aircooler

Hello any and all,

I was wondering if someone could help me diagnose my 88 S4 emissions problem.

When I got the car the PO had almost every vac line installed incorrectly. I re-installed them as per the WSM. I have also pulled the intake manifold to re-paint and replace the flappy actuator (all new gaskets). I had the car's body restored this summer and had the charcoal canister out, and I have since re-installed it.

The car runs great, very strong. It also idles fairly well and smooth. It has stock exhaust that appears in good shape. I have not tested or replaced the O2 sensor.

When I went to get the emissions test done, the car failed miserably! Here are the numbers:

40 km/h_____LIMIT_____READING_____RESULT
--------------------------------------------------------------
HC ppm_____65________216_________FAIL
CO %_______0.36______1.33_________FAIL
NO ppm_____0726______2126________FAIL
RPM__________________1352________VALID
Dilution_______________13.33________VALID


CURB IDLE___LIMIT_____READING_____RESULT
--------------------------------------------------------------
HC ppm_____200_______168_________PASS
CO %_______1_________1.11________FAIL
NO ppm_____N/A_______N/A_________N/A
RPM__________________764_________VALID
Dilution_______________12.70________VALID

I was not able to get new plates with these results and will need to be re-tested after repairs. This is no big deal as here in Canada the snow will soon fly and the salt will soon fall, so the car will be nestled away for the winter.

I am by no means a "tuner" and was wondering if there was anyone with experience in emissions that might have an idea as to the problem, or suggestions as to what to look into?

Thanks in advance

Mitch Johnson

Looks a lot like numbers I might expect without a cat. I can offer no assistance beyond that.

worf928

Based upon the HC reading I'd say that you are running rather rich which leads to discussion of the Mass-air sensor. Do you know if it has been replaced? When the MAS begins to fail it fails by over-reading air which causes the mixture to be too rich. Does the car stall or nearly stall when coming down from high RPMs to idle? (That's another symptom.) Do you have any buds nearby with '87s+ that could loan you a MAS for a little while?

Also, it may very well be time to replace that O2 sensor. The O2 sensor and MAS are just about the only two items that could cause you to run rich.

worf928

Mitch, wouldn't you expect NOx to be higher without cats? His HC and CO numbers are high but the NOx doesn't look too bad.

Donald

I am not familliar w/ Canadian testing, but is the "dilution" value the air:fuel ratio? If it is then I would be looking at the O2 or cat.

ViribusUnits

Are you running a set of chips in the car, maybe a PO's work?

As some other posters ahve said, the cat is jsut there to clean up an engine thats dirty. The 928's EFI is pretty good about keeping it clean normaly.

Your numbers indicate too rich a mixture, and too much spark advance. Something that aftermarket chips do for more power.

Did you get the car good and warm before doing the test? Well, as warm as you could possibly get it. Not overheated, but in the normal opperating range? If it's cold the brains will advance the spark, and enrich the fuel mixture.

Failing chips, do your various temp sensors respond correctly? Most specificaly the Temp II sensor. If it reads cold, then it'll always thing the engine is cold, and you'll have too rich a mixture, and too much spark advance.

Good luck.

aircooler

Thanks for the comments.

I know what I am doing with eng accessories and bolt-ons, body, interior, and electric, but I know nothing of air/fuel and compression. I am a fuel Philistine!

David, you think I am running rich? I have not touched the MAF. No near stall or hesitation either.

I was thinking O2 sensor as well, but this is not my game.

---------------------------------------------------------------------------------------

VB, no chips that I am aware of. Considering the shape I got the car I don't think the PO was into investing in performance upgrades.

As some other posters have said, the cat is just there to clean up an engine that’s dirty. The 928's EFI is pretty good about keeping it clean normally.

Could an incorrectly connected charcoal canister (in/out) cause rich mix?

The cat was hot for the test.

I have not tested the Temp II. I will search the archive for info on this.

Mitch Johnson

David, Like the other parameters, NOx is mixture sensitive and all bets are off on the predictibility.

