Well, I'm back from hunting (got my pig btw), and decided to check something out this evening.

I'm not sure, but I think I may have the wrong ECU. I have a 37820-PM5-A080, and I think the stock ECU for a manual 88-91 CRX DX is a 37820-PM5-A060.

I can't find a breakdown of the ecu's by year/model, but I'd be interested in finding out if anybody knows if the 'A080 is in fact the correct ecu. That could explain things and would be easy to fix if it is the problem....

Is the throttle cable adjusted too tight? The ~4 foot long DPFI throttle cable actually shrinks and grows depending on engine bay temperature. If you adjust the cable to zero play when the engine is cold, it will hold the throttle open once things are warmed up.

It was suggested earlier that some of the sensors may be different. The are not.

The things that first came to mind for me was the possibility of a lose ground connection on the thermostat housing (a common occurrence when reassembling things after major engine work), or maybe you bumped and cracked a sensor when pulling the old or installing the new transmission.

Throttle cable is slack when at idle so that's not it. Besides the problem is definitely worse when its cold out.

I did some checking and the ECU is correct for a manual trans CRX. So unless it's a MPFI ecu and not a DPFI ecu, that's not the problem (still haven't figured out how to tell that).

I'll be checking the sensors for damage this weekend, but I'm pretty sure that isn't the case. I'm pretty careful about that kind of thing.

All PM5 (the middle group of 3 characters) ECUs are designed to operate the DPFI D15B2 engine.

The MPFI D16A6 engine (found in the CRX Si, Civic EX, Si and AWD wagon) is controlled by either a manual transmission version of the PM6, an automatic transmission version of the PM6 (as far as I can tell, found only in the automatic AWD Civic wagon) or a PS9 ECU (found in automatic Civic EX sedans).

The MPFI DOHC (dual over head cam) ZC engine is driven by a manual or automatic transmission version of the PM7 ECU. The ZC was never available in the US market... The closest thing we got was the D16A1 found in the '88-89 Acura Integra, which has significantly lower power output and a different bell housing/engine mount bolt pattern.

The MPFI D15B6 (found in the CRX HF) is controlled by a PM8 ECU... No automatic variants.

The DPFI D15B1 (found in the base model Civic hatchback) is controlled by a PM9 ECU... I'm pretty sure there are automatic variants, but not 100%.

Thanks Bobski. That jibes with the replies I got on the honda forums.. So I can eliminate the possibility of the wrong ecu.

I've been out of town all week till last night, and will be leaving again tomorrow. So no time to work on the car. Probably until next week I'll just be playing "armchair mechanic"... So if anybody has any other ideas, please let fly. Maybe I'll have a laundry list of things to check when I get the chance. For now all I have is to check for bad connections/broken wires, and to check the sensor values for a bad sensor. I think I've accounted for everything else.

Does your TB have a FITV? (Fast Idle Thermal Valve)

https://forums.trinituner.com/tech/fitv.jpg

Plug the bottom port with your finger inside of the throttlebody with the engine warmed up and running. If it idles normal after doing that than your FITV needs to be replaced or repaired.

That's an MPFI throttle body... DPFI engines don't have an FITV.

Correct. I wish it was that easy. In my experience, these first generation fuel injection systems are all a pain in the butt. My old '85 mercedes (Bosch Jetronic fuel injection) has a similar symptom and with that car, it can be caused by any number of problems. I'm converting it to EFI eventually, so it's not worth messing with.

.... Maybe I'll just bite the bullet and do the MPFI conversion on my CRX.....

Clean/replace the PCV valve? It could be getting stuck open, acting as a vacuum leak.

I wouldn't mind doing that but I've actually been unable to locate the PCV valve to date. I have one that I purchased when I got the car, but there's literally no matching part anywhere on the engine.

Other than that, I have a little update. I thought I had found the problem a few days ago when I noticed that the linkage seemed to be binding in the last few hundredths of an inch of travel before it closed. When the idle is high, I can manually close the throttle linkage and it will drop to base idle. But lubing the linkage didn't get rid of the problem, and I noticed that if I get the idle down, I can shut off the engine and then restart it immediately without touching the throttle, and it'll be back up again. I checked the two vacuum motors on the pass. side of the TB and they don't seem to be bumping the throttle, so unless there's another solenoid that I haven't found, or unless the TPS itself is binding up I don't think the sticky linkage is the problem.

Also, the idle is consistently hunting by about 50-75 rpms. With the airbox open, I can literally hear the injectors firing for a second or so, then stopping, then starting again, then stopping..... I'm starting to wonder about the O2 sensor maybe getting a bad value for the mixture and throwing things off. The wires are intact, but I haven't removed it to inspect it yet...