Cast iron heads can & do crack -- a problem that is more prevalent, yet apparently not limited to, the aluminum heads

This post is meant primarily as a follow-up to prior threads about recurring hot running & rough running problems with my 84 300D (now with ~206,000 miles). I hope that people searching through the archives looking for information may learn from my experience.

A (not so) brief recap: during hot weather last year my car exhibited signs of running hot, yet not severely overheating. A thorough refurbishing of the cooling system helped to reduce the temperatures, but it still ran warmer than normal -- usually in the range of 87 degrees C and up to a maximum of 102 under heavy loads and high ambient temperatures.

Diagnostic procedures were inconclusive. Compression was very good with only the two rear cylinders showing any signs of deviation, both being approximately 20-25 PSI lower than the front three. Doing a leak down test (audible only) indicated no problem.

One time, and one time only, I noticed just the slightest hint of an oily film floating on top of the coolant in the reservoir -- not even enough to form a droplet. I correlated this with the lower compression on the two rear cylinders and concluded that the head gasket was beginning to let go. Although it is not generally considered standard procedure to do so, I re-torqued the cylinder head bolts & thoroughly flushed the cooling system. There was never again any hint of oil in the coolant. I knew that this was a temporary fix, just to buy some time.

More recently it had developed problems with misfiring for a few seconds upon startup, difficulty starting while hot, accompanied by excessive gray smoke (from partially burned fuel -- not steam). Being that the compression is good, & I was not losing any coolant, this seemed like a fuel delivery problem. I resealed the delivery valves in the injection pump, checked pump timing, & installed new (rebuilt) injectors to no avail.

One thing I had ignored is that the cooling system remained pressurized after sitting overnight. I was not losing any coolant, and I had read several threads where people had indicated that their diesels did the same thing & this was normal -- nothing to worry about. This always seemed wrong to me, however being a novice with diesels I simply let it go as being an idiosyncrasy of the 5-cylinder diesel engine.

Then about a month ago I did start losing coolant, albeit slowly. There were no external leaks so it was obvious where the coolant was going. There were no indications of coolant contamination in the oil -- the coolant was going into the combustion chambers while it was running, but the system would still remain pressurized after sitting overnight. I figured that the combustion pressure pushed its way past the gasket into the cooling system, but it was still tight enough that the relatively lower pressure in the cooling system didn't push the coolant back through to the cylinders, at least while it was not running, thus the remaining pressure in the morning.

As much as there is one neighbor that I don't mind pissing off too much (:p), it became rather embarrassing to fumigate the neighborhood (especially when it isn't mosquito season) -- so due to the excessive smoke problem & now the coolant loss, I decided to bite the bullet and pull the cylinder head. Boy, was I in for a surprise. The cylinder head had numerous cracks -- in between intake & exhaust valve seats, and also between valve seats and the pre-chambers. Needless to say, that casting has no value now, other than as a boat anchor.

So… a word of caution to those of you with a cooling system that remains pressurized overnight – it’s NOT normal. By ignoring it, you may cause more damage in the long run. I believe that in my case the head gasket started leaking into the coolant -- this accounted for the residual cooling system pressure in the morning. By ignoring this symptom I created a chain of events that led to greater damage. My hypothesis is that the combustion gases forcefully displaced the coolant in the cylinder head, leading to localized overheating, which in turn caused the severe cracking of the cylinder head. IF I had not ignored the ‘residual pressure in the morning’ symptom, I may have gotten away with a simple change of the head gasket (personally, I would do a valve job once the head is off no matter what) but instead, my cylinder head had to be replaced. Oh well, live & learn… I just hope that this can save other people from having to learn the hard way.

RTH

P.S. I've obtained another head, had it completely rebuilt, and I will be installing it during the next week along with a new timing chain, rails, various seals, all the small coolant hoses, etc., etc.