kcfong2

I'm trying to figure out a "hot" condition on my 81 US 5-spd. I've checked and seen others report something similar, but their cars were 86+ with LH and EKZ(?), whereas mine is a simple L-jet.

Here's what happened today. The outside temp was about 80F. I had just driven home from work (about 25 miles) and the latter part of the drive was stop and go traffic. My AC was on. I did my usual routine and drove to a 24-hour fitness. The lot was packed, so I was again in stop and go just trying to find a parking spot. As I idled, the car just stalled. I restarted, move 20 feet forward and had to stop again for people... while waiting, again it stalled. I restarted and found a parking spot, where I sat and revved the engine. When I let off the gas, it was idling at around 700, very rough and then would stall unless I gassed it. When I gassed it, it would rev smoothly. I turned it off, did my work out and returned about an hour later. The car started and ran fine.

This same symptom happened to me once before, again while warm/hot.

I have "rebuilt" this engine with TLC over the past year after getting it from my dad. It's got 82K original miles... New O2 sensor, new fuel pump and check valve, new injector relay and fuel pump relay. New green wire. New coil, new rotor, new cap, new plug wires, new ballast resistors, new Temp II, good Thermo Time switch, new ICU, cleaned/rebuilt injectors from WitchHunter, all fuel regulators tested (no leaks), new fuel lines, new vacuum lines, reset the "trace" on the AFM, new AFM o-ring, new engine wiring harness ($$$).

Timing is spot on, and it passed California Smog Test with flying colors.

I should also mention (but it might be unrelated), that I get a slight stutter on soft acceleration between 2500-3000RPM, but I've just gotten used to living with it. The other day, on top of the hot stall, the car wouldn't restart after sitting in a parking lot for 30 min. It took about 5 min of coaxing and then started.

All these make me believe vapor lock or lack of fuel. The temp gauge registered above the first "tick", and under normal driving, the needle sits at the first "tick" (sorry, don't know what temp the "tick" represents, or whether it's even the same across model years).

I'm at a complete loss... the only things I can think of now are bad "computer" (but I"m not familiar enough with this car yet). Bad AFM? I checked the trace, and reset it to create a new one. Faulty injector harness (but it works...).

Any ideas from an 81 or earlier 928 owner?

kcfong2

Not to sound redundant, but here's a rephrasing of the above with more detail. It's actually an email I sent to someone:

My 81 US-Spec 5-spd, 82K original miles.

Recall over the past year the following has been done/replaced on the engine (countless other things done to suspension and body/interior):

Injectors sent to WitchHunter for cleaning/balancing.
Green wire replaced.
Rotor and Cap replaced.
New coil
New engine harness
New plug wires
New fuel pump and check valve.
New fuel pump relay
New fuel injection relay
Tested Thermo Time Switch (OK)
New Temp II sensor
Ballast resistors (got new ones from you, but old ones actually tested okay on a bench test).
New ICU.
Fuel lines replaced.
Vacuum lines replaced.
Reset AFM armature to create a new trace (although old one was not worn).
New alternator
Tested dampers and FPR.. no fuel leakage (diaphrams good).
Timing spot on (passed California smog test with flying colors) New O2 sensor All filters changed, oil changed.
Outside temperature sensor (driver side wheel well) replaced or checked out ok.
Fuel tank strainer replaced (what a PITA).
Injector connectors replaced where bad or cracked. Confirmed continuity of all injector connections (don't know good Ohm values, but all had continuity if I recall correctly... been awhile).

SYMPTOM:

Reference: The temp gauge has four markings (ticks).

The car starts and idles fine at cold and warm startup. On cold, with AC off, it idles at about 1100RPM and settles down to about 800/850. Smooth idle, no pulsing (hunting?).

I can drive it on a cool/moderate day (80F).

After a good drive in the city with stop and go, the idle sits at about 850, but I notice a pulsing (only about 100RPM, but noticeable). Almost like the old day hot rods with a hot cam where you'd always get a "rough idle". Mind you, it's not bad, but it's noticeable. I sprayed carb cleaner all throughout the engine, around the AFM, dampers, etc, and I get no change in idle, so if it's a vacuum leak, I cannot find it.

The engine temp, on freeway driving, sits around the first tick of the temp gauge.

