gonna try this tomorrow. only concern is that the top radiator hose connects to the thermostat at the top front of engine. The bottom hose goes from the water pump to lower radiator hose. if I force water through the top then it will come out the water pump not the lower radiator. top and bottom will be disconnected. is that why you mean?

 

These are all indicative of a failed headgasket, but a chemical test (testing the coolant for the presence of even the minutest amount of hydrocarbons) will flag a failing HG.
It's possible that the system is being pressurized by trace amounts of blow-by or combustion gases. It's fairly cheap to buy a kit that will last 10-15 tests to rule this out.
This is not the same test as a HG pressure test.

I'm also liking the air bubble in the system idea above - in order for the coolant to vacate the aux tank, it has to go somewhere. So the tank is pressurized, the coolant moves to fill air voids in the block, which again puts them in compression. Once the cap is released, the pressurized air bubble expands and pushes the coolant back into the aux tank...or something like that.
Something is still moving the coolant from the aux tank into the system, whether it be the aux pump or some kind of gas pressure.

 

Yeah, but that failure mode doesn't always manifest itself - a HG can fail in many ways.
The HG serves to separate three things (coolant, combustion and oil) from one another.
Off the top of my head a HG can have six failure modes; letting

1 combustion gas into coolant (pressurizing the system),
2 coolant into combustion (making steam),
3 combustion into oil (making coke)
4 oil into combustion (making smoke)
5 oil into coolant (making mayonnaise)
6 coolant into oil (making milkshake)

Well the two last ones are just variations on an emulsion, but you get the jist. Point of my post was to rule out the HG via chemical test, easy diy - a pressure test of the HG isn't all telling.

 

thanks makes sense. ill get a kit for that and check that also. I'm just lost for what it could be and im not trying to spend more on fixing it than I spent purchasing. hopefully the HG test and back flushing can help. if not I give up

 

Steam from the exhaust, sweet smell from the exhaust and rough idle are easy tests. Have you done these?

 

none of that. the only time I smelt coolant was when it was running off from the expansion tank down to the hot exhaust

 

Yeah - I didn't realize the water pump is at the bottom. Presumably water comes out the motor at the top, down through the radiator, and back into the motor at the bottom. The point is to reverse the flow.

If you're clever you might be able to open the thermostat electrically, without removing it.

I would really work to eliminate the possibility of trapped air someplace. Flushing it is not a huge amount of work but no one's going to pretend it's easy.

If it were the head gasket, this would be the first time I've heard of one going bad.

I couldn't tell you exactly where in the diagnostic tool you'd find the temperature readings. Front SAM, maybe?

 

Yeah but the gajillion dollar question is why the air hasn't burped out.

Not sure if this is the same thing, but: There is a bag of silica gel, which is also a dessicant, in the expansion tank. This is to protect the aluminum (?) surfaces inside the motor; the silica dissolves in the coolant very slowly. If the bag ruptures, the silica beads wander through the system. That may be the problem you're talking about. All that should not have anything to do with this fellow's problem.

There may be dessicant in the AC system; there also is in the air compressor.

 

beats me. no silica in the expansion tank. and I have no clue. I just want to drive and not have to stop every 20 mins to release air

 

Just at thought that's at least easy enough to do (and free!). Can you jack the front end up and then open up whatever part of the cooling system is the highest? Run the engine for a bit and the air bubbles "should" work their way to the highest point (the point you opened up) and out. Monitor the coolant level and, if you're running it for a long time, the engine temp. I have an older BMW and if I don't raise the front end a bit, it's really hard to get all the bubbles out. I'm not saying you "should" have to do this... but maybe you do

 

Don't worry about the silica gel. There is probably a small plastic mesh bag of it somewhere inside the expansion tank.

What happens if you simply leave your GL idling, say nose up in a driveway, with the heater on and the expansion tank cap off?

