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Posted

Moses:

 

I have another question about my truck.   I appreciate the time and attention that you have given to my comments over the years, your insight has been helpful and now I want to ask another one.

One morning this week as I was attempting the start the truck and run some chores, there was a loud clunk inside the engine and it froze.   I looked under the hood and gasoline was running out of the carburetor.  Repeated attempts to crank the engine only make the engine twitch.   Clearly something had broken inside the engine and jammed it although it wasn't frozen.

Using a bar I turned the engine backward several revolutions and then forward.  Whatever was jamming the engine released and now it would crank.   The next step was a compression check to determine the condition of cylinders and valves.   Cylinders 3 and 4 displayed lower compression than the rest; 105-120 psi versus 175-180 psi.   I had to disconnect the fuel line to the carburetor as I performed this test because it continued to overflow gasoline as I cranked the engine for the test.

After conducting a compression check, I pulled the head off the engine.   The head gasket was in good shape and valves all seemed undamaged.  Looking at the pushrods (Photo #4) I discovered the intake valve pushrod on #2 cylinder was bent, although that cylinder displayed acceptable compression when I had conducted the compression check. 
 All the cylinder walls appeared undamaged, and if fact, the cross-hatching pattern from the cylinder bore honing performed 10,000 miles ago was still visible.  There was no indication the pistons and the cylinders were damaged, including an absence of marks on the top of #2 piston.

I wondered if the valve had seized in the valve guide, but after compressing the valve spring I could easily slide the valve up and down in the guide.  If it had seized, it certainly wasn't any longer.   The rocker arms and pivot balls all appeared undamaged as well.

So now I am wondering why an intake valve pushrod bent.  This is a non-interference engine, so it seems highly unlikely the piston hit the valve, and in any case, there were no marks on the valve face or piston top to show they had struck each other.  The valve stem slides easily in the bushing and the engine rolls over without difficulty.   Over the years I have taken about 0.025" off the head, which would effectively make the pushrods longer since the valves are moving closer to the block.   But I had assumed the hydraulic lifters would compensate for minor changes in push rod length.   

You will also see the shiny wear patterns on the pushrods (red arrow in Photo #4). That shiny spot corresponds to where the push rods pass through the head.   The photograph only displays the first six pushrods, including the bent rod, but the rest of them look quite similar.    I believe this indicates the rods are clattering against the walls of their respective bore holes during engine operation.

Any thoughts on why a push rod would bend during start up?  Is this a freak event, or is there an underlying reason for it?  Is the wear pattern on the push rods a problem?

 

Cyl 1 &2 -b.JPG

#2 Cyl Valve Springs -1.JPG

Head- cylinders 1 & 2.JPG

Push Rods 1-6.JPG

  • Administrators
Posted

Mr. Rex...You're being very thorough and painstaking in your troubleshooting.  A bent pushrod, as you hint, means too much resistance.  Since you ruled out valve interference with the piston crowns, it might seem reasonable that the lifters have not "pumped up".  I would still check out the lifters even though 0.025" milling would not cause major concern on an engine that has adjustable rocker arms for setting lifter clearance or "preload".  If a lifter(s) has pumped up, it could cause valve spring bind, which can bend a pushrod.  The resistance is at the valve spring in this case.

Given no piston interference marks and no valve stem seizure in the guide, I would examine the lifters, valve springs and the rocker arm relationships with the valve stems.  Sometimes, due to valve seat grinding, cylinder head surfacing, block decking or valve stem tip grinding, the rocker arm tip angles tilt to the degree that the rocker end binds against the valve stem.  Check the rocker arm tips where they contact the valve stems.  Watch the range and angle of rocker arm movement during camshaft lobe lift.

I'm focused on valve spring bind.  Some shops and DIY head builders use shims with stock valve springs to bolster spring tension.  This can compromise valve spring "height" and permit the spring to bind when the valve opens fully.  A long shot but worth a peek.  More likely, the cause would be a pumped up/defective lifter that extends the lifter plunger, holds it there, and causes spring bind due to loss of preload/clearance.

Judging by the degree of pushrod bend, "something" caused the rocker arm or valve to stop moving.  The valve/rocker either did not move or couldn't move far enough.  The result is a severely bent pushrod.

Check off the items mentioned, including valve and rocker arm relationships and angles that could create rocker arm bind.  Valve spring bind either from "short" valve springs or pumped up lifters are a strong possibility.

The 250 is chock full of 283 Chevy V8 parts and dimensions, many interchangeable parts, so rules that apply to a small-block V8 can also affect your 250 inline six.

Let us know what your find...

Moses

Posted

Interesting comment about valve spring bind and the use of shims under stock valve springs.   If you look at the photo that I sent of the valve springs you will see shims peeking out from under the springs.  Most of the springs have shims under them.  As I recall, the springs were not replaced during the engine overhaul, but simply shimmed to achieve their desired height and stiffness. 

I will take your advice and remove all the shims, but I did have a question: if the shim is the underlying source of the problem by causing valve spring bind, why did it last for 10,000 miles before manifesting itself?  Wouldn't have been a problem immediately after starting the engine?

