Cold Idle Adjustments

[DavidBrough]

I've decided it's time I addressed my cold idle as this is quite slow and slightly rough at around 800rpm. However, before I start I would like to clarify the operation of the cold idle controls and hopefully someone will be able to confirm or deny the following and add anything I may have left out or misunderstood.

From what I have read in past posts this is controlled solely by the WRD on top of the pump
and requires all the usual things to be right i.e. plugs, points, timing, hot idle setup, linkage adjustments, CSV not leaking etc.

In terms of how things work my understanding is that the IP rack achieves cold start status by being moved forward against a spring which then gradually returns it to its hot state as pressure is released. This movement is facilitated by the cold start solenoid whilst cranking (where fitted) and the WRD thermostat which also opens an auxiliary air intake through its own small filter and a separate link pipe to the inlet manifold. I sort of assume that there must be a shim setting where more or less air/fuel is allowed in and thus rpms increased or decreased during the warm up period. Presumably this will also vary the cut in and out temperatures but as long as they are within spec then best cold idle should be achieved by simply adding or removing shims to the WRD tower. Does this seem correct?

Moving onto the WRD itself my understanding is that the thermostat rod is at its shortest position when cold and extends as it heats up. This being the case then the IP rack must be in its cold start position i.e. forward with the auxiliary air intake through the WRD filter open when cold and static with warm/hot run status only being achieved as the thermostat rod extends and allows the rack spring to push it back into this state also blocking off the auxiliary air intake. Does this sound correct?

So, sharp intake of breath, increased rpm requires more fuel/air so the rack must be pushed further forward which would seem to require the addition of shims to make the WRD thermostat rod appear shorter when cold. Does this sound correct?

I assume it's not possible to alter the cold fuel and air mixtures separately and that a single happy medium has to be achieved which may well involve either the addition or removal of shims by the trial and error method. Hopefully this will involve the removal of shims as I have no doubt that sourcing/making additional ones will be tricky.

Sorry to go on so but I would at least like to try and be scientific about it so all thoughts/comments gratefully received before I get my spanners out and start to wreak havoc.

[awolff280sl]

Hi David, I've spent alot of time on the function of the WRD. I'm not saying that I know what your problem is, but I can clarify some of the info in your posting so that our understanding may be a little better.
Here are some perhaps finer points:
You write in yor 3rd paragraph:
"WRD thermostat which also opens an auxiliary air intake through its own small filter". In actuality, that air intake is always open when the WRD is cold. It is not until the thermostat heats up that the cylindrcal "stopper" is pushed down by the thermostat and is supposed to block air from being sucked in." Your 4th paragraph is correct to my understanding.
In your 5th paragraph, we need to make sure we are talking about the same shims. There are small "washer-shaped" shims on the cylindrcal "plunger" against which the thermostat rod pushes, AND there are eliptical shaped large shims under the WRD housing. Removal of the "plunger" shims will effectively raise the starting point for the "plunger" and increase its travel distance. This will increase the time during which the warm-up mixture is operative. If too many of these shims are removed, the "plunger" may not be able to travel low enough to completely cover the air intake hole even when the thermostat rod is fully extended (first-hand experience).
Removal of the shim(s) under the WRD housing, in my understanding, will not affect the air intake at all, but rather will permit the cylinder or plunger to further depress the rack lever in the FIP and thus lean the mixture across all tempertures.
Hope this helps.

[DavidBrough]

Thanks Andy that’s very helpful.

I hadn’t realised that there are a separate set of shims on the thermostat rod itself as it is quite a long time since I’ve had this apart. I certainly remember the housing shims and from what you say if these affect the air side only then I just need to determine how mine is set by disconnecting the throttle linkage and moving the intake butterfly and pump linkage independently to see if I’m rich or weak and then make the appropriate housing shim adjustment.

Presumably the rod shims will affect the fuel level to a certain point always assuming that the remaining travel allows a full return to the normal hot state.

I think I should be able to check rack travel by inserting a rod or bolt in the front of the rack behind the small plastic cover and seeing how far it will move back and forward with the WRD and solenoid removed and then re check when cold and hot with everything connected. This should also tell me how much thermostat rod movement is required to achieve full rich and hot run rack positions.

I’ll let you know how I get on.

[awolff280sl]

David, maybe I confused you.
1. The separate set of shims is not on the thermostat rod, but on the "plunger" which is a separate assembly that slides up and down within the cylinder. To examine this part you need to remove the thermostat housing (not to be confused with the WRD housing) and pull it up out of the cylinder using a small grasping instrument.
2. The shims under the WRD housing affect the fuel, not the air.

