Idle deterioration after warm up

[Tom in seattle]

My 250 SL starts well from cold and performs well for the first 15 minutes of driving. After that the idle starts to gradually deteriorate until it will die at idle and then becomes nearly impossible to keep running at any rpm.  If I can restart it when hot, I can get it home if I keep the rpm's high enough.  I'm pretty sure I smell a very rich exhaust after warmup. After cooling off it can be restarted and the same cycle of events occur.  The car was not doing this a couple of weeks ago.  The motor has 45,000 miles and a valve job was done commensurate with a head replacement 3,000 miles ago.  It almost seems like some sort of fuel/air mix is being induced after the motor is getting up to full operating temperature.  If I take it out on the highway when not up to full temperature and run it at 4,000 rpm it runs with excellent power but when I pull off to test the idle, all the symptoms I have described will be in full performance (bad or no idle).  It is an automatic trans and will kick down under load or come back to low gear properly when idle can be maintained.  This is a 250 car which has the auxiliary oil/water cooler outside of the block and is a Euro car.  I read about a split linkage test but don't know how to do this if it would help the analysis.  I've also read about a WRD device failure, not sure what that is.  Any guidance you experts can provide will be appreciated. 

Here is what I have done to work this problem:
1.  New cap and rotor.
2.  Pertronix ignition (no points)
3.  At 800 rpm idle, 8 degrees BTDC and advance will go to 30 BTDC but no higher
4.  I removed and shipped the Injection pump off to Jerry Fairchild WHo did work on it but said he thought it was in pretty good shape.  It ran well after being reinstalled.
5.  I checked valve lash  to .003 and .007 , all were proper.
6.  New BP5ES plugs gapped to .035
7.  I know of no air line leaks.
8.  Running temp indicates at 80 Celsius and never gets higher even on a hot day.

[franjo_66]

Hi Tom

I would check or a leaking CSV as this happened on my car and the rich exhaust fumes seem to validate that scenario as your symptoms are what I experienced with my car

Good luck and I'm sure you'll hunt down the issue

Regards
Franjo

[Pawel66]

I am sure experts will kick in, but even with my limited experience - indeed it feels like your car is running way too rich when warm and the two typical reasons that even a weekend mechanic can check is the leaking CSD valve (as mentioned above) and the WRD.

It is all on the forum. Search, but just to help you a bit:

CSD - has a little screw on its side. When engine is warm, with fuel pump running, when you unscrew this screw the fuel should not be coming out from the screw hole. Prepare some towel before you unscrew the screw and put it underneath not to spill fuel too much. The CSD valve is there to inject additional fuel when the cold engine is started. It does it for a few seconds only. When it is leaking it does it all the time, making engine run too rich.

WRD - this is a device on your injection pump. It is sticking up on the firewall side (or the back) of the pump. Thick hoses are attached to it. This device feels the coolant temperature of the engine and adds additonal amount of mixture when the engine is cold. Then when the engine warms up, there is avalve that, to be short, cuts off additional mixture. But when it is stuck, it does not cut off and engine is running rich. This device has a little tin on the firewall side with holes. This tin is an air intake. When WRD works, you can clearly hear air sucked through the holes in the tin. The sound is similar to choke air noise on some engines with carb (this is something that does the choke job, actually). This noise should disappear after a few minutes of engine working. If the noise stays, this means probably your WRD is stuck and engine is running rich.

Split linkage test - disconnect rod from venturi or ffrom IP. You now have fuel delivery and air delivery control separated. If your engine is rich, if you add only fuel by moving IP lever a little bit, the engine will stall. Contrary - with rich engine if you add only air by moving a venturi flap a bit - the revs will go up. Make small movements of the levers. Very intuitive.

All this makes sense based on what you said: the idle was good before. Because if the car started doing it after something was done to fuel/air system - you just may have to go through linkage tour from A to Z. Search for it  - too long to explain in two sentences. A very very basic/important part of set up.

I hope I covered the simple things properly. Let's engage the more experienced for more serious diagnostics .

[ja17]

Checking the fuel pressure when the problem is occurring may give you a clue.

[Tom in seattle]

Thanks for the "heads up" on the cold start valve (CSV).  I followed the test procedures I found in the documentation.  No fuel came out of the small screw when removed with the fuel pump operating.  I removed the solenoid and it was easy to move the small rod also I inserted a similar sized rod into the CSV body and found the valve moved easily and returned easily.  Without completely disassembling the whole CSV assembly I'm reasonably convinced the CSV is not failing.

So now I'm going to move onto the WRD.....wish me luck ans thank you.

[Pawel66]

WRD is simpler - just listen to it. Split linkage - very quick too.
If Joe says check the fuel pressure - that is the more labour requiring test I would invest my time into.

[Tom in seattle]

I wish to thank the contributors to this thread (Pawel66, ja17,franfo_66).  I have proceeded a great deal toward my solution.  I can't say I really understand exactly why my car was dying  after a brief period of warmup but here is what cured the apparent problem:  I subscribed to full membership to access the "linkage tour".  Installed all ignition tuneup parts and set the linkage and tuneup to advised specs.  No success!  Ja17 suspected fuel pressure so I started thinking about rebuilding the fuel pump but guessed  a check of the fuel filter was a first order item.  I ordered 2 and pulled the filter out.  I can't believe the engine would run at all!  The filter element had to be the one delivered in Europe on delivery.  It was black, the top and bottom had separated from the barrel which had largely disintegrated as well.  It certainly was not filtering fuel and probably, for reasons I don't understand, was blocking fuel only after the warmup device shut off.  After replacement of a new filter it pooped out on me once and then started running pretty well.  I now have a couple hours of run time and feel I can get further than a mile from home without my wife standing in reserve with a rescue vehicle.  Thanks for the help and this great website of volunteers.  It's nuts, but we do it together.

[glenn]

Hi  Check the other fuel filters too.  Get the crud out of the gas tank.   The Willnot Run Device(WRD) has to expand and cut back the fuel and stop the extra air.

[ja17]

The supply hose leading from the tank to the electric fuel pump and goes to the intake fitting of the electric fuel pump has a small fine screen filter built into the fitting. It also gets plugged. The fitting can be hard to remove while on the car, but you can remove the hose and scrape some of the dirt and debris out of the screen for the time being. 

[mbzse]

.../...the intake fitting of the electric fuel pump has a small fine screen filter built into the fitting.../...

An illustration of what Joe writes about