronic]

On 10/13/2018 at 1:22 PM, mfendley said:

Checked the fuel pressure, was between 38 and 40 psi.

 

20 hours ago, mfendley said:

with no load on the engine, fuel pressure never dropped below 24 psi.

 

Run a proper fuel pressure test and try to duplicate your results.  With the engine running and no load it should be 39 PSI.  With the vacuum line for the fuel pressure regulator disconnected, it should drop to 31 PSI.  

 

You keep going back to the issue starting with the coil and harness.  It only happens after the computer kicks on, so it's electical.  You've replaced so much of it at this point that I think you have to go over it all again and completely rule out bad new parts and proper gounds with proper wiring.

[mfendley]

On 10/13/2018 at 10:29 AM, HOrnbrod said:

Here's the 94 coil and wiring harness on Ebay. These are rare and tough to find.

 

From the FSM:  Correct ECU for a US 91 w. standard trans., non-Cali, w. RFI:  56027532

 

 

 

SUCCESS!!! The 56027532 (used) arrived today, and I installed it prior to installing the new distributor or fuel pressure regulator. Started right up, so I went for a quick test drive, since we have been able to repeat the issue in 6-10 minutes. Drove it 30 minutes without an issue, so I turned the keys over to my some who have been driving it the last two hours. Other than stopping for gas, the engine has not been off, and has not skipped a beat.

 

Thanks to everyone here for the assistance. I going to find another one of these ECU's - one this this has taught me is to build up a cache of spares, becuase some of the parts are getting hard to find.

[HOrnbrod]

Good news. As always with a new (to your MJ) ECU, all residual memory should be cleared so the ECU/PCM can start from scratch. To clear the residual memory, do the following  procedure:

 

Clearing error codes and removing residual memory

1. Disconnect the POSITIVE battery Terminal and touch it to ground for 30 seconds. (This is to discharge the PCM capacitors, which maintain the Adaptive Memory.
2. Reconnect the Battery Cable
3. Turn Ignition Switch to the "ON" position but DO NOT start the engine
4. Turn Headlight "ON"
5. Turn Headlights "OFF"
6. Turn Ignition Key "OFF"

 

The PCM Adaptive memory has now been flashed, or erased from the PCM.

[Minuit]

This is honestly one of the very few times I've seen an ECU, HO or Renix, be the actual cause of a problem on this forum. Wonder what went wrong with it?

[youngfred]

2 hours ago, HOrnbrod said:

Good news. As always with a new (to your MJ) ECU, all residual memory should be cleared so the ECU/PCM can start from scratch. To clear the residual memory, do the following  procedure:

 

Clearing error codes and removing residual memory

1. Disconnect the POSITIVE battery Terminal and touch it to ground for 30 seconds. (This is to discharge the PCM capacitors, which maintain the Adaptive Memory.
2. Reconnect the Battery Cable
3. Turn Ignition Switch to the "ON" position but DO NOT start the engine
4. Turn Headlight "ON"
5. Turn Headlights "OFF"
6. Turn Ignition Key "OFF"

 

The PCM Adaptive memory has now been flashed, or erased from the PCM.

...

 

 

I just installed that exact ECU that I got from AutoComp Technologies out

of Houston for my 91.

Do I need to clear the residual memory of the ECU from a Remanufacturer, also ?

 

Thanks for the details on clearing residual memory.

 

 

youngfred

 

 

[HOrnbrod]

Yeah, there's residual memory in there from the remanufacturer's QC testing. For sure wipe I'd it clean and let it start gathering fresh info from it's new home. 

 

Just for grins, did you check for codes with the new remanned ECU after installation?

[youngfred]

2 minutes ago, HOrnbrod said:

Yeah, there's residual memory in there from the remanufacturer's QC testing. For sure wipe I'd it clean and let it start gathering fresh info from it's new home. 

...

 

 

Thanks Mr. H,

 

When I was looking for an ECU for my 91,

I could only find California ones.

 

I asked NAPA about installing the Cali one

in my non-Cali, and was told that it would work

but that the 'Check Engine Light' would not go off,

if I installed.

 

He then referred me to AutoComp Technologies

and sure enough, they sent one -arrived within two days.

