Pulsing revs

[laf]

Hi there,

I've got a strange problem on my 1996 SL500 R129. The revs pulse when I maintain a speed with very little throttle. It's done it since I bought it 15 months ago.

I might be doing 40mph, tiny amount of throttle and after a short while, the revs will drop 100rpm then recover immediately: it's like the rev counter bobs. It will then continue doing this every couple of seconds until I move the throttle (even as little as 1mm).

It does it in any gear at any speed and with or without cruise control.

I've taken it to a local independent who've checked it over. The computer is showing no problems. They thought perhaps it was the air mass meter so I disconnected it. The pulsing was still there but the rev drop was more like 200rpm instead. I reconnected it and it's back to "normal" bobbing. The air filters are clean, the engine oil's changed a few weeks ago. The car was services about 5000 miles ago tops.

They also say it's running lean as the 9 month old plugs are still really white.

Does anyone have any ideas?

I hope so,
Lee

 

[pomm001]

Hi there,

I've got a strange problem on my 1996 SL500 R129. The revs pulse when I maintain a speed with very little throttle. It's done it since I bought it 15 months ago.

I might be doing 40mph, tiny amount of throttle and after a short while, the revs will drop 100rpm then recover immediately: it's like the rev counter bobs. It will then continue doing this every couple of seconds until I move the throttle (even as little as 1mm).

It does it in any gear at any speed and with or without cruise control.

I've taken it to a local independent who've checked it over. The computer is showing no problems. They thought perhaps it was the air mass meter so I disconnected it. The pulsing was still there but the rev drop was more like 200rpm instead. I reconnected it and it's back to "normal" bobbing. The air filters are clean, the engine oil's changed a few weeks ago. The car was services about 5000 miles ago tops.

They also say it's running lean as the 9 month old plugs are still really white.

Does anyone have any ideas?

I hope so,
Lee

I had a similar problem with my E class
when i was in light traffic, say on the M25 with the throttle only just open i would have a slight surge it was only just noticable, slighly more throttle and it would go away. The rev counter would drop maybe 100 rpm up and down.
If someone else drove the vehicle they would not tend to notice it.

A very long story short, after i did a unrelated transmission oil and filter change it went away, it may not be connected to yours, but it maybe worth a try.

 

[television]

I would have thought a small vacuum leak in/on the pipes on the inlet manifold

 

[laf]

Thanks guys. I'll get both of these things checked out.

Cheers,
Lee

 

[danhunni]

air mass

hi there i also had the same problem but i had a guy come to look at my car and he was looking around the air mass sensor and he said it was fine but the pipe connecting to it had a big split in it and was cable tied up so he is off the buy one from merc i think there about 10 to 20 pounds about that anyway he also said that this is a common problem with these (a class) if the air mass in taking in air from some other place than through the right pipes it is not picking up on the air mass sensor so were it had a crack it the pipe it was taking in unreconized air with the sensor hope this helps

regards
dan

 

[CITYBOY]

I had a similar problem,took it to an indie for a diagnostic check,and on check in at reception,was told probably dont need diognostics ,because he thinks its a split pipe,based on what I told him,and so it turned out to be. The thing is,you said you had already taken it to an indie,he should have known it was a split pipe,based on his experience,
John

 

[kth286]

always do a vacuum test with those types of fault.