I had a customer bring in a nice 300GD 460 Gwagon last week with a low performance problem. I revved it up and it smoked out the neighborhood. I removed the oil cap and it idled with a chuffing sound and puffs coming from the cam cover. "Oh,well" I thought,"a ring job at the least and a broken piston at the worst " .
I removed the head and put it aside, and then popped the pistons out. All five cylinders were badly glazed with zero cross hatch left and the rings,although free were worn so much they sat below the ring land height..in the groove.
The bearings are perfect and look like new after 200,000 kms. So I have deglazed the bores and have new rings coming.
So..I decided to check the head and the usual stuff. When I undid the #5 exhaust valve spring the valve stem was all floppy, the guides are all locked to the stems and are moving up and down in the head. This was the cause of the chuffing sound and the blowby as the exhaust gasses were directed up into the cam box.
The cause as far as I can make out is the previous owner was driving around with bad injectors and taking too many short trips . 200,000 K's over nearly 30 years aint much and the car lived in the snow country in australia .
The wear in the rings allowed a lot burnt oil residue to get stuck to the valve stems and this caused them to lock to the guides pulling them free in the head.
What is surprising is the owner had just completed an 18 hour drive in it and said he was averaging 100KMH (62mph ) for most of the trip. There was no tappet noise and the only odd sound was occasional nailing tap at idle . The rest of the time the car seemed to run OK although slow and smokey.