OK, so check out this video:

'86.5 running after cold start*

I've had a "runs rough when cold" issue off and on for a while, but this is as bad as I've ever seen it. I've been driving less with all the lockdown stuff, so it had been a few days, and this is how it runs. I drove it around for a bit, it will surge and then hesitate, as if it's starting to get the rest of the cylinders involved, and then they stop firing again, although it seemed to be getting better as it warmed up. I got to work, was inside for a couple hours, came out and it ran like a top.Perfect. The next day I started it up and it ran perfect then too. Not sure what will happen this morning, but I'll bet money if I wait another day it will for sure run like crap again.

Wires, caps and rotors replaced when I bought the car, 5-ish hears ago. Injectors replaced shortly after that. Refurbished MAF a few years ago, intake refresh maybe a year ago or slightly more, plugs same, temp II sensor a few months ago (it had been running rough more often, although not this bad, and it seemed that the temp II fixed that but, now we have this, which has happened 3 or 4 times now, in the last couple weeks).

Nothing I've done recently should have anything to do with it...clutch, rear axles, slave cylinder.

*For me to get the video to play, I have to let it spin and load until I get the playback controls at the bottom, at which time I have to press pause and then play. Only then will it play.

 

Did you replace the engine speed (aka CPS) sensor when you did the intake refresh?

 

sounds like a misfire check the plug wires for proper routing and the coil wires that they are not rubbing run it in the dark look for sparks on the wires

 

I did not. I was thinking the PO replaced as part of his "make it run before sale" effort, but it may have just been the connector. I don't remember. I'm guessing that was one of those things you do at that time whether you need it or not, just so you don't end up needing it within a year? Would a failing CPS be so intermittent?

 

If I can get it to run like that at night I'll check that. Good idea. Naturally, I took it out today and it ran great.

 

google bosch crank position sensor for 928 look for the white collar on the sensor where the meets. if yours is tan or brown OR if yours doesnt have this then you probably need to replace it
NOTE if you take it out to inspect it and it doesnt say Bosch on it replace it.

 

Short answer: it can be.

Poor connectivity of signal wires and especially the grounding sheath - for example, when one just replaces the plug - can result in a signal so noisy that the EZK can’t pick out the correct engine speed. I have also diagnosed bad CPSs that looked perfect.

However, your description seems more consistent with one or more of the coils and/or coil amps failing. The latter definitely can go through an intermittent phase where the work for a bit.

 

Are those Nology plug wires? If so, get them off of there and install a good set of proper Beru wires with the right ends. Those Nology wires work great in my 914, but terribly in other cars I've tried them in and I've had one completely come apart in my hands so don't think they're very well made.

 

I have proper Beru wires on the car. As far as I know, the coils are all original, however, as are the amps (I'm assuming the latter refers to the bits up at the front of the engine compartment, not sure how else to describe them).

I'll also take a closer look at that CPS.

 

One thing that you can do to diagnose a poor CPS connection is:

take a piece - about 18 inches long - of safety wire , or mechanics wire, and put a loop around the harness plug connection. Run the other end of the wire to the edge of the engine compartment.
Put everything back together.
Then with the engine idling, jiggle your mechanics wire so as to jiggle the CPS connection. If the idle fluctuates or if the engine dies when you jiggle the wire, you found the source of your problem: wiring. Maybe the replacement plug. Maybe the sender side wire. Worst case: engine harness.

 

I didn't get very far diagnosing this, because I drove it every couple days and it didn't come back (unless it's the same root cause as a more subtle issue I posted another thread about, which it may be). Anyway, I took the car out for a drive last night, let it warm up, it was doing OK...still had that more subtle issue with the random misfires or whatever but pretty typical, but then right as I pulled into my driveway, the engine just died. Subsequent attempts to restart would either just crank, or it would catch momentarily, sound terrible (much like the video at the top of this thread) for a second or so and then immediately die again. I am guessing that whatever the original intermittent problem was, it has become more permanent. Which might be a good thing in terms of chasing down the root cause.

FWIW, I pulled the fuel pump relay and put in a jumper, and I can hear it running, and I can hear fuel moving about. The pump sounds a little noisy perhaps? But it doe seem to be working, and the car's behavior doesn't change when the jumper is in place.

There were three suggestions in this thread before that are intriguing: CPS, coils and the ignition amps at the front. CPS sounds like the most painful to deal with, so I thought I'd try to rule out the other two first. Short of simply buying new parts and replacing them, is there a good way to diagnose either of these? Given the comments, I'm particularly wondering about the ignition amps. In another thread I think someone mentioned opening them up but I thought I read horror stories about getting them back together again. Any thoughts on this?

 

The ignition amps will eventually fail as the thermal compound on them dried out long ago and now glues them to the heatsink.

I had mine intermittently fail on my 89 and replacing them got rid of the rough running/loss of power (you're 4 cylinders down if one dies).

They're fairly inexpensive at $100 for the pair and easy to replace, remove the heatsink and pop the old ones off with a screwdriver then install the new ones with fresh thermal compound.

The only way to test them is if 1 is still good so if you unplug one the car either dies or experiences no changes in running.

As for the coils the WSM outlines how to test them with their resistance. Easy enough to do, also check the ground straps on the coils as well. When they corrode or break they can cause all kinds of havoc.

 

I forgot about the resistance test for the coils, I'll do that, and I'll go take a closer look at those ignition amps. I think I'm past the point where I can do the unplug test...I'm a little suspicious that both of them would be dead but I'm tempted to just replace all of this stuff.

 

For grins, I tested my Temp II (~6 months old) and it measures within spec.

I pulled the cover off of those ignition amps, nothing looks obviously wrong. I didn't pull the modules themselves up, so I couldn't evaluate the thermal paste. In any case, if I can't think of anything else to try I may just replace those, since they're not expensive and they're definitely old.

The coil wires look reasonable. No corrosion visible when I pulled them off the coils. The (Beru) wire set is 4-5 years old. I didn't see any evidence of any shorting through the insulation, anything like that.

I measured the driver's side coil resistance: Terminals 1-15 measured 1.8 ohms, 1-4 measured 5.6K ohms. Now...that 1-15 measurement is a little off (1-4 is within spec) but it's close, and I suspect my meter is introducing some error there, at such low values I suspect it's not that accurate. If that were the problem, I would expect a number that's not particularly close, but someone please correct me if I'm wrong. It's raining on and off and my car is outside, so I didn't get a chance to measure the other side, but I think the car would start and run, albeit poorly, on one coil, wouldn't it? So I suspect this is not a coil problem.

I think I'm going to have to check that CPS next. Question: If the CPS is bad, would I see the tach bounce when I crank it? Because I still do see that.

 

I think tach bounce means the CPS is 'seeing' the rotation.