running rich, fouled o2 sensor?

[blk924s]

My 88 924s has been running quite rich for a couple weeks now and is idling rough due to misfiring and occasional minor hunting. I had had a huge air leak past the ISV which I fixed and the car ran absolutely great for a week or 2, and then I noticed the current problem when I revved it up and black fuel smelling smoke billowed from the muffler.
I checked emissions an verified that the hydrocarbons are high, and then checked fuel pressure and it is right on at idle and under load.
I bought a faze air/fuel guage and wired it in, and found something interesting. Before the O2 sensor warms up, the guage reads just lean of ideal. As it warms the guage reads leaner and leaner until it is fully in the red. Under load the red lean LED shines even brighter, even though there is clearly unburned fuel dumping out the exhaust. Since the guage reads the O2 signal and the sensor clearly things the harder I press the gas the leaner it gets, does this mean the O2 sensor is faulty and is telling the DME give me all the fuel you've got?

[1982911sc]

My Fuel Pressure Regulator tested fine at idle and under load, but on WOT, under boost is where it was failing-very hard to test on full boost. Something you may want to consider, or at least swap out and see if it cures your problem.

[onZedge]

To cause a lean reading AND unburnt fuel smell symptom, I would suspect an ignition problem. If a cylinder does not fire the unburnt fuel and un-consumed oxygen go out the exhaust resulting in a high hydrocarbon reading AND a lean O2 reading. Also something to consider is a stuck open or not fully closing exhaust valve (maybe a broken valve spring).

[jcommin]

Fuel pressure regulator. Mine was running rich, gas millage not up to norm. My Porsche mechanic found the problem quickly. He pulled the vacuum hose off the switch and smelled raw gas. Porsche no longer stocks the part so I bought it from Pelican. Replaced the diaphragm damper too.

Car runs great!!!! What he told me: If the O2 sensor was bad the car would struggle to maintain a constant idle

[Techno Duck]

I would also suspect the fuel pressure regulator. You may have to change the O2 sensor aswell, running excessively rich will cause them to go bad much faster.

[924Sman]

Could be someone before you decided to play with the A/F mixture screw located on the AFM causing a poor mixture. Many think this adjustment controls idle....not! Many adjust it w/o using a O2 tester and that will kill a good A/F ratio in a snap.

Dal

[jcommin]

924Sman is right on. This like like the chicken / egg. Not fully understanding the problem, start playing with the A/F screw to compensate for a regualtor and/or O2 sensor. Things go well for a while and the car gets worse. Before you know it everything is out of wack. I had to re-adjust the A/F screw after I replaced the regualtor and diaphram.

[onZedge]

Guys, please re-read blk924s's original post. He confirmed high hydrocarbons with an emissions test AND high oxygen (lean) with a NEW O2 sensor. Bad FPR or incorrect FQS setting doesn't explain both of his symptoms. Unburnt fuel/air charge does. Either a cylinder is not firing or a bad exhaust valve is allowing the fuel/air charge to be pumped into the exhaust during the compression stroke.

[blk924s]

Sorry to take so long to respond. My o2 sensor is 2 years old so its not brand new, but I wouldn't expect it to fail since my car has ran pretty well for thoes 2 years. The cap over the screw on the air flow meter has never been drilled, and the plastic top doesn't apear to have been removed and re-glued. The FPR is also 2 years old- one bought from pelican- but just because its a new part doesn't mean its not bad i know. Well I quess its time for a cly leakage test...

[Ernest Hill]

i was about to post a similar question for my 86 951, new ignition and fuel system and im still rich. electrically my car checks out so im wondering next about a stuck fuel injector? could that be a mutual problem?

[onZedge]

blk924s: Did you do a compression test yet?

[F18Rep]

You guys probably have this all figured out but I've been thinking about the new-to-me idea of deleting the fuel dampner. You almost never hear of them going bad but maybe they do and I just haven't picked up on it. So lets just suppose one had a pin hole leak in its diaphram. Wouldn't gas get sucked into the small vacuum line, especially at idle - resulting in a super-rich mixture? It would be easy to check, maybe just pull of the vacuum line and see if gas is leaking. Just an idea...Bruce

[blk924s]

I really wanted to do a CLT, but I just havn't been able to yet. I did decide to just replace my O2 anyways, and as I pulled the old one out I shook it a bit and heard the cracked element jingling around inside. I put a new sensor in - the less expensive type you have to solder the wires together- and my A/F gauge works perfectly. Not to mention, it is smooth and doesn't miss at idle, the power seems up a bit, and I think its a little less rich.

What I noticed is that under normal acceleration the gauge swings between lean and rich like it is supposed to. However, at idle, constant throttle, and full throttle my A/F gauge reads almost full rich. I understand full throttle being richer, but shouldn't it be leaner at idle and constant? Also, isnt the stoichiometric A/F ratio most desirable for maximum power and efficiency? Thanks for your help so far


-by the way ernest hill, I recently had my injectors flow tested, and the were within 4% of each other. this cost me 30$ at a shop here in tucson.

[onZedge]

Hmm, your first post led me to believe you installed a new Faze O2 sensor (you didn't exactly say when you did it). Now that it runs better, did you get the emissions re-checked and do you still have an unburnt fuel smell?

[ronin]

Originally Posted by Ernest Hill

i was about to post a similar question for my 86 951, new ignition and fuel system and im still rich. electrically my car checks out so im wondering next about a stuck fuel injector? could that be a mutual problem?

quite possibly. have the injectors cleaned and benched. most problems I see of that nature are related to injector issues, with vac leaks running neck and neck

[Ernest Hill]

suddenly remembered my first 944 breakdown: dunno how your car is running or your fuel flow (not pressure), but I can say that I have replaced fuel pressure regulators and dampeners 2 of 3 944s and my 951 right around 115K; both will cause odd problems that defy testing. Tired Voice of Experience: if your Porsche is running better but not right you did not fix it. Signed for Ronin, Fat-bloated-idiot.

[blk924s]

WOoohoo im gonna do a CLT today. Just for fun I'll try swaping my FPR with the used good unit ive got, and see what my gauge does. I havn't tested emissions again, but I know that for sure it isn't pouring black fuel smoke out the pipe anymore. Before, when i would floor it at night with headlights behind me, it was like i'd hit the smoke screen button.

[ronin]

Originally Posted by Ernest Hill

Signed for Ronin, Fat-bloated-idiot.

pardon me?