James Bailey

only the 85-86 had an adjustment...."My concern is that my CO% is literally at 0% so I am guessing adjusting my mixture would help as well. I will try and search how to do this one." Although the mass air has an adjustment screw under a plug BUT BIG BUT it is not wired to your or any other 1987 or newer 928...
It is all in the computers....

ptuomov

Based on following this forum, I'd say that 3/4 of the similar problems have been resolved by either MAF or O2 sensor change. Someone with actual experience may want to voice a more informed opinion.

Everything on the smog sheet points towards the car running very lean but not having any other problems. Small errors in MAF or fuel pressure will be compensated by "adaptation" that we just talked about in the ST2 thread, so it would have to be a big error in MAF or fuel pressure. It's more likely that the problem is a faulty O2 sensor causing LH to run lean. I am not contributing anything new to the thread with that suggestion.

x98boardwell

I thought only the timing was controlled in the computer (relating to this issue) and that we were able to adjust the mixture slightly to increase or decrease fuel into the combustion chamber richening the mixture and also cooling head temps... so I'm inaccurate in this thinking? How do we adjust anything then.

Would it then be only air leakage or fuel pressure related or potentially air leaking past an injector or something... if not the catalytic converters or O2 sensor?

James Bailey

That was the point.....no adjustments. Now with the sharktuner you can get inside the brains and tweek all kinds of things . But you are correct in that you may either have an air leak (unmetered air below the mass air flow sensor) or a faulty sensor giving the brains bad information .....possibly the mass air flow possibly NOT !!

Tom in Austin

If it were me and I didn't know how old the O2 sensor was, I'd put in a new one, borrow someone's MAF, and give it another try. There are certainly many possible factors, but these are the big two sensors that could mislead the computers into running too lean.

Hilton

^-- exactly this.

Checking for intake air leaks can be done with some home-depot plumbing bits and compressed air.. even just spraying brake cleaner or starter fluid around the intake with the engine running will often show the presence of a leak by a rise in engine revs.

For the MAF, you can send yours off to be tested, however, they do have a finite lifetime, and starting out your ownership with a newly rebuilt MAF that's good for a decade can be a good thing.

Mongo

Looking at those numbers, the car is pretty lean. Most of my smog recordings had HC and CO levels that were high and I barely passed (like a difference 4ppm HC). Then again, my aftermarkets cats are those high flow ones.

That aftermarket cat you have is functioning, but it looks like the O2 loop and or MAF is out of spec, just as everyone else mentioned. Anyone on this board close to him that can lend him a MAF just to see if it passes?

O2 sensor is a cheap replacement that couldn't hurt other than loosening the exhaust up while on your back with nothing but concrete under you.

Mongo

Just out of curiousity, have you done a recent spark plug change? Part of me is thinking colder spark plugs may help lower those combustion chamber temps.

ptuomov

...

Mongo

If I'm wrong on that let me know. I haven't pondered the whole hot versus cold plug concept other than the fact that high performance engines like cold plugs.

GregBBRD

Welcome to my world, non residents of California!

Very common result for a S4, in California. Very low CO and high NOX.

While a bad O2 sensor can certainly drive down the CO, new ones tend to do the very same thing....usually resulting in the same results.

A bad MAF can certainly contribute to the problem, but by the time they get bad enough to "outrange" the O2 sensor, they usually run very poorly when cold.....so unless the car runs really poor when cold, a new MAF will generally result in the same problems.

I'd monitor the O2 sensor and see what it is putting out, with a digital volt meter, when hooked up and the car hot. If it is ranging between .2 and .8 volts, forget about the MAF and a new O2 sensor fixing it.

An intake leak will obviously cause the problem.....but the intake leak has to be big enough to outrange the O2 sensor. Again, this will show up when you check what the O2 sensor is doing. If it fluctuates between .2 and .8 volts, don't bother looking for a tiny intake leak.

A single slightly funky fuel injector will cause the problem.....and will not necessarily show up on the O2 sensor test.

And I have seen vehicles that had several heat range hotter plugs drive up the NOX....but generally not one single jump in heat range.

"Sharktuning" will reduce the numbers.....you can retard the timing to reduce the combustion temperature. You can also change the target mixture some, which will cheat the O2 sensor some.

