2008 GL420 Mystery Coolant Leak

mattmee489

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Hi All,

I currently have a 2008 GL420 4.0 V8 Diesel and the coolant level constantly needs topped up every time its driven and no one can find the issue. My mechanic has looked and found the coolant cap was broken so it was replaced (didn't work or make a difference). The thermostat was also replaced again it did nothing.

The coolant level only goes down when the car is driven under pressure. If I leave it ticking over its fine. I have even checked to see if the cooling system was air locked but no bubbles appeared after 30 minutes of running.

I am now thinking its a water pump or the dreaded head gasket.

I have also noticed that the coolant system is always under severe pressure even if I leave the jeep for a day or two (this is probably why the first coolant cap broke).

Has anyone ever had a problem like this before?

Any help would be great appreciated.

Thanks
 

LostKiwi

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'93 500SL-32, '01 W210 Estate E240 (RIP), 02 R230 SL500, 04 Smart Roadster Coupe, 11 R350CDi
If the cooling system is under high pressure (as in hoses are rock hard when the engine is running and up to temp) then it's almost certainly either head gasket or cracked head.
 

Mark A

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58 ML-320CDI.
Oil cooler seals (down in the v of the engine) were leaking on my 420. Also worth a look at the water pump, get them to remove the belt and check for play.

Mark
 

MRMRHILLS

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Mercedes GL420, 2008, 4.0L V8 Diesel (OM629)
Hi, did you ever get to the bottom of this issue?
I have the exact same issue and have had it for a couple of years. Never overheats, but high pressure has caused some gaskets to leak, it will vent fluid out of expansion tank after driving for a while, especially if revs go above 2000rpm. I have replaced waterpump, thermostat, expansion tank and numerous caps. At the moment I have a low pressure cap (1.4bar instead of 2bar) to try and lower operating pressure but obviously doesn't fix problem.
I have also tried the 'sniffer test' but never changes colour, fluid goes from blue to yellow if I hold it at exhaust pipe so it definately works, it kind of suggests head gasket isn't leaking.
My current theories include:
1. Head gasket only leaking under load as bolts are stressed - Tightening head bolts might fix it but potential to snap bolts so uncertain for now
2. Blocked radiator causing high pressure drop and possibly leading to pump cavitation.- easier fix but probably waste of money as it seems too simple.
I have seen a couple of similar problems on Internet searches but haven't found anyone who has actually fixed the issue yet.
Any advice would be much appreciated.
 
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mattmee489

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Hello, Mr MrHills

I did eventually find the cause and unfortunately it was the head gasket. It ended up at over £4k to fix the head gasket alone.

To add insult to injury in my ownership the head gasket, torque convertor, front diff, drive shaft, wheel bearing, ABS Sensor, front suspension all failed. After this all the electrics started playing up with an SRS fault plus the wiper blades stopped working.

If I'm entirely honest this is possibly the worse car I have ever owned. No mechanical in the country wanted to work on it.

In Total to fix the car was 10k plus in the last two years and I ended up selling it for 5k two months ago with the SRS and electrical faults still on. What's worse is the fact I might have drove the car maybe 500 miles in my entire ownership and that was mainly to different mechanics and back.

I would have been better off financially when I bought the jeep two years ago to park it up and walk away.

Only piece of advise is don't buy one.
 

Mark A

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58 ML-320CDI.
I hope I don’t regret this, but my 420 is still going strong after 9yrs and 140K.

Don't think I have heard of a head gasket going on these before (certainly not a common problem) - the most common leak is the EGR cooler.
 
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mattmee489

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Well you've had the car a long time so you know its history which is always a good thing! I spoke with a few Mercedes mechanics and they have said that apparently it is a common enough problem however there aren't many GL420's on the road. The reason its not heard is because very few people have these jeeps. Mine went at 139k miles (that's just after i bought it)

Also when doing the sniff test make sure the car is up to temperature. I left my ticking over at the mechanics yard for an hour and it passed the sniff test (when cold) but when I drove it for 30 minutes and brought it back, the cap blew off (under serious pressure) and it failed the sniff test.

Really strange and I really wanted to like it but maybe I just had a serious pup of a jeep!
 

ajlsl600

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clk3202001,sl6002003 with everything regrettably sold ,A class 170cdi auto. NG/TF1800 ML250
Keeping correct coolant balance the key to head gasket and seal issues with modern engines with alli components well worth a flush and replace on older high milage motors.
 

MRMRHILLS

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Mercedes GL420, 2008, 4.0L V8 Diesel (OM629)
Not really what I wanted to hear, but not entirely unexpected.
It is always a risk buying this sort of car and I would probably also advise against buying one if you need to pay a stealer to fix everything. I generally do everything myself and started on the head but couldnt get bolts off the turbo so gave up before braking too much. Didn't want to get too far and find it was something simple. Will probably have another go at some point.
They are nice cars and apart from this issue I have only had to do a pressure sensor, a couple of air springs and transmission speed sensors. Not too bad in 2.5 years and 20k miles, car has now done 125k miles.
Hopefully it won't brake again,
Thanks for advive anyway
 


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