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RPMs staying high after foot off

Started by Jenno1981, 12 February 2023, 04:14 AM

Jenno1981

Hi guys,

1978 450SEL

Had the car out on an average day last week. It had been running fine, no running issues.

Started normal, accelerated and thought RPMs were a bit higher than normal. Car shifted through gears fine.

As soon as I took my foot off the accelerator the RPMs stayed at the point I took my foot off (say 2000 for example). The car maintained the RPMs and speed I was going even though my foot was off the accelerator.

The car was literally driving itself - is there anything they can't do?

Coming up to a stop I put my foot on the brake, car would stop and RPMs dropped but we're still higher than normal (approx 1000).

Pulled up to destination, put it in Park, RPMs went UP and stayed at 2500...

This continued for 2 x half hour drives regardless of engine temp.

Before I hand it over to the mechanic, would be great to hear your thoughts and if it's a semi-basic fix I'll do it.

Cheers,
Jenno
1978 450SEL - Green
1999 ML430 - Also green 🤷�♂️

Nabstud

Some basics...
Have a quick look at the throttle and see if it is stuck anywhere, either in cab or under bonnet.
You have cruise control? maybe an issue there.
1975 280S - Australian delivered

rumb

#2
Gas pedal and all linkages, ATF is good lubricant.
Vacuum leak? AAV stuck?
'68 250S
'77 6.9 Euro
'91 300SE,
'98 SL500
'14 CLS550,
'16 AMG GTS
'21 E450 Cabrio

Jenno1981

Thankyou for suggestions gents.
- The throttle moves freely
- No cruise control
- I am due to lube linkages, so I'll do that
- Had a preliminary look at Vacuum in relation to these central locking. Maybe I didn't put it back the way I found it, good starting point 👍

Thanks again
1978 450SEL - Green
1999 ML430 - Also green 🤷�♂️

rumb

I was thinking engine vacuum leak.  old hoses, vac lines.  Spray carb cleaner around hoses and such while engine is running and see if the speed decreases or changes.
'68 250S
'77 6.9 Euro
'91 300SE,
'98 SL500
'14 CLS550,
'16 AMG GTS
'21 E450 Cabrio

raueda1

Is it possible that the airflow sensor &/or plunger in the fuel distributor are gummed up and hence sluggish?  That is, hanging open in the position they were in when foot was taken off the gas and only slowly returning to position at idle?  Or throttle plate gummed up and sticking?  You said it's free but is it really no resistance or damping whatsoever?  I can easily imagine a scenario where gummed up fuel turns into a kind of adhesive-like gunk that impedes free travel of all the throttle elements.  Maybe disconnect the linkage and see if throttle flap and air sensor plate moves perfectly freely.  The trouble with this theory is that throttle and airflow sensor should be working in concert.  I doubt the car would run well (or at all) if this isn't the case.  Still, worth checking and rechecking.  Cheers,
-Dave
Now:  1976 6.9 Euro, 2015 GL550
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

Jenno1981

I always forget to update my posts with a result but not this time.

Broken check valve off the engine on the way to the central locking system. It was just freely sucking in air, pinched it off and Revs started to drop.

Just trying to locate new blue/black check valves if anyone has any or knows where I can get in Australia.

Cheers,
Jenno
1978 450SEL - Green
1999 ML430 - Also green 🤷�♂️