s50015

Hey all,

Long time lurker, first time poster. Hoping somebody here can shine some light on this very odd issue.

Intro:

I have a very confusing issue on this 2002 S500 (W220). A few months back, i was cleaning under the hood and accidentally pulled on a vacuum line to the Secondary Air Pump system and broke two vacuum lines. I was too irritated to fix it right away and forgot about it over time. Except for the small vacuum leak due to the open tube and a check engine light for the Secondary Air Injection system, car ran fine with no other codes.

Problem/Confusion:

Here is the problem : I am in Texas and we have yearly emission testing which is due now on this car. It won't pass with the CEL. If I leave the secondary air injection vacuum tube disconnected, the car will run fine and I/M Readiness indicators will all go ready except when it comes to the Secondary Air system, it fails and sets the Check Engine light after driving for a day or sometimes 2-3 days. If I connect the vacuum hoses to the SA system, the engine will set Multiple Misfire, Random Misfire and randomly anything from cylinder 3, 4 , 7 misfire within MINUTES of driving.

I can hear the air pump running immediately after cold start and then occasionally during extended idling... so I know the pump is working. I couldn't hear any other vacuum leaks once the SA lines (vacuum) are connected.

What is happening ? How can i fix this? I don't see how air injection (which should be affecting only exhaust pre-cat), is causing misfire and the ECU is NOT detecting any misfire at all every over months of driving with SAI vacuum lines unhooked.

Please guide me.

Jack_88

I wonder if the engine adapted to running with that line disconnected and now hooking it back up throws it off? If that's the case, shutting it down when it misfires and restarting it until it readapts would be the likely solution. Any other codes other than the misfires?

wallyp

The Secondary Air Injection system pumps a pretty small amount of air into the exhaust ports of the cylinder heads when needed to slightly reduce emissions. (The W220 is an Ultra-Low Emissions car.)

That is all that it does. Whether or not the engine has the air in the exhaust has no effect on the running of the engine.

To me, that means that Jack 88 is almost certainly correct. The ECU has adapted to the extra air (often called "false air", meaning that it is not measured by the MAF sensor), and is injecting extra fuel during idle. When you stop the vacuum leaks, the extra fuel causes rich misfiring at idle on those cylinders. (The ECU senses the voltage at the plugs to detect misfiring.)

Hook everything back up, and drive the car. The misfiring at idle will not damage anything, the car will eventually correct the misfueling, and will stop setting the codes. You may need to clear the codes after a few days, and give the emission test readiness code time to be set correctly.

I long ago realized that my car is a lot smarter than I am, just like my phone.

s50015

No other codes other that P04 something for SA if the lines are disconnected (not immediate after code clearing but it'll show up after 2-3 days)

Misfire codes show up within minutes of driving with SA vacuum lines connected.

Wouldn't doing a CEL reset make the engine relearn everything since all emission systems are reset in the process? If so, that is what I am already doing, right?

Jack_88

Resetting the check engine light does not reset adaptions. There are separate procedures for that. Your options are to either take to a shop who knows how to reset the adaptions or plug everything back in, and drive the car until it learns on its own(shouldn't take too long.)