Severe Blow-by w/only 104K miles!

I sold my '94 K2500. Current: 1993 C3500 6.5 Auto 4x4 1-ton SRW 104K miles. Obvious oil drips/seepage on chassis & turbo area. Have put nearly 2K miles on it. I recently purchased vehicle in South Carolina and on 12/13/2025 I was driving back home to California. At stop sign I noticed plumes of smoke. Idle was less than smooth. Observed blow-by coming out of some type of boss (with a machined hole about the size of a nickel) on driver side just behind crank pulley, (Initially I thought: blown head gasket due to the puffing/pulsating pattern) as well as dip stick and valve cover grommet at CDR. I placed oil fill cap on top of fill tube and it stayed in place! Minimal blow-by out of fill tube! Although drivable, I had the vehicle towed 316 miles$$$$ from Alabama back to SC. (Atlanta traffic is worse than Socal/NorCal).

Seeking a diagnosis and advice. THANKS!

With so few miles,we'd need some (a lot) more info on its life story. It might be a fine candidate for the MMO (Marvel Mystery Oil) test, to free up rings, valve guides, lifters, etc. If it came on suddenly and isn't making new noises, it may be just as simple as that.

The "Boss" on the drivers side near the balancer ?????

This would be the port for the crank position sensor...
A 1993 engine should not have this......
After this many years the engine may have been swapped

There should be a threaded hole on that boss area that held the sensor in.
Plug the hole with a used / junk sensor and screw in a bolt...(These engines use metric bolts so be advised when selecting a fastener)

Getting that hole plugged will stop the issue.

Like Maverick said.....Some Mystery oil is worth a try....

With the oil cap off there should not be a huge amount of blow by.....Raising the rpm to about 1000 and the vapors should go away....

Plug the hole in the cover...

Same family since new. Grandpa sold it grandson. Grandson claims engine is original. I drove it from SC to Florida and back towing a trailer with a car aboard and had zero issues. How much MMO should I add to the crank case? And how many miles should I drive before draining? THANKS!

I agree about a '93 not have such boss. What year would apply? Grandson claims original engine. How much MMO to add to crankcase? Thanks!

I suggest following the MMO label instructions. I haven't used it lately, but have no negative experience with it years ago. Sea Foam is another option with similar application instructions, but I wouldn't use both at the same time in the same manner (not both in crankcase or both in fuel).

An original CPK boss on a late 1993 would not be unusual. The cover may have been a preparation for the upcoming EFI 1994 models. It should not be leaking, though. It should be an unbored boss. As well, it wouldn't be unusual for an original engine to have been serviced and the timing cover replaced for a variety of reasons, which is common during water pump or timing set service. Depending on the time of service, or availability of parts at the time, a later model timing cover may have been used. They are backward-compatible.

Another possibility for the driver side would be the timing pickup holder, but it should have no connection with the crankcase, and nothing to leak. It's possible that either could simply be a collection area for an oil leak from another source.

OK. Thanks again. I'll try the MMO.

Update: Have yet to attempt any remedies, however, I just started engine and observed puffs of smoke out of filler tube. Definitely blow-by coming out!