 

 

youngfred

[HOrnbrod]

It would be interesting to see how a NOS Cali ECU would perform on a non-Cali system. They are quite plentiful and are available for reasonable prices, even NOS ones. Since they have different Mopar part numbers than the non-Cali ECUs, there might be internal differences, but I doubt it as 1991 was the first year of OBD1 and things were a bit primitive then. I think wiping it clean then running it in a non-Cali 91 XJ/MJ it would settle in just fine w/o throwing any codes.

[mfendley]

8 hours ago, Minuit said:

This is honestly one of the very few times I've seen an ECU, HO or Renix, be the actual cause of a problem on this forum. Wonder what went wrong with it?

 

I did find the the CPS sheath and wire had come into contact with the exhaust manifold and melted down to one of the wires (didn't check which one before discarding). When I did the resistance check, it was 77K ohms, compared to the 91M ohms of the new one. I wonder if this now shorted to ground signal wire connected to the ECU caused a component to fail? This was a Cardone unit that I had purchased from Rock Auto, so I wasn't 100% sure it was the best fit for the vehicle. It had been swapped out when my son went through water after IRMA and shorted out the distributor and computer.

[gogmorgo]

Wouldn't surprise me either if the CEL wouldn't illuminate for a Cali ecu in a non-Cali application. Even the best parts guys won't know every single bit of programming on every ECU. In many cases like that it's an educated guess, unless they have specifically done that, put the wrong ecu into an XJ. 

But the other reason it wouldn't surprise me if the cel stays off is it doesn't always seem to come on even when the ecu throws codes.

[youngfred]

4 hours ago, gogmorgo said:

Wouldn't surprise me either if the CEL wouldn't illuminate for a Cali ecu in a non-Cali application. Even the best parts guys won't know every single bit of programming on every ECU. In many cases like that it's an educated guess, unless they have specifically done that, put the wrong ecu into an XJ. 

But the other reason it wouldn't surprise me if the cel stats off is it doesn't always seem to come on even when the ecu throws codes.

...

 

 

At first, I thought that you were doing - a drumpf imitation.

 

When I asked the NAPA guy about installing a Cali ECU in

a non-Cali, his reply was, " The CEL will always stay 'ON' ''.

That's not good!

 

I've had good luck at NAPA.

The 'boys and girls' at the Chains, seem to always give me - 'that look'.

HUH???

 

Now, I find the part that I need, on-line/checking store stock -

screen print(photo included), and hand it to them,

and point to the Highlighted Part Number.

 

Saves-a-lot-of-time.

 

 

youngfred

[gogmorgo]

I talk to Napa at least once a day for work. They're usually pretty good, but there are some things you just can't know until you try it out. The good parts guys know a lot, but they're not mechanics or engineers, not that that's the end-all of knowledge either. 

All I'm saying is you can't know how something will behave in an unconventional application unless you know exactly how the thing behaves, or have done it already. Stored codes don't trigger CELs on an OBD1 unless there's something wrong at that exact second, so unless there's extra emissions equipment controlled by the ecu that it wouldn't be seeing on a non-Cali Jeep, I'm not sure how the ECU would know it's in the wrong Jeep. The only California-only parts I can find in the manual are ECU, catalytic converter, and emissions stickers. I'm not seeing wiring harness differences, or extra/different sensors, or anything. There's no downstream O2 sensor (at least not in the manual) so the ecu can't tell what the cat's doing, and I don't expect it can read the sticker either.

Maybe the guys at Napa have put an OBD1 Cali-only ecu in an OBD1 non-Cali Jeep and observed it, maybe they're basing it off experience putting a Cali ECU in some other non-Cali vehicle, who knows. But it doesn't make sense to me that it would trigger the CEL. But hey, I could be wrong.

[youngfred]

...

 

 

The only thing I thought of, when he said that,

was - processing lots-of-ECU-customer-returns.

 

 

youngfred

[gogmorgo]

I don't know of many places that would allow you to return an ecu after it's been installed, either, or many other electronics, simply because you don't know that the new ecu didn't get cooked by whatever problem was originally on the vehicle. Or if the problem had anything to do with the ecu. Maybe I'm just cynical, though.