Hello! I have looked up enough threads and I cannot find any that can help me with my problem.
I have a 1990 GXL with 103k miles (about 5k on rebuilt engine). No major mods, just a Koyorad, exhaust, and suspension.

The Problem: It started going into limp mode when I would drive it somewhere, park it, and then start it up when it was still warm. It starts and idles fine, but limits the car to 2k rpms. I did a bunch of research and had everyone tell me it has to be my OMP considering the symptoms and the fact that it’s an S5. To temporarily fix the issue, I would unplug the battery for a few minutes and clear the issue. This whole time I never had a CEL and I never pulled a code from the diagnostic connector. The problem progressively got worse when it would go into limp mode about 20 minutes after driving around, aswell as starting when still warm.

What I’ve Done: Replaced the ECU with another S5 ECU that has a Rtek 1.0 chip.
Unplugging the OMP would throw a CEL prior to the replacing the ECU. After replacing the ECU, unplugging the OMP does not throw a CEL.

Does anyone have any idea as to where I should start looking for an issue? I can’t seem to find anyone who has any clue what’s going on.

 

Sure. Download and read the Factory Service Manual fuel and emissions section for your S5 engine. It will tell you how to pull engine codes from your stock computer. It's really easy, you only need a piece of wire to do it.. Once you have the code, let us know what you have and we will go from there.

Manuals found at
Foxed.ca
Rotaryheads.com

 

How to check the codes
RX7 FD & Series 5 Error Codes

The rtek for s5 does not throw a code or go into limp mode by design. Does the car still run bad with the rtek ecu?

you may consider opening up your ecu and seeing if the omp fried the ECU. It will do very bizarre things if it has.

 

I tried that and no codes came up. The CEL flashes and then turns off.

 

I opened up the ECU, and I’m by no means an expert or trained to see when something looks wrongs, but nothing looked out of the ordinary.

 

Ok, couple things.

Like KompressorLogic said, the RTEK ecus disabled the engine protection (limp mode) with regards to the OMP. This way guys could disconnect their OMP and run premix. I am assuming that you are still using your OMP, especially since you were running the stock ecu. If you reinstall your stock ecu and pull the codes, you'll likely find the issue with the OMP. If you aren't running premix, and your OMP isn't working properly, then your apex seals aren't receiving the proper amount of lubrication and will start scarring and galling the housings. I'm not a premix advocate, I like using the OMP as intended, but in this case better safe than sorry. Run premix fuel until you know what the issue is so your engine stays in working order.

 

I’ve been premixing since before I replaced the ECU just to be safe. Also I tried pulling codes with the stock ECU and there were none. With the chipped ECU, I still go into limp mode. So, I was thinking maybe it isn’t the OMP?

 

Try electrically unplugging the map/pressure sensor. And see if it runs better, Ive had a bad one of those cause super bizarre things before but not throw a code