Vintage Mustang Forums banner

1 - 7 of 7 Posts

·
Registered
Joined
·
895 Posts
Discussion Starter #1 (Edited)
Warning: wall-o-text
Summary: bent pushrod, no clue why
$16,000 question: CAN a pushrod bend on its own?

351 cleveland 2v with hydraulic lifters, mostly or all stock.

Two weeks ago while driving (normally) the engine started missing really violently. After the obvious checks, I did compression testing, 150+ psi (cold engine) on 7 cylinders, 20 psi on cyl 6, added oil, 50 psi. Removed the valve cover and found the exhaust valve spring had broken, wrapped itself up some, and the lack of travel forced the pushrod to bend.

Yesterday, replaced the valve spring and pushrod, set valve lash and verified it ran true and strong. Drove it home 12 miles in medium traffic, some on/off throttle, engine ran perfect. It's worth noting that I used a leakdown tester to seat the valve while replacing the spring, it held (what I presume to be) normal pressure, the needles scaled the same, at 100 psi the leak dial showed close to zero.

This morning, after a couple blocks, it all of a sudden starts missing again. I reluctantly keep driving to work because at work is the only place I can get it on a rack with proper tools and diagnose it (I work at BMW Seattle as a tech.). Get the valve cover off and the rocker on exhaust cyl 6 had worked loose and the pushrod jumped off the rocker. This I am almost certain is my fault, I didn't have enough time left in the day putting the engine back together to properly let the engine warm up before setting valve lash (hydraulic lifters), I set it cold and my inexperience bit me Im sure.

The bigger concern is the intake valve pushrod on the same cylinder is all bent to ****. It appears that something caused it to bend, in turn the small opening for it in the head caused it to snag and bend even further (looks S shaped). Performing another leakdown test, the valves appear to be ok, CLT still showing 0-5% at 100 psi, only air heard from the crankcase.

Running out of time and with no spare parts tonight I put it back together minus the cylinder 6 intake and exhaust rods and rockers. Driving it out of the shop it sounded near normal, like a normal 1 down cylinder on a v8 should, no problems on other cylinders.

I'm looking for speculation on what people may think took out the intake pushrod, logically I have no clue what did it. My next planned step is to replace both pushrods (for safety, even though the new one doesnt appear damaged), and with a dial indicator make sure my cam lobes appear good, not sure how to verify the lifters are OK though. I can't see how there would be any valve/seat damage without showing up on the leakdown test. I can't think of anything that would have logically caused the pushrod to bend other than it just failing on its own coincidentally. Also how can I verify proper hydraulic lifter operation (plunger movement at cold and hot).

I'm trying to do whatever possible to avoid taking the head off as it's my daily driver and my only car, on top of that, I only truly have 1 day every week where I could have the car in my shop to do that kind of work. (operating work hours have to go to the customer cars, darn lol).

To those who have adjusted valve lash on hydraulic lifters, what kind of resistance should I feel when setting the lifter preload. Granted I messed up and set it cold, but I don't know how hard it should be to turn the rocker bolt to compress the lifter plunger. I thought I was doing a proper adjustment aside from the engine being bone cold. I tightened down the rocker until I could no longer turn the pushrod with my fingers. I then observed how far I could turn the bolt potentially (I assumed I was compressing the plunger at this point), backed it back off and then tightened the bolt by half the potential angle. I was confused at the time because the total angle I could turn (until it felt like it was bottomed out) was only about 60 degrees, so I really only tightened the rocker by 30 degrees past pushrod freeplay, but of course the engine started up and ran strong so I thought nothing of it. Should the lifter plunger be THAT hard to compress when preloading? I work on cars, it honestly felt like it was bottoming out so I didn't try to continue turning the bolt.

I appreciate any and all help, Im trying to get the car running again as soon as possible as its my DD. I hope to get my hands on replacement pushrods and a dial indicator tomorrow or monday.

Big question of the day, can a stock pushrod bend on its own? If yes, then Im overthinking this and it's failure is coincidental to the other problem. Seems unlikely but Id sure love it if that were the case.
 

·
Registered
Joined
·
41 Posts
A bent pushrod is caused by a mechanical failure, cracked or worn rocker arm, collapsing lifter, loose adjusting nut, etc...

The most probable cause? worn camshaft lobe. I would check the cam lobe lift on every cylinder.
 

·
Registered
Joined
·
10,287 Posts
It seems to me that to bend a push rod it takes a seized valve.

I would check to be sure the valve is not binding and is not stuck.

I suggest taking the head off and disassembling and examining the valve.

If the valve is sticking you surely have caused other damage.
 

·
Registered
Joined
·
7,103 Posts
I tend to agree with PPrince, but, before pulling the head, pull the intake. Remove all of the plugs. Renew the PR, re-set lash. Crank the engine over with the starter to watch the valve/lifter/rocker action. You may have to pull the intake anyway, but you may get lucky and see an issue with the lifter, before moving on.
I can understand an exhaust valve sticking but not the intake.
Good Luck!
 

·
Registered
Joined
·
1,184 Posts
Mostly stock? are you using studs in the rockers?
A stock 351c uses apositive stop rocker so the is no valve lash adjustment.
Pushrod lenght is what is used to change preload.
If the exh rocker dropped off that could have caused the intake to bend the pushrod from compression not allowing the valve to open.
Replace both pushrods and lock rhe rockers down and retest.
 

·
Registered
Joined
·
895 Posts
Discussion Starter #7
A stock 351c uses apositive stop rocker so the is no valve lash adjustment.
Pushrod lenght is what is used to change preload.
Ahh bingo, I did not know this. The rocker was definitely not torque'd or even tightened not knowing this. One thing I also noticed is I somehow happened to order a .062" undersized pushrod, and the stock rod was too faded to notice the correct part number. So the valve wouldnt have been opening all the way even if I did get the rocker down fully anyway.

If the exh rocker dropped off that could have caused the intake to bend the pushrod from compression not allowing the valve to open.
Replace both pushrods and lock rhe rockers down and retest.
Right again, I totally forgot about the scavenging cycle that would have held the intake valve closed.

Got the correct replacement rods in, luckily stocked locally (they were scheduled for recall/disposal as they were on the autozone shelf too long lol, but I got a manager to help me out) and its running right again.

Thanks for the insight
 
1 - 7 of 7 Posts
Top