1G 1G dsm MPI relay or ECU problems

[sakroda]

Hello everyone,

I have a 1990 AWD DSM running a 1991 ECU. Up until now, the car has been running perfectly with no issues, but recently it started leaving me stranded.

The problem is that while driving, the engine suddenly shuts off and the Check Engine Light comes on. I pull over and try to restart the car, but the Check Engine Light stays on continuously and the engine won't start.

I have a spare 1990 ECU, so I install it and the car starts immediately. Then I shut the engine off, reinstall the 1991 ECU, and it starts and runs again. However, I never know when it will shut off next.

One thing I noticed is that after switching back to the 1991 ECU, the engine usually shuts off again after about 3 minutes. After that, if I restart it, it may run normally again for a while before the problem repeats.

I also noticed that the MPI relay is making clicking noises when this happens. When i try to connect ecmlink it s not connecting to ecu at this issue.I don t get dtc code only light is on all the time.

Unfortunately, because of the country I live in, it is almost impossible to find replacement DSM parts. I do have an identical-looking MPI relay from a Hyundai. Has anyone tried using a Hyundai MPI relay in a DSM? Would it work, or could it cause different problems?

The strange thing is that I'm not even sure the MPI relay is actually the cause. Tonight I'm going to perform the factory service manual tests on the relay and wiring, but this issue is extremely frustrating.
I saw a pin i think it s pin 2 it melted the plastic inside of box.

Has anyone experienced something similar with a 1991 ECU in a 1990 DSM? Any ideas on what I should check next?

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[pma1123]

Show us some pics of the ECU boards; this MPI clicking stuff sounds a lot like erratic behavior with leaking power filter capacitors. If your car has been working great on a 91 ECU for a while till recently, I'd start there.

I'll let others explain the exacting differences with the 90/91 ECU swap. I recall there is 2 wires in the ECU harness plug that get swapped and also using 91+ power transistor to make the 91 ECU tach signal work in the 90 car. Just mentioning it for the sake of understanding; this stuff might already be changed on your car.

 

[dwb]

I assume you swapped pins 6 & 14 on the ecu harness for the 90-91 conversion.
Do you see the check engine light for a few seconds after turning on the ignition?
Have you checked out the capacitors and board on the 91 ecu for leaking and corrosion? Maybe snap a clear photo of the board and post it here.
Relays don't fail very often. Usually it's the power or ground source.

 

[sakroda]

Show us some pics of the ECU boards; this MPI clicking stuff sounds a lot like erratic behavior with leaking power filter capacitors.

I realize that my soldering is not well I have socketed by myself

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I assume you swapped pins 6 & 14 on the ecu harness for the 90-91 conversion.
Do you see the check engine light for a few seconds after turning on the ignition?
Have you checked out the capacitors and board on the 91 ecu for leaking and corrosion? Maybe snap a clear photo of the board and post it here.
Relays don't fail very often. Usually it's the power or ground source.

I've owned my 1G DSM since 2020, so it's been about 6 years now. Thanks to this forum, I've learned a tremendous amount over the years.

Yes, I've already completed all of the required 1990-to-1991 ECU swap modifications, including swapping ECU pins 6 and 14, as well as the 1991 throttle body wiring changes. As I mentioned, I've spent countless hours reading DSMtuners, so I'm fairly familiar with these swaps.

My next step is to test the MPI relay according to the factory service manual.

The MPI relay currently in the car had been opened and repaired by one of the previous owners. I also have an MPI relay from a Hyundai, and it appears to be identical—even the relay coils and connector are the same. However, I'm suspicious of cold solder joints. After inspecting the DSM relay closely, I noticed that some of the terminals don't appear to have been soldered very well.

I also built another EPROM ECU using a 1991 Hyundai Elantra ECU as the donor board and converted it into a 1990 EPROM ECU. It works perfectly, and the car runs without any issues using that ECU. The only downside is that it's a 64K ECU, so the address bus is different and I can't use ECMLink with it.

That's why I'm leaning toward the problem being related specifically to my 1991 ECU or possibly the MPI relay, rather than the rest of the car.

 

[dwb]

@steve might be able to see something on that board, but it's definitely a suspect imo.

I think the tale tale sign here is the issue disappears with another ecu. Unless you have an intermittent broken wire, what else are you touching when swapping ecus?

Makes me wonder why the mpi relay was repaired previously. Was it really repaired or were they just messing with it because they were experiencing the same issue.