Bill Ball

We didn't pull them all before decided to install the O2 sensor. The passenger side is a bit of a pain with the stock tool. So, we left it for later after we saw all the driver side plugs were correct and nice looking.

But, yes, the plug to the non-firing cylinder 1 looked wet, not with fuel or oil fouling, but with a grimy, black, watery slurry. I know the plug was firing, but on first glance it looked like it wasn't. This was probably blow-back of exhaust system vapor through the exhaust valves, condensing in a cylinder that was not expelling any hot gases. At least that was my guess, and the dangling injector plug reinforced that.

Bill51sdr

When's the re-test Bill? I hope the Nox problem is that simple

Bill Ball

I think Mike (Surgiblade) said today or tomorrow.

IcemanG17

Bill
Hopefully Dr Mike will post his results! It will be interesting to see how this problem effected an otherwise clean running 928!

Bill Ball

Mike sent me a note. Car is running great, but it failed again with high NO. Off to DEVEK.

dr bob

MAF, brain, new cats, injector cleaning, copper plugs instead of platinum. in reverse of that order, of course.

perrys4

Where is the O2 sensor Dr. Bob?

Bill Ball

Well, I hope they don't have to do all of that. Anyway, we did the O2 already. It's expensive to just wholesale replace parts, but unless you have an exhaust gas analyzer handy, it's hard to know just what to do. I thought about cats and MAF, but not having a spare MAF, I couldn't help him with that. I would think this would be an odd cat failure, but the cats are original, as they are on my car, for that matter.

dr bob

Start off with the copper plugs if it has platinums in it now. That helped some (but not enough for what you are reading...) on my high-NOx problem.

Try a couple bottles of Sta-Bil as recommended in another thread for injection cleaning.

Test the MAF per posted guidlines and procedures. Sub in a known-good MAF and see if that will do the trick.

Deteriorating LH brains can cause lean conditions. Read the oxy sensor with a scope or voltmeter to see the LH 'hunting' as it sees small changes in sensor voltage. No variation means there is something out of range. Might be the brain, or other fuel system component.

Verify the fuel pressure. Almost undoubtedly not an issue but you never know. Cheap to check. I found a fuel pressure test gauge set at our favorite cheapo tool store (H-F) a monyh or two ago, about $7 with a slew of adapters that don't fit a 928. But... There is adapter there that has a 1/4-20 straight thread on it with an o-ring. Easy to modify a spare rail cap with that thread and an o-ring inside to seal it while measuring. Then put yopur original back on when finished and no possible leaks from Mickey M. permanent gauge install!

Swap in a known-good brain if the other tests point to that.

A set of new cats will clean the car up enough to pass.

There's a low-octane jumper that retards timing slightly for areas that only have poor fuel. That timing retard may be just enough to lower chamber temps below the NOx temp threshold.

Verify that the cooling system is working right. Hot motor can allow high chamber temps needed for NOx production.

Replace any aftermarket chips in the LH and EZK with stock. Advanced timing for additional performance can cause a lot of extra NOx to be formed. Usually but not always, the better chips also richen mixture to take advantage of the new timing advance curve. Can't depend on that though.

Make sure the knock sensors are working.

Make sure the crank position sensor is working correctly.

Verify the initial timing setting with a timing light.

Old wires may cause crossfire between cylinders, as will carbon tracking inside an old distributor cap. Recent tune up? Might be due.

Verify that the cam timing is correct. Cylinder filling and pressurization is very much a function of cam timing.

Make Sure You Don't Have Any Vacuum Leaks!!!

Bill Ball

Nice list, dr. bob. We did the ones we could in my driveway:
- checked plugs - copper, newish, gapped OK (I set them on the narrow side of the range)
- checked vacuum lines (all the ones that should hold vacuum did) and breather hoses (all good).
- installed O2 sensor
- found non-firing injector and fixed it.

We coud have swapped in my MAF, but w/o a gas analyzer, I wouldn't have known whether it helped. same with the brains. I meant to pour in a can of injector cleaner but forgot. I have a fuel pressure gauge, but it didn't occur to me as a cause of high NO. No overheating was observed. Bad knock (or Hall) sensors would retard timing anyway. Crank position sensor was good as the car ran - a bad one would prevent the EZK from sending out any spark signal. Cam timing could be off, but the car was fine 2 years ago and no work has been done in that area. I didn't think to check it under that circumstance. Plug wires were new (2 years) full Beru. I would have expected high HC with a wire issue.

Anyway, that's a nice list for DEVEK and a large check.

ZEUS+

Sometimes high no can result for no obvious reason. If all of the usual suspects for high no are functioning properly it may be necessary to "tune to pass". Without proper equipment this is difficult. A simple and effective approach is by running a cooler thermostat and/or cooler plugs. I hope this can help.

Bill Ball

I believe that DEVEK works with a smog shop that will give them assistance troubleshooting the cause. Jeez, it would be nice to have a gas analyzer. eBay, here I come!

perrys4

Your wife must really love you letting you buy all this cool stuff.

ZEUS+

I hope that does not increase the service charge.

Bill Ball

Well, unless they have some awfully good luck, since I don't think it is plugs, wires or any of the easy stuff, this is going to be expensive...new cats, MAF, etc. Without a gas analyzer right there at your side, I think the most efficient but most costly procedure would be to change everything even if it was changed 2 years ago.