[HawaiiGuy777]

Hello again want to make a thread kind of narrowed down some issues. 1990 GMC S15 4.3, I have replaced alot of stuff on this truck, fuel lines sensors, rebuilt tbi etc. Truck can run ok ish. So i unplug tan and black wire to shoot timing, do the timing truck is running decently with the wire unplugged. As soon as off the truck plug back in ESC bypass wire(tan/black) erase codes by unplugging battery. Soon as i start it up give it like 30 seconds to find rpm and such goes from running fine to running like complete crap like the timing is to advanced or retarded. Missing and such. Anyone can tell me what to check next? I’ve got a brand new computer in there with new eproms. It seems like to me there is noise in a signal wire that’s going to the ecm to control the ELECTRONIC SPARK CONTROL system nd telling it to advance/retard the timing a whole lot to where it’s heavily miss firing. Idk if the ground from the ecm to the distributor is grounding out or what have you. I need help! Lol

 

[94Krippler]

Just had a problem like this on my 94, i ended up having a bad brand new spark plug wire and I had wires 2 and 4 mixed up on the cap. Double check your wires positions and ohm your wires if you got a multi meter. 90 should have an isolated esc module, did you try a different one or a new one?

 

[ala_frosty]

I’ve got a brand new computer in there with new eproms.

New doesn't mean good. Grab your old ECU and plug it in and see how it runs. If it's exactly the same, then I suspect you've got an O2 sensor issue that's causing trouble when the ECU goes into closed-loop.

 

[HawaiiGuy777]

New doesn't mean good. Grab your old ECU and plug it in and see how it runs. If it's exactly the same, then I suspect you've got an O2 sensor issue that's causing trouble when the ECU goes into closed-loop.

where would i start to track the o2 sensor problem? Because i plugged back in my old ecu nd it does run the same.

 

[TomA]

Did you check the timing with the wire connected? What's it reading?

 

[ala_frosty]

where would i start to track the o2 sensor problem? Because i plugged back in my old ecu nd it does run the same.

If you know someone with an oscilloscope, you can hook up to the two wires across the O2 sensors and look at the readings. If not, then check continuity of your O2 sensor wires between the ECU and the sensors. I can't remember whether these are supplied from the battery or the ECU, but check wiring continuity to whichever. If that's all good, then you probably have one or two bad O2 sensors, but the 'scope would let you know for sure. Pine Hollow Auto Diagnostics and South Main Auto on YouTube have a bunch of videos where they look at O2 wave forms on various vehicles so you can see roughly what you're looking for under which conditions. O2 sensors are a lot cheaper than oscilloscopes.