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I found myself in a serious quandry about my 1992 500SEL. I love the car. It is in amazingly good shape. I don't think it's ever been 'smoked' in. With 145000 miles, it really runs pretty good considering its age. Until recently...

An odd and intermittent problem with coolant has been giving me fits. Some time ago, my 5.0 threw a frost plug from the middle of one of the heads. It happened during a third 0-80MPH timing run in the span of about 10 minutes. Wow! A 1" diameter hole in the water jacket created a HUGE cloud of steam for about 30 seconds. I thought I blew it up but it kept running just fine. I had it towed to prevent further damage due to overheating. As they say..."You play, now you pay..."

Since fixing the frost plug, the coolant levels would not behave. It appeared to be getting over-filled and it would spew about a pint then stabilize. The side tank was always under more pressure than expected so I replaced the cap with a new one. I stopped filling the side tank beyond 1/2 full and the problem virtually subsided. I found I could drive it for many heat/cool cycles before it would report low coolant. Driving it all day caused no change in the absolute level of coolant. After a few heat/cool cycles I would have to add a few cups of coolant and it would be good for another few weeks.

I always smelled coolant when around the car. Never from inside the car. Never could I find a leak. Even on a hoist where I could really examine it, nothing. But I could remove the radiator cap, stuff my nose down the hole, and I would smell more exhaust than ethaline glycol. Not a good sign.

About every 10th time I went to use the car, I found that it was hard to start (missing) and it would spew clouds of gray smoke for the first minute or two of driving. Every time this happened, the gray smoke would subside but it would immediately needed coolant. Then the problem would resolve and it would be good for another couple of weeks.

ARRRGGGGHHHH!!!!

I suspected the head gasket from the first incident with the frost plug and the smell of exhaust in the expansion tank. But it was an odd manifestation. I contacted several mechanics. They all seemed to point to a head gasket. It was a matter of time before the dam would burst and I'd be looking for a tow. Nobody really want to fix this old girl. I could not get a serious bid from anybody although one BMW/MB mechanic offered to swap the motor with a used one.

It wasn't just the gasket causing my angst. Timing chain slap could be heard every time I started it until the tensioner pumped up with oil. It ran fine but the clunking sounded like a John Deere for the first 5 seconds after start-up. The car also has a problem with ASR and Limp Home mode on every cold start. Although the engine harnesses have been replaced, the 'pig-tail' on the Electronic Accelerator has not been repaired yet--a potentially expensive repair. And a rear self-leveling strut is squeaking like an old bed spring. All the 'fixes' on this car are approaching and even exceeding its real value.

What to do? The option of just parting it out was becoming a realistic approach. What a shame. A car that has been garaged its whole life. A clean interior and near flawless sheet metal--cared for like a work of art, just scrapped? A travesty!

My wife asks an astute question. "So you it fix this time, what's next?" I just stare at my feet and shrug my shoulders. I can't answer for certain.

I contacted Patrick Hamilton, another member on this forum, for his advice. Pat was encouraging me to take this on and fix it myself. Without the significant labor charge, the car is a good candidate for repair. My confidence level is lacking for such a project. I'm a bean counter, not a mechanic. For one thing, I lack some of the tools. I haven't done any serious wrenching for 30 years. (And never on a MB--an MGB yes, MB no;)

Checking some of the other W140 DIY sites I am not finding any significant help with the details of walking through the steps for head gasket replacement on the 5.0 V8. AllData was somewhat helpful but assumes I know what I'm doing--a foolish assumption.

Pat offered to help. He's been through this enough times to know that for him its just a matter of 'turning the crank' and making it happen. I just returned from Pat's home after delivering my ailing W140. He's agreed to take on my basket case as a personal favor. I've been encouraging Pat to document this project on this website for future reference. I'm hoping he chooses to document the steps on this forum so that others may be encouraged to do the repairs to keep the fleet going. This adds extra work for him. To make it happen, I'm willing to help out with some of the narrative if he's willing to provide the pictures and explanations.

Are there others on this list with similar quandries?
 

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I would first try to find out where is your coolant going (before you start pulling engine apart)...sounds like its going into one of the cylinders somehow, as you say probably gasket...but the loss sounds bit too fast (gasket wont cause that much loss so fast and you would probably have cloud of white smoke comming from exhaust) ...

and as last resort

Find yourself nice low milage M119 and chuck it in.....done, problem solved :) (aprox 1.5K for the engine and about $500-1000 labour)
 

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Hey gentlemen,

Milo i agree with you on replacing the motor if it had a oiling or bottom end problem, but not for a head gasket problem. I feel that buying a used motor is also buying someone elses problems as well. We know that the car is sound, certainly in great shape. After some diagnostic work, determine what the problem is and dive in head first. After the car is done it will be a beautiful car. I will be taking pictures of the car as the work progresses to show just what is involved in doing the head gaskets on this car. I wish i would have done this when i did my V12, im sure people would have been interested in seeing that. Let the fun begin!!!
 

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Discussion Starter #4
Hey gentlemen,

Milo i agree with you on replacing the motor if it had a oiling or bottom end problem, but not for a head gasket problem. I feel that buying a used motor is also buying someone elses problems as well. We know that the car is sound, certainly in great shape. After some diagnostic work, determine what the problem is and dive in head first. After the car is done it will be a beautiful car. I will be taking pictures of the car as the work progresses to show just what is involved in doing the head gaskets on this car. I wish i would have done this when i did my V12, im sure people would have been interested in seeing that. Let the fun begin!!!
I like your approach Pat. There is no substitute for proper diagnostic work. Milo, I have observed gray smoke but only about every 10th time I go to start the car. My suspicion is that it only leaks copious amounts of fluid into a cylinder when it is stopped in the middle of a compression stroke. I've also observed a 'burping' and 'girgling' sound coming from the engine when I open the reserve tank to add fluids the next morning. As far as using fluid, I was able to drive the 350 miles to deliver the car to Pat without adding fluids, but I never pushed it beyond 75MPH on that trip. I've observed an apparent correlation to losing fluids and weird burping behavior every time I get frisky with the accelerator. Problems manifest after full or near-full throttle episodes.

And yes Pat, I would very much like to follow your progress in pictures. Do you think we should start a new thread or just keep adding to this one? I think it should be a thread titled "5.0 liter head gasket replacement" so others can more easily reference it.
 
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