New Cam, Timing Chain & Head. #1 No Compression

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WobWokwood

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May 8, 2021
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Tinker AFB, OK
Greetings.
I have a 1989 YJ 4.2 L with TBI. It was my daily for about 2 years, the. I had it shipped. Jeep didn’t run right, I think the distributor moved somehow and I bent a few valves and pushrods.

Head removed, took to the machine shop. All new valves installed. I also swapped the camshaft and timing chain. Now I have no compression on number one and I’m wondering if I’m 180 out on my cam.

After building it back up, trying to start, backfiring and flamage coming through the throttle body. Not trying to bend valves again.

I ran compression on number, absolutely nothing. Can’t find TDC to begin timing if I have no compression. Pissed if I have to tear it all apart again but I’ll do it to make it right.

I’m an aircraft mechanic by trade, this was my first engine pull and reinstall.
 
If im not mistaken the 258,2.5 and 4.0 are non interference engines. That being said slipped or a broken timing chain or gears should have not caused valves and pistons to collide but compression would be affected on several cylinders if not all possibly. far as no compression,back firing and fireballs it sounds like the cam and ignition timing is off. Might try removing the rocker assembly which should close all the valves 100% and do a leak down test. That would tell at least if there is a bad valve,gasket or??. Not to be a black cloud,but new and/or rebuilt means a 50/50 change its right. If the leak down comes out good then on to plan B. IIRC Ive read many guys complain about bad timing gear marks which caused timing off 180 degrees. If that's the case you'd have to pull the TC cover off and reset cam to piston position to start and go from there. Forgive any mistakes, er...Ive had a few this afternoon. ;)
 
The harmonic balancer should have a nice visible line on it to indicate TDC. That lines up with the timing cover tab or casting where it shows degrees of timing. I can say have heard of the outer ring slipping a bit on the inner part which would cause bad timing marks basically but its pretty uncommon.. Keep in mind there are 2 TDC's on a 4 stroke engine. One TDC setting is where #1 piston is top dead on the compression stroke. If you rotate the engine by the crank 1360 degree revolution you now align the balancer mark to TDC again but have it somewhere in either the intake, exhaust stroke or valve overlap. Probably would take 2 people but If you pull the plugs and stick your finger over cyl #1 plug hole you'll feel the compression building. Once it hits the timing marks as it builds you know its on the compression stroke.
But...If I understand you correctly after it was tore down and reassembled you ran a comp test and had 0 in #1. That would indicate deeper troubles as you note. Going back here if you pulled the valve cover and rotated the engine by hand by the crank bolt aligning the TDC points twice ,one of them should give you completely closed valves on #1,compression stroke. Being the cam may be off you might find at no time when those marks are aligned the valves are fully closed,but one or both are being held open slightly. Then in fact you have cam alignment issues and it needs to be torn into again. Hope this all makes sense.
 
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First things first, why did the motor bend the valves in first place?
Was it just the valves on number 1 that were bent?
If the timing marks for the cam were off, there would be no compression on any of cylinders.
They would all be evenly timed "wrong".
 
Gilaguy23 said:
If im not mistaken the 258,2.5 and 4.0 are non interference engines. That being said slipped or a broken timing chain or gears should have not caused valves and pistons to collide but compression would be affected on several cylinders if not all possibly. far as no compression,back firing and fireballs it sounds like the cam and ignition timing is off. Might try removing the rocker assembly which should close all the valves 100% and do a leak down test. That would tell at least if there is a bad valve,gasket or??. Not to be a black cloud,but new and/or rebuilt means a 50/50 change its right. If the leak down comes out good then on to plan B. IIRC Ive read many guys complain about bad timing gear marks which caused timing off 180 degrees. If that's the case you'd have to pull the TC cover off and reset cam to piston position to start and go from there. Forgive any mistakes, er...Ive had a few this afternoon. ;)
Click to expand...
So when I installed the cam shaft I had the #1 at TDC. But when I put the timing chain on the two dots aligned perfectly but the harmonic balancer did not. Then I read a thread that the harmonic balancer sometimes has an incorrect mark depending on part numbers and to mark it yourself. Not sure how I could be off if the head was compeltely off when I did this and both dots lined up on cam and crank.
 
Harmonic balancers are known to slip when they they wear out.
An old mechanics trick is to mark both parts and then re-inspect for slippage later.
Unfortunately, if yours won't run, that's not going to help for you.
 
jeepjoe43 said:
First things first, why did the motor bend the valves in first place?
Was it just the valves on number 1 that were bent?
If the timing marks for the cam were off, there would be no compression on any of cylinders.
They would all be evenly timed "wrong".
Click to expand...
I had bent intake valve on 1,2 and 6. Replaced them all and had the head sent to a machine shop. Not sure why they bent, I had timing issues with it and couldn’t figure it out it had sat for about 4 years.
 
jeepjoe43 said:
Harmonic balancers are known to slip when they they wear out.
An old mechanics trick is to mark both parts and then re-inspect for slippage later.
Unfortunately, if yours won't run, that's not going to help for you.
Click to expand...
I marked it to check for slippage I thought I might have to change it later but right off the rip I had nothing. I haven’t checked compression on the other cylinders yet but I thought it odd I had nothing in 1. I’m going to remove the rockers and see if I have anything with the valves compeltely closed for sure.
 
Pretty sure 4.0 is a non-interference engine, meaning the pistons and valves would never hit each other even if valves were stuck all the way open.
 
So if you have verified #1 piston is top dead and the balancer marks are way off then id mark the balancer for reference where TDC is. I cant recall clearly but when I did my timing set in my 4 banger I had read several posts where guys said the dots on the new timing set were 180 off or there was more than a couple dots confusing the issue. Keep in mind if you are rotating the engine by hand and expecting to see every mark line us every rotation you wont. It will be every other rotation. valves have to open,close and stay shut each rotation of the crank. Off the top f my head I cant think of how you can set #1 to tdc and rotate the cam without the chain to the correct spot for combustion in #1. Then if the chain/sprockets are put back on you would be good to go for valve timing.
 
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