CLK 270cdi. Low fuel pressure

[SteveB73]

Hi all,

My CLK broke down last week on way to work. Engine just stopped coming off a roundabout. AA arrived and when plugged in it was showing 20bar in the fuel rail. The guy suggested that it was the low pressure pump which I have changed, and after much cranking rail is still only reading 20 bar. Is it likely HP pump? I know the guy I bought car from a yr ago said he’d rebuilt it and there is no apparent leaking. Any ideas where to start would be appreciated. I’ve rented a car for the week but need to decide on whether to repair or replace..

There are no engine management codes present. Fuel pressure or otherwise?

Car has done 260k miles and been giving some black smoke under acceleration.

 

[alexanderfoti]

I know this is odd, but does it have fuel in it?

 

[SteveB73]

Yes I filled the tank before I left for work.

 

[alexanderfoti]

Yes I filled the tank before I left for work.

If you have fuel pressure from the low pressure pump, when turning the engine over, then culprits are:

High pressure pump
Quantity control valve

HP pump is most likely if it has been rebuilt in a DIY manner.
The pressure will be lost via the leak off lines.

 

[SteveB73]

Could it be air still in rail? Someone suggested to me to crack off an injector to make sure the air is escaping effectively..

 

[SteveB73]

The control valve is the one on the end of the rail I assume? This had been replaced with an aftermarket item a while ago..?

 

[alexanderfoti]

Could it be air still in rail? Someone suggested to me to crack off an injector to make sure the air is escaping effectively..

Do not do this on Common rail engines, this only for direct injection PD, or IDI engines, where an air lock can cause no fuel to flow. All this does on CR engines is introduce air into the rail, and potentially cause lethal injection of diesel into your blood.

If you have fuel at the LP side, then the air will be pushed out of the rail and out via the leak off while cranking. It will take 20 seconds or so to get it all out, any longer than that and its not the issue.

 

[alexanderfoti]

The control valve is the one on the end of the rail I assume? This had been replaced with an aftermarket item a while ago..?

Yes, then that is unlikely to be your issue.

 

[SteveB73]

So what can go wrong with the hp pump? There is no apparent leakage?

Also why would there be no codes present relating to fuel pressure?

 

[thebiglad]

So what can go wrong with the hp pump? There is no apparent leakage?

All pumps are similar in that there is a motor bit and a compressing bit. It is not at all unknown for the motor to still turn but creating little or no compression, this is often due to seals within the pump.

I don't know how much a new (not rejuvenated, gefirtled or fiddled about with) pump ideally from MB then you'll know that link in the chain is OK.

Assuming this is the same part that would fit in your vehicle here is an ebay ad for an MB part : https://www.ebay.co.uk/itm/MERCEDES...964096&hash=item4d9a305f93:g:R6QAAOSwgYpdWB1e

At this price it's a no brainer, imo.

 

[umblecumbuz]

If one of the injectors is faulty - passing too much fuel - then this could result in low rail pressure.
The culprit does not have to be the high pressure pump. Don't go changing bits until all avenues have been researched.
A leak-off test will help to isolate any faulty injector.

 

[SteveB73]

Assuming this is the same part that would fit in your vehicle here is an ebay ad for an MB part : https://www.ebay.co.uk/itm/MERCEDES...964096&hash=item4d9a305f93:g:R6QAAOSwgYpdWB1e

At this price it's a no brainer, imo.

That is cheap for sure. Last I looked they’re around the 500 quid mark

 

[SteveB73]

If one of the injectors is faulty - passing too much fuel - then this could result in low rail pressure.
The culprit does not have to be the high pressure pump. Don't go changing bits until all avenues have been researched.
A leak-off test will help to isolate any faulty injector.

I do have a leak off test kit.

Unfortunately I’ve just been over to car and battery is apparently flat from all cranking previously. I’ll have to get it recharged.

The leak off test will give adequate results just from cranking?

 

[umblecumbuz]

... The leak off test will give adequate results just from cranking?

Rail needs to be pressurized beyond cranking, so engine should be running for test to be representative. Not saying cranking would not detect a serious leaking injector, but not heard of it done before. Worth a try, as engine won't start.

 

[mersum1es]

Some engine models will shut down if injector open circuit is detected. Check all injector connectors, sometimes pins will retract inside the plug.

If I recall right your car has CR3 fuel system, fuel quantity valve is next to hp pump, quite common fail though never heard happens mid of driving.

Yes, leak off test can be done just by crankong too. A minute crank.

Just out of fuel issues, it might be crank position sensor, but it should be seen as a fault code. However pingpong handheld diagnostic may not read your car properly.

 

[SteveB73]

Thanks for all the suggestions guys.

I think I’m going to try a rebuild on HP pump first as the kit is £30 or so and I’ve done it before on a w220. Then leak off when re installed. I figure it has no pressure so start with the bit that’s supposed to make pressure.