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  #1  
Old 06-09-2014, 05:31 PM
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engine knock at idle

Hi All

R reg, 1998 E300 TD, 606 engine, 49000 genuine miles, FMBSH

Engine knocks at idle and up to 1250 rpm. rev it up and knock goes away. Let the revs drop and the knock returns at idle. No smoke when driven hard, plenty of power and starts first time every time.
When started first thing in the morning when engine is cold, its quiet for the first couple of minutes then starts to knock.
Done a diesel purge last week using liquid molly and the engine ran like brand new no knocking what so ever.
Ran it on diesel and the knock returned. It suffers from air in the fuel lines and today i bypassed the fuel heater and the pre filter to try and find the fault but made no difference. air bubbles can still be seen in a couple of the clear lines.

If you drive the car normally it runs smooth as silk except for this knock at idle and 1250 rpm.

Crack number 3 injector and knock seems to go away.

Could the pump be faulty?

This knocking did not develop over time, it suddenly started one morning (5 weeks ago) when glow plug light illuminated and engine done a lot of knocking before it finally started.

in the past 5 weeks the following has been replaced with genuine MB parts:

glow plugs.
all filters.
engine oil. and hyd lifter treatment
injectors.
all clear fuel lines.
leak off pipes.
crush washes and O rings in fuel valves 3 and 4.

chain tensioner has been checked and is ok. All dampers and harmonic balancer ok. Cylinder compression test ok all cylinders are 28 bar, MB spec is 18 to 35 bar.
Crank and cam timings are spot on.

Is it possible that the timing advancement mechanism on the pump sprocket has gone faulty, following the bad starting. Its my dads car and he said that the engine was knocking its head off before it was firing on all cylinders.

Ant ideas would be appreciated


Last edited by runningman; 06-10-2014 at 02:46 PM.
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  #2  
Old 06-09-2014, 05:46 PM
KarTek's Avatar
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Hi, my '98 is giving me fits right now with air in the lines. It causes a lot of "pinging" and knocking problems. I would say try to find the source of the air before moving on.

One other thing I would try is to re-torque the #3 delivery valve holder. Back it off a 1/4 turn and then torque it back down while the engine is hot. I have had good results with this to stop idle knock and shake.

Timing advance mechanism? I'll go out on a limb and say it's virtually impossible for it to fail. When a GP goes bad, these engines sound awful, clacking, shaking and smoking until the cylinder comes up to temp.
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  #3  
Old 06-09-2014, 05:55 PM
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I went through this.

I read your list, there are a couple more hidden o-rings that can introduce air.

First is the pre-filter. Mine had some contamination on the bore that prevented a good seal. You can also very carefully bend the tabs down on the retainer to put a little more pressure down on the fitting at the top of the pre-filter well.

Second is the o-ring on the back of the shut off valve to the body of the IP. This is easy to change without removing the manifold. Drain and remove the washer tank and you have nice access to the shutoff valve.
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Old 06-09-2014, 05:59 PM
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After you are sure all the air is gone. Swap injectors to see if the issue moves. That's of course if the problem is still there.

Delivery valve for the original cylinder should be done as well if the problem persists on the one cylinder. After the injector swap. Even if you feel like doing them all do the suspected offender first. That way there is less chance of adding confusion to the mix.
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Old 06-10-2014, 09:54 AM
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is there a pump located in the fuel tank to help pressurize the fuel pump?
im wondering if the tank screen is gunged up because of the little mileage the car has covered.
Also when i release fuel filler cap i can hear the air being sucked in to tank because of the vacuum. But i cant see any vent pipes under the body.
Ive read that some cars were fitted with vent pipes and some were not.
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Old 06-10-2014, 12:09 PM
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The fuel pump is a suction pump located on the side of the IP.

Follow the fuel path and fix the air leaks first.

fuelflow.JPG (image)
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Old 06-12-2014, 11:15 AM
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drained fuel tank and removed screen and checked pipe work and all was spotlessly clean.
Replaced O ring on FSOV, all problematic faulty pipes and O rings have now been replaced.
You can still see air bubbles in the lower most fuel line, the one that runs from the underside of FSOV to the lift pump.

Engine still knocks at idle and up to 1250 rpm ish.

The bubbles don't bother me, its the knock at idle that's driving me mad. Maybe its not the bubbles causing the knock but a faulty diapham in the lift pump.
Worst case is a faulty IP, I doubt it but I could be wrong.

Buying a complete doner car tomorrow for £200 and use it for spares as engine runs nice and sweet. Still taxed and MOTd but gearbox wont select 4th gear.


Can anyone think of anything else
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Old 06-12-2014, 12:12 PM
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Quote:
Originally Posted by runningman View Post
drained fuel tank and removed screen and checked pipe work and all was spotlessly clean.
Replaced O ring on FSOV, all problematic faulty pipes and O rings have now been replaced.
You can still see air bubbles in the lower most fuel line, the one that runs from the underside of FSOV to the lift pump.

Engine still knocks at idle and up to 1250 rpm ish.

The bubbles don't bother me, its the knock at idle that's driving me mad. Maybe its not the bubbles causing the knock but a faulty diapham in the lift pump.
Worst case is a faulty IP, I doubt it but I could be wrong.

Buying a complete doner car tomorrow for £200 and use it for spares as engine runs nice and sweet. Still taxed and MOTd but gearbox wont select 4th gear.


Can anyone think of anything else
You might want to check the belt tensioner shock. I had what I thought was a knock at idle, but it ended up being the top rubber mounting grommet on the shock. It was allowing the metal part of the shock to hit up against the engine. It only happened at idle......changed out the shock, and noise was gone. Good luck
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Old 06-12-2014, 01:04 PM
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At this point, I'd try re-torquing the #3 delivery valve holder. Do it with the pump/engine hot so you won't have problems with it loosening up with expansion.
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Benz Fleet:
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  #10  
Old 06-12-2014, 10:18 PM
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Personally, I'd go back to the air leaks, its a 606, a simple air leak can be perceived by many to be a zillion other issues. If one is methodical they are so easy to resolve that's its foolish not to take care them first and take that item off the list.

Then move on.
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  #11  
Old 04-22-2015, 11:30 AM
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Sorry to bump an old thread.

OP: are you still around? did you ever get this resolved?

Kartek: You're thinking he could be getting air in on after the IP? Should the splined fittings for the delivery valves be torqued with the engine hot?
I'm kind of confused how you get air in the fuel through the delivery valves since that's on the pressure side of the IP.

I have a similar air in fuel issue where I see tiny clusters of bubbles in the clear lines from the lift pump to screw on filter. The clear line from the screw on filter to the IP is too dark to see through. Theoretically though, air should dissipate in the screw on filter and go back the return, right? Either way, I'm planning on rebuilding the fuel thermostat and replacing fuel lines tank strainer, but I'm waiting on parts.

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