The vehicle is a 420 SEL, 1986 version. On this car, there is a smog pump located on the passenger side, just under the alternator. The air that discharges from this pump travels through a rubber tube to a "stop valve". From there, the air goes through another rubber tube to a "check valve". The "check valve" is screwed into a connection on the "timing chain cover" which presumably routes the air to a passage in the engine and then it somehow finds its way to the exhaust manifold.

A few months ago, the rubber tube connecting the smog pump and the "stop valve" wouldn't stay connected at the pump. Upon first cranking, the hose would pop off and I couldn't determine if the problem was with the "stop valve" or the ''check valve". I don't know how to run a test on either of these valves and the "Mercedes Service Manual CD" doesn't cover this topic.

If I should decide to dismantle the smog pump system, I have surmised that the "check valve" would have to remain in place at the timing chain cover to prevent exhaust gasses from backing up into the engine compartment.

I would appreciate someone explaining the details of this system. Thanks.

I'd like some education on this, too.

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1994 C280, dark green metallic
105k miles

1991 190E 2.6, Black
191,500 miles
(sold to another forum member)

2003 Chevy Tahoe LT, Redfire Metallic
105k miles

1989 Mustang GT Cobra Convertible
43k miles

This may not be what you want to hear, but there's a lot of history on the "air pump' in the archives.

Key in -> air pump

then click "titles only"

Look for posts from Arthur Dalton, someguyfrommaryland, M.B.DOC; stevebfl. I believe there's one started by Engatwork as well. If you dig hard enough, you may be able to figure out how to fake out the system so that you don't drive a CEL if you disconnect it. Of course this is illegal and I personally would never dream of doing such a thing.

M.B.DOCs comments in this thread state the devices basic purpose:

What's an Air Injection Pump designed for?

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Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"

Hi Bill.
Let me take a crack at it: The check valve that is between your timing cover and air pump diverts air to either the intake ports of the intake valves or directs air to the air cleaner. When you 1st start the car, the solenoid engages to direct air to the intake thus, quickening the rate the catalyst heats to reduce cold-start emissions. The o2sensor heats quicker, sends a signal in which the diverter is then sw. to direct air from your pump to the air cleaner. Once the the car is at running temp, the air just stays directed to the air cleaner...I think. Being that the air pump has a clutch, it may engage off at some point while the engine is at temp. I removed my whole system when I redid my 380; I tapped the holes in the intake ports to 7mm and put metric hex plugs w/ loctite in. The car nontheless still passed del. strict sniff. If you remove the parts you describe and plug the front timing w/ a22mm fitting, it will be fine......for a while. Because the carbon buildup within those intake ports will be sig in time if no air ever pushes through. You would see this if you ever removed your intake to change gaskets or do any other service. I was told this would happen from a German mb tech. I also saw in the cd of the dowelled plugs that the european models had in place of the plugs I made. The dealer doesn't carry these plugs. les

I'm curious now...I removed my pump on my 380SE years ago. I cut a disc and inserted it into the fitting on the intake, effectively plugging it off, as you describe. What would the effect of the carbon build-up in those ports be on engine performance?

I searched, and found an earlier post in the archives from D H Nugent:

"Not sure about MBenz in particular, but most such air pumps pump air into the exhaust right after the exhaust valve. The theory is that any unburnt hydrocarbons (especially if they were unburnt as a result of too rich a mixture) will burst into flame when exposed to an oxygen source since they are so hot (~1400F).
This is basically a burn-off of exhaust - outside the combustion chamber - so it adds no power when present and takes away none when it isn't operating.
The only downside I can think of is that if your mixture is too rich and the air pump doesn't burn off the unburnt hydrocarbons, they'll end up in your catalytic converter (if you have one/more) and that will shorten the life of the expen$ive cat(s)."

So these 'ports' are actually leading into the exhaust manifold, and the possible effect would be to overload the cat IF the engine was running rich??

I can't see that either, since the pump, according to all the posts I've read, only works for a minute or so during warm-up, so after that, there would be no air being ingested through the ports anyway.

I think the $#@%^& things are an overpriced mechanical nightmare totally useless except for passing visual emissions tests. Glad I don't have testing where I live!

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1986 560SL
2002 Toyota Camry
1993 Lexus

You are right, sorry. I meant the exaust ports, Wrote that after a 12hr at the hospital. I guess "if" carbon builds up back through the exaust channel holes where the air comes in, it could cause some fouling in time. But probably not I suppose. les