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Too high idle at warm-up in Park (P)

Started by Berggreen, 08 February 2022, 04:53 AM

Berggreen

Hi w116 friends

My 350SE from 11/1977 with M116.985 engine and K-jetronic injection has a peculiar behavior after cold-start and during warm-up, but only when it is in Park (P).

It starts perfectly cold, but during warm-up, the idle speed increases gradually and continues to increase until it reaches around 1800 rpm. But as soon as I move the gear selector from P to D, the idle drops to a normal 700-750 rpm.

When the engine has reached normal operating temperature, the idle still increases, when I move the selector to P, but not more than a couple of 100 rpms, however it varies how much it increases. Sometimes more - sometimes less. I also experience, when the engine is warm, that the rpms drops to a too low level in R, but that I can avoid it by first reving the engine in P and then move to R.

I have adjusted the idle CO% to around 1% with the engine at normal operating temperature, and even if there is slight difference in rpm between P and D, it does not affect the CO% to any large degree. Thus, I guess the mixture on the K-jetronic is adjusted correctly.

Do any of you have an idea, what is the problem/error here, and what I need to do to correct this problem?

Cheers and thanks,

Christian :)
02/1971 MB 280SL (aut.) - olive green 291H
11/1970 MB 280SL (man.) - silver 180G
12/1977 MB 350SE (aut.) - deep green 825H
03/1969 Volvo 1800S (man.) - dark green 94

rumb

Sounds like an issue with the WUR.  It should start at a higher idle and then decrease as it warms up. Could also be air leaks somewhere.

Good video on youtube and also older posts here about WUR.
'68 250S
'77 6.9 Euro
'91 300SE,
'98 SL500
'14 CLS550,
'16 AMG GTS

Berggreen

Quote from: rumb on 08 February 2022, 10:12 AM
Sounds like an issue with the WUR.  It should start at a higher idle and then decrease as it warms up. Could also be air leaks somewhere.

Good video on youtube and also older posts here about WUR.

Thanks, yes I had the same thought. But then I would not expect it to be linked to which gear I am in. If it was an issue with the WUR or false air, then I would not expect it to be affected by whether I am in P or D. Or am I overlooking something?

Cheers,

Christian
02/1971 MB 280SL (aut.) - olive green 291H
11/1970 MB 280SL (man.) - silver 180G
12/1977 MB 350SE (aut.) - deep green 825H
03/1969 Volvo 1800S (man.) - dark green 94

rumb

It seems to me the bigger issue is the WUR.  That is easy enough to work through and eliminate as an issue.  One step at a time.
'68 250S
'77 6.9 Euro
'91 300SE,
'98 SL500
'14 CLS550,
'16 AMG GTS

Berggreen

Quote from: rumb on 08 February 2022, 12:03 PM
It seems to me the bigger issue is the WUR.  That is easy enough to work through and eliminate as an issue.  One step at a time.

True, though I did change the WUR last year. But it was not a new WUR and the question is if the one I installed was better than the one it replaced.  ;)

Could it be the mixture controller ("the spider"), which is too worn?

Cheers,

Christian
02/1971 MB 280SL (aut.) - olive green 291H
11/1970 MB 280SL (man.) - silver 180G
12/1977 MB 350SE (aut.) - deep green 825H
03/1969 Volvo 1800S (man.) - dark green 94

rumb

#5
Sorry my brain wasn't engaging this morning.  I meant to say the Auxiliary Air Valve.  AAV


https://forum.w116.org/mechanicals/is-my-aav-in-order/msg151637/#msg151637

https://www.youtube.com/watch?v=aZBGpX9hETc
'68 250S
'77 6.9 Euro
'91 300SE,
'98 SL500
'14 CLS550,
'16 AMG GTS

Jan S

The drop from 1800 to 750 rpms when gear is shifted from P to D is strange ..... can't help you there

I'm also having high idle during warm-up. I have checked fuel system pressure, control pressure cold and during warm up (WUR). I have cleaned and tested the aux air valve, replaced the thermostat. All fine and good now. But idle is still high.

Vacuum leaks are next on my list.
1975-mod W116 450 SE with 6.9 engine

Berggreen

Quote from: rumb on 08 February 2022, 02:27 PM
Sorry my brain wasn't engaging this morning.  I meant to say the Auxiliary Air Valve.  AAV


https://forum.w116.org/mechanicals/is-my-aav-in-order/msg151637/#msg151637

https://www.youtube.com/watch?v=aZBGpX9hETc

Thanks for the link. I will check the AAV, though I did change that one too last year. But again, as it is not available new anymore, it was to another used one. But will check it according to the instructions given in this link. :)

Cheers,

Christian
02/1971 MB 280SL (aut.) - olive green 291H
11/1970 MB 280SL (man.) - silver 180G
12/1977 MB 350SE (aut.) - deep green 825H
03/1969 Volvo 1800S (man.) - dark green 94

Berggreen

Quote from: Jan S on 08 February 2022, 05:58 PM
The drop from 1800 to 750 rpms when gear is shifted from P to D is strange ..... can't help you there

I'm also having high idle during warm-up. I have checked fuel system pressure, control pressure cold and during warm up (WUR). I have cleaned and tested the aux air valve, replaced the thermostat. All fine and good now. But idle is still high.

