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Old 09-23-2010, 02:13 PM
Boots Boots is offline
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Bit of a saga this one folks!

Car as decribed below 940, D24T, otherwise in rude health!

I hope someone can shed some light on my issues as this diesel newbie's about out of ideas...

The car's not driveable due to the problems now so I'd appreciate any advice - it's my transport to work!

Since I got the car about 10 months ago, it has run short of fuel when run at full throttle for long periods eg. overtaking. Power drops off for a second then comes back. It only did this if you used the last 5% of throttle though. Below that it was running fine.

About a week ago, it took longer than usual to fire in the morning, about 30 seconds cranking. Normally it kicks off 2nd or 3rd cylinder to see compression. I didn't think much more of it and later that day it started normally.

Over the course of the next couple of days, this delay in firing got steadily worse till it was taking a minute or so of cranking before it got any fuel.

This car has a piece of clear fuel line between the filter and the pump and on inspection, indeed you could see the fuel draining back the moment you stopped the motor.

Anybody tell me what's supposed to stop this happpening?

I looked at the bosch diagram for the pump and figured the pressure regulating valve may well double up as the non-return function. (My first mistake?) Popped it out and the lower o-ring looked pretty tired and deformed. I thought maybe this had been partially leaking for a while causing the pump to lose internal pressure at high revs and now, having lost the will to seal completely, may be allowing the fuel to drain back down at switch off.

Note: apart from the starting issue, the car was still driving ok at this point.

I had an o-ring kit at work and replaced the one on the bottom of the p-r valve with a new one. I didn't have the exact size, it was a tiny bit thinner in section than the original but at least it was round and springy not egg-shaped and stiff.

Well, it drove home fine, still with the fuel starvation at full throttle but otherwise ok. The next morning, loads of cranking to get going again and then the real issues started! Lost power gradually over a mile or so then stalled completely at a junction. Popped hood after cranking for several minutes - loads of air in fuel feed to pump. Waited a few minutes, cranked it some more and it fired up. Lumpy as hell but I was late for work and as long as I kept it above 2500rpm it would run! Got to work just.

After work, fired it up and watched the fuel line. Lots of bubbles, seemed more if you revved high. Running lumpy as hell but kept this up for 5 mins or so to see if the air would clear. Nope. Still loads of bubbles. Took filter off and inspected seals and housing - all fine. (Filter was new 4 mths ago.)

Borrowed works van to go home in! Next evening, popped flow and return off the fuel tank, stopped off return and slowly put up to 1 bar on flow line - no leaks anywhere. Lines, filter and pump good to 1 bar. wtf?

After much scratching of head, decided the smaller o-ring must be causing real trouble.

Anyone know what the pump would do if the pressure regulating valve had flow from one side to the other all the time due to too small an o-ring? I thought maybe this had caused the high pressure side to try and draw fuel in through the return line and when that ran out of fuel, draw in air?

Anyhow, I got an o-ring the right size today and fitted that. Now it starts after much cranking but runs really weird. Mainly all it will do is a sort of intermittent fire - brrmp, rest, brrmp, rest and so on. Whilst it is doing this there is no real response to any throttle position. If you crank it with the throttle full down and catch it unawares, it fires right up, all 6 and from 2500 upwards it behaves properly. No missing. Turbo kicks in etc. The moment you let it down towards idle it goes back to the intermittent fire crap and won't listen to the throttle until you turn it off and start again! No air in the fuel line now though until you turn it off and then about 5 inches of froth come back down the line from the pump.

Have I actually fixed it and I'm just being a diesel virgin i.e. need to bleed the pump properly or is it proper broken?

Sorry this was so long-winded but I thought the more info I put down the better chance of sorting the problem.

