Problem With Cold Start Valve?

[dtuttle123]

Could you help me figure this out or where to look next?  This just happened for the first time the other day.

When I start from a cold start, my 250SL starts right up, and idles about 750 RPM, after 5 minutes or so - as the engine gets warmer - the idle jumps to about 1800 RPM and stays there.   This is repeated each time from a cold start, and won't come done to the 750 RPM.

Could this be the CSW or the solenoid?  Or where to look next?

Thanks!

[glenn]

7500 rpm???  Do you mean 750 rpm?

Got to get the idle down to 750/800 rpm when engine is at temp, valves adjusted right, timing right, butterfly shut, CSV shut, etc. etc. If it's idling above 750/800(idle air screw adjusted to get max rpm), it is getting too much fuel-period.  Reduce fuel flow from the FI pump.

FI pump injects in a range from 1 oz or so per min at idle to a cup or so(8-10 oz) per min at high load(burning 4 gals per hour at 75 mph).  That means each of the six injectors are passing a pretty small amount of gasoline each squirt and ignition firing at 400 times each minute- 1/2400th of an ounce.

The FI pump has at least a 10/1 control ratio-pretty good from an analog control aspect!

[dtuttle123]

Ooops - yes 750 RPM - heavy fingers. 

The mystery is that this happen 'all of a sudden' - the engine had been running very well - no degregation of performance over time - it was working fine one day, and then the next day....

[clembeauch63712]

Hi Doug;  Im no expert but how about an issue with the warm run device?? That's the thingy on top of the rear top of the Inj. pump. I might think that the spring or something hung up inside it. I would read the tech section on the testing or overhaul .. I might tap it slightly/lightly with a screw drive plastic handle when in that fast run condition and see if it may make a difference? Maybe you will get lucky or some change in RPM.. good luck; Clem

[dtuttle123]

Hi Clem - just read up on the Warm Running Device in the tech manual, and this is where I will investigate next.  The device engages at about 150 degrees according to the manual - which is just about when the car has been running for 5-7 minutes (when the idle changes from 750 to 1800).

Good suggestion - thanks!

[Jordan]

Hey Doug, I too am no expert but could it be that the air valve on the throttle body is starting to stick open?  Just a guess.  You don't say if this happens with or without you touching the accelerator once the car is started.  Since this has never happened before it sounds like maybe something has started to stick, or alternatively has failed entirely.  Good luck keep us up to date on the final fix so we know what to look for if it happens to one of us. 

[ja17]

Hello,

Use a hose like a stethoscope to nake sure that the warm running device WRD is closing all the way when the engine is warm.  Other possibilities are the linkage hanging up, the vacuum dash pot on the venturi (if your car has one) has failed or lost its vacuum connection, or a major vacuum leak .   

My first guess would be the vacuum dash pot if your car has one. Look in the "linkage tour" for a description of this device.

[dtuttle123]

Thanks Jordan & Joe.  I tried tapping the WRD a couple of times to see if that was hanging up, but no luck.  I hear the 'hissing' when first started, but will try the hose after it kicks up to 1800 RPM to see if can still hear the hissing (hard to hear over the engine noise).

Joe, I did look at the linkage, and vacuum dash pot (I have one), and one thing that I noticed is that the linkage is engaging with the dashpot piston - but just bearly.  As I move the linkage by hand - I would have thought that there might be more travel of the dash pot piston - but I have nothing to compare it to.

[Jordan]

Doug, if you have a hard time hearing the air from the WRD it might be simpler just to remove the filter housing.  Then you can just stick you finger over the hole to see if you still have suction when it is warm.  Like you I couldn't hear a thing with the engine running, even with a stethoscope, so I just unscrewed the air filter (19mm I believe).

[ja17]

Hello,
The purpose of the dash pot(vacuum throttle control) is to increase the in gear idle. The the slip type linkage rod should open 1.0 to 1.5 mm when the automatic transmission is in gear.

If the vacuum dash pot is pushing the linkage open all the time this could be your problem. You may want to take a good look at it at idle in park after warm up. It should not be moving the linkage lever on the venturi, off its stop.

[Benz Dr.]

Since the engine slows down after warm up but then starts to speed up again, I think it's something other than what's been suggested. Check those things of course but also look for other less obvious problems.

I've seen where the flyweights inside of the distributor will hang up and stay open or the return springs are weak or broken causing the distributor to remain in an advanced position. My own car has a problem with idle speed but there's nothing wrong with the linkage or distributor. It will idle at the correct speed but it will also idle at about 1,400 RPM.

I know what's wrong with it which fairly obscure. Anyone what to make a guess at this?

[dtuttle123]

Benz Dr. -

Here's what's happening- may be a little different than you are thinking.  From a cold start - car starts right up - and idles at 750 RPM.  If you press the accelerator it will accelerate of course, but then drop to 750 RPM. So the linkage isn't hanging up.

After about 5 minutes, when the engine is warm (150-180 degrees) - if you tap the throttle, it zooms up to 1800 RPM, and stays there (at idle).  The linkages appear to moving very freely - nothing hanging up.

I think the mystery is two fold:  First - one day it worked fine, and the next day I had this problem - like something has 'failed'.   Secondly, it seems to be temperature related - so in the first 5 minutes - the idle is 750 RPM, and then after 5 minutes, the idle is 1800 RPM (with a warm engine).

Does that help?   This is a manual, 250SL by the way.

Thanks for your help!!!  I really appreciate the support!

[wwheeler]

At a cold start, shouldn't the idle be more like 1000 to 1100 RPM? 750 is the idle for a warm engine.

Have you checked the mixture (split linkage) when cold @ 750 RPM and warm @ 1800 RPM? Might give you a clue where to look.

BTW, my WRD air filter is 22mm. I bought a cheap combo wrench and ground it on the sides to make it skinny to fit on the narrow hex of the WRD filter. I check the WRD function every so often to make sure it works.

[Benz Dr.]

That's the problem with questions like this; there's too many possibilities to narrow it down to just one thing. If it's not throttle linkage related, then it's somewhere else. There's only so many places that can have an effect on idle speed and you need to find which one it is. I've already given you a clue and something to check. Have you checked the distributor to see if the rotor springs back after you give it a twist forward?

  On my car the gear set that run the distributor is loose and it will idle up higher that it should. The rotor will spring back but there's some extra play after that. The rotor should spring back to the same position every time you let go of it or it's not working right. Any play at all will cause a very eratic idle speed.

[dwilli3038]

Could it be a vacuum leak someplace? If the expansion of metal opens up spmeplace for vacuum to leak it will increase the rpm

[amg65ri]

i have to agree with benz DR... if i have learned anything about these cars (i have a 250sl 4 speed manual as well) its that any slight adjustment can be good, unoticeable, or a step in the wrong direction. The real problem is that the unoticeable one may cause issues later, like when the car is warming up or after it cools down. This is when you start chasing the next issue (which you may have caused) and before you know it you have 2 issues or 3 or 4...

The linkage guide is very good in walking you through idle issues in a methodical way. Try not to tinker or make too many random adjustements until you get a baseline. I mark and note the baseline so as i adjust things I can quickly reset and start over.