[DavidBrough]

Thanks Andy, I think light is begining to dawn and all will make much more sense when I take it appart and start moving things around.

[wwheeler]

Is it possible to lean or richen the cold mixture only?

I know the large oval shims affect the mixture at all engine speed ranges and I assume this includes cold operation. And now I understand that the plunger shims change the position of the plunger and therefore the amount of time that the WRD is active. As the plunger in the WRD moves up or down, it changes the rack (fuel) and the amount of air drawn in at the same time? It may let in more air but it also increases the fuel and increasing the idle speed.

So, it sounds like the cold WRD mixture is a set air/fuel ratio and can't be changed. Is this correct?

[awolff280sl]

I think you are correct that the cold WRD mixture is a fixed air/fuel ratio.

Referring to this image below: 

The distance between the "air valve" (which controls the amount of air getting in) and the bottom of the "plunger" (which moves the FIP rack) is fixed and not affected by the shims (which are on top of the air valve).

[Dash808]

I took my WRD apart the other day and there were not any smaller shims on top of the air valve.   I did have the 3 larger oval shims on the bottom however.  I wasn't aware there could be any other shims until now. 

Also, what is the trick to taking the air filter off the side of the air valve??  Mine will just turn and turn and turn...

[Dash808]

OK I can't read.  I see where the shims on the air valve can go now.  I was thinking air valve housing because.... i can't read.

But still wondering about taking the air filter off.  Originally it was pretty snug and would only move 1/4 turn either way.  Now It will turn 1/2 turn, get snug, turn again, get snug, turn again.... over and over but won't come off. 

Has anyone encountered this before? 

[ja17]

Hello Chan,
Are you turning the skinny bolt (19mm early cars, or 22mm on later) or are you just spinning the round filter canister?

[Dash808]

Hey Joe,

I have the 22mm bolt, and yes I was spinning the round filter.  I thought it would simply unscrew, but if the bolt has to come off then that would explain it.    Now I need a skinny wrench.

[wwheeler]

Now it's my turn with the cold idle problems.

Just installed my freshly rebuilt IP. The minor rebuild was for small things but did include replacing the WRD thermostat. I have started the engine a few times since but still have the cold idle problems. Just like David, the engine runs slow (700-800 RPM) and a little rough. If I split the linkage and add air when cold, the idle speed increases significantly and smooths out. I also noticed that the time the cold is cycle is active is much, much shorter than before the rebuild.

The new WRD air filter makes a strong suction sound when cold and disappears when the coolant housing gets warm. If I remove plunger shims, it should increase the cold idle speed (adding more fuel and air) and also the length of the cold idle cycle. But the mixture will still be rich which according to earlier replies, cannot be changed. Is the cold idle supposed to be that much more rich? Its not eye-watering rich but can be easily leaned as stated above.

Before the IP rebuild, the engine would run very rich and slower during the cold cycle. The engine would shake so much that I would split the linkage and add air to smooth it out until it warmed up. Clearly, things are better but maybe not quite as good as it should be. I recall the cold idle speed should be about 1100 RPM. Correct? How sensitive are the plunger shims?

Any thoughts?

[awolff280sl]

Wallace, I also went through frustrations in getting the WRD to function the way I want it to, much trial and error. The method that worked for me was to initially  remove all of the shims on top of the air shut off valve. This resulted in a situation where the valve was not pushed down far enough by the thermostat to completely closer the air intake hole in the cylinder wall, and air was still being sucked in. I then sequentailly added shims until I just acheived complete elimination of air being sucked in, and I made note of the number of these shims required to acheived this. This was a PITA because I would have to wait until the engine re-cooled before I could add shims. Once I knew the number of shims that it took to shut off the air intake, then any additional shims affected only the amount of fuel that was passing through the FIP. But here's where things got tricky and I think I may have just stumbled upon the correct # of air valve shims by accident. Because at this point the engine is just about , if not fully warmed, and now the amount of fuel into the FIP depends not only upon the shims on the air valve, but also upon the # of those large shims under the WRD housiing AS WELL AS that knurled idle set screw on the back of the pump. I know that I played around with all 3 (the air valve shims, the WRD housing shims, the idle screw), and I used a CO meter. Alot of trial and error. I learned how sensitive those air valve shims are by doing it this way (the hard way?), but I'm sorry that I can't figure how to relate this, other than to say that you get a feel for it if you go through this process. Hope this helps.

