We are working on a 1979 Mercedes 300SD, and are stumped. It came into our shop for rough idle with intermittent misfiring, and we found 3 injectors which were bad. We replaced them, and the vehicle was running MUCH better on a road test, however that only lasted for a few minutes. When we pulled it into the shop, it started to misfire, but this time it's a dead miss on cylinder 5, not intermittent at all. Cracking the injector line, there is no fuel from the pump. We can find no blockage anywhere, and can only surmise there is an internal problem in the pump. We are not wanting to spend $1000 on a rebuilt pump until we are sure that is necessary. Any suggestions?

Have you checked the fuel pressure going to the pump?

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Yes. It has around 20 psi.

Remove the check valve on the injection pump for the 5th cylinder and look for the piston in the IP moving up and down.
Also check for fuel coming out of that element.

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76 240 D. Bought in 1998 for $25.
85 300D. Got it for free with a bad engine. ( Sold )
60 Unimog 404. What was left of it, was given to me. Now powerd by 617A.
88 560 SEL. Bought without engine and trans. Now powerd by 617A.
67 250 SE. Cuope. For resto or sale.
64 220SE. For resto.

The piston IS moving up and down. There is no fuel pumping out.

Dirt in delivery valve on that #5 injection pump element? If the element where worn out I would not expect it to become actually intermittent. One test for a marginal element is to fuel the injection pump with the much higher viscosity vegatable oil. If the cylinder then comes on line you are pretty well sure the element is worn.



How and where did you read the fuel pressure? This is still a good candidate. Clamp off the return line at the relief valve area or before any other escape route for the fuel pressure. . See if the cylinder comes on line. Since #5 seems to be the best candidate to act up say if the relief valve on the injection pump where stuck open.

The reason I am questioning you on the fuel pressure reading. On an older example like the one you have it would be unusual to have a twenty pound pressure reading in the base of the injection pump. A twenty pound reading at the fuel or lift pump area might indicate you are loosing too much pressure down the line.

You can deadhead that style of lift pump with no negative consequences when you clamp off the return line and for a test you are doing this. I assume the relief valve is discharging fuel even at an idle? If not the clamp off test is pointless.

Do you find when pumping the primer pump that it gets harder to push and you eventually hear the relief valve squeeling sort of. Or does the resistance of the primer pump not changing as you pump it?

You could source a used pump as they seldom fail if it comes to it. First of all though as in my last post double check the fuel supply system.

It is possibly the element in the injection pump of course. But it should have come back on line once the injection pump cooled down and the viscosity of the fuel became heavier.

If it will not discharge with the injector line off resulting in no real pressure required then or no load if you wish. I doubt it is the injection pumps cylinder and bore on the number five element. Even a marginal element is going to normally pump fuel into a no load situation. If you suspect the car was run on an alternate fuel like waste vegatable oil mention it.

Here's a WAG: I'd be interested in knowing if perhaps the owner tried some snake oil remedies prior to bringing it to your shop.

Possible that something was run through the fuel system that disagreed with the pump internals?

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Mac
2002 e320 4matic estate│1985 300d│1980 300td
Previous: 1979 & 1982 & 1983 300sd │ 1982 240d

“Let's take a drive into the middle of nowhere with a packet of Marlboro lights and talk about our lives.” ― Joseph Heller, Catch-22