how long to replace head gasket? - PeachParts Mercedes-Benz Forum

JHZR2 · #1 09-12-2002, 01:58 PM

Hi,

I have never replaced a head gasket, but I fear that the one in my 83 300D may be getting tired. I figure I should start making preparations to do it in the upcoming months, and so want to be as prepared as possible.
-The first thing I would liek to know is how long does it usually take to do this? I figure Ill get an estimate, tack on a few extra hours for newbie learning, and have some idea.
-The next question is, do I need to get a head job while its off? Is it usually necessary in a 190K car to need new valves, valve seats, springs, etc? I figure Ill clean it all up before replacing, but I want to minimize outsourced labor to keep costs down, and also minimize car downtime.
-Do I need to remove all the belts, hoses, fan, etc to take the head off? How much disassembly is involved?
-While the head is off, and Im digging around, I figure it would be smart to replace the timing chain and guides/tensioner. Is this a simple thing to do with head off, or are these two jobs somewhat unrelated (I wouldnt think so, I would guess ill have to take the chain off at least somewhat anyway).
-Any other replacements/rebuilds that are generally DIY while im in there? I like to get all my parts at once so I save on shipping, and like to have an idea now what Ill be looking at spending.
-Any special tools to do this?

Thanks for any info/advice I can get.

JMH

#2 09-12-2002, 02:58 PM

I am also curious about this. How long will these last?

gsxr · #3 09-12-2002, 03:03 PM

Hi JMH,

I have done this on a newer diesel (1987 300D) which is different than yours, but the basic idea is the same. Anyway:

1- The usual shop labor quote is between 12-20 hours for experienced mechanics to do the job, start to finish. I would plan on 20-30 hours, if you have no major problems, and have the factory service manual (not just the Haynes) by your side as you do the work. It took me roughly 40 hours or so but I spent a LOT of time cleaning up the engine compartment and being *really* careful about everything. I could probably do it again in 20-25. Your engine is a bit simpler, and may be quicker, I'm not sure.

2- A valve job would he a good idea. At least have it checked by a machine shop while it's off the car. The valves shouldn't need replacement but may take minor lapping or something.

3- I believe you need to remove the belts & stuff but I'm not positive. On my car I removed the serpentine belt just to have more room to work.

4- Yes, replace the timing chain. You will want to replace the guides & tensioner while the head is off, but the chain itself can be done separately, and is best replaced with the head installed.

5- While it's apart you could have the injectors tested. Also set the injection pump timing (wait until AFTER you replace the chain, or you'll need to do it twice.) If you get the pump lock tool (about $25 from the dealer) you don't need to mess with the "drip" method.

6- You will need the special 27mm injector tool, which also is handy for the crankshaft center bolt (to turn the engine). (Some regular 27mm deep sockets work, other don't.) You will also need the tools to R&R the prechambers, unless you don't mess with the head, then you might just leave them in there. I can't think of any other special tools needed off hand... read the manual and see what it says.

BTW - what makes you think the gasket is failing? Oil in coolant or vice-versa? High pressure in the cooling system when cold? Overheating? It's not very common for head gaskets to fail on the iron-head diesels...

Good luck,

JHZR2 · #4 09-12-2002, 03:20 PM

yes, oil drips in the top of the coolant recovery bottle. Makes my finger oily if I stick it in there, although it isnt very serious, as if I do this a few times, all the oil appears to be gone and it is coolant again. I might be edispersing it, but I like to think it isnt much oil.
The funny thing is the car doesnt use much oil since Ive solved the drips, perhaps next to none, and its temperature is fine, so Im not sure if I should worry about it or not. Gasket swelling agents didnt help though, and I dont want to use brake fluid in the crankcase to make the gasdkets really pucker...

gsxr · #5 09-12-2002, 03:34 PM

Oh- oil in coolant - not good. If you feel lucky, you could try "K&W Metallic Block Seal" before tearing the head off. If you do, follow the directions EXACTLY or it won't work properly. It's a pain to use, requires complete flush before using, and requires a 24-hour drying time with an empty system afterwards. It did cure one car of mine for a few years, and seems to have fixed the second one (for now). It's about $8/can at your local McParts. Don't use any other brand or type of sealer, just the K&W in the copper colored can. Afterwards you should de-oil the cooling system and refill with fresh MB coolant. But either way, don't let it go too long, it does need to be fixed!

leathermang · #6 09-12-2002, 03:40 PM

Everything GSXR said was great... but I would like to say more strongly,,, Have the shop manual with you,,, and read the whole procedure before you start....

It is great to start preparing for this job.. but from your description it could be 100k before you need to take it off...Have fun getting your ducks in order and feeling good about that... don't rush doing the job until you really need to....

In the meantime concentrate on keeping the valves set,the chain stretch measured ( and fixed if necessary ) and the injection timing set... these are all good practice for when you have to take the head off and put it back together...

"Gasket swelling stuff".... I believe it only applies to rubber gaskets... I have never heard of anyone claiming it would reseal a head gasket...

Also, if you don't keep your engine clean anyway.. this is the time to start getting it cleaner and cleaner.. it really makes the job more fun and satisfying if you can concentrate on the mechanicals...and not the mess...

engatwork · #7 09-12-2002, 05:55 PM

I went through the head in my 300D and looking back on it I believe I would have left it alone. The only thing that I was seeing was the cooling system was staying pressurized over night - everything else was operating fine. I pulled the head, replaced guides/seals and put everything back together. Since that time it has not run as well as it did prior to the job. I suspect that the "stronger" head brought out the "weakness" in the lower end. My plans are to pull the engine over the next couple/few weeks and make it right.
I think, if you have all the parts you need that you could remove/replace a cylinder head in one daywithout any problem. That is if you did not have to do any head work. In my case, that means dropping it off at my machine shop for a week or so. The machine shop bill came to $250 when I had mine done.

leathermang · #8 09-12-2002, 06:48 PM

Jim, If your engine is not as strong as before your head work then something is not as it should be... I know it is not fun to retackle a job like that... but you can take one item at a time... like the injection timing and recheck it.... this mystical " weakness in the lower end" is just that.... unless you have a bearing go out... or something like that... there is no way for the lower end to show up its " weakness ".....so buck up your courage and go back in there so you will enjoy your car again... not have that nagging feeling something is not right.. because in fact ... something is not....

engatwork · #9 09-12-2002, 08:07 PM

Sorry for changing thread comments. I checked the comp and it was only around 160 psi on #5. Around 250 on 1-4 and when I pressurized the #5 cyl I could hear/feel air blowing out of the oil fill hole. I will do the drip timing test this weekend before tearing anything down. It "cold starts" badly - I have to keep giving it fuel to keep it running when cold and after it has warmed up it idles very rough. In addition, there is alot of smoke that looks like unburnt fuel. If the timing looks ok I'll pull the head and try to see if there is anything obvious.
thanks

leathermang · #10 09-12-2002, 08:49 PM

Well Jim, that is pretty low on compression assuming it was a good test... my point was just that snapping up the head did not cause this...it was already this way,,, or worse if the valves were not right on the money...Did you put on a new chain at the time of this work ?

gsxr · #11 09-12-2002, 09:04 PM

Ooops. I forgot something. You DO need one important tool. The head bolts are the internal 12-point type. Due to clearance issues, I don't know if aftermarket tools will work. (On the OM60x engines they won't, you need the special extra-long MB tool.) Anyway, the MB tool is a nice Hazet unit, and not terribly expensive - ~$20 or so, IIRC.