TJ 4.0 engine gurus

[mrblaine]

2006 4.0, presents with a rough idle and terrible miss. After some seriously painful diagnostics, we found a divot in the #3 piston and a bent exhaust valve that is not closing.

Several smart folks have offered up theories as to what happened, I have no idea which are correct. Anyone have any ideas how you can bend a single valve and exactly nothing else is wrong except the push rod is a bit worse for wear.

 

[macleanflood]

How does the rocker and push rod look? How about the cam?

I'll take leaking fuel injector on #3, hydrolocked the cylinder for $500. Although that might have bent the rod...

-Mac

 

[sab]

Well, Mr. Blaine, I'm far from a 4.0L guru, but the first thing that comes to my mind is a bad lifter that leaks too much, increasing valve lash enough to float the valve and cause piston impact. But again, I know very little about the 4.0L, so take that for what it's worth. Was it ticking when it ran last?

 

[mrblaine]

How does the rocker and push rod look? How about the cam?

I'll take leaking fuel injector on #3, hydrolocked the cylinder for $500. Although that might have bent the rod...

-Mac

As stated, there was a comprehensive diagnostic done and the ONLY issue that shows how the valve bent is the divot in top of the piston from the edge of the valve.

The only way you can tell anything is wrong is the valve is stuck open. No other damage whatsoever except the wobble in the push rod.

 

[macleanflood]

Well the as @sab said the only thing that could push a valve farther is a rocker.

Did the comprehensive diagnostic include testing spring pressure on the exhaust valve spring?

Or measuring the cam lobes?

Are they oem valves, pistons, springs, rockers, etc? Or anything aftermarket?

-Mac

 

[mrblaine]

Well, Mr. Blaine, I'm far from a 4.0L guru, but the first thing that comes to my mind is a bad lifter that leaks too much, increasing valve lash enough to float the valve and cause piston impact. But again, I know very little about the 4.0L, so take that for what it's worth. Was it ticking when it ran last?

Given what little I understand about this and watching the piston move with a bore scope, I'm not following along very well. Extra lash would be extra looseness which to me means that the lifter doesn't open the valve as far. It has been suggested that the lifter lost prime, collapsed, and that did it which again means I don't understand how it work since that means the lifter should be shorter which in turn means it move the push rod as far which means the rocker doesn't move as far so the valve shouldn't be able to be pushed down far enough to smack the top of the piston.

 

[mrblaine]

Well the as @sab said the only thing that could push a valve farther is a rocker.

Did the comprehensive diagnostic include testing spring pressure on the exhaust valve spring?

Or measuring the cam lobes?

Are they oem valves, pistons, springs, rockers, etc? Or anything aftermarket?

-Mac

The very exhausting diagnostic was to find the cause of the misfire. We had no indication that there needed to be a compression test until near the end of that process.

When I say comprehensive, I'm referring to things like now knowing that the data link connector needs a bus wire to communicate with the PCM in socket 12, not socket 14.

The rig is bone stock with zero other than OEM parts in the motor as it came from the factory.

 

[NashvilleTJ]

I can’t recall - is the 4.0 an interference engine?

 

[TheBoogieman]

I can’t recall - is the 4.0 an interference engine?

No.

 

[sab]

Extra lash would be extra looseness which to me means that the lifter doesn't open the valve as far. It has been suggested that the lifter lost prime, collapsed, and that did it which again means I don't understand how it work since that means the lifter should be shorter which in turn means it move the push rod as far which means the rocker doesn't move as far so the valve shouldn't be able to be pushed down far enough to smack the top of the piston.

Here's the important part of what I said:

Well, Mr. Blaine, I'm far from a 4.0L guru, but the first thing that comes to my mind is a bad lifter that leaks too much, increasing valve lash enough to float the valve and cause piston impact. But again, I know very little about the 4.0L, so take that for what it's worth. Was it ticking when it ran last?

What can happen when there's an excessive amount of lash is that at high engine speeds, the inertia and harmonics override the valve spring's capability to close the valve, letting it hang open too long - long enough that the valve is still in the combustion dome when the piston reaches TDC. When an engineer designs valve springs, the harmonic frequencies of the springs are taken into consideration to avoid harmonic issues that can cause valve float. In fact, for a while (maybe still - I've been out of racing for two decades now, and don't pay attention) F1 engines had pneumatic valve springs because they were revving so high that conventional steel or titanium springs wouldn't work.

I just randomly found this video, and the guy does a pretty good job of explaining it, but he starts at the basics, which you already know:

 

[mrblaine]

I can’t recall - is the 4.0 an interference engine?

Not even close.

 

[sab]

This may help you understand, also. Think of that cam lobe coming around, and there is .25" of space between the top of the pushrod and the rocker. As the lobe comes around, it projects the pushrod upward with some velocity due to that space that isn't supposed to be there. The pushrod now has momentum it normally doesn't have, and it smacks the rocker with more momentum. The rocker than rotates with extra momentum and thrusts the valve open with more force than normal. The valve then opens too far (assuming the springs don't coil bind), and it's still hanging out there when the piston comes up. Or, if the spring hits coil bind, the valve can actually bounce back and forth between open and close while the piston is rising.

If the lifter is the problem, you can have damage to every component in that system due to the inertial slamming that occurs at each contact point.

Edited to add: I used .25" for effect. I have no idea how much space can occur with a leaky lifter in the 4.0L.