 

back flushed it and nothing. Bought a kit that pressure test and fill system. Did two pressure test and both held up for 30 minutes each. filled the system (with vacuum system). warmed it up and drove for a little. got three blocks and surely enough check coolant level came on. opened the cap coolant came right back up. Chemical test didn't detect anything either. I did see a post on here for a Europe model g420 CDI with the exact same symptoms and it was a head gasket leak. so now im absolutely worried. Would it be wise to even attempt to change that myself of just get a used engine and do a complete swap??

here's the link for the one that has the same exact issues as mine.

https://mbworld.org/forums/gl-class-...ng-system.html

 

I'm only at the fringe of my knowledge here, so maybe someone with more knoweldge can comment, but... it would seem to me that since the problem you're having is happening so quickly, then it would be a pretty darn good head gasket issue. Which would then mean you would have detected exhaust gasses in the coolant with the chemical test, would be seeing bubbles in the coolant with the cap off or you would be seeing white smoke from the tailpipe and/or smelling coolant in the exhaust. Did you try lifting the front of the car to better bleed air?

 

Having never done a hg swap on this, and with the engines overall reputation for never having head gasket issues I’d lean towards an known good engine - however, just avoid the early 2007 production motors. But - getting a little ahead of ourselves here, more attention needed to rule in or out the headgasket.
simple test: If you unplug your aux pump and the reservoir still empties, then that is not the source of pressuring the overflow tank. I think only combustion gases can do that, and that points to HG.

 

Keep in mind that the 420 is a diesel engine (cdi). That means the combustion chamber is always developing high pressure as long as the crankshaft is turning. So the symptoms are difficult to compare directly.

There are ways in which the head gasket can be leaking. The cylinder head covers the combustion chamber, and allows the passage of coolant and oil. The gasket isolates these three from each other, and each from the outside. Since you are not seeing gross external leakage, we can rule out "outside". This leaves
1. Chamber <=> oil
2. Oil <=> coolant
3. Chamber <=> coolant

#1 would manifest as all sorts of awful problems. Oil coming out the exhaust, a failed engine because of gas bubbles in the oil.
#2 would be obvious from both the oil and coolant. They would trade places regularly.
#3 would have fairly specific symptoms. People have mentioned testing the coolant, but there are also blatant external symptoms.

The gas engine will develop a vacuum in the chamber if you take your foot off the gas while moving forward. If you are traveling at speed at which you expect the system would be fully pressurized, or over-pressurized as you fear, and you take your foot off the gas, the pressurized coolant will spray into the chamber. You will see a fog of coolant behind you. Or you will smell coolant in the exhaust gases when the motor is hot from driving (and is over-pressurized) and the motor is idling. In that case as well the chamber pressures are minimal, especially during the intake and exhaust strokes.

What's more, if you left the motor sitting, off, in its pressurized state, the coolant should leak into the chamber and down into the oil ... it would make a real mess.

I don't know the design of the aux pump, but it seems unlikely it is able to develop enough pressure to make a difference. Centrifugal pumps just don't push that hard and I can't imagine MB using a diaphragm or piston pump in this application.

You have done a ton of stuff, but I want to confirm the very most basic: Does the motor exhibit the symptoms when you simply leave it idling, nose up (say in a driveway), with the heater on? Walk us through from a cold start, expansion tank half full, cap off, to fully warm.

 

well its a wrap. had two different mechanics to including a relative that an ASE euro mechanic that worked for Mercedes and BMW verify its the head gasket. I guess im gonna be buying a used one, Question is can I go it myself if I have access to a lift and do anyone have a service manual or knows how to do it. a video, write-up or manual would help. I don't have none.

 

Glad the diagnosis is sorted. I think YouTube has a few vids on how to drop the front subframe from the x164/w164 series and it’s the preferred way.

get a good known used 2008 MY and up m273 motor, and have them give you the engine number before they ship it to be sure it isn’t affected by the soft balancer shaft sprocket issue.Mercedes-Benz with the improved parts have engine serial numbers that are higher than:

• M272 Engine Serial No. 2729..30 468993
• M273 Engine Serial No. 2739..30 088611

If your current engine has no other issues than a failing headgasket, you could do both while you’ve got it out and not have to separate the engine and tranny - or sell your original engine and come out ok on money but with a lot of labor invested.

 

tell me about it. I thought about doing the head gasket myself, but after seeing how everything is done it scares the hell outta me touching anything dealing with the timing. And this is coming from a guy who built (rice burners) and is very mechanically friendly. This scares everyone off. but I know I damm sure aint about to pay no $8k-12K to do no head job.

