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It appears the little 250ci I-6 Chevy in my '46 2-ton has seen it's better days. When I first transplanted it in, it ran great, save for what I think is a wrist pin tapping at any speed above idle*. But the last few times I've had it out, I've had a bear of a time keeping it tuned. Can never seem to dial in the idle to where it stays the same from one day to the next and the timing advance curve acting stupid. Now today I take the truck out and it stalls every time I let off the gas pedal. Going down the road it seems to run great until I put a good load on the engine. That's where the spark rattle starts.

So I limp it to my mom's, back it in the garage and attempt to turn the idle speed screw in to gain some more idle RPM's since it kept dying. It's turned in as far as it will go and the engine couldn't be running 350 rpm's!

And that's when I notice the oil. On top of the valve cover where the oil fill is. I go to the passenger side of the engine and the oil is bubbling out the hole for the dipstick tube. I pull the PCV valve and stick my finger on it. No vacuum. Not a bit. So I pull the line off the vacuum advance. Nothing. The big line directly off the intake manifold for the power brakes? Barely noticeable. I look for any flammable squirty stuff to spray around the intake manifold to check for a vacuum leak but mom was fresh out. Crap.

So now I'm thinking the engine sucked in the intake manifold gasket. That's the only explanation I can come up with for the zero vacuum condition at all the ports, but it doesn't explain the crankcase pressure.

Any thoughts anyone?



*Wrist pin "diagnosis": Top-rear of engine. Rhythmic tapping at any engine speed above idle. Noise increases dramatically on decel, especially when coasting against engine downhill or down shifting. Cannot adjust the noise away buy adjusting valves. All push rods are straight. Thought possibly collapsed lifter so replaced them all. No change. 40 psi oil pressure.

First, do you get smoke or puffing out of the valve cover when you remove the oil cap? Have you checked the oil to see if it is getting gas or a_freeze in it. If the oil is OK & you have excess blowby, you have a blown piston & no use going further here. You will need an engine o-haul.
We have other ideas but you should check those first.

Sometimes timing is affected by the distributor housing getting worn. Then the shaft has movement in the housing and the point gap will vary.
John

No water or gas in oil so thinking I can safely rule out a head gasket. Engine actually ran the strongest it's ever ran going down the road. Never know there was a problem at all (other than that off-idle tapping) until ya go to stop and the engine would stall (oh, and the mushy brakes).

The oil cap, along with a rubber grommet in the side of the valve cover below the oil cap were actually partially blown out of said valve cover when I lifted the hood, though I honestly can't say I noticed any smoke. With all the crankcase pressure, I'm guessing I knocked a hole in, or at least cracked a piston somehow. It's the zero vacuum condition that has me puzzled. I've never ran across that before on a gas engine. My plan is to pull the guy out this weekend and strip it down for an autopsy. I'll post what I find.

This actually isn't a bad thing. I was planning on pulling this engine apart to get to the bottom of the tapping. The fact that it's so sick now has just really piqued my interest!

Just out of curiosity, how far can the cylinders on one of these 250's be safely bored? It's a 1974 with the non-integrated head if that matters.

I think most engines can go .060 over on a bore.....

Use caution when going back together, the left front head bolt does not get the same torque as the rest of them. In fact, it maybe all four corners, but left front is the weakest. Once you get it apart, you will see there is very little iron under the head bolt since the water pump hole is right there. I have a broke block where the corner pulled out, I torqued it just like the rest of them. It ran OK, just always leaked coolant at the spot.

I will look around and see what the torque is suppose to be, seems my Chevy manual was different from my Pontiac manual even though they use the same engine.

Joe

That is very good to know! I was gonna look up the torque specs once I got the thing tore apart and figured out what it's malfunction is. Unfortunately, despite the great weather, my work schedule hasn't allowed me to even pull it out of the truck. Hopefully in a day or two I'll have something to report.

These are really tough engines, its going to be interesting to see what you find. Zero vacuum and still running, thats a first for me !

Joe

I broke down, pulled the engine and jerked the head off of it. Results???

Burnt intake/exhaust gasket, causing massive vacuum leak. Guessing a good portion of the air was being pulled into the engine from the failed gasket instead of the carb.

Pistons #3, #4 and especially #6 rock in their cylinders. #6 rocks roughly .040"-.050", #3 and #4 maybe half that much. Stupid tapping FOUND!

Combustion chamber for #4 is wet and possibly cracked. At the very least, a worn valve guide. (though I never noticed any blue or white smoke?)

A foreign object was introduced to #3 at some point in time. The piston, combustion chamber and intake valve are hammered to crap. Small piece of steel embedded in piston crown (chunk of compression ring?).

The piston rock indicates to me we have some egg-shaped cylinders, which means they'll need to be bored. New pistons/rings. Block and head decked. Valve guides (if the head isn't cracked). Valves. Prob cam, lifters, springs, pushrods, rockers. Gettin expensive quick, ain't it? And I haven't even pulled the oil pan off yet! LOL. Pics below.


Crap. Not where I wanted to be.

Piston

Combustion Chamber

Nice truck..... I think I'd try and find another 6 cylinder for it, than going to a V-8. Just fits the look of the truck.

John

Thanks John. I've found a couple of 250's I'm gonna go look at this weekend. At least one of them is supposed to be a good runner. Figure between the 3 engines, I should be able to put together something half-ways respectable. LOL

Problem with inlines, they cost more to rebuild the V8. Keep looking for a better engine, one thats just needs a freshening up, not a total rebuild.
That piston doesn't look to bad, my race car ate a screw at 5600 rpm, it took out four pistons and the block, I salvaged the head. Its amazing what a little piece of something can tear up.

Glad you found something, to much really!

Joe

Yeah. The one came out of an early-70's Nova last fall. 66,000 miles. Non-integrated head. Supposedly ran like a top. Pulled it to slide a small-block in it's place. The other has a couple hot-rod goodies like an Offenhauser 4-barrel intake and an "RV" cam but hasn't ran in 10 years. Figure they are at least worthy of throwing a compression gauge to and seeing what he has.