1991 300-SEL - No Start Issue Solved

[Mike Murrell]

It's going to be a problem with fuel/ignition/compression - mine turned out to be fuel. Car description in title and signature.

Didn't think it was compression as car has always run well and nothing had been going on that would relate to that.

Started with fuel analysis first.

Decided to cut to the chase and see if it would start if I primed fuel distributor with a very small amount of fuel. Did that and car fired up and ran for a sec. or two. Did it again - ran for a sec. or two. W/O this fuel dist. priming - car would not start. I have a fuel delivery problem.

- FYI - fresh battery
- Test fuel pump relay - car has MAS unit - jumper ports 1 & 2 - will not start - it's not the FPR
- Test voltage at FPR - use 12V test light - probe in MAS port 1 - clip on engine hanger at rear of engine. Lights up - I have voltage at FPR.
- Test pump for voltage - same test light - probe at + term. on FP - clip on - term. on FP - ignition to 1st position - test light illuminates for a sec or two and goes out - stays on while engine is being cranked. I have voltage at FP.
- Test pump for amperage. Back under the hood at the MAS unit. VOM on 10A scale - red lead in MAS port 1 and black lead in MAS port 2. Battery connected. I see a reading. Key is not in ignition - HOT setup at ports 1 & 2 in MAS. Crank engine - amp reading drops a hair. I used 2 different meters. My Sears meter - the one that's used for duty cycle reads in the 3A range - In hind sight it may have been right. My $4 Harbor Freight meter reads in the 7A range which is OK as per 6A-10A spec in FSM. In my experience, these HF meters read high, but not that high. I know I have a pump/filter issue - want to dig further.

Siphon gas tank as much as I can with Mityvac - pull short 3" hose off inlet to pump - other end is connected to elbow - catch residual in catch pan. Tank drained fairly quickly when I pulled inlet feed to FP. I doubt I have a clogged fuel tank screen. With hose still off - pour some more gas(small amt.) into tank and watch it drain into pan. Happens pretty quick. Fuel tank strainer OK as far as I am concerned and there was no sediment whatsoever in the gas drained.

Pour a couple of gallons fresh gas into tank after reconnecting inlet FP hose. Pull cap nut off outlet side of FP - short, but usable length of threaded end exposed. Connect hose and snug down with hose clamp. Other end of hose in dry catch pan. Crank engine a number of times. No fuel is coming out. Pump is shot. Pull entire fuel pump/filter/accumulator assembly. Drain fuel filter - lots of wattery looking crud. It's possible the filter clogged and killed pump.

Being anal - I'm not done. Further test pump off car on bench - use 12V battery and 2 sets of leads. Feel/hear nothing when I power pump. Put mechanics stethscope to pump during one test - hear nothing but a very faint motor-like sound.

The coroner has officially declared the pump to be dead and it's companion fuel filter to be no better. Order new pump/filter. Neither came with gaskets known online as seal rings. MB part # 007-603-012-102 . Need 2 for pump - need 2 for filter. Swap out the old with the new - reinstall FP/FF/Accumulator Assy. Accumulator not changed. No reason to believe it's bad - no hot start issues and I can pull this thing down and reassemble pretty quick + I can use that money for other things.

Key in ignition - turn to first position to build pressure - wait for buzzer to go off. Repeat. Crank engine Not much happening. Crank some more and it catches. Ran a little ragged at first, but smoothed out quickly.

Next up - O2 sensor - throwing code 22.