Strange cold idle behaviour

[dante53]

My '65 230SL starts immediately when cold but the idle goes up and down (so much down that the engine seems to stall and sometimes it does) for about 20 seconds. Then it stabilizes around 1000 RPM and goes progressively down to 750 RPM while the engine gets warmer. So, it seems to behave fine except for the severe hunting in the first seconds.
Warm start and idle are trouble free. Screwing out the WRD air filter does not change the situation. Not even the idle level. The air suction is present during cold starts and stops as it should as soon as the engine warms up .
Spark plugs reveal that the mixture is on the rich side but you must consider that I'm using NKG BP7, which maybe are way too cold. Leaning the mixture only lowers the idle and does not fix the initial hunting at cold starts

Any suggestion?

[jedwards]

Hunting is usually a sign of excess leanness I believe.  Mixture during warm up is mainly determined by the WRD shims, primarily the oval ones. Can you tell us the number and thickness of oval shims under your WRD? If you have too many, the WRD will not be enriching your cold start mixture sufficiently.
Others will certainly may have greater knowledge on this subject.

[dante53]

Thank you jedwards. I did realize that hunting is associated with leanness. It was not easy to realize because on my k-jetronic 911 idle hunting means that the mixture is too rich. I can't say how many oval shims I have under my WRD but as far as I know (but I may well be wrong) adding or removing oval shims changes the mixture at all RPMs and my engine is already running on the rich side at working temperature.

[Cees Klumper]

At the risk of adding misinformation, but I believe the shims under the WRD do not affect mixture once the engine reaches operating temperature. The shims under the barometric compensator do. As I understand it, the shims under the WRD affect the amount of time, and/or the mixture, during warm-up only. Others correct me if I'm wrong please.

[dante53]

I searched and read a lot of posts on this matter  before posting this one. I understood that the round shims in the WRD plunger controls how long the auxiliary air valve stays open while the oval ones  control how much the rack moves forward leaning or enrichening the mixture. So, adding or removing round shims should work only when the engine is cold. Adding or removing oval shims, on the contrary, should have a permanent effect  similar, if not identical, to what you get shimming the BC. Is this correct or just my misunderstanding?

[ja17]

Dante,

Try splitting the linkage at the injection pump and cross-over rod, while your engine is surging. Push down slightly on the linkage going to the injection pump while the engine is surging. Next let the linkage return and press the linkage cross-over rod slightly. Which procedure eliminates the surging? (Split Linkage Test)

Also, the oval shims will adjust the idle mixture only until full warm up. You can use the Split Linkage Test to check the fuel mixture before warm up, and at different engine RPMs after the engine is warm. You should test the whole warm and hot cycle of the engine first.

As always make sure that everything else is in good working order before doing any injection adjustments. How is your main fuel filter?

[Rolf-Dieter ✝︎]

Dante, I take it you checked thru the Technical Manual under .... Warm Running Device also known as WRD.

If yes please disregard my message.

[alchemist]

As I understand adding oval shims increass the richness of the mixture. Removing shims leans the mixture. Perhaps, you need to remove some of the ovel shims. I hope this helps.

[dante53]

Hello Joe. Thank you. I will follow your suggestion and do a split linkage test. It will not be easy and I'll need an assistant as in 20-30 seconds from start the idle normalizes, even though not as high as one may expect (900-1000 rpm instead of 1100-1200). Fuel filters are OK. They were all checked when I cleaned an coated the tank. The only one I did not check is the CSV mesh filter but I think it's ok too because the engine always starts at first crank, cold or hot. My suspects are more oriented on the cold start enrichment solenoid. The place I live in has a very mild climate (almost never under +4/5  C in winter) so my solenoid could not be working properly and yet give only minor issues as the the one I'm chasing.

[dante53]

I did the split test immediatly after a cold start. Giving more fuel cures the problem: higher idle and no hunting. Giving more air and engine stumbles and stalls. So, mixture appears to be too lean at cold start. Should I add one or more oval shims to the WRD or should first investigate if the enrichment solenoid is working properly?

[ja17]

Dante, good job on diagnosing the lean running condition. Adding an oval shim will richen the mixture a bit during warm up.  You should do "the linkage tour" first, in case improperly adjusted linkage rods are causing the lean situation.

Keep us up to date, ask quest ions.

Joe

[George Des]

One other area you may want to check out is your brake booster and brake booster line. I had some of these same issues and once I had the brake booster rebuilt most of them were resolved. Prior to the rebuild, I could never achieve a consistent idle at the relatively low RPM range called out in the specs for a std transmission model. Now everything is fine.
George

[dante53]

Hi George, thank you for your advise.

[garymand]

Unfortunately on my fip, the oval shims DO affect the mixture past 80 C.  It was a big surprise.  I had just gotten to a good balance and mixture adjustment on the regulator screws and got consistant performance for many startups and 100's of miles.  The cold start was lean so I added a few shims to get a close cold start, but then at temp, the mixture was very rich -many clicks on the rack.  I didn't expect that and found it a bit startling.  But it wasn't that bad, I just had to balance back and forth with shims and rod clicks.  Its within a few clicks now, But I have to wait until the car comes from the paint shop before attaching the final adjustments. 

In overview of what I have experiencde on this one car, once you get the WRD closing the air valve at 80C and have it shimmed for a good cold startup, adjust the fip for how you want the motor to perform: rich for best performance, lean for better economy.  If the rack was adjusted, then check the wrd during cold start and reshim.  This adjustment will effect the rack, so think about if the wrd adjusted to rich or lean.  Then be prepared to compensate a few clicks on the rack.  It sounds like it is ann interitive process were you add a shim and have to lean a bit the rack.  Perhaps the trick is to go a little extra rich with shims and then the rack will bring the cold start too rich back to closer to where it is needed.  And I hope ther isn't something odd about my fip that the wrd is still affecting the mixture up past 80 C and maybe to 90, not sure how far it goes before reaching an internal stop.

[wwheeler]

Gary,

To make you feel better, I have noticed a very slight hot mixture difference when I have changed the oval shims. In theory, I don't think it should make any difference. Maybe it is because there is almost 50 years of wear in all of the hundreds of little parts in the system, that it doesn't follow theory. So maybe some engines are effected more than others?