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1500rpm idle at cold start

Started by TJ 450, 21 November 2007, 11:16 AM

TJ 450

So, I have this problem with my 450 whereby at a cold start, the idle shoots up to 1500rpm, then settles down to specification when warm. The car was serviced last week, WUR was tested OK, they commented "Vacuum leak at rear of manifold, further investigation required". Surely an external vacuum leak that large would result in a fast idle across the board  ??? I have looked at the "deceleration air valve?" with suspicion, but I'm unsure of it's overall function. I have noted that it appears to be in parallel with the throttle body. Is it responsible for increasing the revs at initial start-up, and is possibly sticking?

Last weekend I went over all the vacuum connections in the engine bay, checking for leaks again. I replaced all dodgy fittings I could see, including the ones at the back of the manifold, in an attempt to fix the problem, but to no avail. The connection at the modulator on the trans. is good (although there is a small amount of fluid coming through the diaphragm).

Regards,
Tim
1976 450SEL 6.9 1432
1969 300SEL 6.3 1394
2003 ML500

Andrew280SEL

Hi TJ,

I personally thought that the warm up speed of 1500 RPM was correct... others may correct me on this of course.

Certainly, at cold start up, it is designed to rev up and then as the engine gets warmer the idle speed drops down normal. :)
'79 280SEL- 560,000 Kms
'73 350SE- getting an AMG facelift
'79 450SEL 6.9

WGB

My 4.5 does exactly the same thing - goes to about 1500 when cold and a little irregular and lumpy - settles to regular after a couple of minutes and then warms and behaves itself.

My 6.9 is similar but engine stays smooth and revs are nearer 1000 than 1500.

Bill

SELfor50

Mine does the same... I always thought it was my car.  Until I adjusted it so that when cold it would idle at about 800-1000 rpms.  but then when it warmed up it would die cause idle was too low.  Re-adjusted and now it's fine.
Cold start = 13-1500rpm's
Warm = 6-800

Nothing to really worry bout... I think.  ???

Cam.  :)

TJ 450

Thanks for the replies. It looks like it's all good then  :)

Bill, didn't you mention you had a K-Jet manual?
I still haven't seen any mention of the deceleration air valve. It's behind the Air Flow meter/Fuel Distributor and is mounted on the throttle linkage with a vac line going to it. So far I can't work out what it actually does and how to test it.

Regards,
Tim
1976 450SEL 6.9 1432
1969 300SEL 6.3 1394
2003 ML500

WGB

I'll try and have a look tonight for you.

Bill

torana68

The silver 450 had a restrictor in the air line from the little air valve thingy , the green one goes to about 1450 or so , you can reduce it by restricting the air from the air thingy, if you like, I assume you dont live where there is ice and occasional snow!
Roger

TJ 450

That would be excellent, Bill. Thanks for that  :)

Tim
1976 450SEL 6.9 1432
1969 300SEL 6.3 1394
2003 ML500

WGB

I think what you are talking about is called the pressure damper in the CIS manual and sits between the fuel distributer and the right hand bank below the throttle linkage.

It has a vacuum line that goes to the right hand cam-cover breather and lines that go to the Fuel distributer and the warmup/full load enrichment compensater.

I believe that it both dampens the fuel flow pulsations and also increases the fuel pressure for cold starts and full acceleration to enrich the mixture. If you had a problem with it there would either be a leak or fueling too rich or too lean. Testing of it would be by measuring fuel pressure delivery under different conditions with a gauge.

If it is running alright I would leave it well alone.

I believe the fast idle is part of the normal  start up and is caused by the auxillary air valve being wide open as the idle speed is controlled by the air entering the system not the amount of fuel. The wide open valve is to allow it to start and run in extreme cold  and our milder climes do not need this level of air and hence the idle is a bit quicker than necessary.

As the auxillary air valve starts to heat up and the bellows start to force the slide closed the air entering is reduced and the idle slows down. The fuel itself is injected as a consequence of the amount of air entering the system with cold start injector and warm up regulator keeping the appropriate level of enrichment in addition to that supplied by the fuel distributer.

Quote from: torana68 on 22 November 2007, 12:47 AM
The silver 450 had a restrictor in the air line from the little air valve thingy , the green one goes to about 1450 or so , you can reduce it by restricting the air from the air thingy, if you like, I assume you dont live where there is ice and occasional snow!
Roger

You could always put a restrictor in the rubber air line from the auxillary air-valve and that would reduce the idle speed at cold start if it really worried you.

I started up my 6.9 tonight and it idled at 1200 revs from cold start.

Hope this helps a bit.

Bill

koan

Quote from: WGB on 22 November 2007, 07:49 AM

I think what you are talking about is called the pressure damper in the CIS manual and sits between the fuel distributer and the right hand bank below the throttle linkage.


Nor sure, I think it's the coasting bypass valve which we talked about in this thread here .

I'll have a look at my engine pics and see if I have any of both items.

koan
Boogity, Boogity, Boogity, Amen!

koan


Here are a couple of pics, first showing both the fuel damper and the coasting bypass valve, second, just the coasting bypass valve.


koan
Boogity, Boogity, Boogity, Amen!

TJ 450

Interesting stuff, Bill. That all sounds good.

Thanks Koan.
Now that I know the correct term, it is the coasting bypass valve I'm talking about. I was thinking it might be causing problems by being possibly stuck open, or causing a vacuum leak.

The car is otherwise running very well, although it does have a lumpy idle when warm.


I'll have a look at that thread...

Thanks,
Tim
1976 450SEL 6.9 1432
1969 300SEL 6.3 1394
2003 ML500

koan

#12
The test for the valve is with the engine running pull the small vacuum hose off and seal it with a finger, in this condition the valve should pass air and increase engine RPM by 2-300, replacing hose should restore normal RPM.

As well as emission control it also serves to increase idle RPM for a few seconds on a warm engine start. Should be able to hear the change.

A mechanic told me few work and no one cares much about them.

(the valve operation is described in several M-B books, earlier books show air flow through the valve in a particular direction, this is reversed in the in the newest book I have and is actually the way it is connected.)

koan
Boogity, Boogity, Boogity, Amen!