Just did a full engine rebuild and we're up and running. Problem: Check engine light comes on revealing a Code 11 (air injection pump problem). California car, so I can read the codes myself via LED light near battery. No symptoms, car is running great. Was having trouble with check engine light prior to the rebuild, but not as consistent. Not sure what those codes were as I didn't know how to check them at that time. Air injection pump comes on for 1-2 minutes when engine is cold and then shuts off (as I think it is supposed to). One thing I noticed is that it will sometimes come on even after the engine is warm for only a few seconds and then shut off.

Need a little help diagnosing my problem. When I put the engine back in the car after rebuild, had a little trouble hooking up all the vacuum lines in the engine compartment and I think one or two of them may be incorrect. Does anyone with my same car or engine have a photo(s) of their engine compartment with the air cleaner off they can post on the forum or email to me? I am particularly interested in vauum lines running from EGR valve and air injection pump over to the intake manifold area.

Anyone else ever dealt with this? Input would be greately appreciated. Car has new oxygen sensor, by the way.

The air injection code is usually set during a test made with the engine hot. It is this way because the way it tells it tells whether the air is induced is by the O2 sensor voltage dropping below 40mv.

The vacuum necessary in the air inj is that which goes to the check valve that is between the pump and the exhaust. this check valve shuts off the channel to prevent air from being aspirated into the exhaust. Added air is a no-no to a warm O2 sensor system.

The vacuum will go to a switchover valve that turns the vacuum on at the same time the pump comes on. That check valve must have vacuum and the it must open when the pump works and the O2 sensor voltage should quickly drop to 40mv if one were to jumper the air pump clutch.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Thanks for the quick response, Steve. I'm still new at this mechanic stuff, but did learn a lot with the recent overhaul. I just want to clarify what I should be checking.

I have one vacuum line running from the air injection pump to the passenger side firewall into what appears to be a switch of some sort. Another line, which is part of a "Y" tube, comes out of the top of that same switch and goes into a blue and black check valve and then runs the length of the rocker cover (along with the spark plug wires) to the intake side of the engine. I think it is then supposed to plug into the intake manifold, but I'm not sure I have it hooked up right. Back at the passenger side firewall, there is another "switch" just to the left of the previously mentioned one which connects to the other leg of the "Y" tube at the top, causing it to merge with the line that runs over to the intake manifold. Further down on that switch, another vacuum line goes into a black and yellow check valve and then into the EGR valve.

Based on what you said in your post, which way should be air flow in both check valves be going? Where should the vacuum line going over to the intake side connect? I'm not sure about the condition of the two check valves I mentioned. Do they sometimes go bad? The blue/black one seems somewhat restricted and the yellow/black one seems to work intermittently. Not sure if this makes a difference or not.

I'm sorry but I can't follow such descriptions without having a diagram in front of us. The reason I explained the portion of vacuum I did was that it doesn't matter how it gets there only that it exists at the same time the air pump works. That is what you need to test. Tee a gauge to the check valve and verify that the valve gets vacuum when the air pump works cold. Then bypass the circuit to the air pump clutch when warm and see if the O2 sensor voltage drops when the pump is turned on. This won't work when cold because the O2 sensor will be inactive when cold.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

If it helps, here's a picture of my M103 from an '88 300SEL.

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Mike Heath
1988 560SL Black/Palomino
1988 300SEL Black Pearl/Burgandy
1984 500SEC Anthracite Grey/Palomino

maheath
you have them hook up right i have a 92 2.6
right switch over valve goes to airpump/left switchover valve goes to egr. the hose that goes thru the sparkplgwire cover goes to constant vaccume. enjoy jose