1990 XJ 4.6L Stroker Valves Seizing

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JordErik
Posts: 3
Joined: July 2nd, 2021, 8:45 am
Stroker Displacement: 4.0
Vehicle Year: 1990
Vehicle Make: Jeep
Vehicle Model: XJ

1990 XJ 4.6L Stroker Valves Seizing

Post by JordErik »

Hey guys

First of all, id like to give a shout out to Russ Pottenger for all of his help with my build!

I feel bad being "that guy" who makes his first thread a "please help" thread, but here we are.

My build is using:
1990 4.0L block 0.030 over
4 counterweight crank from an 88 4.2L
7120 head cut for LS valves
Qualcast 21-2N valves in the head
Russ's 0.030" pistons
Scat 6.125" rods
Hastings rings
Comp 68-235-4 cam
Comp 9.600" 0.080" hole pushrods
New stock stamped rockers
Standard Melling oil pump
Double roller timing set
HO throttle body bored 60mm through
99+ improved intake manifold
Bosch 0280155746 injectors
AEM wideband gauge to watch air/fuel

Ive been having issues with valves seizing/sticking in my stroker build. The problem arises when ive taken it out for the first drive after assembly. The first time, i started the engine and did the cam break in, it got a little hot (vapor locked) so we shut it down and burped some air out of the system, let it cool down, topped up the coolant and finished the cam break in and it never got above 100°C. I took it out for a drive a few days later, air fuel ratio was rich (12-13:1) except when idling (around 16:1) and after 3-4 mins it stalled and died, wouldnt idle and was shaking and knocking when i tried to start it. Towed it home and started disassembly and found that exhaust valve #5 was seized almost 1/4" open, cylinder walls in #1, 3 and 4 were scarred and later found out the cam bearings were half eaten.

The machine shop rehoned the block, put new cam bearings in and reamed the valve guides 0.003" (he previously reamed them 0.002"), and put new exh valves in because the other 5 were starting to seize as well, and i reassembled the pistons with new rings.

I found out that the cam bearings were eaten because of cam walk after reading SilverXJ's build thread (many thanks to SilverXJ for documenting his build) so i turned a nylon pin/button for the cam and had the end play dialed in to 0.008". New rings on the pistons were filed to a 0.025" gap and deburred. I swapped in a 49psi fuel pressure regulator to avoid any lean condition for the 2nd attempt, and with my injectors the flow rate worked out to 21.1 lbs/hr. By the time i was ready for the 2nd start up, my Renix Engine Monitor from Nick Risley arrived, so i was able to use that to run engine off of the wideband o2 sensor (found out my stock one was dead) and got her running properly around a 14.5:1 air/fuel.

The start up went great, ran well, got her up to temperature and no overheating at all, oil pressure around 40 psi at idle and around 60 with throttle. I left the front end jacked up for 4 days while i was back at work to burp out any potential remaining air. So, on the first drive since the 2nd assembly, i got about 10 mins away before it stumbled and died like last time. The block temp never got above 92°C but the temp gauge from the sender in the head jumped up around 110°C right before it died. I rolled to the side of the road and let her cool down for 30 mins while i pulled the spark plugs. She started up and was running well enough to limp it back home, but the valve train sounded pretty noisy now. I scoped the cylinders and #2 is scarred up now.

The machine shop said he would ream the valve guides out another 0.0005" to 0.0035" when i bring the head back and he thinks its a lack of oiling issue with the valve train, but i saw that all rockers were full of oil when i pulled the valve cover and checked the torque on the head bolts before the drive.

My plan for the 3rd attempt will be to ream the valve guides bigger. I did the open cooling system swap, but kept my old thermostat housing, so ill use the later one with the 3/8 npt bung and put the temp switch for the e fan in there so that it responds much quicker if the head gets hot. Run a bottle of Water Wetter in the coolant. And any advice you guys can provide will be of much help.

My questions are, how big do you think we can go with reaming the valve guides? I know the spec is 0.001-0.003 clearance, but what measurement would you not exceed?

Do you guys also think its an oiling issue with the valve stems in their guides? Or a heating issue?

Let me know if ive left out any relevant information, or photos that would help. Thanks for reading.
I6FAN
I made it to triple digits!
I made it to triple digits!
Posts: 167
Joined: March 28th, 2010, 9:31 am
Stroker Displacement: 4.2
Vehicle Year: 1987
Vehicle Make: jeep
Vehicle Model: wrangler

Re: 1990 XJ 4.6L Stroker Valves Seizing

Post by I6FAN »

I believe there was an oil pump fitment issue as well - the block flange surface had a machining ridge that interfered with the aftermarket oil pump seating properly. Sounds like the valve seizing might be an effect of something else, and not a direct cause(?). How was the valve train geometry?
JordErik
Posts: 3
Joined: July 2nd, 2021, 8:45 am
Stroker Displacement: 4.0
Vehicle Year: 1990
Vehicle Make: Jeep
Vehicle Model: XJ

Re: 1990 XJ 4.6L Stroker Valves Seizing

Post by JordErik »

I made sure to check the oil pump mating surface when i installed it and i didnt find any interferences. I ordered the shorter push rods after i found that my lifters were preloaded approx 0.100", they were still a little too much so i shimmed the rockers with some shim stock turned on the lathe. After everything looked good i marked the valve stems with a sharpie and turned the crank through a few revolutions and they all looked good, rocker arms were rubbing the sharpie off the middle section with a little variation from valve to valve.
cstephens85
Noob
Noob
Posts: 19
Joined: December 26th, 2018, 8:19 am
Stroker Displacement: 4.6L
Vehicle Year: 1988
Vehicle Make: Jeep
Vehicle Model: Comanche

Re: 1990 XJ 4.6L Stroker Valves Seizing

Post by cstephens85 »

Unfortunately, I don't have any advice, but wondering how are the cylinder walls getting scored if there is an issue with the valves/valve guides?
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