I find the ratio of the test result to that of the limit to be interesting. Roughtly 3 times the limit for all of the failures....hmmmmm.

Watercooler, are you certain your cat is good or not bypassed? I still say the cat is not functioning.

aircooler

I have not had the exhaust off the car to see if the cat was "rodded out". This is something that I was hoping to avoid if possible.

worf928

Mitch, you are right. I for some reason mentally swapped the columns on the NOx - it too is out of wack.

Watercooler: I've not heard of the CC causing this problem. In any case even if something was pouring fuel into the system the LH-Jet system should compensate if everything is operating properly.

dr bob

Some basics... and how soon we forget!

HC is directly related to combustion efficiency. That means that the ignition system is the biggest contributor to high HC readings. A cylinder misfires, the fuel passes through unburned, and bingo, hydrocarbon readings are high.

CO percentage is the direct result of fuel mixture. As the mixture goes richer, the CO goes up. You can pul an ignition wire (don't do this on your 928...) and see virtually no change in CO reading.

NOx is formed when you have hot spots in the combustion chamber. Various methods have been tried over the years to "manage" production of NOx. Your 928 has a hemispherical combustin chamber with no squish areas that might have the higher pressures that cause extra NOx. After that, a lean mixture, old or poor quality fuel are the probable causes.

Assuming that all the basic stuff is good, the catalyst is there to clean up the rough edges, so to speak. It takes care of load transients, occasional lapses in engine management accuracy, and some start-up conditions. When one fails, you can easily see some of the readings you report.


On to what you are seeing:

There are rumors that a perfectly tuned 928 doesn't need cats to pass emissions tests. Old wives pass these rumors on to the flock, and they are believed. But it's still a good idea to get all your basic tune-up stuff back to original. That means plugs and wires, caps and rotors if they aren't recent. Plug selection may affect NOx readings, so consider using the original copper Bosch plugs rather than platinum.

If the oxygen sensor is more than 50k miles old, a new one will help you some on both smog and driveability. You can buy a rather generic Bosch sensor and solder-splice your connector on to it, or go for the complete unit that has has the right connector already installed for some extra $$. This is a DIY task that includes dropping the exhaust at the header flanges, so get two new gaskets for reassembly.

After that, you may be looking at a failed catalyst. If you can find somebody or a place that has a gas analyzer you can use, there are test pipes that run up into the engine bay from the exhaust ahead of the cats. On my '89 they are at the rear of the cam covers just under the air cleaner housing, with blue silicone-rubber caps on them. Attach the sniffer there and get a set of readings, then sniff the tailpipe under the same conditions. The difference is what the cat is taking out. If there isn't much difference, the cat isn't doing you much good and it's time for a new one.


Other posts talk about airflow metering issues, but those typically affect power and driveability as well. I'd satrt off with the basic stuff, since that will have to be brought up to spec no matter what else you find down the line.

Good luck!

Lizard928

I would agree with Bob, when was the last time a basic tune up was done? O2 sensor, spark plugs and leads, air filter,

WallyP

You may have a two-stage problem.

A cat will clean the exhaust very well - provided the air/fuel artio isn't too far from stoichiometric.

There could be several reasons that the mixture may not be controlled, including bad oxygen sensor, bad/misadjusted idle switch or WOT switch, bad Temp II sensor, etc.

If you get the mixture under control, a good cat should drop the emissions well under the requirements. A bad cat will have little effect.

aircooler

Thanks for your comments Wally and Dr Bob,

I will purchase an O2 sensor and install it.

Before I sent the car to the body shop this summer I did quite a bit of routine maint. New:

Plugs
Wires
Caps and rotors
Flappy actuator
Air filter
Smog pump filter
Engine oil + filter
Trans oil + filter
Water pump
T belt
Re-built tensioner
Rad flush
Injector service

I will most likely not be able to purchase a new O2 sensor in and get a re-test before the spring. I will post the results then.

While I have the cat off the car, is there anything that you might recommend I check or do in this case?

Thanks again for the help

Donald

At the risk of repeating myself: I suspect the "dilution" reading is air:fuel. What is the target mixture on that MY?