strange issue with 78 280z running - I'm at the end of my rope!

[240260280z]

If you want to dig deeper there are heaps of ECU info here: http://www.classiczcars.com/forums/fuel-injection-s30/44688-280z-fuel-injection-good-system.html

[Captain Obvious]

Are you sure you aren't drowning your engine when the ECU gets the WOT signal? You keep thinking that fuel is being cut because the engine misfires... Maybe you're dumping so much fuel at WOT that it just can't light it?

[FastWoman]

OK, fuel isn't cut at WOT above 3k with the TPS unplugged. It IS cut at WOT above 3k with the TPS connected. Therefore we can assume there is a connection (perhaps a very high impedance one) between pins 2 and 18 when the TPS is connected to the ECU and the throttle is wide open. Somehow, some way, THIS IS YOUR PROBLEM. We know the TPS mediates this connection, but we cannot assume we have isolated the source of this connection to either the TPS, the ECU, or the wiring inbetween.

We have these possibilities that I can see:

(1) There is a high impedance salt bridge between 2 and 18 in the TPS. (See suggestions above for washing electrolytes from the TPS.) What resistance range did you use on your multimeter? If you used something like a 200 Ohm range, you're not likely to see the sort of salt bridge I'm talking about. You should use the highest possible resistance range on your meter.

(2) Your middle contact (18) floats off idle and below WOT (which is really just a "healthy" throttle, not necessarily to the floor). Perhaps the contact is hovering close enough to the 2 contact to bump it when the engine is vibrating hard. If the middle contact floats pretty evenly between 2 and 3, I'd say there's very little likelihood of the contact bumping. I've never heard of it.

(3) I'm tentatively ruling out a wiring problem that shorts pin 3 to pin 2 or that reverses pins 2 and 3, based on your observation that 2 and 18 have continuity at idle and at no other throttle position. However, I'm ASSUMING you mean that 2 has continuity WITH 18 and not with something else. I suggest, again, that you reconfirm 2 (idle contact) runs to 2, 3 (WOT contact) runs to 3, and 18 (center contact) runs to 18, and that none of these is shorted to ground.

(4) You re-flowed the solder on the ECU boards. I suggested a while ago, but I don't think you've investigated the possibility, that you might have introduced a solder bridge between pins 2 and 3 on the ECU board. That would be easy to do. If you have done this, then at either idle or WOT, there would be continuity between pins 2, 3, and 18. Otherwise there would only be continuity between pins 2 and 3. Of course unplugging the TPS would eliminate any continuity between 2/3 and 18, so the ECU wouldn't enter into a fuel cut state above 3k. I suggest you test for continuity between pins 2 and 3 of the ECU (not the 35 pin connector). If continuity exists, then there's your problem.

(5) I suppose it's possible there's a solder bridge upstream from the 35 pin connector and its board contacts. Without a schematic of the ECU, it would be difficult to identify possible trouble spots. The adjacent contacts 2 and 3 sound as likely a trouble spot as I can imagine. BTW, I don't think anybody has a schematic. The ECU uses proprietary chips in oil cans, so even if you can map out all the traces and small components, god only knows what's inside the oil cans.

[gheiser70]

Captain O - I don't get a misfire at WOT, the engine stops running like a switch was flipped. Tach remains stable however.

Fast - I'll try to clean the TPS as you suggested. I've taken the ECU apart and checked for solder bridging per your recommendation. Checked with the Multi meter where the pins hit the board and the pins themselves. No continuity between 2&3.

I was checking at 2000K ohm range (for the TPS switch checks), still showed open.

The inside of the TPS looks visually OK. I agree with you that the chance of contact jumping is small. I'll try to get some measurements but the idle contacts are well apart in WOT mode.

I'll also double check the wiring.

Greg

[FastWoman]

I suppose I should add...

(6) There's a possibility that the "logic" state for the idle circuit has become unresponsive and is either permanently high or permanently low. That is, your ECU might be bad. I've never heard of one failing in this way, though.

[Captain Obvious]

Captain O - I don't get a misfire at WOT, the engine stops running like a switch was flipped.

I guess what I'm saying is that if you're dumping so much fuel that the mixture won't light and you're misfiring on all the cylinders 100% of the time, then wouldn't that feel like a switch was flipped?

You could wire a small 12V bulb across one of your injectors and hang it out of the engine bay taped to the cowl or something. Drive with it attached and see what the bulb brightness does as you drive.

If fuel is being cut, the bulb will go dark. If fuel is being poured in there at great volume, the bulb will glow bright.

[240260280z]

After more thought I think the WOT fuel enrichment circuit in the ECU has a bad component that kills the pulse shaping when activated.

[FastWoman]

Er... Please ignore my #6 (last post). Obviously the involvement of a connected TPS falsifies that possibility.

Thinking about Captain's theory, assuming the ECU is delivering WOT enrichment correctly, there is nothing different >3kRPM (vs. <3kRPM) that would suddenly deliver excessive fuel and choke out the engine. However, if you look at it from a standpoint of engine load, perhaps 3k at WOT is where the engine begins to suck in enough air to move the AFM vane to a dead spot in the potentiometer, thereby mucking up fuel delivery. Maybe the 3k mark is just a cruel coincidence.

There's one thing I'd like to know: Does this happen only under load? Can you recreate the issue out of gear in your driveway? Can you lay the pedal to the floor and have the engine bounce around the 3k mark as though it's rev limited? If you can do this, then it would be interesting to plug a lightbulb in to one of the injector plugs and observe its behavior when this happens.

[gheiser70]

Fastwoman, I think your salt bridge insight may have been the answer. Yesterday I scrubbed the TPS with a thoothbrush and good old soap and water, plus some white vinegar for good measure. The rinsed everything in distilled water. I also did the same for the TPS connector in the engine bay. I then dried the components with compressed air. Drive way testing shows no cutout when revving to WOT. I haven't had a chance to road test, but I may today. As for the other question, the cut out does (did) take place whether or not the engine is under load, ie, the engine would cut in the drive way at WOT as well as on the road.

Now I just need to get out on the road, get her good and warm and see what happens.

[gheiser70]

So - More good news.

20 min road test with multiple WOT runs (I may have exceeded the speed limit -) NO cutting out was observed, just smooth acceleration. So, it appears the salt bridge theory was the answer.

Thanks to Fastwoman, and all others on the board that helped me through this troubleshooting adventure.

I'll keep you guys updated if anything changes, but right now I'm going to enjoy the sweet sound of the inline six humming along!

Thanks again to all.

Greg

[rossiz]

wow... you guys amaze me.

salt bridge??!? who knew!

[FastWoman]

I'm glad we ferreted out the problem! Damn that Ohio road salt! It's still taking its toll on my Saturn.

It's odd they'd make this circuit soooo sensitive. The conductivity of the short had to be > 2 MOhm! Maybe they wanted to pass as little current as possible to minimize corrosion in the contacts -- to make them last longer.

Anyway, congrats on getting your Z back on the road! Enjoy the ride!