W140 surges at low idle
 

For the past week or so, My 1995 S320 rythmically surges when stopped at a light, or at a slow crawl (stop and go traffic). On the tach it bounces from about 600 to 800 RPM or so, every second or two. You can feel it in the motion of the car if I am moving very slowly.

There doesn't seem to be any noticable affect over about 1000 RPM.

Any thoughts on what might be going on, and what I should check?

Thanks much,

I had a similar problem in my 500SL and traced it to--I thought--the black Climate Control Unit which mounts behind the glove box. Replaced that and the problem quit.

Awhile back though my voltage regulator started acting up and I replaced it. Wondering if the CCU box...which MB now wants over $1,500.00 for...might have been affected by the voltage regulator, I reinstalled the original CCU box and it works like a charm.

So, bottom line to this is I think the voltage regulator was causing my problem. Perhaps yours could be so simple.

Seems to be more noticable once the car has warmed up...?

This might be some good reading for you..

http://www.peachparts.com/shopforum/tech-help/65863-aiflow-meter-pot-source-bosch-cis.html

Also search for surge, surges, and surging..

Many will reference the air flow meter, and a worn potentiometer on the side..

I would look at:

1. vacum leak
2. Mass Air Meter
3. Throttle actuator
4. Engine wiring harness

For sublettd...
 

I'm curious about your comment about replacing the A/C component behind the glove box and relating it to the engine idle issue...
Are you talking about the Valve Block? It has about 15 vacuum hoses on it...
How did you trace the problem to the valve block.? Was it a vacuum leak?

I'm curious cuz the climate control on my W140 does strange things now and then and my idle occassionally wanders... have checked various components of the climate control system and am left with the valve block as a possible problem area... never thought about the two being related.

Can you fill me in on how and why you thought yours was bad?

Thanks, KenP

Ken,

I'd pull up to a traffic light and my CCU would go bzzzzzzzzzttt and the idle would surge two or three hundred rpm at the same time. I would also hear the contacts sticking at times while driving, but couldn't detect an rpm surge then. It was only when I was stopped that the surge was noticeable. Figured out it was the CCU by pulling the glove box and had a buddy ride with me to lunch and put his hand on the various boxes mounted to the firewall. The CCU turned out to be the culprit. This CCU is NOT the Heater and A/C controls in the dash, but is a "black box" that mounts to the firewall. Believe some folks might tend to confuse the two.

Interesting part about this was when I first began tracking the problem and checked the price at the MB dealership it was something like $154.00. Figured what the heck and went to order it. They'd had a price increase to the tune of $1,500.00+. Started searching the internet for one and found a guy at TriStarpete.com who came up with a "suitable substitute" for $125.00. It had three prongs on the right side vs the four on mine, but seemed to work. Turned out that what I lost by not having the fourth prong was the a/c cycling. That's why I decided to try it again after replacing the voltage regulator. Glad I did.

Hope this helps.

Thanks
 

Don,
Thanks, much appreciate the response.
KenP

OK, so I took the car to the dealer for diagnosis - here is what they said:

"Checked for surging when idling and found codes for air flow meter and oxygen sensors. Checked Oxygen sensors - tested OK. Checked computer and check for vaccum leaks and found throttle body actuator leaking also found air flow meter may be faulty but could also be symptom of air leak.

Recommend replacing throttle body first."

Now the bad news:

Throttle Body part: $1536.00 + labour $515.20 = $2054
Air Flow Meter part: 427.00, labour $184.00 = 611

($Canadian)

Yikes.

What would be the impact if I delay fixing this for some time? Dealer told me horror stories - engine would be getting unfiltered air, and may wipe enough oil off cylinder walls to cause damage...

Comments???