As I drive more in city, the idle seems to get slightly rougher. Turning on the AC, the RPM does increase, so I know that RPM "thingy" on the cross bar is doing it's thing. The electric fan is running when I experience this rougher idle.

On TWO occasions how, yesterday being the latest, I did a lot of stop and go freeway traffic, made it to my gym, and then had to "wait" to park, as it was packed. The engine stalled on me. I started it right back up (kicked over immediately), drove 20 more feet, got stuck in traffic again, and again it stalled. Again, started right back up, found a parking space, and gently revved the engine... it revved smoothly, but then when I let off the gas, the idle was terrible. Went to the gym, came back an hour later, and she started, and idled with no problems.

This happened to me a week prior when I went shopping, so similar situation.

Also, two times when I came back to a hot car, and cranked her over... she fired right up, but then immediately died. Had to wait for 5-10 min before she fired up again (she would crank but not fire up). Almost like vapor lock. Again, outside temp about 80F.

This is definitely something related to temperature, but I can't figure out what. I should mention I'm at sea level, not high up in any mountains.

Any ideas? I'd be more than happy to throw more parts at it, but I'm at a loss as to what parts now. A new AFM? Disconnect the Cold Start Valve? A bad "computer" (but I don't know what to replace here... it's an 81 L-Jetronic). More harness replacements? Arghhh, she's so fun to drive when she works right, but so frustrating otherwise. That's my car, not my wife...

Thanks in advance!

Billu

Mrmerlin

how much fuel is in the tank when this is happening

kcfong2

I had about 3/4 tank when it stalled on me a couple of days ago. I don't recall what the tank was the first time it did it a couple of weeks ago.

I should probably check for fuel pressure in one of the rails, but as I'm still learning about this car, I'm not sure how to install a gauge... I don't see how I can mount a gauge as my rails don't have any threaded end IIRC.

I'll also have to follow-up on Billu's hall-sender in the distributor... that's all new stuff to me, and I'm still not sure how temperature can affect that. Wish there was a "lending library" of 928 parts where I could "check out" a distributor or AFM just to help debug these problems :-)

kcfong2

I'll have to read up on the hall sensor in the distributor... that's all new to me. With regards to vapor lock (which I initially thought this was), are you aware of any aftermarket fuel cooler for the 928? At the same time, if it is vapor lock, I'm wondering why it's problematic now, but never before (when my dad owned/drove this car).

I might put a lower temp thermostat to kick in the engine fan earlier...

Mrmerlin

After reading this I would check this part
Reset AFM armature to create a new trace (although old one was not worn).

It has to be worn.
I would see if the spring on the contact wiper is tight add some deoxit 100 to the wiper.
maybe you moved the board and it has a crack or loose solder joint and the heat soak kills the signal.

Best bet swap in another part and dont open it till you have tested it

kcfong2

Can anyone with a 1981 US with L-Jet do me a big favor PLEASE?

If you would, please disconnect (unplug) the connector to the Temp II switch and then measure the resistance on both leads of the connector (NOT the switch) to ground (chassis). That would be the Bosch FI-style injector plug on the harness side. Measure each lead to ground and let me know what you get. I'm getting 0-4 ohms on one lead and an open circuit on the other... at one point, I *think* I got 7.9K ohms, but can't replicate that. Remember, test resistance from lead to ground. I'm trying to figure out if I have a break in the harness and don't want to start cutting into it yet... hoping that someone can test theirs for me so I have a benchmark. There is nothing I could find in the WSM, so I don't know what a "normal" reading is. BTW, I did my test with the car OFF.

Thanks in advance to anyone who can test and report for me.

kcfong2

Checked the harness this weekend... got consistently 7.8K ohms on one lead (yellow wire) to ground, and 0 ohms on the brown to ground (brown I assume was ground to begin with). Replaced the plug anyway as it was getting brittle.

So again, as I have no reference on what is "normal" and cannot find anything in the WSM, could someone with an '81 measure their resistance on the two contacts on the plug side of the Temp II harness (don't measure the sensor, measure the leads on the plug).

Thanks a bunch!

Kurtglmft

I think your on the right track with the temp II. Weird things happen when they go out of spec. I had different symptoms, but my old temp II was just a little off compared to spec and a new one (I did a side by side comparison on the bench). I found this really helpful... I think the info you are looking for is on p8.

http://jenniskens.livedsl.nl/Technic...%20Andrade.pdf