[SMTCapeCod]

OK, I just searched the "All star technical discussions" thread for keyword 928 and came up empty...good to know the '8s are so much more reliable.

Thought I'd test the waters.

Noticed an occasional flutter in engine output at constant highway speed- actually suspected wheel or driveshaft vibration dampener except that it was too intermittent.. Drove for a bit longer, noticed an occasional miss emerging when at idle at traffic lights etc. It became more pronounced until I eventually had to pop it into neutral and brush the throttle from time to time to avoid a stall. It is more of a miss or stumble, not ragged in the way a cammed car would be, smooth then *hiccup*...

This follows a couple hundred miles of driving after a full refresh of hoses, gaskets, sensors, switches from the heads up (and t-belt job, and motor mounts).

What it oughtn't to be (but still could)
MAF is rebuilt/100 miles or so
Temp II is new,
Crank position sensor is new,
Idle speed valve is new,
As mentioned, all fuel lines and vacuum hoses are new
Wires are new,
Spark plugs are new,
Caps and rotors are new,
O2 sensor is new,
Final stage ignition amps are new,
Throttle switch is new, clicks properly, connected with new harness,
Injection & ignition relays replaced
Grounds recently serviced
Fuel filter is newish, pump sounds fine,
Fuel pressure regulator is new.

When I say the above parts are new- bear in mind they weren't installed immediately prior to the symptoms, they had been running fine for a few tens/hundreds of miles.

Additional info- pulled air cleaner housing.
1. Simply waving a hand over the MAF at idle can cause the car to stall...I mentioned the MAF was rebuilt and had been running for a while, right?
2. In addition to the air intake whoosh sound, I can hear a hum that seems to emanate from around the maf.
3. Using a stethoscope, I could not locate/isolate the humming sound. I did hear knocking at the fuel dampeners, that I did not hear at the fuel rails, block or bell housing. Seriously.

Runs slightly better with O2 sensor disconnected.
I've ordered the fuel pressure dampeners, though I didn't see/smell fuel at the vacuum ports.

So, a foray into the Tech forum for a 928 issue.....I picked a voltmeter and have the factory test protocols for the injection & ignition (electronics make my head hurt) but anyone have some advice or care to venture a guess?


Edit: I should probably add: 1986.5 928S, automatic
I'm suspecting-> vacuum leak somewhere, though none could be detected from inspecting with intake in place...smoke machine would be awful nice...or MAF, despite that it was just rebuilt.

[BlackTalon]

Search for some of OldT's 928 threads. He did a ton of work on the one he owned for a few years. I think he replaced a few miles of vacuum hoses and a ton or two of electrical wiring while troubleshoot a few issues.

[catron1]

http://www.dwaynesgarage.norcal928.org/

[SMTCapeCod]

Yuh, I've watched Dwayne's garage evolve over the years, its a terrific effort and a great aid for first timers digging into different systems on the car.

I don't recall any diagnostic info there that gets at my question though.

[Eplebnista]

Am I correct in concluding that this gets worse as things get warmer under the hood?

Have you experimented with a good "hot soak" - i.e. getting it fully warmed up or hot and then parking it for 10 minutes with the engine off so underhood temps really spike?

My first guess is that you have an electronic component or sensor suffering from heat stroke. That is a WAG. I think you run better with the O2 disconnected because you are going into the computer's open-loop or limp home mode and that is probably biased toward running rich.

Hope this helps.

[SMTCapeCod]

Thanks!

Last time I drove any distance in the car was to work and back in stop-and-go, so that was a good heat soak. I didn't try diagnostics afterward. Going to delve into it tomorrow. I agree with the O2 sensor/default map assessment. That's part of why I'm thinking/fearing MAF. From what I understand, actual LH computer failures *usually* manifest differently.

BTW- so I guess you've noticed the Panamera (Hugh's) and Cup III wheels (D-phus classifieds) with suitable offsets for sale here....?

[SMTCapeCod]

Man, *Crickets*.

OK, located one small and one marginal vacuum leak. Not a cure.
Replaced fuel pressure dampeners, not it.
MAF currently being re-checked for calibration. With MAF disconnected the car was hunting at idle and exhaust smelled rich.

18JUL update- MAF checked out OK. Dagnabbit.