W140 Air Injection Pump Replace / Check Engine Light

[oestek]

I've got a '95 S500 with 191k miles and the Check Engine light on. The local dealer diagnosed the problem and replaced the deteriorated shaped rubber hose on the Air Injection Pump and reset the light, but the light promptly came back on. Dealer said "it's the Air Injection Pump" and siad it needs to be replaced.

Dealer price is around $1400 for the pump, plus labor. (Whew!)

I found the pump at a reasonable price, (reman, $641.98, $350 core) and I am confident I can handle the replacement of the pump. Alldata DIY has a nice procedure and diagrams.

Question is, can I reset the Check Engine light without a scan tool when I complete the job? What's the trick? I've read on this board that you can unplug the light module on an SL to reset it... is there a similar trick on the W140? If so, which module?

Thanks in advance,

- Kevin

[ken_xman]

I believe you can disconnect the battery for 1/2 hr to clear the CEL.

[oestek]

Thanks for the tip.

I bought this car used, and I don't have the radio code. If I do disconnect the battery, will I have to re-code the radio?

[Peter Guenther]

YES, the dealer can provide it sometimes if they want to and the radio has not been replaced.

[oestek]

Thanks - I'll have to call them and see.

[suginami]

You can also get the radio code directly from becker.

They have a website with a forum.

www.beckerautosound.com

[benzaddict SG]

Be careful, it may not be the air pump. I would recommend performing a couple quick checks before you spend the money on the air pump. On a cold start, take the rubber hose off of the air pump and feel the amount of air volume coming out of the pump, they usually either work or they don't-not much grey area. If you have air flow from the pump, then the air passages in the cylinder head are most likely plugged. We are starting to see this quite a bit on high mileage M119 v-8's, where the air injection passages get plugged with soot and gunk just like the EGR tubes, which means the head needs to be removed and cleaned. Not a happy diagnosis if that is your problem, but I'm sure you'd rather know before you spend money on a pump and still have the same problem. Good luck.

[oestek]

Thanks for the tip. Do you know specifically what triggers the CE light in the air pump system? I am trying to understand the system so that I can perform any and all checks that I can before taking the plunge. I will try your check next week. I know the pump does make some noise now, kind of like a subtle bottom-end motor knock, but I suppose it could be a sign that the pump has restricted output as you suggest. Then again, it could be about to grenade. Thanks again for the tip!

- Kevin

[benzaddict SG]

The system uses the o2 sensor to diagnose the air injection system. After the car has warmed up, it will activate the air pump for approx 60 seconds and watches the o2 sensor voltage-as it blows air into the exhaust system it will drop the o2 sensor voltage-it needs to drop below 40mv within 60 seconds or it will trigger the light. When the air injection system is working correctly it will drop below immediately, if there is a minor restriction it may take a while: 15-30 seconds. The same is true of the o2 sensor is getting lazy, then it may take a while for the voltage to drop, it may not drop at all if the o2 sensor is really bad. Good luck.

[oestek]

My AIR pump seems to only engage on start-up, but I'll double check that today. Sounds like I may have an aging 02 sensor or 2. Is there a bench test for the 02?

[ken_xman]

Bad story. My father in-laws 98 S500 had the same carbon build up mentioned earlier. It started with the CEL, dealer thought it was O2 sens... nope, then some valve...nope, then air pump... nope.
It was the carbon in the airways. They pulled the heads, everything internal was worn from the heat build up. Heads to the MB machine shop + all new parts.
The head work alone is currently $7,000+
The dealer agreed to eat all the pre trial work + all parts on the head work, and 1/2 the labor.
This is a 65k mile car, and has a history of the same CEL issue.

Good news is if someone wants a 98 S500 Blue, with new heads, he will be shortly selling it for $25,000

[Stan Pittman]

Does the block passages in the head refer to the right head only or to both heads?

[Will_w202]

Originally Posted by benzaddict SG

Be careful, it may not be the air pump. I would recommend performing a couple quick checks before you spend the money on the air pump. On a cold start, take the rubber hose off of the air pump and feel the amount of air volume coming out of the pump, they usually either work or they don't-not much grey area. If you have air flow from the pump, then the air passages in the cylinder head are most likely plugged. We are starting to see this quite a bit on high mileage M119 v-8's, where the air injection passages get plugged with soot and gunk just like the EGR tubes, which means the head needs to be removed and cleaned. Not a happy diagnosis if that is your problem, but I'm sure you'd rather know before you spend money on a pump and still have the same problem. Good luck.

Is this specific to m119s or should I check for buildup on my m111? My air pump blips on and off for the first 30 mins of driving. I notice it at stoplights, etc. Quick whistle, on and off, rhythmically.

[wbain5280]

Where is all the soot coming from? I had soot buildup in the head air passages on my 420SEL and the EGR pipe as well. The factory has to know why this is happening.