1990 300ce failed smog - PeachParts Mercedes-Benz Forum

jlc · #1 12-07-2009, 12:49 AM

Hi all. I apologise for what may turn into a long post/rant, but I'll try to be very descriptive.
First the numbers:
%co2 %o2 hc (ppm) co (%)
test rpm meas meas max gp meas max gp meas results
idle 702 12.70 1.8 120 270 431 1.00 2.50 0.26 g pol
2500 2455 13.08 0.9 140 290 261 1.00 2.50 0.68 fail

So first I hooked up my sears multimeter set to the FREQ%duty setting to the x11 connection (red#3, black #2) and I get 81.35 when the key is on/engine off or at idle. No change.

Then I hooked up my self made code reader (thanks Arthur!) to the x11/4 and I get 16 flashes. According to the factory service manual this is "EGR switchover valve (Y27), open/short circuit." I installed another(used) switchover valve and the promlem persists.

The EGR valve itself is functional when I apply a vacuum pump, and I cleaned the pipe @ 75k miles when I changed the head gasket. The car now has 110k miles.

At first I thought it was 17 flashes so I changed the o2 sensor and of course that didn't help. The old one had 40k miles on it.

One more thing that confuses me: When I took the car to get tested the check engine light worked at start-up as it should, and then go out. Two days later it doesn't light up at all.

Any ideas or advice would be greatly appreciated. Thanks in advance.

wbain5280 · #2 12-07-2009, 01:08 AM

Most likely the EGR pipes are clogged. Remove them and the valve and dig out the carbon soot.
Is your engine the SOC M103 or the DOHC M104?

This may be illuminating

http://www.continentalimports.com/ser_ic20134.html

jlc · #3 12-07-2009, 04:24 PM

Thanks Warren. I removed the pipe at both ends and it actually looks pretty clean inside and I could blow air through it without restriction, so I'm thinking my problem is elsewhere. I'll print and read the link you sent as it looks helpful.

I forgot to mention the car has the M104 engine, and it a 49 state vehicle now living and trying to pass smog in CA (its done it before).

Jeff

olpos · #4 12-07-2009, 09:56 PM

Your test is a two speed test and does not check the egr or nox.Leave the egr alone and find the real problem.
Does your car have the oe cat converter?
Can you put your fail numbers alone .hard to read what you have.
Rich

jlc · #5 12-08-2009, 03:50 PM

Hi Rich. You're right, my #'s are hard to read.
Basically I failed the HC both at idle and at 2500rpm.
The max at idle allowed is 120, mine was 431.
The max at 2500 allowed is 140, mine was 261.

My catalytic conv. is not oe, it was replaced 2 years and about 30k miles ago. I was kind of suspecting that myself, but I'm still not sure where my engine fault code (EGR) is coming from.

Thanks for any ideas.
Jeff

jhodg5ck · #6 12-08-2009, 04:29 PM

Well, to get you through this year I would do an oil change right before you go in w/ 20/50.. Keep it 1 qt low.. Drive over in a gear lower then you should be, keeping RPM's HIGH..this will get the cat Good and hot.
I'd also pull your air filter, one less restriction to help get efficency up.

Who made your cat? Did you replace your O2 @ the same time?

Jonathan

olpos · #7 12-08-2009, 10:19 PM

See if some one can try adjusting your mixture a touch richer but cat may be toast.going with a slight mixture adjust ,either way may drop the hcs.My experiance with MB'S (and bmws)
is alot of aftermarkets cats last 2 years.on my car this is the case.Even the shop i use to put a new cat on says to smog the same day.The only thing that may save you is you are not on a dyno measuring nox .you may find someone to get your mix to pass.
Your car has a 3 wire 02.This part MUST be direct fit AND BOSCH.and pretty new.I work as a smog repair person and do a lot of 02's.As your co is not terrible 02 may be working ok.
A little ,slight adjust,toward rich may drop your hc's alot.
Rich

gmercoleza · #8 12-09-2009, 08:27 AM

HC is unburnt fuel. How are your plugs and wires (and cap/rotor if it's a M103)? I would check those first to establish a baseline.

jlc · #9 12-09-2009, 11:59 PM

Thanks again Rich. I installed a new Bosch o2 sensor so I'm ok there. I need to find a place where I can tweak the mixture screw while hooked up to the machine to see if I can get it to pass. If not I'm thinking I'll replace the cat. conv. I'll report my progress.

Hi gmercoleza. My cap and rotor have about 35k miles and the wires checked out good last time I looked. The plugs have about 10k so they should be good as well.

dhjenkins · #10 12-10-2009, 08:53 AM

Unburned HC's are usually the result of too much gas, not enough air or inadequate spark.

A quick fix might be to introduce a small vacuum leak without making any changes to the mixture settings to allow "extra" air into the chamber.

You could also advance your timing a tad (if possible) to increase burn time.

gmercoleza · #11 12-10-2009, 08:59 AM

Pop the hood and take a spray bottle with plain water, then "mist" the ignition wires as the engine is running. Do this when it's very dark out. You might be surprised at the arcing you see. This happened to me on a set of ignition wires that were newer and still looked fine. Micro cracks develop in the insulation leading to misfires and occasional stumbles. I must admit this was on a set of Bosch wires from AutoZoo.

Just a suggestion.