jcorenman

Missing under load sounds like ignition, something arcing or shorting on the high voltage side. The spark voltage goes up with load-- increased combustion pressure requires more voltage to fire the spark plug-- and if something else "fires" first, then no spark.

And an ignition misfire will show as "lean" on an AFR gauge, which matches the symptoms. (A misfire leaves unburned fuel plus unburned O2, and AFR is measured with an O2 sensor-- not a fuel sensor).

Jim said the coils were new, correct? Both of them? I would go through everything on the secondary (high voltage) side of the ignition circuit-- anything that hasn't been changed/swapped since the problem started.

And also check the 12v side of the coil, make sure it is really 12v with the engine running (should be a direct connection to bus 15). And check the ground connections to the coil drivers.

Good hunting!

WallyP

I would lean toward a fueling issue for a couple of reasons:
With two individual ignition systems, a failure of one system should show up on the ignition monitoring relay - that's why it is there.
There are few failure points that will cause intermittent operation of both systems. Perhaps ignition switch or wiring fault in the primary side.
No mention of a wavering or falling tach needle, which should follow most of the faults that affect both systems.

Last time I was at Harbor Freight, I was tempted to buy their electric oil pressure gauge. Stick the sending unit on the fuel rail, run a wire in thru the passenger window, hang the gauge on the ash tray, and have real-time monitoring of fuel pressure. Might be something to consider in your case.

heinrich

Hi Jim, sorry you're having no fun!!! If it were me, I'd replace the o2 sensor just for grins at this point. Seen this sort of thing with frayed O2 sens wires.

Very best wishes,
Heinrich

928er

I'm going to stick with the in tank pump. my last one only 2 years and about 10K miles....

SeanR

If he doesn't do this by Saturday, I'll have a jumper on hand for him.

About a year old

If it was just a small, one or two cylinder misfire, I'd lean towards this, seen it before. But not 1/2 the engine. It acts just like 4 cylinder mode which is why I kept leaning towards the ignition system.

Have not, will do so. Thanks.

Need to get the ST hooked up so we can get a better look at what is actually going on, being the cheap *** that I am, I don't have one handy and Rogers are all out on loan. I also wonder about a bad coil, since only one has been replaced.

All the injector clips are in place. Thanks Bill.

Will get another coil and replace what ever one wasn't. I've not personally checked the coil voltage or messed with the coils. Oddly enough, when I had the car for some work it was for A/C, oil leak and found a vacuum leak. We thought the vac leak was the cause of another running issue. Since this never happened to me, I had no cause for further investigation. I hope to work though it this weekend with him. Thanks a bunch.

I got to watch the tach while Jim was driving and there was no wavering or anything else odd on the dash as the car sputtered under load. The IMR was also not lighting up. Tried two different relays and neither time they lit. I like your idea on the HF oil pressure gauge and setting it up like this. I'm going to pick one up, even if we don't use it on Jims. Thank you.

At least this one runs, sorry to read about the issues you are having. I'll re-check the O2.

Yes, two different ones and none lit up.

soontobered84

It IS possible to purchase defective new electronics. Although a new/rebuilt MAF was fitted, a bad MAF has not been ruled out positively. I would replace the MAF with a known good MAF to absolutely rule that out before I went much further.

Jadz928

Jim,
The symtoms sound like a bad MAF, very similar to my old '84 Euro. Biggest diff. I didn't hear the term "bucking", where it stumbles heavily... then w/ enough throttle, revs.

Other thing that coming to mind when I think of 2 coils, 2 ign. circuits are the 2 coil amps. I have a feeling they are nearing the end of their life-cycle, in general.
If you haven't replaced them (from what I read, you haven't), change them.

Landseer

Not much more to add

Weird, hidden harness break.

CE panel obscure melt or disconnect.

ROG100

Rebuilt MAF fitted and nothing changed regarding the problem - car ran generally better overall as one would excpect.

killav

Jim,

I will be camping this weekend but will be semi-local (up at Ray Roberts in Sanger). I will get the Sharktuner and wideband out of my car and see you Saturday at the GTG. We can have it hooked up in 10 minutes (can somebody bring a jack?) and see what is going on with your AFR.

Jim M.

Okay guys, an update. Drove the car to and from work today. NO bogging on either trip, but it would not idle until fully warmed up (end of a 20 minute drive) but even then at only about 3-400 RPM. My PEMS are set to GT idle of 850-900 RPM.

Tonight I checked the ground wires to the coils 0.4 ohms for each. The coil wires are factory and were ohmed when I put them on 30 days ago, along with new caps and rotors. One coil is new, drove the car with no change so moved the new coil to the other side and re-installed the 1st coil on the 1st side. Still no change so I believe both coils are good. Also checked the Temp II with the car still warm (4 hrs after shutdown) RH contact reads 688 ohms and the LH reads 687. I'll check them again in the morning when everything is cool. Also checked the injectors, all wire clips are present and the rubber boots are still soft and pliable. I believe this harness grounds at the back LH side of the block. I cleaned that ground last night, although it looked good with no corrosion and was tight.

When it was bogging the tach and speedo indications are always normal with no flucuations so I don't believe the ignition switch is faulty.

We have a sharktuner locally so I may have to get it back on and try to log the A/F ratio better than the ARM-1.

atb

My S4 had the 4 cylinder mode symptons, but it's an '88.

Turned out that one of the wires going to the output stage had worn though its casing behind the fuse panel. It was intermittent because it would only short out when it contacted metal.

If everything is new and checks out Jim, it may be worth poking around the fuse panel and just making sure all the wires are in order.

JWise

Jim - I have a ST2 and could come down Saturday if necessary. Just let me know...

Jim M.

Jarrod, thanks, we have a sharktuner locally plus I have to work Saturday so I may be leaving the GTG early. May just swap cars with someone if we get our sharktuner on it. Maybe get Mike to give me his turbo to try while he messes with the sharktuner. :-))))

docmirror

I need to get an oscilloscope on the outputs of the injector lines for both batches. It sounds like an intermittent short in the injector harness. I'm avail on Thursday, but going out of town for the weekend. Let me know. If that tests out good, we can monitor the ignition final stage inputs/outputs and make sure there's nothing funny there.

A failing ignition problem will create either the IMS shutdown, or it will backfire with unburnt fuel in the exhaust. I'm pretty sure this is a fuel metering problem. It can also be causes by incorrect resistance on the injectors themselves. Either a short, or an open circuit can affect all four injectors in the batch. That will make it run funny, without any IMS trigger or backfiring.

<edit: I think my Oscope is at Rogers house if you want to use it yourself. I can find the test points for you, or we can connect and do it together.>