Sometimes my 1995 S500 will crank but not start, and after waiting some time it starts up OK. Other times it seems to start after slightly more than the usual cranking. I already changed the distributer caps. Could my problem be a bad fuel pump relay or a bad fuel pump itself, and is there a way of checking where the problem is without replacing the components?

crank position sensor?

I'm just putting this out there as no one has answered yet, but I am not a mechanic. Maybe do a search for CPS and see what comes up. Sorry, I can't be of more help.

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RT


1984 300D---Sold
1995 E420 - 106K - Brilliant Silver Metallic/Orion Gray --- Sold
1984 300SD - 151K Manganese Brown Metallic/Mostly Black --- Sold
1982 240D - 79K - Manila Beige/Palomino---Sold

Without knowing much I would think that you might have a fuel problem. Assuming that your spark plugs and other tune up items are in decent shape, I would check your fuel pressure.

1st I would take a fuel pressure test to determine a possible blockage or a bad pump itself. Considering this checks out.....

2nd I would to a leakdown test on the fuel line. Sometimes if you let your car sit (overnight or shorter) you can loose all of your fuel pressure through a small leak. If this happens then your car will take longer to pessure up to be able to start.

Again it is hard to answer this question with such little information. Maybe you can be more descriptive of when it starts poorly, doesnt start and actually starts well. Include time in your descriptions.

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1998 E320 150,000
1980 300SD 240,000
1965 190D 79,000

+1

I am getting a spark at the plug, and I can hear the fuel pump relay click in but cannot find the fuel pump to check whether it is turning. I am not sure whether the fuel pump is being activated. Where is this pump?

First thing you need to do (or have done) is check the fuel pressure at the injector rail. If you have pressure and the system holds thaty pressure when it is turned off then the pump, relay and pressure regulator are OK.
It would also help if you downloaded the DTC's.
Could be the wiring harness....if that hasn't been changed it should be.

Underneath the car, about 3/4 of the way back on the right hand side. It's hiding under a black plastic cover just forward of the differential.

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"It's not about how fast you can go, but how well you can go fast."

Bob in Richmond

'97 S320 (LWB), Ruby Red Metallic, 73k miles
'97 S420V, Smoke Silver Metallic, 155k miles