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Liqui Moly Diesel Purge Gone Wrong - Now barely starting

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Old Apr 7th, 2021, 16:18   #1
ca2n
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Default Liqui Moly Diesel Purge Gone Wrong - Now barely starting

Hello all,

Hope everyone's doing well.

I attempted a Liqui Moly Diesel Purge recently. As the title suggests, it didn't go too well, and am looking for advice on what to do. Prior to attempting the diesel purge, my D5 163 ran just fine (albeit with ECM-6805 saying hello occasionally), always a quick starter.

I'll try to detail as much as I can, so be warned, this might be a long read.

TL;DR: Did Liqui Moly Diesel Purge, now D5 barely starts.

After plumbing the fuel lines into a jar of Liqui Moly Diesel Purge, I managed to get the engine started. But not before:
  1. Destroying the small-length hose on the injector return line during the removal; replaced with a portion of the hose I bought for the diesel purge process.
  2. Priming the inlet hose and diesel purge filter to the LP/HP pump, including pulling a vacuum via the return line.
  3. A few cranking attempts before the engine "happily" ran on the Liqui Moly Diesel Purge.

+ YouTube Video
ERROR: If you can see this, then YouTube is down or you don't have Flash installed.


After about 30 minutes of idling with the diesel purge, I tried to gradually raise and hold a steady RPM. I only got to around 1100 rpm before the engine died. A buzzing could be heard from the LP/HP pump area for around 30 to 40 seconds after the engine died.

At that point, I decided to give up on the diesel purge process and plumb the fuel lines back to the original setup.

It took a few cranking attempts to get the engine running after returning everything back to stock. When it did eventually fire, I noticed that starting was not as quick as it was before, taking just that bit longer of cranking before firing. I experienced something similar during a fuel starvation issue not too long ago, so I wasn't too concerned, yet, as I was just playing around with the fuel system after all.

This is a graphical representation of the engine parameters after that start:


I let the engine idle for around 4 minutes and revved the engine a couple of times after that. At around 350 seconds into the log, I tried to gradually raise the revs, but just before reaching 1000 rpm, the engine died, just like it did whilst running on the Liqui Moly Diesel Purge.

I cranked the engine again, and it fired back up. I tried to gradually raise the revs again hoping to replicate the engine dying but it didn't happen again. I gave it a few more revs just after the 500-second mark and everything seemed to be just fine.

I ended by shutting off the engine and restarting a couple of times, and again, everything appeared to be just fine. Because of the slightly longer cranking time before firing and the engine seemingly starved of fuel at low-ish revs, I didn't take it for a test run, for fear of stalling in the middle of the road.

Fast-forward 18 hours later, it was early morning, not much traffic to hold up in case things go wrong, I thought I'd take it for a short run to the shops. The slightly longer cranking was still there, but everything else seemed to be just fine (data below):



Went into the shops and came back out 5 minutes later. Cranked the engine 1st time: no start. Cranked 2nd time: still no start. Cranked 3rd time for a longer period: started. Unfortunately, I forgot to log the engine parameters this time. Quickly drove back home and the car sat still for about 5 hours.

I came back out to the car to investigate further, this time not forgetting to log the engine parameters:



The engine fired on the first try and I let it run for around 170 seconds, gradually raising the revs at around 90 seconds in.

I then turned the engine off and tried to restart it again. But after 7 starting attempts, the engine barely fired for a second or two but died immediately afterward. As you can see, the fuel rail pressure during this didn't indicate more than 200 bar during this time.

I've had a look around (and still reading) the forums and several issues have been mentioned (loose ECU connections, FP sensors FP regulators). I'm inclined to think I might have a leak somewhere in the plumbing to the LP/HP pump and/or rail, as I've been messing about with them and especially after discovering this on the inlet hose to the LP/HP pump:

+ YouTube Video
ERROR: If you can see this, then YouTube is down or you don't have Flash installed.


Is the hose supposed to be 'twistable' like that? I imagine that would allow air to be sucked into the LP/HP pump, no?

That said, I'm more than happy to be corrected and pointed in the right direction.

Lastly, I hope this doesn't put off others who are interested in doing a LM diesel purge. It's been done successfully by many others with good results, just not on this occasion.

Please help!
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Old Apr 7th, 2021, 16:56   #2
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I'll start with the absolute basics.
Clean all fuel unions with brake cleaner so they are bone dry and look for leaks.
Can you hear the in tank pump running with ignition on?
When was the fuel filter last replaced and with what brand?

HP pump appears to working fine so I'm guessing the issue is around fuel delivery/regulation.

There's either not enough fuel available due to flow/restriction somewhere or fuel is available and not being regulated correctly at the HP pump.

Given the work on the unions didn't go smoothly, it adds an extra level of possibilities.
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Old Apr 7th, 2021, 23:09   #3
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Thanks, Si, for your reply.

