Miss at idle/low RPMs when hot

[MikeL43]

My very late 250 SL runs great when cold but has a miss at Idle/low RPMs when hot especially after sitting at a stop light for several minutes.  From the smell of the exhaust I believe the miss is the result of a rich idle.  I have been through all of the recommendations in the linkage tour, CSV check (I blocked off fuel line to the CSV to try to eliminate the problem), changed the coil, points (to Pertronix then back to points-no change in problem), replaced the plugs (NGK BP5ES), replaced the plug and coil wires with copper core wires, adjusted the valves, adjusted the timing and seem to have exhausted all of the hints I could find on the forum.  As a last resort I even adjusted the low RPM adjuster on the injection pump (12 turns CCW) which improved things very slightly but makes me doubt that is the problem.  I double/triple checked all of the vacuum lines and had the brake booster rebuilt.  I recently got on Joe Alexander's Engine Starting Aid Tour (thanks Joe, your contributions to the forum have been extremely helpful) which basically deals with problems relating to cold starts.  I live in Florida and haven't had any problems re cold starts but have nevertheless been conducting the tests recommended by JA on the TTS.  I used a voltmeter to check the TTS and everything checked out though the voltage at the G terminal was only 9 V and the voltage at the CSV solenoid was only 4.75.  The car runs perfectly except when idling for a few minutes.  I suspect I must have screwed up the results on one of the many tests as there doesn't seem to be many other options.  Any suggestions?   

[Tyler S]

Check that you have sufficient coolant flow through the WRD thermostat. If it is partially blocked you wont get the hot coolant flow through at idle speeds. This will richen the mixture. Another thing to check would be the fuel injectors. If one is leaking down then the engine will load up at idle. How is your compression? Is it even across all cylinders?

[MikeL43]

My compression is between 140 and 145 through all 6 cylinders which seems to be where a healthy engine should be.  I have read the plugs and they appear to be about right, if anything a little light in the brown color area.  I haven't checked the coolant flow through the WRD and will probably do so tomorrow.  I'll certainly let the forum know if I find a cure for the miss.  I haven't checked the injectors since it runs great at speeds other than at lengthy idle and I did not realize a malfunctioning injector would cause the engine to load up at idle.  Guess that's next on  my check list.  TIA

[MikeL43]

I have solved my miss at idle when hot problem and Tyler S. gets the credit as he suggested that the injectors might bleed down when idling thus causing an over rich miss.  I bit the bullet ad bought 6 new injectors (not cheap at about $110 each) and that solved the problem. Idle speed still "hunts" a bit as it warms up, from about 1100 RPM when cold to about 750 when hot but as this car is almost 50 years old I don't think I should expect perfection.  Thanks to the forum for all the help.   

[Tyler S]

Glad you got it sorted! 

[Tyler S]

Mike, now that you have replaced the leaking injectors, there is a chance your mixture at idle is now lean. You may want to go back and adjust your idle mixture and idle air screw.
The high idle when cold is normal as long as it settles down when at operating temperature.

[Pinder]

Tyler you definitely know your stuff! makes my membership worth while!

[MikeL43]

I like Tyler S's recommendation to richen the idle speed adjustment on the injector pump as I also considered that now that I have replaced the injectors (great minds and all that stuff).  Unfortunately I am having problems turning the adjuster (clockwise to richen, right) which seems to require a lot more effort than it should.  I am beginning to suspect my previous attempts to lean out the mixture didn't actually engage the adjuster screw.  I have adapted a 14 MM socket with a nut inside to take up the slack in the spring and allow the socket to push the adjuster screw into the IP and am slowly working on richening the mixture.  Just to complicate matters further I have developed an erratic vacuum leak in the  brake booster  (fine when cold, leaks when hot) that I had rebuilt by USA Rebuilts about a year and a half ago.  I plan to call them and if they won't work with me on this I will be looking for another company to do the next rebuild.