Vacuum leaks are next on my list.

Thanks, I guess it is normal to have increased idle during warm-up, but it should not exceed more than 11-1200 rpm in the beginning and then gradually reduce down to 700-750 rpms with the warming up of the engine. So 1800 rpm and only in P is definitely too much and very puzzling with the difference in rpms between P and D...... :/

Cheers,

Christian
02/1971 MB 280SL (aut.) - olive green 291H
11/1970 MB 280SL (man.) - silver 180G
12/1977 MB 350SE (aut.) - deep green 825H
03/1969 Volvo 1800S (man.) - dark green 94

ethanrmerc

I am having the same exact issue with the difference in RPMs in park compared to drive... Were you ever able to pinpoint this issue?
1973 450SE

raueda1

Quote from: ethanrmerc on 05 November 2023, 07:23 PMI am having the same exact issue with the difference in RPMs in park compared to drive... Were you ever able to pinpoint this issue?
An interesting post, I missed it first time around.  I'm curious too.  As far as I understand there's some wrong stuff posted here.  When everything is working 1000% right the idle speed start at about 750 and stay there through the warmup process in neutral or park, and drop a little when in gear.  Well, ok, maybe a tiny bit higher when cold but drop to 750 after a few moments.  To be clear, "everything" means WUR, aux air valve, vacuum system, distributor, vac modulator on transmission.  Sounds to me like there are multiple things going wrong and that they interact. 

I'd start by focusing on the engine warmup cycle and warm idle control, but take the transmission out of the equation. I greatly doubt that it's all working as it should.  For this purpose maybe cap the vacuum modulator line and go back to basics on the engine itself.  When warmup and idle get fixed then see what's going on with the transmission.  Another candidate for wandering idle is the distributor.  It's supposed to govern idle too.  Take a look at what your timing is doing while the car goes through its shenanigans.  Is it OK?  The internal mechanism can get gummed up over time as lubricant transforms into sticky sludge.  I'd rebuild on principle (tutorial here).  This is something that should be done on all these cars IMO. Good luck and cheers,
-Dave
Now:  1976 6.9 Euro, 2015 GL550
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

revilla

Hi
Late reaction here. Maybe this is a closed exchange. But fwiw. I think the behavior described on your first post is absolutely normal for your car. AAV and WUR will promote those high RPMs for about 6-7 minutes especially during winter days when  cold air temperature (0-10 deg C) is present. But reduce to about 800 gradually until engine's warm (80-90 deg C). When switching from P to D or R it's normal too the RPMs drop as the transmission is now engaged but breaks are applied so car is still static.
I wouldn't worry about the behavior described.
Robert
W116 1977 280SEL & 1979 280SE
[img width=150 height=100][url="http://revillagodfroy.com/MB%201977%20280%20SEL/album/thumbs/IMG_0371.JPG"]http://revillagodfroy.com/MB%201977%20280%20SEL/album/thumbs/IMG_0371.JPG[/url][/img]

raueda1

Quote from: revilla on 26 December 2023, 11:59 PMHi
Late reaction here. Maybe this is a closed exchange. But fwiw. I think the behavior described on your first post is absolutely normal for your car. AAV and WUR will promote those high RPMs for about 6-7 minutes especially during winter days when  cold air temperature (0-10 deg C) is present. But reduce to about 800 gradually until engine's warm (80-90 deg C). When switching from P to D or R it's normal too the RPMs drop as the transmission is now engaged but breaks are applied so car is still static.
I wouldn't worry about the behavior described.
I've got to respectfully disagree.  Per the OP:
QuoteIt starts perfectly cold, but during warm-up, the idle speed increases gradually and continues to increase until it reaches around 1800 rpm. But as soon as I move the gear selector from P to D, the idle drops to a normal 700-750 rpm.
1800rpm is pretty fast, much faster than anything I've experienced or heard of during the warmup cycle. 1000 rpm?  OK, maybe.  And from OP's account it sounds like rpm drops from 1800 to normal (750, 800, whatever) when it's in gear.  That's a huge drop from putting in D or R. 

And then there's this:
QuoteI also experience, when the engine is warm, that the rpms drops to a too low level in R, but that I can avoid it by first reving the engine in P and then move to R.
I'd start by looking for vacuum problems and sluggish distributor.  Distributor has a major effect on idle.  Vacuum issues or gummed up distributor can make you tear your hair out.  Check the distributor vac lines.  Are they pulling and working as they should?  And does distributor respond accordingly?  If OK start thinking about WUR.  Otherwise get distributor right first.  It can be the cause of many inexplicable problems that come and go. Good luck and cheers,
-Dave
Now:  1976 6.9 Euro, 2015 GL550
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

Randys01

Bit of a left of field here but....
I'm wondering if there is some correlation between the brake booster and the gearbox.  Eh???
Is it the actual gear selection  that coincides with a drop in rpm OR is more the fact you doubtless put your foot on the brake pedal so the car doesn't bolt off ?

daantjie

I agree with looking at vacuum leaks as possible culprit.   I always start there as many KJET issues are vacuum related in my experience.
Daniel
1977 450 SEL 6.9 - Astralsilber