O2 sensor voltage question
 

Got this '90 964 with a surging idle, like rich surge, when the idle surges higher and smoother, the O2 voltage is 0.86. When it dips down and almost dies, it's 0.01. This according to the bosch hammer. No DME faults shown. This is a new bosch sensor. Didn't cure the issue. Any ideas? Brain freeze this AM.

Voltage seems kind of "normal" for symptoms as I interpret them. That is assuming fuel starvation at time of faltering.

I scanned the 964/993 forum for non optimal idle.

Peeps had trouble when they drilled holes in their airbox.

Reference to the air flapper door being hung up in one - but no resolution posted

Mechanically driven idle switch. On off function should be audible by moving throttle linkage.

IAC was referenced

Only what I read. I don't know a thing about these cars. Good luck.

Wild butt guess fuel pressure too high hence rich and exceeding RPM at which DME
will cut fuel with idle switch closed?
Need more information.

another WAG, air leak - intake or exhaust?

I never thought I live to see the day when JW asks for help.

I once had a problem like that with a CIS car on a 86 Ferrari 328. It was not possible tweak the O2 sensor voltage to 500mV. So, a new sensor solved that problem. I know that John says this is a new Bosch sensor but... is it Chinois? Did you also look at the DME connector where the o2 signals go in? Any issue there?

Just thought i'd try to speed the diagnosis by asking here, 'cause I don't know everything. I'll get back to it monday and check fuel pressure again after it's warmed up for a few minutes, which is when it starts acting funky. Starts well and cold run is fine. Just did the complete service. Swapped a test computer in, no diff. Definitely running too rich. The injectors passed the flow test and don't dribble. Head temp sensor is recent as well as the reference sensor. Concentrating on the rich run issue now.

Any chance it could be the altitude correction switch? I know the adjustment factor is small, like 3%, but maybe it's worth checking out.

FP regulator membrane could be ruptured causing fuel to leak into the intake at idle.

Thankyou Sir, I'll check the small hose.

Didnt the early 964s have the issue with the voltage on the mass airflow flapper contacts that causes all sorts of idling and rich/lean issues, etc? Been a long time, but i seem to remember something about that?

The way the AFM is located the connector faces the firewall and the wires get stressed.
There was a Porsche service bulletin to use a zip-tie to clamp the wire near the connector to the AFM onto itself so it wouldn't touch the rear firewall and cause intermittent running issues.

Ingo

wiring from o2 to ECU or pin.
how does it run with O2 not connected

Not much different, except the CO machine shows higher CO and HC than when it's attached. I remember one years ago where I had to remove the O2 wire from the computer plug to make it run right. Something in the loom was giving that wire interference. There's a thread about that one somewhere too. Sometimes these cars are rather annoying electrically. I think the current issue is fuel though.

Ingo nailed it. Fuel pressure regulator leaking into the intake.

nice to know.

how did you verify it.

Pull off the small vacuum hose and start the engine for a few seconds and watch the fuel dribble out of the regulator. Visible under the left side of the intake. Intake manifold removal to replace it.

Yeah, another one for the book.

that's one you will never forget

wow, nice Ingo.. I can't remember ever seeing/hearing about that particular issue..

A common problem on the BMWs (325/528) of the late '80s and also the 928/911 3.2 of that time period,
i.e. they all used the same basic type of fuel pressure regulator. Besides carrying a spare DME relay,
911 3.2 owners should probably carry a small plug that can be inserted into that vacuum line,
(if on a long drive) and that problem occurs. Obviously, the vacuum line must be re-connected
to prevent at fire. The only short term problem will be a less responsive acceleration.