Hoping someone can advise, Boots.
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  #2  
Old 09-23-2010, 04:45 PM
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Jason Jason is offline
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Well, you have a couple things to check. First I would try a new fuel filter. If it is getting clogged up, the pump could be sucking air from the front seal becuase it is building up a vacume in the fuel supply. Even after 4 months, if there is crap in your tank its very possible that the filter is getting clogged. I would also install a vacume gauge inline between the filter and the pump. With the new filter installed, if your getting a vacume of more than 1 or 2 inches there is a restriction somewhere else like at the fuel tank pickup. I'm not sure if your model has a screen in the tank over the pickup but some do. If the filter makes no difference and you dont have excessive vacume I would do this next: Get a piece of fuel hose and a jar of diesel fuel. You need to hold the jar above the injection pump and try to run it that way. If it runs well, you probably have a leaking front shaft seal that is sucking in air rather than fuel. By doing this, the pump is being gravity fed and not relying on the vien pump to suck fuel into the pump. These can leak air into the system without necesarily leaking fuel out. If that is the case and it runs smoothly, than you need to install a new front shaft seal, it is found behind the drive pulley/sprocket on the pump. If this doesn't make a difference, I would start checking injectors. I have had one with a piece of crap hang a nozzle open and compression gas from the engine was being forced back into the pump and filling it with air, making it run like crap and very hard to start once you shut it off. You would get a big blast of foam from the feedline when you shut it off. I know the delivery valves are a one way valve, but they are not a totally positive seal and air can go backwards into the pump, I've had it happen to me. That's about all I can think of at the moment. If you don't have a pop tester or access to one, you can remove one line at a time from the injectors and hold it to the side, diesel will be squirting out, but if the injector is fualty as I'm describing, short little blasts of air will be shooting out of the injector inlet.

Hope this is helpfull.

Jason
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Old 09-26-2010, 06:07 AM
Boots Boots is offline
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Thanks for that Jason. I will give these things a try. It's done about 5k miles since I switched to bio so the filter may well be clogged. I took a bit of a gamble that the pump seals were ok for bio as I believe 1991 (when the car was registered) was about the changeover period to Neoprene seals. I figured I had got away with it as no diesel was leaking out but maybe not... I'll post what sorts it when I find out!

Boots.
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Old 10-02-2010, 04:47 PM
pgringo pgringo is offline
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try idling at home with the fuel filler cap off. one of my diesels runs like poo sometimes if i don't open the filler cap every so often to let some vacuum escape.
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Old 11-06-2010, 11:56 AM
RLDSL RLDSL is offline
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Fuel tank vacuum can be a big problem on these, The tank vent can get clogged with mud or by muddaubbers , the loose fuel cap trick is a good indicator of this.
Very often the ve pump gets worn in the lift section. Sometimes adding a low pressure electric feed pump will be a relitively long term fix for this ( bigger engines with the same pump come from the factory with an external lift pump, but this engine relies entirely on the forward lift section of the injection pump, which usually goes before the rest of teh pump. It used to be common to install a check valve in teh feed line to prevent the drainback, but an epump works even better as it also solves the fuel starvation issue that comes from a worn lift section and can get sometimes a few extra yearsr out of an injection pump. The little Facet cube pumps that most parts stores sell work great, they have a low pressure one that is only around 4 psi that just feeds teh thing. In an e pump doesnt at least get it running half decent, then the injection pump is done and needs a rebuild.
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Old 11-06-2010, 03:02 PM
m-reg m-reg is offline
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would a tank unit from a gasser with the little pre pump be able to flow enough fuel? probly simpler to just splice in parts shop one under the floor pan though. might need one for mine as wont pull full revs these days (since experimenting running waste oil)
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Old 11-06-2010, 08:33 PM
RLDSL RLDSL is offline
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Quote:
Originally Posted by m-reg View Post
would a tank unit from a gasser with the little pre pump be able to flow enough fuel? probly simpler to just splice in parts shop one under the floor pan though. might need one for mine as wont pull full revs these days (since experimenting running waste oil)
Not likely a good choice. Most intank pumps are higher pressure and they , by design are supposed to be cooled by sumbersion ( they dissapate heat with the fuel contact all around in free air they get hot ) and you would have a hard time getting a pre filter in front of it that you could clean or replace. You can pop a simple inline filter before a parts store cube pump to keep crud out of it and they are cheap and easily replacable ( or you can but the mercedes prefilters in 3 packs at a lot of places ) THose little Facet cube pumpsare only about $35 ( they are sold under a few different names at various places, they look like this They are very efficient little pumps. Only time I've ever had one fail is when I rigged one on an old bed tank on a truck and forgot to plumb in a pre filter and locked it with rust, my fault, I should have known better, but got in a hurry.
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Old 11-08-2010, 06:41 PM
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Jason Jason is offline
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I'm using one on my engine for a lift pump to add a little positive pressure and just help out with fueling since the pump is turned up. Facet is selling a diesel specific pump now, its a black composite body with green labeling on it. I have seen them on ebay and other places. Might be worth looking into.

Jason
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Back again with a '84 760 GLE D24T/ZF

SOLD but not forgotten! 1984
760 Sedan, built D24Tic/ T-5 swapped

My engine build: http://www.d24t.com/showthread.php?t...t=engine+build
T-5 swap: http://d24t.com/showthread.php?399-W...to-quot-w-pics!
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