Getting the puddle of oil out of the bottom of the intake system will help, short term....although these engine end to push oil pretty quickly back into the intake, until they get one of my breather systems.

If the engine has a whole bunch of carbon in the combustion chambers, the compression ratio will be up a point or two....which drives up the NOX. A "water injection process" probably does the most for getting rid of carbon. If the carbon is rock hard, like the GTS engines form...it can be very tough to remove. Remove a spark plug and look down the hole or use a bore scope to see how much carbon there is and see how hard it is.

You can try a 1/2 tank of 100 Octane race fuel...to reduce the temperature of combustion......and negate the effects of the oil mixing in, which reduces the effective octane of the fuel.

I've done all of these things, multiple times, to get these vehicles past the California Smog *****.

In the end, a new factory cat (or a low mileage one) would cure this car, instantly. Those factory cats clean up virtually anything. Very high quality stuff.....which is why they cost so much money! A pair of OBD2 replacement cat might fix it....but they will only last a couple of years (tops) and are way more restrictive than the factory cat.

dr bob

Folks looking at the CO number need to keep in mind that these are after-cat numbers. A good cat will scrub CO down to nothing, so trying to guess what the actual mixture is from the after-cat CO is, well, only a guess. A new oxy sensor opens the door to reading the sensor voltage, and getting at least an idea that the LH is responding to the reading and keeping mixture in a controllable range. Now, we also need to remember that CO is a product of combustion, and richer mixture equals more CO. Leaner mixture means higher NOx, in this discussion. There's one oxy sensor looking at the average CO, so it takes one or maybe a few --relatively-- lean cylinders to push NOx through the roof, since the --average-- CO number isn't moved so much by one or two lean cylinders. Cylinders can be lean because of vacuum leaks, particularly around injector seals. One or two underflowing injectors will do it too, and conversely, one or two rich might skew the average enough to cause a couple others to lean out to the point of making NOx. A particular cylinder only needs to be lean enough to have about 2200º on a flame front in the chamber. This can easily be less than enough to cause a knock especially at low smog-dyno loads.

So.... Make sure there are NO vacuum leaks, look at plugs immediately after a hard drive (like: pull to the side of the road aftyer killing the ignition at the end of a moderately-hard pull) and look for lean cylinder indications, install the new oxy sensor and verify that voltage is managed between about 0.2 and 0.8 on the new sensor, read with an analog meter if possible. Put new Bosch copper plugs in, no plats or multi-electrodes. Report back.

ptuomov

Getting a Sharktuner and WBO2 controller would be worth the investment for most people who are interested in these cars. ST2 isn't that expensive compared to time and parts that it will take to figure problems like this without it... at least the time and parts it would take for someone like me to figure them out.

I agree with GB a couple posts above that if the O2 sensor and LH are working properly, it's unlikely to be a small leak or small MAF problem because "adaptation" would compensate for it. Unless it pegs at the limit. ST2 would immediately tell one if the O2 adaptation is pegging at the limit.

Similarly, a WBO2 controller with ST2 would immediately tell whether the narrowband O2 sensor is busted.

x98boardwell

Guys, thanks for all the input. i read Greg's response earlier but was mobile and couldn't respond.

What is the best way to access the O2 sensor to check if its working properly? It appears as though it enters the passenger side compartment near the center console. Would that be the place to check the wiring for volt range? It there a simple way to do this for access purposes while the motor is hot?

If it is fluctuating from .2-.8 volts then it is a good O2 sensor correct? How do they normally act if they are bad... not fluctuating at all?

Thanks again... I will get to the bottom of this one for sure.

Update: I did take fuel rail covers off and intake tubes and airbox to see what I could find. Everything looks to be in good nick. What should I spray on each fuel injector or seals to see if something changes with idle if a leak is present? Should it be something that helps (like a flammable fluid) or just something simple like soapy water? I will look into this of course before I do something like that but wanted to see what the most common thing used is.

Thanks,
Bryan

Rob Edwards

Perhaps I missed it earlier- what is the idle speed when cold, and when hot? Would a big enough airleak to overwhelm the ISV and cause a high idle be enough to make it run so lean?

You say GT conversion- what did that actually consist of? Did they install GT cams but not the GT brains? I can't imagine that the maps between S4 and GT brains are that different in the cells where the smog testing would happen.