I was hoping to get a strong smell of diesel around the unions, suggesting a leak in that area, but there was none. Nevertheless, I'll give the area a clean with brake cleaner and go from there.

I've never been able to firmly establish whether the in tank fuel pump is working or not. Would taking a voltage reading at the fuel pump fuse while key in position II tell whether it's working or not?

The fuel filter was last changed in June 2020, 4000 miles ago, with a Purflux C516 packaged in a Volvo box. There is currently just over half a tank of diesel in the tank and the car is parked level.

What are your thoughts on the last video showing the twisting tube to the inlet of the LP/HP pump? That can't be normal, can it?

I can't deny that I might have broken whatever seal might have been between the rubber tube and the LP/HP nipple (there's even a part number for it 30731749!) while taking off the other end. And since it's a pump inlet, there would be suction in that area, no?
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Old Apr 8th, 2021, 01:52   #4
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That hose should not be free to twist. It could be drawing in air which would stop the car from starting. The movement of the engine when starting could be enough to move the hose, causing the engine to start sometimes and not start other times. Diesel purge is a very effective fuel system cleaner, and it contains enough additives to keep the components in the fuel system well lubricated, but you need to make sure the HP pump doesn't run dry at all. A few seconds is all it takes to cause irreparable damage to a HP fuel pump.

In your case I would hazard a guess that there's air getting into the HP pump via the loose supply hose. The hose clamps onto an adaptor that's threaded into the pump. Hopefully it has just worked loose and the threads are fine. If so then it should just be a case of removing the hose, tightening the adaptor then refitting the hose.
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Old Apr 8th, 2021, 13:18   #5
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Thanks, Dancake, for your reply.

I've just been out to the car to have another look.

By the looks of it, the LP/HP pump nipple was still securely threaded into the LP/HP pump and it was the hose that was twisting around the nipple.

I undid the hose clamp and took the hose off of the nipple, looking to replace with another clamp and tighten the hose-to-nipple connection.

Before I did that, I decided to see if my electric fuel pump was working by feeding the disconnected hose into an empty jar and turning the key to position II.

No fuel came out, neither on the first turn of the key nor on any of the >10 key turns that I performed.

I replaced the fuel pump fuse (15 A, No. 21 in the engine compartment) with a spare, and still no fuel was flowing with key in position II.

I think that pretty much concludes that my electric in-tank fuel pump is not running. It probably hasn't been working for the past 4000+ miles since I couldn't get the engine started after a fuel filter change. On that occasion, an AA technician came out and, with the help of easy-start, got the engine running again. And it probably has something to do with the CEM-3F91 Fuel pump (FP) relay Shorted to supply DTC I have had on VIDA for a while now.

>4000 miles later, I decided to do a diesel purge and have probably disrupted any sealing between the hoses and the LP/HP pump in the process, allowing air into the fuel system.

I could tighten all the hose fittings and force fuel to the LP/HP pump again (as it has been for the past 4000 miles), or I guess the better option would be to look into the electric fuel pump issue.

Can anybody point to where I might find this fuel pump relay? EDIT: It's "located internally in the Central Electronic Module".

Can it be faulty and replaced?

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Last edited by ca2n; Apr 8th, 2021 at 13:47. Reason: Found answer to question
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Old Apr 8th, 2021, 15:25   #6
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The Euro 3 HP pump is a tandem pump so if you can fully prime the system with diesel by vacuum pumping the return line at the front of the engine, it should run fine.
That's why Diesel Purge is possible on a Euro 3 D5 but not Euro 4.

My in tank pump has been dead for a couple of years now.
I keep it above a quarter of a tank and the car runs fine so I wouldn't spend too much time on that for now.
Having a working tank pump is ideal but not essential. .

The fact that you had revving issues during the Diesel Purge does suggest air in the system.

You could clamp the rubber feed line into the HP pump and pull a vacuum on the return and it should hold vacuum or point towards where air is being allowed in/out.
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Old Today, 17:27   #7
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I have a starting D5 again.

The inlet hose to the HP pump was secured using a new hose clamp but not before:
  • Brimming the inlet line with diesel via a vacuum pump,
  • Blocking off the HP pump inlet and drawing vacuum at return line to test for leaks, and finally,
  • Pulling a vacuum via the return line to force diesel from the (brimmed) inlet line to the HP pump.

I thought I had the system sufficiently primed to have a firing engine from a single turn of the key, but it took 10, see below:



I let it idle for around 10 minutes while double-checking for any leaks, followed by some revving. I also stopped and restarted the engine a few times.

I'm still convincing myself to take it out, will report back later.

Here's hoping that the non-starting issue was leak-related and nothing else and that my HP pump is still serviceable.

Thank you, everyone, for your help.
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Old Today, 17:53   #8
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Well done.
There's nothing in those figures that would concern me so hopefully you're sorted long term.
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