WRD Thermostat Question

[Benz280]

Question, could a poorly setup WRD stall the engine once warm, allowing engine to cool for 10 min , engine could start again.   

[Tomnistuff]

Dear Benz280,
I had that problem on my '66 230SL.  After start with the temperature gauge at 40 deg C (the minimum on the dash gauge), the engine would warm up very rich (according to an almost continuous Split Linkage Test while warming up).  As the engine temperature approached 65 deg C , the engine would quickly go very lean and stall. 70 deg C is the temperature at which the WRD air flow is supposed to completely shut off.  After several consistent tests, I realized that the WRD was shutting off the fuel enrichment before it shut off the extra air.  Since the only way to control the WRD fuel flow independent of the WRD air flow (without changing the air flow shut off temperature), is to add or subtract oval shims under the WRD.  Adding oval shims spaces the entire WRD up away from the injection pump and extends the time for the WRD to shut off the WRD fuel flow, without affecting the WRD air flow.  That is because moving the entire WRD housing does not affect the WRD air flow versus temperature.  Removing oval shims lowers the entire WRD closer to the injection pump and shuts off the fuel earlier relative to the air shut off.
Mine was going lean and stalling the engine about a minute or two before the WRD air flow shut off.
I added a few tenths of a millimeter of oval shims below the WRD to make the WRD fuel flow last a little longer and allow the engine to warm up just a few degrees more and therefore tolerate the eventual WRD fuel leanout .  I determined the amount of oval shim thickness needed by doing another cold start and loosening the two lower WRD housing screws and allowing the WRD to back away from the injection pump and reduce its leanout of the fuel enrichment.  I saved the impending stall by loosening the two screws 1-1/4 turns each just as the engine was about to stall.  IT WORKED!  With a 1 mm thread pitch, the 1-1/4 turns equaled 1.25 mm of oval shim thickness required.  A little experimentation and two or three subsequent cold starts and warmups led me to settle on 0.9 mm of oval shim thickness to make the WRD fuel shutoff later and therefore coincide with the WRD air shutoff.
Therefore the answer to your question is, "YES".
Don't hesitate to ask if there is something you don't understand.  I may not know the answer but I'm learning too.
Tom Kizer

[wwheeler]

My 280SE also ran lean just before it completely warmed up. If I added enough oval shims so that it ran well just before the WRD shuts off the air, it ran way too rich during the warm up. So I compromised and found a happy mid ground. Still runs a bit lean just before WRD shut off, but not enough so it that it stalls. And it runs a bit richer when cold, but I don't let it sit around and idle very long after cold start up.   

[Tomnistuff]

Although the leanness just before WRD air shutoff is now no longer “terminal”, my warmup is, like yours, still plug-fouling rich, so I do the same as you probably do.  I work at keeping it alive and not too rich until it’s weaned and can survive without attention.  That normally includes driving it away before either the car or I am really ready.
I have studied the TM but cannot find a way to lean out the WRD functional period and achieve a smooth warmup transition.
What might I be missing?
Tom Kizer

[wwheeler]

If you look at the cylinder where the air valve slides, there is a slotted opening that regulates how much air is let in with the air valve. We know how the fuel is determined. So this slot is a stepped three stage affair with the first opening being the widest and the last is the least wide. As the air valve slides down, it shuts off the fuel and the air is consequently shut off by closing off the slots as it slides down. Eventually, the air valve slides down far enough that all of the slots are closed off and no more air flows.

To solve the lean stage toward the end of the warm up, you need to reduce the air and add more fuel at this stage. I think the easiest way to accomplish this is to open the first two slots (allow more air in) when it is colder and then you are able to make the mixture (adding oval shims ) more rich across the board. That will then make the last of the warm up more rich as well. I tried filing the slotted openings but it is very tight and didn't do much good.

Given the age of these pumps, it is possible that wear in somewhere else in the pump is causing this to show up. Not sure. Just seems like that the engine warm up cycle is not 100% complete on my engine when the WRD shuts off. The WRD shuts off at 170* and cannot go any more else it may not completely shut off in the winter. The lean problem goes away once I drive the car for about 3 or 4 minutes after the temp gauge first reaches 180*.

I suppose you could go with a hotter thermostat and then subtract round air valve shims to make the WRD shut off later. That would then make sure the engine is 100% warmed up before the WRD shuts off. I am not going to do that because it gets very hot in Texas and I do not want to overheat. Oh well, I guess I will just live with it.