P0400 Code '98 C280

[abox5]

Hello,

I have been getting the P0400 code for a while now (about 3 mos). My car has 84K and runs great except for a rough idle while in drive. I have replaced the MAF sensor and cleaned the EGR tube, no help on either account. Does anyone have any other suggestions before I have to go to the dealer and grab my ankles?

Thanks

Abox5

[stevebfl]

The P0400 code is for EGR malfunction - causal chain. Causal chain always used to bother me..... just what the heck is causal chain.

It is one of those weird translations I think. One can almost detect meaning in the brief statement but it is almost silly.

Here is what causal chain means: It means that the controller has monitored all the funtional components and they are working but the expected result doesn't happen.

The DM states that the manifold vacuum must drop atleast 54mb with EGR. The functional test with an appropriate scanner is to activate the sequence and the engine must misfire and drop in RPM during the event.

On the ML I worked on last week with this code the customer had already cleaned the stainless pipe going into the manifold from the EGR valve (The ML we did yesterday the problem was the clogged pipe). I hooked up my SDS and activated the EGR nothing happened. I got out of the car and was pulling the cover off the motor I heard this ringing pop sound and the engine died. Thinking I had knocked something important loose, I looked around, saw nothing and went inside the car and restarted it. Started and ran fine.

I then hooked a vacuum tube to the EGR valve teed to a gauge and sucked on it to see if the diaphram was OK (the engine was off). At about 10inches of vacuum POP there was that sound again. I started the engine and sucked it to 10in and POP the engine died. Ah ha! I had left the EGR activated when I had went under the hood originally and it took a while to pass 10inches of vacuum to the valve, but when it did bango the valve openned and stalled the motor (it normally doesn't go far enough to stall the motor but because it was sticking the vacuum went much higher than normal and when it gave the valve openned much more than normal - BTW normal vacuum to the valve usually peaks at 5-7in, I know this from testing both these MLs just this week.

So... to really test the causal chain, do this. Make a vacuum tester rig out of a long piece of MB plastic tube and hook to a "Y". The first test is to put one side of the tube (with a piece of rubber vac line coupling) to the EGR valve activation port (this can be reached behind the motor there is a small elbow vacuum line attached normally). Place the gauge in one side and suck on the other side. The engine should run poorly at a few inches and get worse the greater the vacuum. The effect should be significant with misfires and/or surging the effect.

If that isn't the case, clean the line this is the common problem. A sticking valve as I saw is rare. Next hook two legs of the tee (Y) to where the small elbo hose came from and the EGR, getting into the circuit but completing the circuit. The other end of the tee will hook to your gauge, with the line being long enough to take the gauge inside the vehicle. Drive the car. The vacuum will be zero at idle and full throttle. It will max out at just above 20% throttel (a guess - vac decreases to zero quickly the more throttle you apply).

Remember the code says everything works but EGR doesn't happen. Clogged tube and sticking valve were the answer in the two I fixed just this week. The controller activates the EGR by activating a switchover valve. The controller is smart enough to gauge the current the solenoid of the vale takes and if it were wrong the code would be different. The controller isn't smart enough to know whether the switchover valve passes vacuum on application. The controller tests the final result with the engines MAP sensor, so a stuck on of those could cause the result but a stuck MAP would be easily seen.