I need an education on the smog pump system

[William]

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.

[brookspw]

Originally Posted by William

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.

[Mike Murrell]

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:

http://www.peachparts.com/shopforum/showthread.php?t=60044&highlight=air+pump

[lesrrt]

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

[donbryce]

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.

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!

[lesrrt]

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

[S-Class Guru]

I have a '91 300 SE, which is a bit different I'm sure.
But my pump has a clutch pulley like the AC compressor, and only runs
about 1 minute after cold start. It pumps air directly into the exhaust manifold through the check valve and sttop valve, to heat the cat up sooner.
It was rattling a bit, so I disconnected the electrical connector, and also the vacuum line that opens the check valve. no issues at all after two years.
Smog check is not affected, since they run the check with a warm engine, and the pump is off anyway.

But to add another thought - my old '87 Vette pumped all the time, but had two switching valves that routed the air to:
- exhaust manifold when engine was cold.
- air cleaner when engine was hot - just somewhere to get rid of the air.
- a little tube with a silencer, leading outside - this was only routed under specific circumstances of quick off throttle, to avoid a backfire or something.
- I killed the whole thing and never saw any issues.

I think on an older, less sophisticated car like an '86, or my '91, or the '87 Vette, you could disconnect the whole thing and the computer would never notice. Effects would be slightly more emissions at cold start.

DG

[Duke2.6]

There are two fundamental emission control architectures. The first and earliest systems developed back in the sixties relied on oxidizing HC and CO with injected air. As standards tightened, oxidizing catalysts were added (mid-seventies) and NOx was controlled by EGR systems, valve overlap (essentially built in EGR since more overlap increases exhaust gas residual, but it also degrades idle quality, where NOx is not an issue and EGR is not needed) and ignition timing map modifications. Retarding timing reduces peak flame front temperature, which reduces NOx and also increases EGT, which helps with oxidation reactions, but retarded timing also increases fuel consumption and reduces output.

The development of "three way catalysts", which promote both oxidation and reduction (chemical reduction) of NOx required a very precise exhaust consituency that is only obtained by a stoichiometric fuel air ratio, which can be maintained with zirconium oxide O2 sensor technology. The first of these systems showed up in the late seventies, and they were pretty much universal by the mid to late eighties. Nearly every car built in the last 20 years relies on this second emission control architecture development and TWC/O2 sensor systems enabled both higher output and better fuel efficiency, so emission control system began to become seemless relative to the older system that sapped both performance and fuel efficiency.

Thus, you CANNOT use air injection on a TWC/O2 sensor car. Emission control is achieved by the relatively low aggregate "engine out" emissions of a stoichiometric mixture and oxidation and reduction by the catalyst.

The big fly in the ointment is the cold start during the emission certification test. As much as 80 percent of total generated emissions are produced in the first two minutes of this 45 minute test, so the OEs often had to add air injection prior to the catalyst and 02 sensor becoming hot enough to go into "closed loop" operation and maintain a stoichiometric mixture with good catalyst efficiency. Some of these cold start air injection systems are mechanically driven pumps and some are electrically driven.

A key stategy for meeting certification standards is "catalyst light off time", which also includes getting the O2 sensor up to temperature as quickly as possible to get the system into closed loop operation. Short light off time eliminates the need for warm up air injection, and one way to accomplish this is to mount the catalyst as close to the exhaust ports as possible, but early catalysts would degrade rapidly under high load operating conditions, so this wasn't an option, and most catalysts were mounted considerably downstream of the exhaust ports - typically under the front seat occupants. As these catalysts age, they become less efficient at lower tempertures and this is why "conditioning" is so important to passing field emission tests. Too cold a catalyst can easily bust a standard even though everything is basically in good working order.

As catalyst technology got better, and they could handle higher operating temperatures, OEs began mounting them in a "closely coupled" arrangement - usually bolting them to the exhaust manifold outlets, and this is common practice today. As a result, most warm-up supplemental air injection has been eliminated due to shorter light off time, but many older cars are stuck with warmup air injection.

Though the air pump many not be needed to pass state inspection tailpipe emission standards, if it is removed because it seized-up, you will likely get busted on the visual. The emission/tuneup label in the engine compartment has codes for the various emission control equipment, and the test tech is supposed to look at the label and verify that all listed equipment is installed and appears to be in working order. If in doubt, they have reference books, which describe every engine configuration ever sold including photos.

Duke