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Greely 01-20-2007 12:43 AM

90 m103 Check Engine Light
This is driving me crazy!!

1990 300e m103 160,000 miles California car.

Car runs smoothly, accelerates well and uses no oil (1qt every 4 or 5000 miles). When it gets a quart low, I usually change it. I always run premium gas (91 octane)

I get an intemittent, occasional slightly rough idle on cold start, but it will smooth out when I depress the accelerator and disappear when the car warms up.

2 months ago I received notice to take it to a test only station for its smog check. The day after I received the notice, the Check Engine Light came on. Fault code 17...O2 sensor signal. I replaced the 02 sensor (Bosch 13942) I didn't test the old one before changing because it had been almost 60,000 miles since the last change. It was due. I also replaced the plugs (Bosch HR9DC), the distributor cap and rotor (both Bosch). I reset the light and drove it for a check engine light.

I took it for the smog test. During the smog test, the light came on again (grrr)...automatic fail. All gasses were within allowances, but the NOx was high, almost to the limit. I checked and replaced a bad EGR valve. It wouldn't hold vacuum.

Also checked all vacuum lines. All good. I reset the light. The next day, the light came on again.

Borrowed a friend's spare o2 sensor.

The check engine light came on again.

I Shorted the r16 resistor to retard the timing. This seemed to help, I went a full day with no light. So I went in to get the smog recheck.

It passed as follows:
15 mph
co2 14.7
O2 0.2
HC Max 105 Meas 50
CO Max 0.69 Meas 0.10
NO Max 757 Meas 344

25 mph
co2 15.0
O2 0.1
HC Max 80 Meas 32
CO Max .56 Meas .06
NO Max 696 Meas 245

On the way home from the smog test, the light came on again. At least it waited this time.

It will always come on if I'm sitting with the car idling for any significant length of time (10+ minutes). Other times, it pops on when I'm in stop and go traffic.

I need some help here, I don't know what to check next. Any direction will be appreciated.

JasonOne 01-20-2007 12:49 AM

Where's the car located? Are you in Southern California? If you are in Los Angeles I would suggest you take it to Mr. MB Motors to get the codes checked.
People on here will definitely help. Also, use the search option to find threads that pertain to this problem.
Welcome to the Shopforum and good luck with 300E's CELight.


Greely 01-20-2007 01:13 AM

Thanks Jason

I'm in Riverside county. Hefty drive from LA.

I've always maintained and repaired my own cars. A friend and I replaced the head gasket and valve seals on this one about five years ago. The electronics, however, are a bugaboo for me. I've read in this forum and others about how they work on this car, but I am at a loss as to what the cause of this could be.

Thanks again

softconsult 01-20-2007 08:56 AM

Wrong plugs.

Do not use resistor plugs in that engine. Numerous posts on this. Do a search. The gist of it is that Bosch has discontinued the proper non-resistor plugs H9DCO. Beru and NGK offer alternatives.


david s poole 01-20-2007 11:14 AM

ok,the check eng light in that car is only supposed to come on when the o2 sensor gets stuck in one place and doesnt change voltage[usually 0.00 or too lean---or 1.00 meaning too rich] and it doesn't happen immediately.usually 3-4 can check this by unplugging o2 sensor with engine running and watch the cel light.poss cause of your problem is the heater in the o2 sensor.i think it is powered by a relay[your car should have the mas relay that i think has this job]if power drops out to o2 sensor and catalyst or pipe cools[cold ambient temps and not enough revs to keep warm enough---you need 300deg centigrade]light may come on----just a thought.

Greely 01-20-2007 04:50 PM

Thanks, Steve, for the plug info. I had an old set of H9DC0 Bosch I'd saved from some years ago. I'd marked the box 7000 miles, so that was probably the mileage on them. They eyeballed in great shape, so I put them in this morning. Car still runs great, but the light still comes on. I'll look for the NGK or Beru plugs next week.