 

[mrblaine]

This may help you understand, also. Think of that cam lobe coming around, and there is .25" of space between the top of the pushrod and the rocker. As the lobe comes around, it projects the pushrod upward with some velocity due to that space that isn't supposed to be there. The pushrod now has momentum it normally doesn't have, and it smacks the rocker with more momentum. The rocker than rotates with extra momentum and thrusts the valve open with more force than normal. The valve then opens too far (assuming the springs don't coil bind), and it's still hanging out there when the piston comes up. Or, if the spring hits coil bind, the valve can actually bounce back and forth between open and close while the piston is rising.

If the lifter is the problem, you can have damage to every component in that system due to the inertial slamming that occurs at each contact point.

Edited to add: I used .25" for effect. I have no idea how much space can occur with a leaky lifter in the 4.0L.

What you describe is what I think happened. I don't know what I don't know other than this thing floated a single valve far enough to smack the piston. Pulling the push rod only shows it is not straight, but not horribly bent either, just a bit of back and forth wobble. There is no damage we can see to the top of the lifter, either end of the push rod, or the bottom of the rocker arm where the push rod rides.

It supposedly happened while driving but the owner reports normal shifts are around 3000 RPM.

 

[Mike_H]

I'm more thinking a small burr or piece of dirt caused the valve to stick open. Its the exhaust valve on number 3, which would be one of the hottest places in the engine (so most thermal change). Kind of a perfect storm, if you have a valve on the large size of tolerance, guide on the small side and a little bit of contamination. It really wouldn't take much... Once it hangs, that piston comes up, smacks the valve and bends the stem, locking it in place. That would also explain why the rest of the valve train looks decent, because its now not doing anything. The pushrod might be slightly bent just because its now loose between its points and if it misses the rocker pocket or lifter pocket, it might put a slight bend in it.

Also not a 4.0L guru, but I've been in my fair share of old tech pushrod engines...but I might be completely wrong about this and that is OK!

 

[macleanflood]

How many miles on the engine?

3k rpm seems pretty low for valve float (unless the springs are worn out.)

And our PCM has a rev limited, right? I think I've bounced off mine a few times snow wheeling.

I think measuring the compressive force of the spring would help confirm that diagnosis.

Interesting stuff. I've watched all the Engine Masters stuff. The one thing I wished they would have done with the show is bring in a new "expert" each week to help answer the question. A single dyno session on a single engine doesn't always definitively answer the question.

-Mac

 

[mrblaine]

I'm more thinking a small burr or piece of dirt caused the valve to stick open. Its the exhaust valve on number 3, which would be one of the hottest places in the engine (so most thermal change). Kind of a perfect storm, if you have a valve on the large size of tolerance, guide on the small side and a little bit of contamination. It really wouldn't take much... Once it hangs, that piston comes up, smacks the valve and bends the stem, locking it in place. That would also explain why the rest of the valve train looks decent, because its now not doing anything. The pushrod might be slightly bent just because its now loose between its points and if it misses the rocker pocket or lifter pocket, it might put a slight bend in it.

Also not a 4.0L guru, but I've been in my fair share of old tech pushrod engines...but I might be completely wrong about this and that is OK!

Oddly, the push rod is not loose at all. I don't disagree with any of this, I just can't figure out how the valve gets down far enough for the piston to smack it on a non interference motor other than valve float.

 

[mrblaine]

How many miles on the engine?

150,000 ish

3k rpm seems pretty low for valve float (unless the springs are worn out.)

And our PCM has a rev limited, right? I think I've bounced off mine a few times snow wheeling.

Rev limits are ignored when you downshift.

I think measuring the compressive force of the spring would help confirm that diagnosis.

Maybe, I won't be the one to find out.

Interesting stuff. I've watched all the Engine Masters stuff. The one thing I wished they would have done with the show is bring in a new "expert" each week to help answer the question. A single dyno session on a single engine doesn't always definitively answer the question.

-Mac

 

[BlueC]

float the valve and cause piston impact.

Floating a valve at high RPM is the way I’ve seen this done.

at high engine speeds, the inertia and harmonics override the valve spring's capability to close the valve, letting it hang open too long - long enough that the valve is still in the combustion dome when the piston reaches TDC.

Looks like you already jumped on the how.

I'm more thinking a small burr or piece of dirt caused the valve to stick open.

I think it would have to be an interference engine to accomplish that.

I just can't figure out how the valve gets down far enough for the piston to smack it on a non interference motor other than valve float.

What does the valve stem look like? Is it clean or varnished? If varnished, that can cause a valve to stick, and they usually hang open quite a bit.

 

[mrblaine]

I think it would have to be an interference engine to accomplish that.



What does the valve stem look like? Is it clean or varnished? If varnished, that can cause a valve to stick, and they usually hang open quite a bit.

Even if the valve stuck from varnish, would it not have to be an interference motor as you referenced in order for the piston to smack the valve? Something caused the valve to move down a lot further than normal to get into the piston.

 

[BlueC]

Even if the valve stuck from varnish, would it not have to be an interference motor as you referenced in order for the piston to smack the valve? Something caused the valve to move down a lot further than normal to get into the piston.

Yes it would. Now I'm trying to think of what would allow the valve to travel further into the cylinder, or the piston to travel further up.

Also, we are assuming this is a stock engine with no significant decking, higher comp pistons or stroking.