Battery got hooked up backwards, (don't ask!)- car now idles very rough after warm-up. Any suggestions?- mechanics are shooting towards the $1300.00 ECM. What about a used ECM with matching numbers, what does it require to be used in mine? Will a new ECM require loading or have to be flashed?

 

But it runs smooth when the motor is cold? Is the current ECU throwing codes? Sounds like you need more explanation for a new ECU than just "rough idle". Rough idle usually indicates leak somewhere.... Why was the battery out in the first place? Was your MS having other issues prior to reverse battery hook-up?

 

It runs fine when it's cold, it also seems to run fine if you can baby the accelerator beyond 1k rpm. The ECU isn't showing any codes, even if you purposely disconnect various sensors like IAC or throttle position. The battery was out because it was dead, and at 5 years old, needed to be replaced. The only problem I had before was a shudder or vibration during acceleration from 40 mph and an occasional check engine light. I've recently seen that the shudder at that speed is very common with this model. The check engine light never bothered me, at 135,000 miles, it's almost expected, and showed no additional problems. The dealer is now saying that there is another computer that has to be changed along with the ECM, I can't just change one. The mass air flow sensor was changed, it helped it run smooth for about 20 minutes, then it went right back to running rough again- thanks for the help!

 

Hmm - Im a skeptic to believe by hooking the battery backward that you will fry not only 1 but 2 major components!!! I guess it could happen but Im still wondering if its something more simple. I was hoping that the rough running was due to the computer re-mapping itself after having its memory cleared. Unfortunately, I cant think of anything off the top of my head. Maybe someone on the board has some ideas. I would be careful throwing big $$ parts at the problem unless I was confident that it would solve the problem. What happened when the battery was hooked up? Was the car started? Was it on while it was installed?

 

Replace the fuel pump module located in the front passenger kick panel. I've seen this many times when i worked for Mitsubishi. Apparently, jumping the Sport in reverse polarity is a common occurance and this fuel pump ECU is one of the 1st to go (along with the ALT fuse at the B+ cable). Good luck!

Joel, CA

 

I'm skeptic for the same reasons- I don't want to spend big money on it unless it's going to fix it! Nothing was done while the battery was backwards. The evidence towards ECU is that I don't get any codes at all when it's running rough, or if I purposely disconnect sensors???? I'll double check the fuel pump relay, and I am going to try a IAC servo first. If I change the engine ECU, I also have to change the immobilizer ECU and ignition key????

 

Interesting data Joel. I did not think that fuel pumps in general cared about polarity. But then again, you said fuel pump ECU! I would definitely make sure all fuses are in tact first! Where is that ALT fuse located? I would think that any modern car would have its electronics protected against reverse polarity. If it really does fry major components the battery manufacturers should key the batteries more obvious than just 2 slightly different OD posts.....

 

The ALT fuse was fried, it is a 100 amp fuse in-line to the fuse box under the hood, big blue one, 2.99 from Autozone. I would have thought that would have protected everything else, but evidently something got through before that blew. I've been thinking all along that they oughta safeguard the terminals somehow from just such a ocurrence.

 

I presume that the ALT fuse was replaced previously OR did you just discover this? Did the fuse help your overall problem?

 

he is 100 percent correct.