Ian S4

I just hooked-up a 'scope to my car's oxygen sensor, and ran the car at idle, 1500, and 2000 rpm.

Mongo

I don't have sound for the video but what happened when the scope pegged at 1 for a little bit before backing off? Did you rev it real high?

That O2 is definitely working.

Ian S4

At 0:09 I went from idle to 1500. At 0:22 I increased to 2000. At around 0:37 I drifted up a little to about 2100. And I backed off at 0:42.

My car (87 S4) just passed California smog check yesterday with excellent results.

Mongo

Great! Can I borrow yours for my test? J/K by the way..

x98boardwell

Guys, thanks for all the input. Been working on my other project so now I will address the voltage tomorrow. That was a great write up... handling it tomorrow. will report back.

Here is my other one. Just finished powdercoating the calipers and adding RUF wheels.

x98boardwell

So, unfortunately I had already installed the new O2 sensor but the good news is at least I know it will be working for this test.. maybe cured the problem I don't know fully yet but this is what I do know.

- Installed O2 sensor and checked with a volt meter. It was fluctuating perfectly between .2 and .8 volts so it's working. I ran the car up to operating temperature but did not drive it. Does this mean my MAF is working fine and that I don't have a major air leak? Could it have just been a faulty O2 sensor (possibly the aftermarket cat's but trying to eliminate other possibilities) that was causing my issue or could it still be an internal combustion chamber problem (running too hot internally) to create this issue?

I plan on re-smogging it this week when I have time. If it still is failing I may just replace the cats with a stock system to see if that helps (borrow one to try) and then go from there.

Any other ideas or could I have fixed the problem since it appears to be okay?

Lastly, I changed the plugs. Here is a picture of my old ones. I didn't number them but you get the idea based on my diagram.

Thanks,
Bryan

dr bob

If the car was driven hard IMMEDIATELY before the plugs were pukked, the two center cylinders on the right would get my attention since yjey are so white compared with the others. If the car doesn't pass this time, look at the injectors on those two as a potential problem.

BC

Those are some VERY big NOx numbers.

Since you have some room to play with on the HC side, maybe try e85 for a half a tank just to see what happens. Obviously something is amiss and you will want to fix it right but getting it smogged, registered, etc, is a good idea - and it buys you time to work on it the right way.

BC

I would wonder how the car is actually running - right now. Is it a bit weak in the knees? Did you mention that the idle was high?

Mongo

Maybe he needs both a MAF and new injectors?

x98boardwell

I'm confused a little.

Based on my recent info above after replacing the O2 sensor doesn't the MAV seem to be working since its cycling between .2 and .8 volts as mentioned o how to check and how to interpret the data?

Remember, I haven't failed since the last test after doing what I said above. My main question was based on this info I posted today does it appear that the MAF is doing its job? I should have checked the O2 sensor prior to removing but now only have the new data.

Idle when cold is around 900 or so. It doesn't seem to be weak either but I have nothing to compare it to and this is my first 928 so my thoughts on how it is performing could be skewed a little. It seems to run very well but by looking at my numbers I had/have something going on.

Is there another way to test if the MAF is working properly besides testing the O2 sensor or replacing it to check from another known working MAF?

Thanks,
Bryan

BC

Obviously Greg is correct. He is even correct for cars OTHER than the 928 - I just spent two months knowing another car was failing because of cats and I didn't want the hassle (MB, expensive, not modularly replaceable like the 928). Well, it got a bit better, then much much worse with things like o2 sensors, coils, spark plugs and MAF sensors. Finally I had to give in and replace the cats. Yes - a 2000% reduction in my bad numbers (I was high on HC).


The reason it's still good time spent to play around with this is that it's not exactly all that same with the 928, but mostly is. As cars have gotten more modern, the OEMs have laid harder and harder on the CATS to do the job. Run a NEW car without cats and its dirtier than a 928 was without cats. HUGE HC numbers, and high monoxide numbers as well.

Can you get a used good pair of cats just to see?

jcorenman

What it means is that the combination of O2-sensor, MAF, LH, and injectors is getting the right amount of fuel to the engine.

Remember that the O2-sensor is just there to tell the LH whether the mixture is rich or lean. It can lie, or tell the truth. The LH, based on that info and the reading from the MAF (ditto), fiddles with the fuel to make the mixture alternately a bit rich, and then a bit lean. Cats like that. So that's all working, and what you are seeing on the meter is the effect of that rich-lean-rich cycling.

So yes, this means your MAF is not so far out of calibration that the LH cannot compensate.

If the MAF were far out of calibration and reading low, then the LH would not be able to inject enough fuel to reach a proper mixture, assuming a good O2-sensor. (Aging MAF's typically cause the engine to run lean, usually).

And if the O2-sensor were bad, then it would always indicate "rich", causing the LH to reduce fuel. (Aging O2-sensors usually cause the engine to run lean, mostly).

You can't be sure that it was the O2-sensor, because you didn't test it before swapping it out. So what you know is that the new one is good, but you know nothing about the old one. One variable is eliminated.

It could still be weak cats, or one or more weak injectors. Bob's point, above, is a an echo of a previous comment: If you have a weak injector then that cylinder runs lean and generates a lot of NOx. (Not to mention robbing you of horsepower).

And, as noted, aftermarket cats are marginal. (Someone smart once wrote that the best test for the quality of a cat was how much the metal recycler would pay for them). If the motor is running optimally then an aftermarket cat will clean it up just fine, but if things are a little wacky then they don't have enough scrubbing power to take care of it.

Good plan. The factory cats (if in decent shape) will take care of a lot of crap and get you through. That does not mean, however, that everything is running 100%. If there are more than 60K-80K miles on the MAF then you would benefit from a freshly rebuilt one.

And based on the plugs and Bob's comments, pulling the injectors and having them cleaned and flow-tested would also be a good idea. Or just run a couple tankloads of Technichron (just before an oil change) and see if that cleans things up.

What plugs did you put in? (Hint: The correct answer is WR7DC+).

Yep, it can be that way around these cars.

You mentioned GT chips, that would raise the idle to 775. The tach can be off also, especially at low RPM's. (If you have a fancy timing light, it may read out RPM). MAF's can also cause high-idle issues, I have not figured out why.

And yes MAF's can be tested. Louie Ott can do that, he is also the agent for JDS Porsche rebuilt MAF's. I also have a setup for testing MAF's (I do a lot of work with Sharktuners) and can do that but it will cost a couple of six-packs. Or, depending on miles, just put it on your to-do list.

Cheers, Jim

Chalkboss

Jim, your excellent explanation of the fuel control is what I have been looking for. How is a MAF rebuilt? To my novice eye I see some metal wires and a plug connection. Very simple, I wonder what is done to refurb them. Mine has 87 K miles and never been touched (S4).

dr bob

If te car has a problem delivering enough fuel through a couple injectors, I'm not sure running it leaner on e85 will help. I know you've donea lot of research on this. Is there something you can share?