First off, I want to say that I know nothing about cars. Any information mentioned is directly from a mechanic. I recently purchased a 1984 300SD. Before buying it I had a prepurchase inspection done in which they told me about a problem with one of the cylinders where it had absolutely no compression. The shop (MBI Motors in Portland, OR) tech did some further tests and told me something to the effect that it was not leaking from the bottom so it was probably just an issue of the heads needing rebuilt, a $1500 to $2000 job. He also assured me that the car was in no danger of dying on me, the only consequences would be poorer performance and possibly issues starting if the outside temperature was very cold.

Long story short, I went back to the seller and negotiated the price down to $1500 as I figured $3000 to $3500 wouldn't be a bad price for a 300SD.

I was talking about it to someone who works on these cars and he mentioned it might just be a stuck valve or something due to carbon build up. He recommended sticking automatic transmission fluid in the gas tank (with a full tank) which may break down the carbon and cause the cylinder to start working again. This coincided with MBI's diagnosis in that they mentioned that there was a slim chance the cylinder may clear itself and start working again.

My question is two-fold. First, is the suggestion of carbon buildup a possibility? If so, would ATF break down the carbon like the other guy said? If ATF is not the way to go, what would work?

Thanks

Welcome to the forum.

It's possible that carbon built up on the seating face of the valve........because it was stuck open.......and now it won't seat properly.

ATF won't do a thing to fix this problem.

You need to pull the head and have the valves done. Typically, the guides and seals are replaced at the same time. I had the prechambers replaced at the same time.

It's important that the head go to a very well qualified shop.

I sent mine to Metric Motors in CA. Anybody local was just playing with themselves.

Your local mechanic can R & R the head for about $800. or so. The cost at Metric will be between $400. and $1100. depending on how much needs to be done.

Yes, the valves on these cars are unlike most of the other cars on the road. They require a manual measurement with a feeler gauge and adjustment via bolt and lock nut on the rocker arm. Most people, even those who own these cars, don't know that this needs to be done fairly often - MB specifies every 15,000 miles I think.

So, what happens when they are not adjusted properly is the clearance decreases - the valve train gets "tighter". When it gets tight enough, the valve will not close properly.

Start with the easy things first - adjust the valves clearances or have someone adjust them. If it were my car I would set them just a bit "loose" so they would be sure to continue seating properly as the carbon attached to the valve seats wears off. And I would check them again on the 15,000 mile maintenance schedule.

This will work if the valve has not been burnt somehow - give it a shot.

Ken300D

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles

I just had my head rebuilt at M&B Cylinder Head it is in Delta park. I brought him the new valves and stem seals and he did the rest. I bought the car with the head already in the shop. He checked it over, apprently it was slightly warped. He checked it for cracks etc, suppossedly shaved the head, put in the new valves and coated it with some crap, can't remember what it was. That is all I can remember. Anyway that was $200 bucks. Don't know how much it should have been but that is what I paid. Again head was already in the shop. You have to hound the guy to get the job done but he knows what he is doing.

It may have taken so long as the head had been in there for 5 months waiting for the guy I bought the car from to pay for it.

Hopefully it doesn't come to that for you though.

Good luck and welcome to the forum. If you know nothing about cars you soon will

the automatic tranny fluid might help. it wont hurt anything.

and in the end you might have to go brians route, but it wont hurt to try adjusting the valves and driving it for a while.

did the inspectors give you actual compression numbers. "no compression" if literally true would be unusual and probably indicate a broken valve. ask them for the actual compression numbers and share with us, please.

tom w

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Thanks for the advice. Since it won't hurt, I'll go ahead and dump some ATF in the tank. I'll also look into the valve adjustment, since it sounds like that shouldn't cost me an arm and a leg. As far as the compression for the other cylinders, IIRC two were high 300's and two were low 400's. I have the paperwork around somewhere but I can't find it.

Thanks again, at least I'm armed with a little more info to proceed. Besides ATF, is there any other products that would do a better job of breaking down carbon?

Mamono

If you're in Portland you are lucky to be right close to what might be the best MB shop on the west cost. "Baring Service" 503 222 1366, Gene is the guy to talk to. They are good and know the old cast iron engs.

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1985 Euro 240D 5 spd 140K
1979 240D 5 spd, 40K on engine rebuild
1994 Dodge/Cummins, 5 spd, 121K
1964 Allice Chalmers D15 tractor
2014 Kubota L3800 tractor
1964 VW bug

"Lifes too short to drive a boring car"