Surging problem and cause

[kevinde1]

Well I put in the new AC Delco ICM and same result. Injectors completely shut off at idle. Yes the distributor and ICM was replaced by the previous owner and is new. It's some aftermarket brand not sure what. I tested the pick-up coil with my volt meter and it seems to be fine. I'm stumped on this one. I guess it could be the ECM but has anybody ever seen such a problem? Seems like when the ECM fails they wont do anything at all, but I don't have a lot of experience with them. Does anybody have a diagram of the ECM harness so I can start testing it?

 

[kevinde1]

I have read and heard from numerous sources that there was a lot of problems with this these TBI's (ECM's failing, ICM's failing, Distributor problems). Does anybody know what those problems were caused from and if they can be fixed without converting to a carb? I would hate to replace an ECM only for it to go bad in few months. Thanks for your replies

 

[kevinde1]

https://www.turboimagehost.com/p/31480984/6293071.jpg.html

Ok using the ID chart above I did some test on the wires at the ECM harness yesterday with the engine running. I didn't get to go through every wire but I did test the ones highlighted on the chart. My EST REF LOW (BLK/RED) was 0 as it should be, EST REFERENCE HI (PPL/WHT) was 1.4 chart shows 1.6, EST CONTROL (WHT) was 0.20 should be 1.0, EST BYPASS (TAN/BLK) was 0.00 should be 4.75. I'm going to run this test again today after I do a computer reset to see if it shows something different, but at this point it seems that my ECM is bad. Does anybody have any thoughts on this?

 

[kevinde1]

Found a good article about GM's ICM

http://www.gmtuners.com/tech/CTS_ICM.htm

http://www.gmtuners.com/tech/modes.htm

 

[1987 GMC Jimmy]

It sounds like it should be tripping a Code 42 with those readings unless the ECM is messed up. Is your tan/black EST bypass wire on the driver side firewall hooked up? Is there any difference in how it runs with it unplugged if it was plugged? Also, have you looked into the ESC module? My brother's '93 truck was doing a really bad surge that it couldn't recover from sometimes, and I diagnosed a problem with the ESC. A replacement of the PCM had to happen in that case (ESC moved to the inside of the computer in '92 and on automatic TBI's). Yours is gonna be on top of a carrier on the passenger side of the intake. I don't know if playing with that would be a fruitless endeavor, and you could try it, but it is sounding more like the ECM to me.

 

[kevinde1]

Well at times it did seem to make a difference plugging in the tan/black wire.
but yesterday there was no difference. What is the ESC? is that where my egr vacuum plugs in?

 

[kevinde1]

Nevermind I found it. I will have to check that out today when I get home. I plan on having this thing figured out this weekend. Fingers crossed

 

[1987 GMC Jimmy]

Well at times it did seem to make a difference plugging in the tan/black wire.
but yesterday there was no difference. What is the ESC? is that where my egr vacuum plugs in?

Okay, well that bypass wire has to be plugged in unless you're setting/adjusting the base timing. And if it's ever unplugged and Code 42 is set, you should disconnect the battery for thirty seconds to let the computer reset after plugging it back in. My Jimmy ran really fruity when I didn't do that. If it was unplugged, the vehicle was ran, and it didn't set a Code 42, you either have a wiring problem or a bad ECM. The ESC (Electronic Spark Control) module is the little square disk looking thing above the EGR solenoid on that carrier. It's got a flat connector with a bunch of wires. It works with the knock sensor to adjust timing. I'd look more into the aforementioned EST stuff, though. I'd plug that wire up, reset the computer, and see what happens. If it still runs bad, I'd unplug the bypass and see if you get a Code 42. If that wire was clipped, frayed, or chewed, you should have a Code 42 all the time. The fact that you don't really makes me suspect the ECM. Maybe this was covered earlier, but are your ECM fuses okay?

 

[kevinde1]

Yea I had the wire disconnected before I started testing the wires at the ECM and didn't reset the computer so I was kind of thinking that may be why I wasn't getting any volts after I plugged it in. Do you know what kind of test I need to do on the ESC to tell if its good or not? I've never got code 42 but I wasn't checking it at the time either. Any other time I checked for codes it never came up, so I'm thinking reset the computer and try again. ECM fuses are fine.

 

[1987 GMC Jimmy]

Yea I had the wire disconnected before I started testing the wires at the ECM and didn't reset the computer so I was kind of thinking that may be why I wasn't getting any volts after I plugged it in. Do you know what kind of test I need to do on the ESC to tell if its good or not? I've never got code 42 but I wasn't checking it at the time either. Any other time I checked for codes it never came up, so I'm thinking reset the computer and try again. ECM fuses are fine.

I don't know how to test the ESC other than unplugging it and taking note of a difference in running. I couldn't find any tests or voltage parameters, either. What I can offer you is this. You should get an SES light as soon as you unplug that EST weatherpack connector. Is your bulb working? It should self-test when you first turn the key and then go off. It should have also come on solid if you turned the key to on with the computer out of it. If that's not happening, and the SES bulb is good, I would further want to look to the computer for my problems. Maybe @chengny has the ESC voltages. Here's a diagram of the ESC.

 

[kevinde1]

Ok thanks, I have that diagram but still thanks. Yes my check engine light works and I have confirmed it by unplugging MAP sensor. I don't get any codes when everything is hooked up properly.

 

[1987 GMC Jimmy]

Alright. Well, when it comes to ECM stuff, they can fail partially. I've had several experiences. Those experiences, however, were on the earlier CCC computers and the later OBDI powertrain computers. The 7747 ECM's commonly found in '87-'91 trucks usually do very well and are the most popular model for tuning because they're so simple. The fact that the computer finds nothing wrong when you unplug the EST bypass bothers me, and it should bother you, as well. I think you're getting closer to the problem, for sure.

 

[kevinde1]

It could have been on when it was unplugged and I just didn't notice I've had all the sensors unplugged at one time or another but I'm gonna check tomorrow to make sure. Got my EMC in today so going to over all my grounds again give it a try tomorrow.

 

[kevinde1]

Alright new ECM is in and no change but here's what I found.
Checking the voltage at the ECM with key on voltage is at 4.6v which should be 4.9v although with engine running it is .7v and it should be 2.0v, another thing TCC voltage 0v and should be 14v running and no speed sensor signal 0v should vary from .60v , the EST control was reading high 2.2v should be 1.0v
Where is the TCC control and Speed Sensor? Does speed sensor signal come from there?

 

[1987 GMC Jimmy]

The TCC is in the transmission, and the connector for it is on the driver side on the lip above the pan next to where the governor is located. Make sure that's plugged in, and make sure non of the wires are chewed or cut. The VSS buffer is a little green plastic thing on the back of the instrument cluster. It maybe unplugged or bad, but I've never heard of them going bad. I doubt that those things would affect what's going on with your vehicle. Those are definitely things that should be rectified, sure, but there's something else. How's the ground for the ECM, and how are all the grounds in general? It sounds like the ECM ground is poor or you're loosing power somewhere.