P38A MPG Worries.

Happy New Year ladies and gents!

I was wondering if anyone could shed a little light on on why the MPG has started plummeting on my 2001 diesel.

The car has always returned roughly 29-30mpg but in the last couple of months this has taken a sharp tumble to about 22.5-23mpg (despite the car being used almost exclusively on the motorway, sitting between 55-60mph).

It’s knocking on the door of 200k Miles now but runs very well and has always had oil changes at 5k intervals (more recently at 3k intervals).

My suspicion has been on boost pressure, as the car feels even slower than usual, but I’m not entirely sure. Are there any common issues with the diesel that would lead to such a dramatic drop in economy?

Cheers in advance.

Check the Map pipe to sensor is clear and not leaking, and spigot under inlet manifold is clear, is the turbo working properly get some one too rev it up while you squeeze the in let hose
and check to see if the spill off pipes on injectors are leaking.

The first place I would look is the leak off pipes.

alexmcrobbie said:

My suspicion has been on boost pressure, as the car feels even slower than usual, but I’m not entirely sure. Are there any common issues with the diesel that would lead to such a dramatic drop in economy?

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In tank pump ...Bad leak off pipes would cause bad or poor starting

As others say, loss of power in particular on hills = In tank pump either failed or failing. Loss of frugality maybe extra throttle required to maintain power because of pump and or leaks.

Datatek said:

The first place I would look is the leak off pipes.

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So I’ve just a quick look at the bleed off pipes. They all look good (I replaced them last year after a significant fuel leak).

Strangely though, there’s some fuel/oil? and small bubbles coming from where injector number 1 is seated. I’ve attached a link to a video showing this.

I’m potentially being stupid here, but would have thought that a failing pump wouldn’t have resulted in additional fuel use if it’s struggling to deliver the desired amount in the first place?

The MPG is dipping so low that it’s making me consider changing for a 4.6 P38...

alexmcrobbie said:

So I’ve just a quick look at the bleed off pipes. They all look good (I replaced them last year after a significant fuel leak).

Strangely though, there’s some fuel/oil? and small bubbles coming from where injector number 1 is seated. I’ve attached a link to a video showing this.

I’m potentially being stupid here, but would have thought that a failing pump wouldn’t have resulted in additional fuel use if it’s struggling to deliver the desired amount in the first place?

The MPG is dipping so low that it’s making me consider changing for a 4.6 P38...

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That is leaking from the spill pipes. Look at the others number one is the only one that will well all the others if the spills are leaking just run down the block.

wammers said:

That is leaking from the spill pipes. Look at the others number one is the only one that will well all the others if the spills are leaking just run down the block.

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+1 to that. Also if the in tank pump has failed it does affect fuel consumption, I can only guess that it's because of loss of efficiency as the engine is struggling.
Easy enough to check the in tank pump, drop the fuel filter, place a bowl under and either turn on the ignition when the pump will run while the glow plug light is on, or on earlier models you need to crank the engine or jumper the fuel pump relay.

Datatek said:

+1 to that. Also if the in tank pump has failed it does affect fuel consumption, I can only guess that it's because of loss of efficiency as the engine is struggling.
Easy enough to check the in tank pump, drop the fuel filter, place a bowl under and either turn on the ignition when the pump will run while the glow plug light is on, or on earlier models you need to crank the engine or jumper the fuel pump relay.

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Only works on cranking 1999 and on.

wammers said:

Only works on cranking 1999 and on.

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3 diodes on post 99 fuse boxes, one operated from the cranking circuit, one from the alternator D+ and the third from the glow plug circuit, at least that's what my copy of RAVE shows

wammers said:

That is leaking from the spill pipes. Look at the others number one is the only one that will well all the others if the spills are leaking just run down the block.

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Having now replaced the leak off pipes (again) with 3.2mm i.d. fuel pipe I can confirm that these were not leaking. There are distinct bubbles rising from where the injector meets the head. I have taken another video (link embedded) which perhaps more clearly shows where the bubbles are rising from.

A trick with the leak off pipes is to put some heat shrink in the ends or cable tie them it helps to stop the ends splitting.
I changed mine three times in short time till I cable tied them then no issues been on there years now.

brianp38dse said:

A trick with the leak off pipes is to put some heat shrink in the ends or cable tie them it helps to stop the ends splitting.
I changed mine three times in short time till I cable tied them then no issues been on there years now.

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Another option is to heat the braided exterior which can be made to melt back a little providing a hard outer case on the last 2 or 3 mm. Seems to have worked on mine.

Datatek said:

3 diodes on post 99 fuse boxes, one operated from the cranking circuit, one from the alternator D+ and the third from the glow plug circuit, at least that's what my copy of RAVE shows

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Only works with glows from lead six and with engine running prior to 1999. Also when cranking from 1999 and on

alexmcrobbie said:

Having now replaced the leak off pipes (again) with 3.2mm i.d. fuel pipe I can confirm that these were not leaking. There are distinct bubbles rising from where the injector meets the head. I have taken another video (link embedded) which perhaps more clearly shows where the bubbles are rising from.

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That is combustion gas escaping past the injector seal, fuel cannot leak up from the injector it is not possible. Your spills are leaking either that or the injector top.

Do these use a copper washer type seal between the injector & the head?

If they do this type of leak is well known on Sprinter vans.
The result in those is a black liquid bubbling up past the injector body as some of what's injected into the cylinder & the products of combustion which includes vapourised liquid gets forced past the seal by the compression/ignition which will harden to a black mess referred to as 'the black death'.
May also be accompanied by a 'chuffing' noise & a strong smell of diesel in the cab.
Resealed two of the injectors on my Sprinter the week before Christmas.
Performance drops & as you push the pedal harder to compensate. fuel consumption gets worse.

Ratae said:

Do these use a copper washer type seal between the injector & the head?

If they do this type of leak is well known on Sprinter vans.
The result in those is a black liquid bubbling up past the injector body as some of what's injected into the cylinder & the products of combustion which includes vapourised liquid gets forced past the seal by the compression/ignition which will harden to a black mess referred to as 'the black death'.
May also be accompanied by a 'chuffing' noise & a strong smell of diesel in the cab.
Resealed two of the injectors on my Sprinter the week before Christmas.
Performance drops & as you push the pedal harder to compensate. fuel consumption gets worse.

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Yes they use a copper washer which can be reused if you anneal it. Heat it to cherry red then drop it in water.

Thanks.

The genuine Mercedes ones are very hard & really need annealing before being used or you may get a repeat performance.
On the Mercedes they usually have some surface damage caused by the blowby & have been slightly compressed so best replaced - they're pennies & it would be a false economy although I can see it as a field repair - & if you're unlucky the Mercedes head will also have some damage to the seat so may need cleaning up or recutting.

Used to anneal rifle cartridge case necks for reloading & that was the cherry red & tip over into water method.

Datatek said:

+1 to that. Also if the in tank pump has failed it does affect fuel consumption, I can only guess that it's because of loss of efficiency as the engine is struggling.
Easy enough to check the in tank pump, drop the fuel filter, place a bowl under and either turn on the ignition when the pump will run while the glow plug light is on, or on earlier models you need to crank the engine or jumper the fuel pump relay.

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I'll give the in tank pump a check as well.

Is there a flow rate I should expect to observe, or is the test just to determine if the pump is working at all?

Check out the technical archive,top left of page think it was wammers posted the relevant info.