Help us help you. By posting the year, make, model and engine near the beginning of your help request, followed by the symptoms (no start, high idle, misfire etc.) Along with any prevalent Diagnostic Trouble Codes, aka DTCs, other forum members will be able to help you get to a solution more quickly and easily!
I have a 94 nissan pickup 4 cyl 4 wheel drive, and it runs great until the engine warms up. The check engine light is on but I have been unsuccessful at getting the code to flash out. The plugs and wires were changed not very long ago and I put my u scope on it and the secondary waveforms look ok for the most part. Every now and then the burn line would be near the peak of the firing line, but it would stay level. It didn't look like a typical "jumping out" early shorted secondary winding. It was just the burn line all of the sudden jumped way up to near the top of the firing kv line. The primary current ramp is a nice perfect triangle with no vertical shorts.
For kicks I scoped the o2 and found it appeared to stay rich, but on throttle snaps would dive lean and oscillate a few times then settle back rich. So am I right in thinking the o2 is functioning and I do have a rich condition? Or could it be mis reporting causing the computer to pull fuel and give me a lean miss?
Should I be leaning towards a leaking injector, or a dirty maf causing the engine to think that it is getting more air than it is actually getting?
I was going to scope the maf voltage but have not found any sources for what the reading should be.
I suggest using a water bottle to see if there are any vacuum leaks. Perhaps something is getting warm and warping, allowing air to enter manifold causing a lean condition.
The water bottle technique used to look for vacuum leaks can be found in numerous ScannerDanner videos.
I'm also bothered that you can't communicate with PCM.
You have to do whatever it takes to get the codes out of it. I second the vacuum issue & would also look at EGR, if it has one. Really have to get codes from it first. That's what the check engine light is there for. Gives you direction, where you have none.
Ok I finally got the computer to flash out the codes to me. It took forever especially since I was using the wrong procedure forever, story of my life. Anyway, it gave me 41 which according to what I am seeing is the intake air temp sensor. So I got to looking and both wires are broken off right at the sensor. But they look like they have been broken for years, and the problems just started recently. Would the air temp sensor cause conditions like I am seeing?.
I would fix that issue for sure. I've seen some injector issues with these old trucks. I would OHM test (LOL) the injector or use the U Scope to do an amperage ramp and see what it looks like as well. OBD1 is a fun park to be in.
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