acm123

Hello Guys,
I am new to the forum and was referred to you guys from 928 international. I am having an issue with a 1989 928 s4 going lean at about 4000 rpms and higher. It is a consistent 15.0 to 1. The fuel pumps were replaced and still no change. Anyone know or seen a problem like this before? Your help is greatly appreciated :-)

Bilal928S4

Need more info.
Was the fuel filter change?
When did this start happening?
What was the last thing that was done before it started behaving like this?
What is the maintenance history on the car?
Is it an S4 automatic transmission?
More q's will come once you answer these, I am sure.
Welcome to rennlist.

acm123

Need more info.
Was the fuel filter change? yes it was changed when the pumps were replaced
When did this start happening? Did not know it was an issue until it was put on a dyno lol
What was the last thing that was done before it started behaving like this? Not sure since the vehicle has never been tuned or really pushed to redline before (kinda baby the car lol)
What is the maintenance history on the car? Have done all maintenance according to porsche expect the fuel pumps and filter that were just done
Is it an S4 automatic transmission? yes it is automatic


Thank you for the quick response :-)

Rob Edwards

When you were on the dyno, was this wideband sensor installed in the factory oxygen sensor bung (ahead of the cats), or in the tailpipe? . If the latter, then the measurement is meaningless.

acm123

Hello Rob,
Why do you say that the 02 sensor in the tailpipe is meaningless. It still gives you a reading no matter what and for a vehicle to go from 12.0 to 15 at wide open throttle it does not matter where the 02 reading is coming from. There is definitely something causing it not to get richer. Unless these cars always run lean?

Thank you

GregBBRD

I'd check the WOT throttle switch, first.

If the WOT switch doesn't function, the O2 sensor will not get "kicked out" and the car will continue to try to run at 14.7 to 1.

GregBBRD

Once the gasses go through the cat, it is difficult to get meaningful readings.

Rob Edwards

You're right, meaningless is too strong a term. Still, putting the O2 sensor after the cat gives you leaner readings than the engine is actually running.

Greg's spot-on that a dead WOT switch will not trigger the WOT enrichment map- definitely worth checking.

kmascotto

just for my own understanding...where is the WOT switch located? Is it part of the TPS?

Rob Edwards

Yes, WOT switch and idle switch are incorporated into the TPS.

There are many threads on testing the TPS, but here's a good one with pics of the pins to test resistance on, in post #8:

https://rennlist.com/forums/928-foru...itch-help.html

kmascotto

Thank you

acm123

Hello Guys,
Well after doing the tests as stated here https://rennlist.com/forums/928-forum...itch-help.html I am getting readings of OL on all circuits?? am I doing something wrong? I would think the vehicle would not even run if I had a TPS issue on all WOT and idle circuits?

acm123

Any input would rock :-)

SeanR

Your link doesn't work for me so I can't say what you did there or what you were testing.

It is very common for the WOT switch to not be engaged either due to the cable assembly not being adjusted correctly or the Throttle Position Switch (TPS) being faulty. It's common to still have the idle contact but not WOT. You need someone with a Bosch Hammer (KTS 300) or similar diagnostic tool to test. You can test via the connection at the LH brain but would need the work shop manual for reference.

Bertrand Daoust

Scroll down post #1 (10th pic) here for how to check the TPS.
Thanks Dwayne.

https://rennlist.com/forums/928-foru...re-w-pics.html

Hope this will help.