I also removed the first side plate and examined the particular lifter that was under the bent push rod.  It slid right out of the bore when I pulled on it.  The lifter looked almost new without any varnish or discernable wear on it, which is not surprising since it only has 10,000 miles on it.  The cam lobe, or what I could see of the cam lobe, looked undamaged as well.  If the lifter was defective or had pumped up, would that be evident when I examined it days after the incident occurred? 

Thanks for your time and attention with your responses. 

V/R,

Rex

Posted

Interesting comment about valve spring bind and the use of shims under the stock valve springs.   Most of the springs were shimmed, and if you look at the photo that I sent earlier you will see them peeking out from under the springs.  As I recall, the springs were not replaced during the engine overhaul, but simply shimmed to achieve their desired height and stiffness. 

I will take your advice and remove all the shims, but I did have a question: if the shim is the underlying source of the problem by causing valve spring bind, why did it last for 10,000 miles before manifesting itself?  Wouldn't it have been a problem immediately after starting the engine?

I also removed the first side plate and examined the particular lifter under the bent push rod.  It slid right out of the bore when I pulled on it.  The lifter looked almost new without any varnish or discernable wear, which is not surprising since it only has 10,000 miles on it.  The cam lobe, or what I could see of the cam lobe, looked undamaged as well.  If the lifter was defective or had pumped up, would that be evident when I examined it days after the incident occurred? 

Is it possible a backfire in the manifold could have slammed the intake valve shut as I was cranking the engine?  Gasoline was slopping out of the carburetor when I opened the hood to see what the matter might be, which indicates that manifold pressure had suddenly spiked.  Could this have disrupted the valve train enough to allow the push rod to pop out from under the rocker and the subsequent engine rotation bent the push rod?  

Thanks for your time and attention with your responses. 

V/R,

Rex

  • Administrators
Posted

Mr. Rex...I see that you added information to your second reply, so I'll answer it here.  See my blue highlight text below:

4 hours ago, Mr Rex said:

Interesting comment about valve spring bind and the use of shims under the stock valve springs.   Most of the springs were shimmed, and if you look at the photo that I sent earlier you will see them peeking out from under the springs.  As I recall, the springs were not replaced during the engine overhaul, but simply shimmed to achieve their desired height and stiffness. 

Common practice, saves the cost of new springs when spring seats are ground or springs are suspected of being weak...from wear and heat.  Some shops arbitrarily shim springs, but the right way to do this is to test the installed spring heights and bench test spring resistance pressure with a tester.  Bad springs get tossed.

I will take your advice and remove all the shims, but I did have a question: if the shim is the underlying source of the problem by causing valve spring bind, why did it last for 10,000 miles before manifesting itself?  Wouldn't it have been a problem immediately after starting the engine?

There is a problem with removing spring shims:  the possibility that the extended spring lengths/heights will be weak and below the test specification for proper valve seating.  Reman shops simply install a full set of new springs.  This must also include setting correct valve seat depth and valve "margin" thickness to be sure the valve keepers set at the right height;  this assures sufficient valve spring height and pressure.  Envision deeply cut valve seats and thinly re-ground valves:  The valve stems and keepers stand high above the spring seat, and this reduces the valve spring pressure.  Valve won't seat properly and can "float".  That's the same as valve float at higher rpm.

I also removed the first side plate and examined the particular lifter under the bent push rod.  It slid right out of the bore when I pulled on it.  The lifter looked almost new without any varnish or discernable wear, which is not surprising since it only has 10,000 miles on it.  The cam lobe, or what I could see of the cam lobe, looked undamaged as well.  If the lifter was defective or had pumped up, would that be evident when I examined it days after the incident occurred?

The lifter pumping up would be holding the lifter plunger at full, extended height (against the retainer spring clip).  It's hard to discern this after a pushrod bends, however, because the pushrod is now short.  The lifter would naturally extend its plunger because there is nothing to establish the right clearance/preload on the plunger.  Pressurized oil would simply "pump up" the plunger.  

Is it possible a backfire in the manifold could have slammed the intake valve shut as I was cranking the engine?  Gasoline was slopping out of the carburetor when I opened the hood to see what the matter might be, which indicates that manifold pressure had suddenly spiked.  Could this have disrupted the valve train enough to allow the push rod to pop out from under the rocker and the subsequent engine rotation bent the push rod? 

What you describe is possible but different.  You described a "clunk", not a backfire, then having to rotate the engine backward to relieve the "bind".  If you follow this clue, cylinder (#2) may have filled with so much fuel that the engine locked up without a backfire.  This is similar to hydrolock with either leaking coolant or water sucked through the intake (deep stream/creek forging).  A backfire would be less likely to prevent an intake valve from opening because normal valve overlap between the exhaust and intake cycle has both valves open.  However, if your engine hydrolocked with gasoline, piston and liquid (gasoline) resistance against the valves could bend the pushrod of the intake valve trying to open.

I would not remove the valve shims.  On the bench, test the valves for leakage by putting solvent in the ports with the springs holding the valves closed.  I would replace the bent pushrod(s), fix the carburetor, start the engine carefully and adjust the valves by the book.  See how the engine runs.

Thanks for your time and attention with your responses. 

V/R,

Rex

 

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