E420 M119 – Engine runs fine cold but loses power and idle drops in traffic

[gautrique]

Hi everyone, I need some help.

I have a 1994 Mercedes-Benz E420 with the M119 V8 engine. Recently, I installed a missing vacuum hose from the EZL ignition control module to the intake manifold. After making that fix, I expected the car to run perfectly.

When the engine is cold or when driving without traffic, it runs great — smooth idle, strong acceleration. However, once I get into traffic and the engine warms up, the RPMs begin to fluctuate, and the car starts losing power.

This issue only happens when the car is in Drive; if I shift to Neutral, the idle immediately stabilizes and everything smooths out.

I’m attaching videos that show the behavior.

Any ideas on what could be causing this? I’m a bit lost and would really appreciate your guidance.

Thank you all very much for your help.

 

[gsxr]

Hello @gautrique,

Your video does show a small fluctuation in idle RPM, but it doesn't appear drastic. This could be due to something like a vacuum leak (check all vacuum hoses, better yet perform a smoke test on the intake manifold)... or, possibly the ETA is having problems (the ETA - Electronic Throttle Actuator - is responsible for maintaining idle RPM). Odd that it only occurs in Drive.

Need more information about the lack of power. Is the engine still smooth, just down on power? Or is the power delivery jerky, or is there a misfire? Does this go away after driving 30+ minutes, or does the lack of power persist at all times after reaching operating temperature?

 

[emerydc8]

I hate to admit this, but I’ve been chasing the same problem on my 95 E420 for years. It went away for a few years but I have no idea why. I can share with you what caused it to return: I was getting a warm idle bump that I eventually traced to a bad plug boot, but in the process of troubleshooting that I replaced the LH module. The dreaded idle surge returned. Just like yours, if I take it out of R or D into N it dampens out within a few seconds. I initially assumed it was a bad LH module since that was the last thing I did before it re-started, so I reinstalled the old module but the problem persisted. I then sent the spare LH module out to get the capacitors replaced hoping that would solve the problem. It didn’t. I think I have replaced everything I can think of that might have some connection to this—some items like the NSS several times. The problem still persists but it has died down to a nuisance now that I can stop it by simply going into N while stopped at a light.

EDIT: Mine may be different than yours because there is no notable power loss and there are times when the idle surge is not so bad.

 

[gsxr]

Thanks for the extra data point, Jon!

Forgot to mention - check fault codes on all modules, clear all codes, and see if any return over the next few hundred miles of driving. I wouldn't expect many (if any) fault codes for what's described above, but it doesn't hurt to verify.