Fuel Pump Relay or CIS-E Unit??
 

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?

Have you tried jumpering the fuel pump relay...? That would be my first (and least expensive!) test.

Jonathan

You mean jumper it when I'm having problems? That would take the relay and the CIS-E control unit out of the equation, so I still wouldn't know what the root cause is. The fuel pumps always run for at least 2 seconds when I switch the ignition off then on again, but they are shut off while the engine is cranking when the problem happens.

Well, no. It will Only take the fuel pump relay out of the equation which I'm wagering is the issue.

Jonathan

I agree, jumper the fuel pump relay to run the fuel pumps. It could be a cold solder joint on the Fuel Pump Relay circuit board.

http://i179.photobucket.com/albums/w...lpumprelay.jpg

http://i179.photobucket.com/albums/w...pumprelayB.jpg

http://i179.photobucket.com/albums/w...pumprelayD.jpg

Ah, you mean jumper the fuel pumps when I have the problem;)

I measured power consumption of the pumps (including the O2 sensor heater) at 7.7 A, within the 6-10 A spec. They've been noisy for over 35k since the car was purchased.

At this point I'm also confident it is the relay. I'll pull the relay and open it up before I spring for a new one. Thanks for the solder example pics:)

No solder joints were bad, but from damage to the plastic housing, it had been opened up at least once before. I just bought a new one and now its all good:D

Glad you got it sorted...My only Hope is that the unit you purchased was either Genuine new, or a Beckman rebuild. The KAE's are Very poorly made (China) and do not hold up over time.

Jonathan

I can't remember, but it was probably the cheap one from worldpac. It did seem like a lower quality part and I barely got it to fit in the plastic holder slot. It doesn't matter though, since the car's days are numbered by windshield frame rust:(