Let's Talk About Lambda Warm-Up Regulators
 

My late-model SCs use a later-type "090" warm-up regulator, or control pressure regulator, that is equipped with an internal leaf-switching device (I think some Mercedes used these as well). This leaf switch has an adjusting screw, but for the life of me, I can't figure out how it functions, other than introduce another current path to heat up the bimettalic strip. This leads me to think that it's got something to do with the pace that the bimettalic strip heats up. I've poked around checking ohms resistance at various points with my multimeter and haven't learned anything conclusive.

Has anyone figured out (Tony?) how this switch functions and how it can be adjusted? The issue I'm seeing is that I can set the WUR on my cab for a solid cold start, but the WUR quickly "ramps up" the pressure to the point where the engine is too lean (you can see this by watching the test gauge). Warm pressure is fine at 3.6 bar. What I would like to do is slow down the heating up of the WUR to provide a smoother transition. Thoughts?

Brian

How old is this WUR? Sounds like the bimetallic strip is worn out and needs to be replaced. Adjusting would be a short lived band-aid. I've got a rebuilt one in stock if interested.

The WUR is in perfect shape. I want to slow the bimetallic strip down -- it ramps up the pressure too quickly. The leaf switch has something to do with this.

Brian

Still looking for thoughts about this.

Here's what's happening: On a cold start (80-90 degrees F ambient), the engine starts fine and goes into a nice high idle at 1300 and then begins gradually decreasing. About a minute or two in, the engine hits this point where, when you apply throttle, it lacks power and backfires through the intake. After about another 30 seconds to a minute, the engine is back to running fine and you can drive away.

What I see from watching the CIS gauge during this whole thing is that it takes probably a minute or two for the control pressure regulator to have gone from cold start pressure to the topside warm running pressure.

The net affect is it's like un-choking (de-richening) a manual-choke-equipped car too soon.

Before anyone says "air leaks," let me say I've been through the entire system via a partial engine drop and systematically removing and plugging the lines for the auxiliary air regulator, auxiliary air valve and the deceleration valve to eliminate them as potential problems. The intake runner boots are new, injector O-rings and sleeves are new. In other words, the engine is tight.

The mixture is spot on, too. Once warm, the engine runs absolutely perfectly.

Brian

Here is my feedback to our questions.......
 

Brian,

Without running the engine, check the control fuel pressure by testing the FP using a suitable wire jumper with in-line fuse. Record the initial reading after the FP starts to run and every 30 seconds afterwards till it reaches max. reading. Do not detach the electrical plug. The control fuel pressure would stabilize between 3 - 4 mins. and the second part of the test is to determine the time the AAR would be fully closed. Allow 4-5 mins. or until the AAR's slide valve is completely closed.

You may not have found a vacuum/air leak during your last inspection but that did not confirm the absence of an air leak in the system. By pressurizing your system, you could test the integrity of the vacuum under question. The air-box in particular could be leaking or not, we don't know until you test it. You think the engine is air tight and you never had a test to verify or confirm your assumption (guess).

Tests results would only confirm or verify your suspicion. Right now there are too many unanswered questions which make the troubleshooting unpredictable. Keep us posted.

Tony

"This leads me to think that it's got something to do with the pace that the bimettalic strip heats up."

That's correct. If you're absolutely sure that no other problem exists
with the CIS system and the warm-up mixture lean process is to fast,
i.e. the control pressure reaches it's max too soon, then the process
can be modified. You can add a small resistor in series with the
heater element in the warm-up regulator. Measure the resistance
of the heater and then start with a resistor about 1/3 that value.
Once you determine the proper resistance, you then need to calculate
the proper wattage for the final value.