macerl

Recent purchaser of 1990 928 GT. Has some superficial issues with which I am working; however, just recently had a more perplexing problem. Was running fine (smooth and strong), and then suddenly started missing and dies at idle. Came off the freeway (after a good strong acceleration getting onto the freeway), put in the clutch at the stop sign, and engine died. Able to restart immediately, and died again when at idle.
- Will not hold idle, but has no problem restarting. While nursing the car home (by keeping throttle on at every stop sign), noted engine occasionally (not constantly) missing.
- Waited a couple of days to start looking into problem, and the car seemed to fix itself. Drove it multiple times on multiple days without a problem. Looked for vacuum leaks, found a serious hole in one line, but it appeared to be an old hole, and by then the car was working fine again. Replaced that vacuum line.
- Then yesterday the exact same issue with engine missing and dying at idle came back.
Any ideas?

worf928

Classic sign of bad mass-air sensor. Possibly in conjunction with a bad O2 sensor.
How many miles?
Has the mass-air sensor been replaced / rebuilt? History of O2 sensor?

It could also be a faulty idle switch in combination with something else. Or faulty ISV in combination with something else.

Mrmerlin

I would suggest to check the battery connections and the inspect the ground strap for corrosion ,
also inspect the fuel pump fuse for burning,
if it is then use deoxit on the fuse blades and insert a new fuse,
make sure its a tight fit when inserting or you may need to bend the contacts

ashmason

I had this exact same behavior on mine about a year ago... it was the idle stabilizer... has it had a top end refresh, or is it all original?

SteveG

If none of the above, then check coil connections and at the disbr, Clean and reinsert. How old are your ign. wires?

Hilton

Lets assume the car isn't already running on 4 cylinders (i.e. you've checked the stuff SteveG brings up, and the car is definitely running on all 8).. the 3 main candidates for dying at idle:

1. Failed idle microswitch on the throttle position sensor
2. Idle Stabiliser Valve sticking
3. old MAF

easy to test the idle switch at the pins on the LH with a multimeter - so start there

ISV testing on-car is tricky unless you have access to a bosch hammer (KT301) or similar. You can try the "fix" but if the ISV is working ok, then the spray "fix" may cause issues as the spray lube attracts dust/carbon/oil.

maf testing needs special equipment to verify calibration under flow conditions, so send to Louis Ott for testing (performance928.com).

worf928

Actually, it's not any harder than checking the idle/wot switch at the ECU pins.

The inspection guide in my sig has instructions.

macerl

Thank you all for suggestions. I will go through your ideas.
High miles > 140 k. Will check paperwork and with previous mechanic on history. I have had the car only a few months. I know serious repair and replacement work was done after previous owner had belt failure.

Mrmerlin

since you just got the car do a ground cleaning and battery cables/ CE panel check first

Hilton

Nice - just had a read, elegantly simple. Thanks (I usually just run the actuator test, but don't always have a Spanner to hand).

Niles

Dave, that's a great write-up. I have one question on the primary vacuum. You said it should measure 17 or 18 in-hg. I had previously heard 21 was the number. Is 17 a minimum? I periodically experience issues similar to the O.P. My vacuum measures 18" and I was concerned a vacuum leak may have been contributing to my issue.

worf928

Thanks Niles.

The only way to know for sure would be to go back in time to the factory with several different hand-pumps and measure vacuum of 928s as they came off the line. There are a lot of variables involved from calibration of the gauges to the static compression ratio of the engine as-new to ring wear on older engines.

So, I can't opine on what the number should be for any specific 928 motor series. I haven't kept records of my measurements, nor do I get too worried about a specific measurement against a target.

That section of the guide (like most of the guide, really) is a 'gross check' that can potentially help uncover other issues or lead you in a particular direction for further research or diagnostics. For example, if you pull the line and fuel drips out. Or if you get no vacuum at all. Or if you get no perceptible idle change when you pull the line.

They way I look at it is 17-18 is a strong vacuum and >18 is a really strong vacuum. If you see vacuum of less than 17 from the primary vacuum system you probably have a big-enough intake leak that you can find it by 'normal' methods such as water-and-soap in a spray bottle with the intake pressurized. Or maybe the line is clogged or split? Or the line junction is clogged or split?

If you see 18 on a gauge you trust and are having issues then by all means you can look for an intake leak, but it may to be very small and hard to find. Your best bet would be to look elsewhere for 'gross' indicators first then come back to intake leaks. Of course, this is without any knowledge of issues you are having or what you've checked to date already.

EDIT: I see you have issues similar to the OP. I would suggest looking to the other advice in this thread before chasing vacuum leaks further.

NC928S4

I feel and am living your frustration!

I just returned from Frenzy 19 on a wing and a prayer with a sputtering engine that barely made it into my garage. Symptoms were engine die at idle and could not run unless throttle was goosed to 1200-1500RPM. By the time I got home(270 miles) power was surging at 2000-2500rpm, exhaust smelled bad and no matter how much gas pedal fluttering I did I could not keep her running.

The consensus at Frenzy was failing ISV but the car definitely took a turn south so I dug into the Worf Diagnostics.

All diagnostics on Temp II, WOT/Throttle switch, ISV and Vent Tank for EZK and LH checked out. Switches, Valve and Relays were clicking and clacking as expected. For grins I swapped in a new 53 Relay for the LH. Frustrated that I found nothing suspicious I buttoned things back up.

Started the car back up and idle was perfect for many minutes. Lots of black carbon emanating from tail pipe initially. Too soon to declare victory but my current thought is there was a flaky electrical connection on either the EZK or LH plug.

928 GT R

I have a 90 GT that exhibited the same symptom. After a great deal of diagnostic work, I sent the MAF to be rebuilt/reprogrammed and that fixed the problem.

It was relatively inexpensive as well. Not sure I am allowed to publicize who did it for me but if you can not find someone, PM me and I'll get it to you.

worf928

Out-of-spec MAS is the easy cause of dying at idle: If it is consistent or consistent with temp/heat soak. Inconsistent dying is a symptom of LH failure.

These, if all else that can be easily tested is good.

Either root cause can be exacerbated by an out-of-spec TEMP-II or O2.