[wwheeler]

Thanks Andy. I figured that is about the only way to do it. When you took off the coolant housing, did you also have to drain the coolant?

I noticed my WRD thermostat was glued in the coolant housing by some gray RTV sealant that the rebuilder used. I guess if the sealant is strong enough, it may hold the coolant and the thermostat in place so I won't have to drain the coolant each time.

In my case, I think I will probably want to remove shims. What idle speed does your engine run at when cold? Did you find that removing air valve shims increased the amount of air and fuel and therefore increasing engine speed?

Sorry for all of the questions.

[awolff280sl]

The coolant was another PITA. As you know, the coolant is in the thermostat housing, and the "seal"  is formed between the "flare" on the thermostat and a "ridge" on the housing. This is where I put some Locktite 574, which is anerobic and hardens under pressure. When I was lucky I could carefully remove the thermostat housing and add shims without disturbing that seal. I had a bucket under the car to catch leaking coolant when I wasn't so lucky and got some leakage. In any event, breaking that seal releases only a relatively small amount of coolant since this is one of the highest parts of the system and I just topped it off at the expansion tank. Messy though. Once I settled on the right # of shims, I cleaned up the mating surfaces and redid the seal.
When I start my engine cold it does this: For about 10 seconds it idles at 800, then fairly quickly climbs to 1100 and stays there until it begins to warm and then slowly drops to 800. I can't exactly remember what removing the air valve shims did, but it makes sense that removing them would increase engine speed. However, if too many are removed casuing the air intake to never fully close, then idle may severely drop off or die once the engine is warmed from too lean a mixture.

[DavidBrough]

Whilst I haven’t got round to making any adjustments on my cold idle as yet I have been looking through my papers and found a test procedure from my BBB CD. Apparently, the cold idle revs should increase by 300/400 rpm when the inlet butterfly is opened slightly for the cold idle mixture to be correct. If the increase is greater than 400 rpm then base shims should be removed with shims being added if the increase is less than 300 rpm. For those with vacuum retard distributors there is a bit of a problem as, for the test to be true, the vacuum pipe has to be relocated to the test grub screw on the venturi body. Apparently, if this is not done the revs will be further affected by the ignition timing making a true reading impossible. Obviously the engine has to be cold throughout the test.

Additionally, I’ve found PTFE tape to be best for sealing the thermostat and WRD housing with the thicker gas tape being best.

[ctaylor738]

Not to pile on, but I am having similar diifficulties with my 230SL.  The air part of my WRD is working fine, but it seems that the pump does not supply extra fuel, which actually leans the mixture when the engine is cold.  The result is that I get a lumpy, slow idle when cold and during warm-up and poor performance under load.  To get the car to start reasonably well, I have a rich idle mix, around 7% when warm.

I actually observed this yesterday, by using the CO tester during warm-up.  Immediately after start-up, CO was 3.2-3.5%.  This is way too lean for warm-up.  The CO gradually came up to 5.4% when the temp hit 80, and stabilized at 6.8-7.1%.  Right after the temp hit 80, a mild idle surge started, and I could watch the CO cycle between about 5.5 and 4.5, roughly in time with the RPM.

I believe that the CO should be around 8% just after starting.

[wwheeler]

One last question about the air valve or plunger shims. I removed mine tonight and found only ONE shim that was .039" thick. It also looked like it was hardened steel. Since I want to remove shims for my problem, I could either replace the .039 shim with a thinner one or machine the one I have. I would be scared to machine it because that is permanent.

Do you know if you can buy pre-made shims of different thicknesses?

[ja17]

Hello Wallace,

Try installing it without that onlly shim first.  You can purchase shimstock in various thicknesses and make some irf you need.

[wwheeler]

Just for my curiosity. If I take out plunger shims, I will be adding more air when cold. But, will I also be adding more fuel as well? Or does the rod from the WRD to the fuel rack only have so much travel (maximum amount of fuel it can deliver). I know at some point you can take out too many shims and the air valve will not shut off when warm.   

[wwheeler]

I had a wild thought about the picture that Andy posted earlier in this thread.

Is it possible that the spring loaded plunger would wear over time decreasing it's length and therefore richening the mixture further when cold. I know the heat feeler will wear, so why wouldn't the spring loaded plunger?

It would seem that if you could change the length, you could change the cold running mixture ratio of the WRD. The plunger shims simply add or lessen the amount of both air and fuel.

What do you think?