x98boardwell

Hello,

I recently purchased a 91' S4 that has had extensive work done to it over the past 10 years to the tune of 70k. It currently has 136,000 miles and looks brand new. The PO rebuilt the motor, painted certain parts of the car, did a GT conversion, replaced almost everything on the vehicle, did a restoration on the entire interior (new leather, carpet, door seals, etc..) just to list a few things which was done between 130,000-136,000... yeah, I know a lot of work.

Today, I took it down to the local smog shop and it failed the NO portion of the test (I have attached a photo below) and I am curious as to what could have been the cause. I know of a few causes for this to happen but I'm unsure as to what it actually is. It could be one of the following things...
- Lean fuel mixture
- Defective EGR system
- Defective catalytic converter
- Engine overheating (this isn't it)

I do know that the car has aftermarket catalytic converters and I will need to look under the car to verify which ones but they are CARB certified so I thought it wouldn't be an issue. I also am not sure the last time it was smogged (1 year ago) if the exhaust was done after or before which would be good to know... The PO can't remember.

Overall the car is amazing and aside from some oil leaks which I will be pulling the motor to address it runs and looks like brand new.

Any help is appreciated in this issue. Here is a pic of the smog certificate from today. I am looking to find what others have seen in this area and would like to try and pinpoint the issue and take it back down quickly to get done... I also still need to go to the DMV to register it.

Cheers,
Bryan

rnixon

If this is the car that I think it is, it was a California seller. In California, the seller is responsible for smogging the car (within the 90 days preceding the sale). See here.

Sorry, I can't help with the diagnosis, but there is a lot of information out there, and no doubt here.

x98boardwell

I am aware, I took it upon myself since it passed a year ago and was on a limited tine schedule.

I did look at the cats below on my lift and they are a generic 50 state legal brand (maybe magnaflow but doesn't say) and have the part numbers D-193-86, 36304 and 0112. Here is what I know..
- Cat's installed 1/3/13
- Smogged last on 3/13/13 (measured 617 on 15 MPH test and 490 on 25 MPH test)

Could these have worn out already or is there likely another problem?

I am familiar with 911's but not these V8 beasts. What else could it be or what have others ran into?

Thanks,
Bryan

rnixon

Have a read here.

x98boardwell

Great info!!! Thanks. Still wondering if mine are bad as it seems as though they worked once and now not but I'm wondering if there is still something else wrong. Great info on the CAT's though.

Any other info on other ideas or is it typically a CAT problem all the time?

Thanks,
Bryan

dr bob

CO Numbers say it's lean. Verify that the oxy sensor is functioning, good gas in the car, etc. Test with HOT cats, like drive it hard enough to get them working and roll directly into the smog test while they are still hot. Do the tests in a lower gear (like second in the automatic); jumper the kickdown switch, tighten the TV cable to the trans if needed o hold it in a lower gear.

There is no EGR on your car. A little oil in the intake will cause this, as will a small number of partially-blocked injectors. It only takes one to blow the test, and you might see it in plug colors.

Bill Ball

As Dr. Bob suggests, I had similar test results in a 91 fixed by a new O2 sensor. Your cats seem OK...for now. Aftermarket cats have a reputation for only lasting a couple of years. I've observed that too. I would also wonder about the MAF. Old MAFs often underestimate airflow and this calls for less fuel than really needed --> lean running as well.

jon928se

IIRC defective cats can't make the NOx higher than it is out the motor. High NOx generally means running lean - first culprit is the MAF but also should change the O2 sensor first as it is cheaper.

Bilal928S4

I have had problems with my car passing emissions here in CT if it has been sitting for a long time. I usually drive it for a couple of hundred miles and put fresh gas into it. It then passes with flying colors. HTH.

worf928

All good suggestions above.

I went through high-NOx failure diagnosis on a customer car. After investigating all the obvious culprits we had the intake system smoke tested. All the fuel injector lower o-ring seals were leaking. That intake leak was enough to drive NOx through the roof.

You can 'fake' a smoke test at home with compressed air and soapy water.

Given a 'PO Rebuld' I would investigate intake leaks before spending money on parts.

x98boardwell

Man, all good ideas.

My priorities are as follows..
1) O2 Sensor
2) MAF potential issue.... I will look and see if he has replaced these
3) Intake leak...

What has been done or added to the motor and their dates.

4/17/13 - Press Regualtor (928 110 198 03)
4/17/13 - Fuel Pressure Dampner (930 110 602 01)
4/17/13 - fuel pressure diaphragm (930 110 602 01)
4/17/13 - Fuel Pressure Diaphragm (928 110 202 01)
4/17/13 - Fuel Pressure Regulator (928 110 198 04)
3/13/03 - There was an intake leak and it was fixed by local Porsche dealer. They also found that the oil fill tube bolts were too long causing a gap between tube and block, they installed correct bolts and no leaks were present. NOTE: This was probably done by the idiot who put the motor back together.
5/24/13 - Found fuel leaking out of all 3 pressure regulators causing a rich condition. They replaced all three.

This is what I can see so far. I will continue to look at if he had replaced the MAF sensor at any point. I wonder with all the Intake work if it's worth looking at right away or just change O2 sensor... thoughts?

Thanks for all the wisdom, this is exactly what I was looking for.

Cheers,
Bryan

x98boardwell

Additional questions.

- What would the best place be to buy the MAF and O2 sensor? Curious on cost as to these would be easier to replace if I can't see that they were ever replaced before I dive into the intake.

Thanks,
Bryan

Bilal928S4

Usually it is always the last thing that was touched.
In your case all the above were changed after the smog test, so any of them could be the culprit.
How many miles has it been run during the last couple of months? Like I mentioned before, sometimes an Italian tune up with new gas or putting in some Stabil will clear everything up.

danglerb

MAF gets rebuilt, but is "real" easy to swap if anybody local has a known good spare to see if that is the issue. I sent mine directly to a rebuilder, but I think it may go through one of main 928 vendors now instead of direct.

O2 sensor once you know the Bosch part number etc needs a google search to find lowest cost. Nothing special about the sensor except the plug to fit a 928, so its possible to use a generic and splice.

Both cases, not sure, talk to one of the 928 specific vendors www.928intl.com or www.928rus.com

I just bought a pair of those 36304 cats for my 85 Euro S, no way should they have such poor numbers. OTOH I am not putting them on my smog fail 928 until after I have injectors back from cleaning and check the AFR with the old exhaust.

rnixon

It depends on cost of the parts and the cost of a smog retest (some places offer a free retest). I'd probably replace the O2 sensor anyway though.

If it's any consolation, I'm expecting similar issues with my GTS (bought in Maine), once she arrives in California.

Good luck!