 

No no - while the old engine is out, then do the headgasket job. And you can sell it with that pedigree or sell it as a core with a known headgasket issue. That being said, if you wanted to dig into the HG job with the engine still in you’ll have very good access to timing components once the radiator is out. YouTube is your friend.

 

I'm confused. You're ready to discard the truck because you talked to two guys that told you it's a head gasket problem. Even though this is the first any of us have heard of a head gasket failure in this (gas, not diesel) motor, and the coolant seems to hold pressure, and you have not reported any other symptoms of a leaking head gasket.

You have listed a bunch of things you have done, including things I have never heard of guys doing, like blowing air through the heater core. Are you doing these things yourself? Did you pay the dealer to change the thermostat, hoses, tank?

I'm sorry to put you on the spot, because you're obviously having a tough time, but it's important to understand something: These vehicles present an opportunity for someone who is mechanically able and is patient and persistent, because the value plummets once they go out of warranty. It simply is not cost effective to pay others to keep it running.

This is a different skill set from building street racers: It is one thing to modify and another to diagnose and repair. For example, if you think you have a leaking head gasket, you should already be figuring out whether the motor is worth saving (is the compression good?) and which cylinder the leak is on. I would verify with certainty the diagnosis before moving on. Diagnosis can be difficult; I suppose if the leaking gasket is acting like a one-way valve, with combustion gases blowing going into the coolant and not vice versa, the external symptoms could be missing.

Why not drive with the coolant cap off? If combustion gases are leaking into the coolant, they should just purge right out. At least this buys you some time.

My sympathies to you for the heartache you're going through. Your decision is not whether this motor is worth repairing, but whether this is the sort of vehicle you want to own. If you're willing to do diagnosis and repairs yourself, and take good care of it, these trucks age well and will serve you nobly for a long time. If not, there's no shame in walking away.

 

I think Eric nails the value proposition of the x164 here...they’re so cheap to pick up and if you DIY; very inexpensive to keep on the road. And what quality and ride you get for that little money!

I think you’ve got the skills to pull this off, however you approach it. My 90s Buick is a piece of crap and a pain to work on compared to the x164.

 

Yeah, honestly, it took me a while to really understand the mindset I needed to adopt to keep my sanity with this thing. I just went on a 3500 mile road trip and the GL was great. I left Boise right at winter solstice and knew if an unexpected storm came up, I'd be able to make it over the Rockies. Yet at the same time I just finished fiddling with the valve block and air pump, struggling in my buddy's driveway in 40 degree weather, trying to figure out what magic I needed to work to get my beloved happy again.

I'm obsessive as hell, tracking my expenditures, and besides gas and insurance, I spend $0.15 per mile maintaining my GL. That's really not bad for a truck I genuinely like driving. At this point it's like the wife who bore your children and you freely admit is now ugly as sin, but you couldn't imagine life without her. Pulling in to the driveway in Texas, I patted the dash and thanked the old girl for getting me there. She can be persnickety, but when she's in a good mood, she's a joy to be around, and besides - she ain't plugged in to no Internet of Things, ready to deliver me to the police for wrongthink. I won't go so far as to call 2007 old school, but it still is something resembling a standalone piece of hardware.

In exchange you have to be patient, flexible, and persistent. I spent several days with a misfire error, only to figure out that I had to clear the code for the motor to run right. In another thread, a guy posted a whole slew of errors, wanting to ignore most and fix Just That One. No, you have to be patient and persistent, determined to get to the bottom of the problems no matter what it takes. These are complicated machines, unfortunately with things made difficult to discern (such as a trip computer that switches to a drawing of a car at a gas pump when it no longer has any idea how much further you can go), but we can assert mastery if we try.

I'm sure our OP has the skills to handle his responsibility. If not, those skills aren't too hard to develop. The question is whether he is willing and able - and some would laugh, stupid enough - to commit.

 

Trust me. I want my truck. this is my second one. personally if im going pull the motor, its better to just replace since there can be a whole lot of other issues going on in there. Being that these block are pretty much a block of aluminum they can eventually crack and create worst problems. Im just seeing what is more cost effective. Stealership wants 12K to put a used motor in. Euro Shop wants 8K. and yes I did most of the work myself based on verification of the diagnosis I had already done. If the dealership told me once nothing wrong, then two days later said its air in the system. Then later said they can't figure it out. also the same thing with the Euro shop. Up until two days ago the only two things I did not do was a chemical test or compression test. But I don't think I need a compression test now that the chemical test verifies there is a leak. and trust me brother, I honestly wish it was something simple or even that this might be misdiagnosed. If you have any other suggestions please tell me. and I only saw two videos on dropping the motor none of which was informative. But I did see them taking it out from the bottom. if the top is an option im getting a cherry picker just for this job.

