924Board.org

[bureau13]

You are definitely correct about #1!

The problem is that it idles great, but at very light throttle, ie driving away slowly, it starts to buck and surge and that corresponds to the lean spikes. It very much is not just a wideband issue.

I then did a smoke test, found and fixed vacuum leaks, same problem. Did another smoke test, found and fixed more vacuum leaks, same problem. Smoke test now seems good. Tested frequency valve operation best I could, I can't really measure duty cycle but it *seems* to be behaving as expected.

I have a recently-installed Mittelmotor distributor running my ignition, which was working great initially. Could something be happening with the ignition? I guess it's possible? I'm not getting into boost or high RPMs at all when the problems happen though. The lean spikes made me think fuel, but I guess you're right that misfires might show up as lean.

Vacuum leak really did seem like the perfect answer. And I had them! And yet still after fixing several of them, nothing has changed.

[Carrera RSR]

FPR cutting the fuel pump
Overboost switch not earthed or failed, cutting the fuel pump
(Wrong map in the dizzy?)
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1980 931 - forged pistons, Piper cam, K27/26 3257 6.10 hybrid turbo, 951 FMIC, custom intake, Mittelmotor dizzy & cam pulley, H&S exhaust, GAZ Gold, Fuch'ed, Quaife
Now www.924board.org/viewtopic.php?t=34690
Then www.924board.org/viewtopic.php?t=31252

[bureau13]

I bypassed the overboost switch, no change. I don't think it's the map, it's the same one I've been running with all along...the FPR cutting the fuel pump issue...now that is a new one to me? How does that happen/how do I test for that? FPR is in the fuel distributor on these cars? Or am I way off?

[bureau13]

I honestly don't know what else to test, but I think a basic CIS test is in order. Now, supposedly my fuel distributor was "rebuilt" before I bought it, but at the very least if I test line and control pressure and it's where it should be (or if it isn't!) then I'll know. I never bothered to get a CIS test set because there was no indication that it wasn't working properly (and in fact I think it WAS working properly) but it's something I should have and know how to do anyway.
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-----------------------------------------
1982 931
1986.5 Silver 928S
2007 Hummer H3 (Daily Driver)

Past lives (I miss them all):
2004 RX-8 (Wrecked)
1993 RX-7 (Sold)
1987 RX-7 Turbo II (Sold)
1985 RX-7 GSL (Stolen)

[Fasteddie313]

Can you make a video of the symptoms in action?
Is it really like lean spikes where it cuts in and out or more like a lean spot in the tune?
Does it do it if you just hold like 4K rpm with no load?
Have you tried driving with the fuel pump jumped or just put a regular 30/87 relay in it?
Does your tach drop out at all?
Check your fuel pump contacts at the fuse, contacts on pumps, ground at the rear ground crown by the left rear tail light?
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80 Turbo - Slightly Modified

[bureau13]

I need to try to do that (the video). It's definitely most obvious if you try to actually drive it, but I can make it happen in neutral. "Spikes" may be a poor choice of words. Driving, I can see the AFR jump up to 15 and above, it doesn't stay there but of course I'm not holding the gas there either. I neutral you can hear a difference and see the AFR go up as well. Last time, I tried it very gradually but I don't remember what RPM it started happening, or if it was repeatable in exactly the same way. I'll get a video of all that.

I have done this with the a jumper in place of the relay, no change. I have not seen the tach drop out, and I will check those contacts and grounds.

Thanks

[Carrera RSR]

[bureau13]

Lol I thought that didn't make a lot of sense! I've never seen FPR used to mean the fuel pump relay, but yes, I have bypassed that with a jumper, it made no difference.

I tested the overboost switch by removing the wire and grounding it. No change.

I will check out the connections to the fuel pump, etc.

Regarding the map, I tweaked the areas you were concerned about, but I don't see how they could be in play here. Those were higher in the rev range, I am not near those RPMs when I have problems. Also, while you are 100% right about the dyno tuning, etc, I was driving with this map with no problems that I could tell. Now, that doesn't mean it wasn't risky running that much advance, but if that map at least seemed fine at first, I don't see how it could cause the problems I'm having now. There is no way for this car to "seem fine" the way it's behaving at the moment!

[bureau13]

This folder contains a video of me slowly bringing the RPMs up in neutral. AFRs are on the right, you can see them start to jump around as the revs get higher. I stop around 3K or so, maybe a little less. There is also a screenshot of the 123Ignition dashboard, so you can see vacuum and ignition advance near the top of that "run."

It might not be so obvious, but the car starts to break up as the AFRs start to jump around. It's way more obvious when trying to drive, but also harder to film

https://photos.app.goo.gl/S8KvudaSFF2U6sWq7
_________________
-----------------------------------------
1982 931
1986.5 Silver 928S
2007 Hummer H3 (Daily Driver)

Past lives (I miss them all):
2004 RX-8 (Wrecked)
1993 RX-7 (Sold)
1987 RX-7 Turbo II (Sold)
1985 RX-7 GSL (Stolen)

[Fasteddie313]

Ignition...

Mine runs fine up into the mid 16s, 15s don’t make it run poorly..
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80 Turbo - Slightly Modified

[bureau13]

Hmmm...would the ignition cause it to be up in the 15s and 16s though? It never used to. Also, something to keep in mind, the problem does seem more severe under some load. I've seen 18s, not steadily but I've seen it.

Given I have this Mittelmotor distributor thing, which removes the DITC and ignition amplifier thingy up behind the headlight, I'm not sure how to dig into it. Plugs and wires are all new. I honestly was wondering if the distributor stopped "doing its job properly," but if you look at my dashboard snapshot, the advance is right about where it should be according to the map, and that map ran fine, right up until it didn't.

Actually the coil is not new. I hate just fixing random things, but it's the one not-new piece of the ignition...does that seem like a plausible item that could cause this?

[Fasteddie313]

Yeah maybe..
Maybe it doesn’t have enough dwell which causes a problem as rpm increases?
I don’t know how that distributor works..

Ignition misses would show lean on the AFR gauge.. Does the exhaust smell like unburnt gasoline when it happens? (Which would also mean unburnt oxygen ie read lean)

Any chance your distributor thing is wrong about where it thinks tdc is?
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80 Turbo - Slightly Modified

[Fasteddie313]

I just tested and it will run all the way up to 18 AFR and not cut out like your car does...

https://youtu.be/wwGYwV9Rqoc

So I’m pretty sure yours is an ignition problem..
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80 Turbo - Slightly Modified

[Carrera RSR]

[Carrera RSR]

Check the rotor in the dizzy is not wobbling. I found a couple that did this and caused misfires at higher rpm as the rotor hit the contacts in the cap.
_________________
1980 931 - forged pistons, Piper cam, K27/26 3257 6.10 hybrid turbo, 951 FMIC, custom intake, Mittelmotor dizzy & cam pulley, H&S exhaust, GAZ Gold, Fuch'ed, Quaife
Now www.924board.org/viewtopic.php?t=34690
Then www.924board.org/viewtopic.php?t=31252