Die For MX

This is more of an ECU logic question.

First of all, I had the fuel pump relay out (the relay for switching from 9.6 volts to the fuel pump to a full 12+ volts under WOT), not the main relay. The other day, I walked outside with the relay in my hand, started the car(since it starts fine without it) then popped the hood and put the relay in its spot. Then I drove up the street after it was warmed up, as soon as I went to WOT it would bog and sound terrible and decellereate. Under vaccum there was no problem at all, completely driveable.

I figured it had to be something with the relay because it ran fine the last time i drove it(4 days prior). I brought the car back and turned it off, then turned it on again just thinking of how maybe the relay didn't activate. Took it out a second time and it did the same thing under WOT. So I was thinking WTF could be wrong.

Then I thought about how ECU's work, and maybe the logic of the ecu doesn't permit this to work. I pulled the negative battery cable, pressed the brake down for 10 seconds, did KOKO, just to make sure it would reset the ECU. I drove it after that and there was no problem whatsoever.

My question is, for those of you who know the logistics of these ECU's, why did this happen?

I don't know if this is correct but this is my conclusion.
-I turned the key forward and started the car with the relay out. Since the ECU didn't detect this relay it couldn't activate it, so it just ruled it out. When I put it back in and turned the car off and then on again, the ecu had already learned that it wasn't there so it didn't give the relay any supply voltage to make the armature of the relay activate to the NO(normally open)/12V side of the relay, assuming that this is a single or double pole double throw (SPDT or DPDT) relay. By resetting the ECU, you're giving it the chance to learn all inputs and process them and thats why it worked.

Does this sound remotely correct? Any criticism on my conclusion?

Die For MX

bump

FDjunkie

Certainly the ECU should have detected the missing relay and based on my chart of error codes it should have then turned on the check engine light with a code 51error. Did you notice the light being on?

Considering that error codes are not cleared automatically it makes since that once a critical error occurs the ECU sort of 'locks out' that circuit until its manually reset; presumably by a competent mechanic that's also checked the offending component(s).

Anyone else have any insights or better explanations?

Die For MX

FDjunkie, sounds very reasonable. I have an XS remapped stock ecu. Never saw a check engine light though, maybe its burnt out? This means I could have other problems that I don't know about. I know my high beam indicator light is burnt out, could be a fuse leading to that set of lights though. Does a beep accompany the check engine light? My FD has always been taken care of pretty well and I've never had to deal with a check engine light in the past. Over the past 2 years, since I've owned it, I've only put 12000 miles on it.
Thanks for the input
- Kevin

DamonB

My car is the same. You can pull the fuel pump relay and start the car but if you don't reset the ecu when replacing the relay the ecu will never reset its fuel control.