Repeat bent pushrod on single cylinder
Vehicle was diagnosed with a dead misfire on cylinder 4. Engine would backfire through the intake at times while driving. I won't get into everything that we have done to diagnose our source issue, just the highlights of the diag process. A compression test revealed cylinder 4 read lower than all other cylinders. Removed passenger valve cover to perform a visual inspection and found the purshrod for the intake valve was bent. Technician removed pushrod, manually pushed down on the lifter and felt little to no resistance. Cylinder head was removed, lifter was removed and found to be stuck collapsed. MDS solenoid was removed from under the intake. The pickup screen was found to be clogged with carbon/ usual lack of maintenance. Oil passage from MDS to lifter was inspected and cleaned. The MDS solenoid, intake lifter and all four intake pushrods were replaced. Cylinder head was sent out to a machine shop. No issue was found with the head. Assembled vehicle and test drove. Backfire returned after a short 5 mile drive.
Misfire returned to cylinder 4, compression test found lower compression on cylinder 4 again. We performed a dynamic/running compression test this time. Found slightly lower running compression when compared to other cylinders. Note: We did not do a snap of the throttle when checking running compression. Removed head, we performed a visual inspection of valve train, pushrods, lifters, etc... Couldn't locate an issue. Sent head back to machine shop thinking something was maybe missed. Again they stated nothing was found but offered to do a valve job just to be sure. Received cylinder head back, assembled vehicle and started engine. Misfire was still present on cylinder 4, backfire at times. Technician let vehicle idle while inspecting data on scan tool, performed a couple of throttle snaps and suddenly engine starting running PERFECT. Great idle and throttle response. The vehicle idled in the shop for over an hour like this. Performed final test drive, misfire returned after about 7 miles.
Misfire returned to cylinder 4. Again performed a compression test, found 4 was lower. Performed a dynamic compression test, running compression was 10 psi lower on cylinder 4. Performed a snap and found reading 105 psi lower. ( No air getting into cylinder four.) Removed valve cover and found pushrod for cylinder 4 bent again.
We have not removed the lifter or cylinder head yet to inspect condition of lifter assembly. Oil level and condition is good. Vehicle has 191233 miles. Any insight would be much appreciated!
Check rotational lock guide for that lifter. When they wear the lifter turns 90 degrees leading to the symptoms you describe. Hopefully you can use a bore scope/camera to check my theory
We removed the head, inspected both lifter assemblies and couldn't locate a sole reason for the bent pushrod. Both of the lifters were installed correctly, flat sides were all in the same direction for that bank. We have no visual signs of oil starvation on the rocker, pushrod or lifter.
We have not measured cam lift but visually all lifters appear to move in the same amount. Is it possible the lifter could stick on that lock guide at times and cause the lifter to not push all the way back down? There is a little wear on the contact side but as stated when we removed the guide all lifters were set.
I haven't worked on Dodges since the '70s, but if the engine uses pushrod guides, I would ensure that there is no wear (excessive side clearance) that allows the rocker arm to rotate and/or if the rocker arm has a recess where it contacts the valve, to maintain alignment over the valve stem tip.
This would happen on Chevy small block engines in which the holes in the cylinder head served as pushrod guides. If a rocker arm can rotate sideways, any upward movement of the pushrod can result in sideways rotation of the rocker arm, bending the pushrod.