ViribusUnits

Dr. Bob, your information about HC is incorrect.

It can be due to a lean miss fire.

It can also be due to a rich mixture. (If there is no O2 to oxidize the fuel, it stays in hydrocarbon form.)

It is also a weak fuction of ignition timeing. Too much timeing advance, and when the cylinder starts burning, the pressures and tempretures of the air/fuel mixture are reletivly low. As a result, the start of the raction does not run to completion. Instead it leaves a bit of fuel and air behind. This fuel and air will not combust any more, because after the partial combustion, the products of the partial combustion get in the way.

ROG100

Update:

New O2 sensor - retested and failed as before - no real change on the NOX reading.

Just picked the car up from Autobahn in Ft Worth. They checked the air pump and all connections.

They say it is definitly not the MAF - however I do have a new one on the way so will try that next - cheaper than new cats.

Jeff the Porsche tech (who I trust 100% and has done all the work on my 928) tells me it is the cats.

I am now looking for a new set of cats - anyone want to lend me some money??????

SharkSkin

Cats have nothing to do with NOX. Check the reason for high combustion temps instead of wasting your money on cats.

ViribusUnits

Shark Skin, cats have ALOUGHT to do with NOx.

A fully tuned 928 engine should make it without a catalytic converter, BUT if the engine is not in a perfect state of tune, the cat will clean up the exauste. This is the reason for the 3 way, 2 bed cats. That is the reason they exist.

However, if the cats are good, you can still fail the test. One of two things could be happening, 1. you have too little fuel, 2. you have to much timeing advance.

1. Check for a vacuum leak in the brake booster, or the HVAC system. It is very rare for a car to have a perfectly ait tight system after it's more than a few years old, so odds are you've got a vacuum leak in there somewhere. Most shops I don't think would think of checking this system. There are probably a million other vacuum lines and valves in the 928, and they all have to be checked. I don't belive your problem is here, as your CO readings don't seem to be excessivly low.

2. Your cars timeing advance is controled by the computer, dirictly. The computer gets the information from a set of memory chips. Do you have any "proformance" chips installed? If so, go back to stock chips. The proformance chips make they're power by advanceing the ignition timeing. Ususaly they seem to try to keep it around stock at or near normal drivieng conditions, BUT not always. Next, check the part numbers to make sure you've got the right computers. If you've got the wrong computer in you car, all bets are off as to how it would react. (Maybe an S4, or GT computer or chipset instead of GTS.)

ROG100

Thanks for all your suggestions.
SharkSkin:
I am no expert at all and some of what you guys are saying is way over my head - sorry about that.
ViribusUnits:
I have done nothiong to the car in the last twelve months since its last test (apart from oil, filters, transmission fluid).
At the last test it passed on everything with very low numbers. Nox was 264 ppm @ 25mph and 450ppm @ 15 mph.
A borrowed cat may be the answer to test the cat therory.
Where do I find the computer to check the numbers - is it to the right of the fuse board?
Thanks again guys.

ViribusUnits

Yea, there are two of them, to the right of the fuse board.

Stirling, the hall sensor? Or do you mean the anti-knock sensor?

Randy V

While I continue to not be convinced that the failure is due to toasted cats, you can buy good used cats from 928 International at very reasonable prices.

There are also numerous after-market cat manufacturers that will have something that will be a good match - at much lower prices than Porsche OEM.

Gretch

Echo above.......GTS cats should not be bad unless you put some leaded fuel through them.........and a lot of it............

simple test, get a vacuum gauge and see what you are pulling at idle. I had problems, traced to vacuum leaks after I confirmed I was only pulling 10. fixed the leak and all is well with vacuum at idle reading 16 - 18

ROG100

Randy:
I already have a nice offer from Jim at 928Int.
Gretch: No leaded not one little drop.
Car has done 78K. 7K since the last test.
Porsche quoted me $2400+tax and thats with a heafty discount. Normal price is about $1000 more.
I will try a borrowed cat to answer the question on the cat theory.

ROG100

Gretch:
I am learning! Where do I gat a vac guage (can you recommend one) and how do I make the test?

SharkSkin

OK, VU, you are correct in pointing out that what I said is not 100% correct, and that cats do reduce oxides of nitrogen.... but that is not their primary purpose. Excessive NOX readings will overwhelm the ability of the cat to break down the NOX very quickly. So, the more accurate statement for me to have made, would be to say that the problem is most likely upstream of the cats, since the cats only play a limited role in reducing NOX. The main thing to check is things that might raise combustion temp. Lean mix, advanced timing. My V8 Dodge truck put out a fraction of the NOX that Roger stated above, and it is carbureted with no cat whatsoever, no air pump so I know it can be done. without changing the cat.

ROG100

I have been surfing and learning about cats. A number of companys will supply a new unit - they say identical to the Porsche unit - for about $350 delivered.
5 Year 50K warranty and 2 guarantee to pass the emissions test.
If I have to go down this path - and I hope I don't - has anyone used one of these aftermarket units? They are all made by Catco which seems to be one of the biggest manufactures of cats.

Kevin in Atlanta

When my 86.5 failed do to high NOx I found a disconnected vacuum line on the throttle body. It ran from there to the diverter valve. I reconnected and passed with flying colors.

dr bob

Just re-looked at this thread and noticed this post.

VBU, if you read my post, the one you are commenting to, it specifically mentions lean misfire and timing as factors in the HC.

Your analysis of how advanced timing affects chamber pressures is, um, wrong. In that situation, the piston is still rising as flame speed peaks. This causes a pressure wave that slows or stops the flame momentarily until the piston starts to move down. That collision of the flame face causes "pnging" in mild cases, and is known as "knock" when you can hear the effects rattling through the rotating components. The collisio of the pressure waves also causes a localized spike in temperature, and it's this phenomena that is the root of NOx production in IC engines.


Though you'd like to know...

ROG100

Well I hope I am not boring you all with my efforts to pass emissions.

Fitted another set of cats today - lent to me by Sterling (many thanks).
Car failed the test again with high NOx readings the same level as before.

So kudos to those who said it was not the cats.

I am next going to recheck for vacuum leaks. Did a physical check of everything I could see without finding anything amiss. It will have to wait a couple of days until I can get some more spare time.

I have a new MAF on its way - been a long time coming - so will also try that.

Please jump in with anything else.

I will keep you posted.

Roger