Canajet

Good morning,
I am hoping you guys can share some expertise on this one:
1991 928 GT...
Recently had the Air Mass Meter and O2 Sensor replaced to fix a coughing/sputtering/Engine light problem. Ran great for 3 weeks, then got it Smogged yesterday and failed! Allowable HC is 120 ppm, mine was 163, with the other categories ok, but close to the limit. Last time it registered just 25 ppm!
Upon leaving, the old problem was back but maybe even worse...died, sputtered, died, very rough vibration, no power. Limped to my mechanic. The Smog guys don't work on Porsches, but recommended a new Catalytic Converter from a muffler shop ($250), which would give a 70% of passing, but they say I'm still left with the coughing/dying problem.

Porsche says they bet the Cat IS the problem of both the Smog and the Coughing, but theirs is $1600 which is far too much right now. Any ideas? Gamble $250? Help!!

Thank you!!

perrys4

Call Precision and talk to greag about cats. He has a guy that used to work for him that has some cats that work very well and he reccomends them highly. There is also 928 Motorsports, I think they have some. Precision# 714-879-9072

Randy V

The high HC levels indicate a rich condition, possibly due to incomplete combustion from bad plugs/coil(s)/ignition wires.

Cats won't correct that.

Canajet

Thanks guys...I had new spark plugs replaced 3 weeks ago, but not coils. I will give Greg a call.

I wonder if that's right that the Cat is the cause of both?!

Thanks again!

Imo000

high HC and the symtoms you described makes me think that maybe the fuel pressure regualtor is acting up. Its running rich for some reason.

What are the other numbers?

Imo000

Whatever you do, don’t start throwing new parts at it. This is how some people spend a fortune without actually fixing the problem.

First get the actual problem properly diagnosed, then decide what to and not to replace.


As Randy said, the cat is not the casue of this problem.

perrys4

When the cats plug the car loses power and they glow red hot. I had a car that this happened on and even the exhaust manifold was glowing red. Good luck to you!

Imo000

Yup, and the car feels like its being chocked but it usually does not sputter or backfire.

Bill51sdr

Try jumpering the ICM relay and see if the sputtering/no power problem goes away. Your engine may be running on 4 cylinders at times. If this works, next step is to find out if you really have a problem triggering the relay.

Canajet

Thanks for the ideas, guys. The other numbers are:

RPM %CO2 %O2 HC (PPM) CO(%)
meas meas meas max ave meas max ave meas RESULT

1053 7.7 9.7 120 29 163 1.00 0.10 0.14 Fail
2403 7.6 9.7 140 20 129 1.00 0.10 0.43 Pass


last time:
960 11.6 4.5 120 29 25 1.00 0.10 0.01 Pass
2508 10.8 5.5 140 20 11 1.00 0.10 0.09 Pass

Thank you!!

tx928s

Check the temps at the convertor inlet and outlet. This will tell you if the cat is working. It will be about 200 degrees hotter at the outlet than the inlet. I use a infared temp gun for checking.

dr bob

High HC is a sign of incomplete combustion, typically from weak/failed ignition or a lean condition as Randy pointed out. It's unburned hydrocarbons passing through. If it's too rich, the CO goes way up. It's possible to have rich misfire, but high CO would be the telltale for that.

Check your "new" plugs to make sure they are the correct ones, installed correctly/tight, and that the wires are seated completely. If you have ancient original wires, you may have disturbed them enough to cause the problem too. Look at the things you did most recently, just before the "problem" surfaced.

ZEUS+

If there were no driveability problems before and during the tests, contaminated oil can cause high HC.

Canajet

Thanks, guys...good ideas!

Mrmerlin

What Bob said, check your last repairs