Another misfire in the Cobra!
 

I was out shopping today and was almost back to my house when the Cobra developed a misfire and a tapping noise in the valvetrain. When I got home I pulled the valve covers which requires removing the upper intake manifold and discovered that one of the rocker arms broke. To be precise it was the exhaust rocker on cylinder 3. To my surprise the pushrod was fine. However, I pulled the intake rocker off the same cylinder and found that the pushrod was bent. So obviously something happened to cylinder 3 but I have no idea what as of yet. I went by Phil's house tonight and picked up his cylinder leak down tester to help determine what else is wrong. If I'm lucky the whole cylinder will be F&#%ed so I will have an excuse to build a 331 so that Eric's car won't be faster than mine.

Holly crap !

damn that sucks lol is it a 4.6 or 5.0

It's a 5.0.

Holy crap is right. The rod is trying to turn itself into the letter S.

oh ok and haha yea it coulda been alot worse tho

It might still be. I still don't know what caused the valves to bind up enough to break a rocker and bend a pushrod. I'm going to do a leak down test tomorrow and go from there.

That's kind of what I'm use to seeing. The shocker was the broken rocker. The only time I've broken a rocker before this was when my dad and I first built the 396 in the Chevelle when I was 17. The guy who built the heads didn't set them up for the lift on the cam we had and the springs went into coil bind and pushed the pushrods right through the back of the stock stamped steel rocker. We then had the heads redone and my dad bought the Crower roller rockers that are still on the car today.

This morning I pulled the plug from cylinder 3 so that I could run the leakage test. The plug looked fine so it doesn't look like anything was inside the cylinder. I did the leakage test and it only has 10% leakage. Not bad for having 80,000 hard miles on the engine. So going to order a new rocker and pushrod and put it back together.

Lucky. Did you see anything wrong with the piston at all? Not sure if you have a borescope or not. Or even just looking in there with a flashlight.

I can't get down there to look into the spark plug hole and I don't have borescope. However, I did compress the valve springs down to make sure they moved freely. The 10% leakage tells me that there isn't a broken piston ring, blown head gasket, or even a hole in the piston. I'm not sure where the piston is at relative to the stroke but I probably should make sure it is at bottom dead center and do the leak down test again. That way I can make sure that there isn't a crack in the cylinder wall.

That's actually 16% leakage, but still good.

I think he's on the road, but big2bird has a bore scope if you want to take a look.

I ordered the new rocker arm and pushrods. I'm just going to bolt it back together when they come in and see how it runs.

If there was a crack in the cyl wall or the piston it would puff air out of the rocker cover like a steam engine.

Can you see the lifters?

When I first assembled my motor I had pushrod guide plate clearance issues. Check yours to make sure they haven't shifted, causing binding.

I thought about that later. I don't have guide plates because I have shaft mount rockers. Take another look at the first picture.

did you inspect the lifters and that portion of the cam to see what could have caused the bind? One lifter might have extended out of the block and thats what bent the pushrod or snapped the rocker, i would investigate further while you have everything apart.

The lifters are fine. I can't see the cam and I don't want to take the front end apart to pull it. I'll put it together and then hand cycle the engine over a couple times to make sure it all looks good. If it does then I'll put the intake back on and do a compression test as well as another leakage test. If they both come out good I'll start it up and see how it runs.

This is so strange. I can't visualize what might have happened.

Phil and I are at the same point. It broke the exhaust rocker and bent the intake pushrod but everything seems fine. The valves move freely and are sealing so they're not bent. The leak down shows that the rings and piston should be fine. I'm just hoping what happened is that the rocker was fatigued and broke and some how caused the intake to bind momentarily which bent the pushrod.

Now I'm thinking you had some intense valve float, which allowed the pushrod to escape from the rocker, causing it to bind and break the rocker...

I did not read Adam's link but what if when the rocker broke it somehow hit or got lodged by the intake rocker and did not allow the intake rocker to move through the entire movement for a cycle of 2 and caused the pushrod to bend.

Possible?

Yes, that's possible. I'm really not that worried about it. I think everything is okay and that I'll be able to put it back together and run it.

Pushrods and rocker arm came in yesterday. I have people telling me that the leakage percent seems high and that I should check some other cylinders so I'm going to try and do it today if I can.

I read that other thread and agree that the rocker arm had to break first like we talked about. Also I never said that 30% was what I normally see on an 80,000 mile motor. I normally see 12% to 20% but anything under 30% the cylinder should be fine. On a burned or tuliped valve I see 60% or more. If the valve were bent or the piston broken it would be even higher. It is always good to check against the other cylinders to see where they are at. The bottom line is that 16% is an acceptable leakage. If the cylinder next to it only reads 10,12,13% what difference does it make. Put the rocker arm and push rod in it, fire it up, and drive the shit out of it for another 50,000 miles.

I didn't know Throttle Crazy was your brother, and you've had your car to 175mph? Holy crap.

Yes and yes.

It's amazing what some of the aftermarket cars on the road can do. We should have more super street challenges with street driven tuner vehicles.

Well I got the Cobra put back together and it's running great again.

:beer:

Take at least 1 car off the Motorgen "Not Running" List! :sm_laughing:

Add a few more months to my car :(

Why?

Pushrods were back ordered a few more weeks, and i need more dental work

So I think this happened again. I ran to the store last night and on may way back something let go. The engine has a knocking sound but I can locate from where and I'm down a cylinder or two. I'm hoping that I just broke another rocker but we'll have to see.

Time for a 347...

Fixt

331 if the bottom end is an issue but I don't think it is. Otherwise it will just be some porting on the heads and intake and then to Pacific Engine for the valve job and new springs.

Too much weight, too much of a combo change