1998 W202 C250 TD air in fuel system

josephcarey1

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Not had to post on here for a long time but now need some advice for my father.

We have a 1998 W202 C250 TD sport automatic. it's been fine for a while but recently after a 20+ minute drive it starts to run a bit rough and we can see air in the fuel pipe in the pipe that goes between the lift pump and the pre filter.

We haven't really done much to the car in terms of fault finding but we did replace the pre filter and also the main filter was replaced too with all new O-rings.

You can let the engine idle but if you rev it and then stop the engine shakes a little and you can see the air bubbles in the pipe.

Does anyone have any ideas what might have failed here or which areas we should check? What are usually the common things that go?

any help or advice is highly appreciated
 

yorkshire1

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are you saying it runs ok then stsarts to play up after 20 mins each time
 
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josephcarey1

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That's absolutely correct. it starts no problem but after like 15 minutes of it running it starts to run lumpy
 

yorkshire1

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That's absolutely correct. it starts no problem but after like 15 minutes of it running it starts to run lumpy

Strange, by the symptoms I would guess the tank venting system could be faulty start by running it with the cap slack/off, also make sure tank is over half full to eliminate other supply issues
 
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josephcarey1

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Thanks for the advice. I'll give it a try and let you know what happens
 
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josephcarey1

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Just as an update I did what was suggested and no difference. take about 3/4 full. took the cap off and it was still drawing in air.

I replace every leak off pipe and the rubber stop and still doing it.

you can see loads of air pull through the pump between the pre filter and the lift pump at high revs, low revs and even on idle too.

does anyone else have any thoughts at all?

Could it maybe be something to do with the lift pump?

Thanks
 

CRH71

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Sadly, now things start getting a bit pricey.

Have you yet replaced all the hard fuel lines (the clear ones, there are 6), the pre-filter plus O-ring and the main fuel filter? My 98 C250TD needed this surgery recently for exactly the same issue. For good measure, I had the delivery valve seals replaced as well.

No air bubbles, any more, I'm pleased to say.

The other place that these can draw in air is the heat exchanger on the head (manifold off job to access) but from the sounds of your symptoms, I'd be checking the rubber/metal fuel line leading up to the filter, in case there's a joint there that's leaking or the pipe itself has degraded - especially check the metal-to-rubber pipe joint.

Failing that, try replacing the pre-filter and O-ring - they're less than three quid all up from Mercedes.

HTH
 

yorkshire1

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using clear or braided hose and as suggested start right back at the steel supply pipe on the inner wing bypass the heat exchanger and the prefilter, and see if that cures problem ie no air in this new section of hose if not youve got air getting into the system between the tank and inner wing, its odd that it runs for 20 mins before problems occur though
 
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josephcarey1

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Hey sorry for the long delay in replying to this thread.

We fixed it - We replaced the pipe that goes to the lift pump and all was fine after that. It was the only thing that didn't replace and it turns out that although the pipe didn't look too bad it was in fact the problem.

Thanks for the help and advice of everyone! Hopefully this information will help someone else in the future. We have since sold the car and still see it being driving around our town on most days :)
 


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