W124 300TD estate - fuel issue at motorway speed.

[paddingtonfire]

Good morning,

I've recently purchased one of the above vehicles, a '92 model. 225k on the clock, OM603.912 engine. The guy who sold it to me said it had been running on veg oil, but changed the filters before I got it, and they do look new. Starts fine, a little bit of a rough idle, but when I put my foot on the accelerator she sounds sweet.

Around town, lovely, it glides, not the fastest by any means, but appears fine. However, when on the motorway at between 50 and 70 mph on a very slightly uphill gradient I get what can only be described as juddering and a loss of power that if I floor the pedal it gets through it, I can maintain speed but the guts go and it's happening regularly. Then, after travelling for some time at motorway speeds the engine tries to die when I slow to a stop. If I keep the revs up it gets through it and returns to normal when trundling around at slower speeds.

Before I put it into a local specialist, I wondered if there was anything I could do first. Could it be the veg oil has played a part, do I need to flush the fuel system or purge it? Is my injector pump playing up? ECU? Relay? Should I change the filters again.... I spoke to one guy who said use the more expensive diesel? Help!

Thanks in advance,

Mike

 

[SilverSaloon]

Hi

I suspect you have clogged filters still - if the veg oil wasnt used for atleast 2000 miles in constant use, and filters changed within that time then there will still be rubbish from the tank coming up blocking the newly fitted filters. As soon as veg oil gets into the tank it will start lifting crud that is lining the tank from nasty diesel bugs. If it went back to diesel shortly afterwards this stuff will still be dislodged and therefore will take a few K to bring it all thorugh, blocking a couple of sets of filters.

i would also suggest you check that all pre-filters and tank strainer are clear

then inspect for airleaks into the system.

my best guess is that at high revs at motorway speed the car is finding it easier to suck air through any bad seals rather than the fuel.

running it on expensive diesel rather than regular will make no difference at all.

is there any veg oil still in the system? if so it could just be a bad batch - i'd wait until its been used up, fill the tank up with 100% standard diesel and see how it goes, and replace the filters and tank trainer again.

The 124 estate in-tank strainer is easy to remove and check, and can be cleaned rather than replaced. There is a how-to i wrote on this forum (search for w124 tank strainer)

 

[bolide]

I believe one byproduct of running vegetable oil is gummed piston rings. Eventually the compression drops so low that the engine won't run

Nick Froome

 

[SilverSaloon]

I believe one byproduct of running vegetable oil is gummed piston rings. Eventually the compression drops so low that the engine won't run

Nick Froome

could be, however i feel its unlikely, more likely a fueling issue.

 

[paddingtonfire]

Evening, thanks chaps, I've not checked he compression, but she starts instantly, and is good at all other times I described. I'll do the filters again, and thinking about it only the top one has been done. Is there anywhere in particular to get them or any make in particular I should look for?

I've driven 60 miles today, and it only did it once after a long stint at 50 mph when it went flat at a roundabout. I've done a couple of tanks of fuel since I got it, so I'll run this one down then do the tank strainer.

Lets hope it's not the rings Nick...

Mike

 

[trapperjohn]

Just before the main fuel filter there is a;so a very small pre fuel filter. This may have been over-looked at past filter changes.

Its in the same top housing as the main fuel filter. Ill root around the forum see if I can find an exact picture for you to work off. They cost buttons and are about the size of you little finger.

Edit. Sorry Derek I see you have covered this.

 

[paddingtonfire]

Thank you John, I'm still orientating myself with everything on the car, any help appreciated, especially where to get all the bits from...!

Mike

 

[trapperjohn]

EuroCarParts or local motor factor.

Ill nip out and take a picture of my fuel filter.

 

[trapperjohn]

Sorry just spotted yours is a 1992 mines a 95 so it will be different. Rats.

Standing at the front of the car lift the bonnet and look down the passengers side of the engine block. You should see a torpedo shaped "thingy" in the fuel line. This is your pre fuel filter. Ill see if I can find one in a parts catty.

 

[trapperjohn]

You are looking for something like this.

Hengst Mercedes-Benz | Euro Car Parts UK?s No.1 Car Parts Retailer

 

[paddingtonfire]

Brilliant cheers John, looks like a trip to Wembley lol.... I'll report back once they're changed.

Just while on the subject of this, will the tank strainer be able to be cleaned up, or are they available to replace. I've been looking around this evening and not found any.

Mike

 

[bolide]

As others have stated it can't be much else than fuel starvation. On the older diesels it is possible for the fuel shutoff to be only half open - that does make them stutter at higher speeds.

I wouldn't use any parts other than MB ones in the fuel system

Nick Froome

 

[paddingtonfire]

Do you mean the shut off on the pump Nick?

Mike

 

[bolide]

The little red lever in the middle of the inlet manifold - IIRC it's vacuum-operated. I don't recall if it's on the pump or not. Long time since I looked at a 603 engine

Nick Froome

 

[Dieselman]

I go for tank strainer being blocked.

 

[a111r]

When you remove the filler cap, is there a 'woosh', indicating it can't breathe properly?
If so, there's probably veg. crud blocking the pin hole breathers in the cap.

 

[paddingtonfire]

Thanks, no there is no whoosh, I've tried a few times over the last couple of days and nothing. I've attached a picture of he small inline pre-filter, I've got some on order ready to change, does the condition of this tell me anything? I noticed that he bottom three retaining clips on he air filter box aren't there. Is there a part number so I can get them replaced?

I read somewhere to stick some auto transmission fluid in the filter to help things along a bit - advisable?

Also with this tank strainer, are they available anywhere?

Ta, Mike

 

[balge]

Tells you the fuel system has some nasty looking black crud in it (not an expert opinion)
All I can say is it's much dirtier than mine ever gets...even during an injector flush (might help btw)
Tank strainer probably available from MB classic centre if I have the right part number here -A 2014700406

A2014700406/en ? Mercedes-Benz Classic Wiki - Online-Lexikon rund um Mercedes-Benz Oldtimer

not sure thy're all the same though

Decent breaker may have one, google three pointed parts

 

[paddingtonfire]

Ok I'll source something strainer-wise... Any recommendations on which cleaner to use? Or should I remove injectors and clean/source replacements?

Mike

 

[balge]

Think I used Liqui-Moly last time, but I think they are all pretty much the same - not sure using ATF is optimal though...
btw the plastic inline filters can be found at most factors for a few quid, might be worth a serious flushing session - but if there is crud in the tank, flushing the injectors will never clear it
cheers

just meant to add the tank strainer should be cleanable, the tank might need flushing though