I have a 1990 560SEL with a warm start problem thats also expanded into short hickups while driving. Narrowing it down with a friend, here is what's happening:
- Starting problems only happen when the engine is warm and has been sitting for more than 10 minutes parked.
- Stalling (only for ~1-2 sec) when driving leaves the tach functioning so it's probably not the ign. control module.
- Ignition coil replaced with no improvement. (old secondary had 8.5kOhm, new has 10.5kOhm)
- When the problem happens while starting, the fuel pumps are NOT running while the starter is running. They do however run for 2 seconds if engine is not started. A clue may be that the fuel pumps will run much longer than 2 seconds at times even without starting the motor (this is intermittent).
- Fuel pressures and leakdown of fuel system have been tested with good results.
- I haven't tested fuel pump current, but they do make more noise than I'm accustomed to.
It appears to be temperature dependent since I've had no cold starting problems. I think the cause must be either an intermittent fault in the fuel pump relay or the CIS-E control unit sending a "fuel cut" signal to the relay when it shouldn't. The CIS-E control unit unit has a temp sensor input which is why I'm including that as a suspect. I don't think it is tach signal related, because my tach still works when the motor cuts out on the highway.
Please, any ideas or suggestions?