 

Try to take this in the spirit of helpfulness: You are going off on tangents like I've never seen around here. First you declared it was a problem with the aux pump, which I had never even heard of. I'm glad you did, because I used it when I wanted to keep the cab warm while I finished my beer, lol. Then you figured out it is the thermostat, the expansion tank, the hoses - none of which people dick with, except it's mentioned as a preventive maintenance thing at 200k miles. Then it's the head gasket - which no one on here has ever seen go bad. Now you've talked yourself into changing the motor, because the block is cracked. I'm not there, but it sure seems to me like you're being awful jumpy. This is why I was saying to be persistent and patient.

If there is one thing good about this truck, it's the drivetrain. We sometimes hear about wheel bearings. I might have heard of someone needing to change the transfer case chain. People blow up the trannies cough-me-cough, but it's generally due to neglect. The motor - I think I remember one guy talking about doing a motor swap. Mine has 170k miles and last I checked the compression was still golden. So it's hard for me to imagine the head gasket having gone out, but if you say so - if I had convinced myself the head gasket was leaking, I would figure out the health of the rest of the motor - do a compression test and a leak-down test. At least localize your problem to one of the two banks. Since you must have a small leak in the head gasket, for it to pass a pressure test and for there to be no coolant stink coming out the exhaust, you aren't in a hurry. Just take off or loosen the coolant cap so the system doesn't pressurize. The combustion gases should burp their way out on their own.

Figure out with certainty exactly what is going on before you go committing to a donor motor. At the same time, research changing the head gasket; I don't think it's as hard as you make it out to be. You most likely only need to change it on one head, unless something really weird has happened to it, like the workers that put the goo on there (I think it's just a silicone caulk) were hung over that day and messed up both sides. Get to the bottom of what's wrong before you drive yourself nuts planning for an engine swap.

 

I hear you 100% Eric. lemme break everything down so im not going back an forth between post explaining.
Bought the truck with 120k. Dude told me he buys and fix cars and sells them back. He said it was in an accident (nothing major) and the only thing that needed replacing was the radiator. I figured for the price and milage he was selling it for even if I had to fix something im still winning. remember I had the same year and model truck before. On my way! home coolant was coming out. I noticed that the expansion tank was leaking from the top and lower part at the seam. Got a new tank. drove it for a week coolant kept going down but not empty. took it to the stealership and they said nothings wrong. they added coolant. drove home. left for work next day check coolant level came on again and that's when I noticed that the auxiliary pump kept running as long as the car was in the on position. took it back to dealership same day. they kept it, did all their test, refill system. everything was good for 2 days got home from work siting in the driveway and boom (loud pop) and the lower coolant hose is off the radiator. got a new hose, put everything back and noticed the radiator was leaking from the Botton of where that hose was. had radiator replaced at the euro shop. same issue with auxiliary pump so they replaced that. then he said he thinks the head gasket is gone but he had to check some other stuff, then he call me back and said it was the oil cooler. needed a new gasket. at that point I said im done spending all that money at the shop so I picked it back up and bought some parts from the dealership and replace radiator, expansion tank, thermostat, hoses, water pump and the belt. all was good for a day then check coolant level again. stated asking advice on forums and about a week or two ago that when I replaced the heater control valve in the back of the engine. refilled the system when I replaced it with a pressure vacuum/filler kit, but 5 mins into drive on highway same thing. the last I did was the chemical test this week and that changed colors. the only thing I haven't done is a compression test. besides everything that was done, the only two things I can think of now is a shut thermostat (but that's new. like two weeks old) and whoever worked on it before I bought it maybe left a rag or something that stuck in there. that's why im asking everyone on here for help. if it seems like im jumping around well yah. but in no means am I upset or anything at anyone here because im being helped. and I am grateful for all the advise I been getting so far. so once again. any suggestions are greatly appreciated. I havent bought a used motor yet. still being hopeful