My 91 LS is having Issues with the computer, At least that is what i think...

It Gives me code 25 Indicating its Running Lean.

I Try to run a Test with a Snap-On Modis Scanner. During the test the engine is supposed to be help at 2,500RPM but at about 2,300 RPM while trying to get it to run at a Closed loop it losses Communication with the computer...

Is my ECM Going bad??
is there bad wiring?

 

Anybody have an idea...?

 

Check with a voltmeter at DLC1, Ox1 and Ox2 to ground. You should see a nice rapid swing from around .2 to .8 volts on both O2's. It can be done at idle or 2500. I bet one of the O2's is weak or sitting around .45 volts steady. The Modis needs supplied power from the cig lighter adapter for the TOY-1. Voltmeter is quicker!

 

new ECU cost me around $1200. Worth it.

 

when you can buy a used working one for ~1/10th that price? not worth it.

 

When you can replace 6$ worth of capacitors, it's really not worth it!

 

I Got a Few ECM from Ecology Auto Wrecking but had the same Issue.. I replaced the O2 sensors and fixed the DTC But still have a Communication Failure With the Scan Tool. So it Must Be the Wiring.

 

Probably not even that much but not everyone can change the capacitors, or they think they can't. It's pretty easy but some people don't want to dive into the electronics. The ECUs for a 91 are only $30 - $40 here so that would be well worth it. But wow $1200, nowhere near worth it.

 

The problem with used ECU's is that they are all about the same age. I suppose if you live in a climate that does not reach the extreme temperatures we see in the South, that would be an option.

 

It gets pretty hot here sometimes but not as bad as you have in Texas, especially this year. But I would much rather change 3 or 4 or 10 at $40 ea. then to change 1 at $1200.

 

I think his communication issue may be with his scanner. His year model is much easier to just use a good ol' DVOM. Anyway, I'm happy he got his problem resolved.

 

But i dont think i would get the same issue with 3 ECU's right?? i mean is that even possible all 3 with the same issue??

 

It is unlikely you would have the same communication issue with the three of them,yes. However, you stated you were using the MODIS platform from Snap-On. I have had the same issue with it and on some I needed to plug the 12v cig lighter adapter into the side of the TOY-1 adapter you are using. Sometimes that wont even work. You can jump terminals on the DLC1 or the DLC2 and use a voltmeter on all of the problems you are likely to encounter. This car is OBD1 and you will not get the data you may be expecting like on an OBD2 car. In my original post I outlined how to check the O2 sensors and you should do that! Its possible the O2 reading portion of your ECU was bad but, I have not seen one fail like that. The capacitors fail on the ECU and leak a brown acidic type of fluid onto the circuit board and this fluid is also conductive. Usually the failure is on the ECT Temp sensor leg or the TPS leg. Sometimes the fluid will bridge the injector drivers and cause an injector to dump fuel all the time. Code 25 will take a bit of time to come back but if it does, check the O2's. Or, you could just check them now!

 

B1S1 was lagging to switch voltage so i changed it.
B1S2 was ok but i changed it anyways.
B2S1 is ok but need to save to buy
B2S2 is Also ok but i also need to save to change.

My CEL Light Hasn't come back since i changed it and its been about 75 mile since the Sensor Swap.

I Was just concerned about the loss of communication.

 

OK, sorry to ramble on..... BTW: save your money on S2's as they only monitor the catalyst. Good luck!