David - I'd tried two different O2 sensors, so I don't think that unit is the problem. I'll test it anyway. If the MAS relay controls the CELight through the O2 sensor, that relay is a likely candidate. A friend of mine has an identical car to mine, but with a blown transmission. He 's told me I can borrow test parts from his car as long as I put them back. I think I'll start with the MAS relay, (after I check my O2 sensor).

By the way, when the CELight comes on, does the car go into a Fixed operation mode? Maybe just my imagination, but the car feels differently when the light is on, not bad, but different.

Thanks for the information and advice. I'll post again with the results.


dakota 01-20-2007 07:30 PM

This may not be the problem, but it's worth a try and won't cost anything. I had the CEL come on on my '90 300E because a sensor connection came loose. The sensor was the one on the underside of the air snorkle going into the air cleaner. Just a thought.

Bruno_300TE 01-20-2007 10:01 PM

A faulty OVP could possibly cause this?
Is the fuel/air mixture regulation well adjusted? (It could be out of range)
Good luck, Bruno

Greely 01-22-2007 03:00 PM

Dakota- I rechecked that sensor, it's in. Thanks for the thought.

Bruno- I should have included this in my orig post. I replaced the OVP appx two years back with the 'new and improved' two fuse OVP. I checked it, no rattles and fuses ok. It could still be this, and I intend on swapping it with a test unit this week.

I have never removed the mixture adjustment tower. I figured because this always occurred during the lambda open loop, I'd leave that as a last resort. The plugs look fine, tan color, no carbon or oil on any. Thanks for your advice.

Question...could this be caused by the water temp sensor? Possibly misreading or slow reading a temperature change? I think the one in the car is the original. Any way to test this?

Other than plugs and distributor cap and rotor, I hate throwing parts at a problem.

michael cole 01-23-2007 03:48 PM

i would double check the wiring attached to the o2 sensor possibly replace.sounds like an intermittant problem which indicates high probability of a pinched wire or faulty connection:)

Mike Freeman 01-23-2007 04:56 PM

O2 sensor?
The oxygen sensor heater current is controlled by the MAS unit. With the engine running check for power at the two pin connector to the sensor.
If there is power to the Sensor add a ground wire from the sensor to chassis.

Greely 05-11-2007 08:50 PM

Check Engine Light Fixed...I think
Check engine light came on occasionally. Finally fixed...I think.

This is my saga

new distributor cap, wires, rotor, plugs...light still came on, they were due anyway.
error code indicated o2 sensor.
New 02 sensor...light still came on
New EGR Valve...this was needed the old one was bad.
Swapped out 02 sensor with another known good one...light still came on.
checked 02 sensor voltage per a post in this forum...light still came on
checked all the vacuum lines (almost)...light still came on.
checked the OVP...ok
swapped out the MAS with a known good one...light still came on.
checked EHA and obvious problems there.
checked the cold start valve...acceptable, no leaks.
checked the duty cycle...high and low rpms...close to 50%...fine.

I decided to live with it for a while.

About a month (3000+ miles) ago, I was changing my oil and air filter. While removing the air cleaner, I bumped the vacuum check valve in the brake booster vacuum line. It broke. The bottom of the valve was badly deteriorated, but the top looked fine. Looks like it may have been cracked (and leaking?) on the bottom side. I really don't know if I sprayed that valve when I was checking for leaks. It sits right on top and is too obvious.

Additionally, when I changed the air filter, I noticed that the rubber gasket on the bottom of the air cleaner had folded in on itself. I have no idea how long its been like that. I've had the air cleaner on and off so often that it could have been a month or a year. But there was obviously not a good seal there.

I fixed the air cleaner gasket and replaced the vacuum line with the check valve about a month ago. As has become my habit, I reset the CEL before I closed the hood. That was a month ago and to this day, not a glimmer out of the CEL.

I don't know if it was a cracked check valve or a bad air cleaner gasket which was causing the light to come on, or whether it was a combination of the two, but the light appears to be fixed. Now maybe I'll buy that new electric antenna which died several months back.

I learned for the umpteenth time to stick to the basics, hopefully my journey through CEL HEL helps somebody else.

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