420SEL Rough Idle & Poor Power After Driving - PeachParts Mercedes-Benz Forum

JamesDean · #1 02-10-2016, 09:34 AM

Hi All,

My 420SEL appears to be having some issues.

Primarily, after driving for about an hour at 80 MPH and coming to a complete stop the car will run pretty rough.

Here is a video, this was taken after I got off the highway. Waiting at a red light. I was on my way to work and there are two more red lights in the video. You can see that, each time, the car comes to a stop the economy gauge drops and the car runs rough. Often times one has to mash on the pedal to get it back up to running order.

According to the x11 port, duty cycle hits upper 90%s. I believe this means it is running very lean.

https://www.youtube.com/watch?v=jvpoA0bKE0w&feature=youtu.be

The spark plugs have been replaced.
The fuel filter has been replaced.
The cap/rotor do not have a lot of miles on them either.

It is very consistent in its behavior. Its never on a short trip. I've only seen it do it on a long trip, like my daily commute (1h).

It does burn a decent amount of oil, in the neighborhood of 1 quart every 300-500 miles.

I started daily driving it back in December, since then I've put about 7,000-8,000 miles on it and have gone through at minimum 2 gallons of oil. Its not leaking, I don't see any puddles or anything. I am pretty sure its just burning it. You can tell immediately on cold start that it is burning it.

Thoughts gents?

lsmalley · #2 02-10-2016, 12:19 PM

Leaky valve stem seals possibly explain the oil consumption. It may also explain roughness if the oil is fouling the plugs. Check to see if the plugs are wet with oil. Also check for vacuum leaks and check the throttle position sensor. If it sticks open it can throw off the idle as well.

JamesDean · #3 02-10-2016, 12:30 PM

Quote:

Originally Posted by lsmalley

Leaky valve stem seals possibly explain the oil consumption. It may also explain roughness if the oil is fouling the plugs. Check to see if the plugs are wet with oil. Also check for vacuum leaks and check the throttle position sensor. If it sticks open it can throw off the idle as well.

I agree on the leaky valve stem seals.

Plugs were just changed and plug #8 was pretty bad.

Not sure about the throttle position sensor. Hmm. Can it be opened up while installed?

JamesDean · #4 02-10-2016, 08:28 PM

Some more videos!

I shot these videos when I got home tonight. I opened up the hood and took a video of the engine running so you could get an idea of the idle. It may not have translated through the camera very well.

My meter said that duty cycle was hitting upper 90%'s. So I figured that it was running very lean.

So either:
a) too much air
b) too little fuel
c) other??

CIS can only regulate fuel pressure. I disconnected the EHA and the car appeared to run better... It did not do its telltale sign as described in the original post but it definitely felt like it was running rich. You could smell it very well.

https://youtu.be/w_C6fa7FAYM

https://youtu.be/0plwqCEyEd0

Hit Man X · #5 02-10-2016, 09:05 PM

Vac leaks create lean running conditions.

At 200K plus, you should have resealed the intake manifold assembly at least once. I did mine about four years ago around 155k, 560 drove totally different after the procedure.

Not sure when M116/7s need valve guides, I have heard 250-300k area. This is a spendy job as you need to install helicoils in the block, on top of the valve work, labor (yours or shops?), and be foolish not to pull timing cover for lower guides.

pmckechnie · #6 02-12-2016, 08:53 AM

My first guess would be the EGR valve hanging up (if it still has one).
BTDT.

PaulM

JamesDean · #7 04-07-2016, 12:44 AM

Partially resolved this problem...We had the catalytic converter replaced and boy did it make a difference.

Doesn't really fix the root cause of the problem though. Car is about due for a valve job. Its been burning oil for a good while.

Now when it starts up in just smokes out the immediate area...

In a few months here I'll have the valve job done on it.

The next problem is an apparent ground issue in the trunk... Not sure how to go about fixing that.