91 420SEL hesitates while warming up
 

Got a problem with my 91 420SEL (85K) miles. When it is dead cold it starts and runs fine but in the warm up process at approx. 70 degree C, it hesitates and does not want to take the gas. Once the temperature goes up a few degrees the problem ceases. It does not reoccur for the rest of the day. It has to sit over night before this condition can be duplicated. Have checked all thermo-valves related to the problem or not and vacuum circuits. It was time to replace the distributor cap, rotor and plugs anyway but this made no difference. Have read on line about others having this condition but never found a remedy. Contributors to lists have said that the CIS-E goes into closed loop at 70 degrees C and it may not be a smooth transition. Seems with my car that it is getting a signal that it has warmed up a little too soon and the engine does not like the fuel mixture. Rather than test the temperature sensor reporting to the CIS-E, I just replaced it. No difference. Does anyone out there have experience with this problem?
Thanks for any help.

Bob /San Carlos, Ca.
78 280CE
91 420SEL

vacuum leaks on the intake,most probably the injector seals and cups.your car is drawing in air that the f.i.cannot see and therefore doesn't provide fuel for it.what you have is a temporary lean condition.most likely if you can gently raise engine speed to 2000rpm you will not notice a problem as vacuum becomes less of an issue as engine speed rises.

Thanks for the quick response David. Sounds like something I can hang my hat on as they say. Any magical way for me to check these before I replace them, such as spray fluid etc?

Bob

Smoke
 

NOT you but the car! USE what ever you can to cause smoke vapor and the leak will suck it in. I use an old BEE keepers smoker with a long hose so I can direct the smoke just where I need it to be, no pun intended. I personally do not like spraying flammables on an engine,and they may cause the failure of the seals you are trying to check.JNT

I've had the same problem with mine. It starts and runs great for the first minute but then there is hesitation off idle during the transition thru warmup (about 5 mins.). I've been thinking the warmup regulator is acting too quickly. Thats an expensive part and the problem is not severe enough to consider replacing it. It can be tested with a CIS fuel pressure gauge during the warmup phase, but I haven't done that yet. Maybe that's a possible culprit?