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Timing issue or bent valves please help out of ideas

2K views 49 replies 6 participants last post by  fishadventure 
#1 ·
First off I did a 4.0 head swap on my 4.2 and now it runs like crap. It's running like I have the firing order wrong but it's correct. It also has the 4.3 tbi and it seems like it keeps trying to backfire through the throttle body. Could my timing be 180 off. I pulled the valve cover off and watch all the rockers they all move the same amount so I don't think one is getting stuck. Thank you
 
#2 ·
You should use a timing light and a compression tester and see what is up. You could be off on the timing because you messed with the timing chain. You could be off a tooth on the distributor or you could have a air/fuel issue.
 
#4 ·
Yes, you very easily could be off 180 on your timing. Also make sure you are putting the wires on in the correct rotation. Rotor spins clockwise.

Also, how much work was done to the 4.0 head? Are your pushrods too long and not allowing the valves to fully close?

You should use a timing light and a compression tester and see what is up. You could be off on the timing because you messed with the timing chain. You could be off a tooth on the distributor or you could have a air/fuel issue.
No reason to mess with the timing chain for a head swap.
 
#10 ·
Yes, you very easily could be off 180 on your timing. Also make sure you are putting the wires on in the correct rotation. Rotor spins clockwise.

Also, how much work was done to the 4.0 head? Are your pushrods too long and not allowing the valves to fully close?

No reason to mess with the timing chain for a head swap.
?
 
#6 ·
It was running great before I tore in to it. It was stock with junkyard tbi. I was thinking about for a second how do I tell if the valves are closing all the way. When I had it sort of running it seemed like it was pushing air back out of the throttle body sometimez. I don't know if it was just trying to backfire or what. Thank you very much for the help at my wit's end trying to get this thing going
 
#9 ·
welderman85 said:
Someone at work asked if a compression test would let me know of the valves were closing all the way. I didn't know the answer so I'll ask here.
If the valves aren't closing fully depending on the amount of not closing fully you will have low compression and no holding compression. So a poor or failed compression test /may/ indicate valves not closing, sealing.

Having said that the "airback" symptom can be / a timing error with the distributor orientation, but you have like five or six variables now that I've thought it out. You need to stop wide thinking this and start on some basics one thing/variable at a time. Pushrods first, distributor and TDC second. Then compression test. Then chasing rabbits.

It's always good to do a compression test but I just think the valves closing or not is too easy to do. ALSO if you didn't loosen all the rockers to start with and baseline them on the new install it could be that simple a fix.
 
#15 ·
It stayed the same I had the head gone through at a machine shop so either something isn't correct with that or my pushrods couldbe to long. I have to reset the distributor anyway because I pulled it out with out really thinking first but I think once I get some time my plan of attack will be reset the distributor, pull the pushrods and make I put that altogether correct. T hen test to make sure the valves are closing. Dose that sound good. And thank you all for helping me figure this out
 
#14 ·
Im working tons of overtime right now so it has to sit for a while. But I was trying to gather all the info I could so once I get a little time I can go at it some good ideas. I think I have it narrowed down to timming or valves staying open. I pulled the distributor and am going to reset everything and stap it back in so I will know that aspect is correct.
 
#16 ·
fishadventure said:
...if you didn't loosen all the rockers to start with and baseline them on the new install it could be that simple a fix.
Well maybe I'm an idiot cuz I just figured out that most of our motors 4 or 6 don't have initial baseline adjustable rockers- the pushrods are a certain length and the hydraulic lifters self set to zero lash. So wrong length pushrods or not having them seated in the lifter cups can cause your symptoms.

As far as the other question, the push rods break in and seat in their respective lifters- that is why you shouldn't mix them up. Oil normally goes up the pushrods and a bad fit stops this needed flow to the cylinder head cuz it bleeds off before it can get there. A bad fit can occur when they're mixed up, on one hand, or on the other hand they will wear to a bad fit if they're mixed cuz they don't rotate properly, and they can actually bind the lifters from rotating, too, killing the cam.

I'd just buy new pushrods. I think it could last a little while but don't think it's a good long term gamble otherwise.
 
#17 ·
I had a little bit of time to play with this issue. First I tried to find TDC compression stroke but I couldn't the air pushing in the plug hole. So I figured I would check the valves on both TDC so I lined up the marks and in did the rockers. The intake was closed but exhaust was open a little then I rolled it to the other stroke and did the same this time the exhaust was open and the was open a little. I think too long of pushrods are to blame are 4.0 pushrods shorter. Thank you
 
#20 ·
No lifter preload as said, just the set torque on the rocker bridge and hydraulic lifters to compensate. At TDC for compression, do both valves fully close if you loosen up the rocker bridge? If so, then you need to order up a length checker. You are Ball-Ball for your pushrods and 9.625" length 5/16th pushrod factory for the 258. You will need a checker that covers that distance +/-

https://www.summitracing.com/parts/tfs-9002/overview/
 
#19 ·
welderman85 said:
Also how to I check the lifter preload? Could that be an issue of mine
You don't. They rockers aren't adjustable - the hydraulic lifters either bleed down pump up for zero lash.

I made an assumption earlier that was incorrect. If the valves are staying open and don't close then it would seem the pushrods are out of spec. Tthe lifters can't oil themselves up enough to be too long.
 
#23 ·
welderman85 said:
So I had an idea to get compression stroke I couldn't fell the air because a valve was open. So I pulled the rockers let the valves close the bam the correct TDC. Then I reinstalled the rockers the intake didn't move but the exhaust opened
I'm not following your linguistics here. That is why I didn't reply to the earlier question on these lines. I can't visualize what you're saying cuz I'm not sure what you're saying...others may be having the same experience. Not trying to be mean or critical, but please re type slow, check it, and repost.
 
#24 ·
Good way to find tdc is to rotate it with a long skinny screw driver or a 2 foot piece of brake line in the spark plug hole (spark plug out) and feel for when the piston is at the top of its stroke. Go slow and make sure you don't bind it in the hole when the piston is going up
 
#25 ·
Sorry I was trying to type and hang out with my little 3 year old so I was trying fast and not really reading it. What I did was pull the rockers off cylinder #1 thinking that would make both valves close for sure. Then I was able to find the correct compression stroke with a finger in the spark plug hole. So I then placed it at TDC with marks on my timing cover and balancer. I then went and put the rockers back on and watched the valves. The intake stayed closed and the exhaust valve opened a little. So I guess I'm thinking that means my pushrods are to long. Is there another way to check this? Thank you
 
#29 ·
With both rockers off of #1 you will get a puff of air at tdc at the end of both strokes, exhaust and compression. As you rotate it with the rockers installed on #1, does the exhaust valve ever fully close at any time? You have verified that both valves close with the rockers off, so we can rule out a bent valve. If the exhaust valve never closes over the 2 revolutions, then you have an issue with one of the three, lifter stuck at full extension, pushrod too long, or rocker arm is incorrect.

How do the rest of the valves look as you spin the motor? You should be able to watch the exhaust valve open and close then the intake open and close then both valves be closed for compression then power strokes. If you have multiple cylinders that have valves not fully closing then it points strongly towards too long of a pushrod.
 
#27 ·
Not to sound dumb but do you mean two revolutions from TDC or one? If I go one full revolution from compression stroke tdc lthe exhaust valve is alot but that's it. I was rolling it over without the valve cover to watch the rockers all of them move the same amount up and down and seem smooth. Not sure if it's the pushrods or what. I had the head at the machine shop and be said he went through it and all was well. It was milled/decked but that's it. Thank you very much for your help
 
#28 ·
welderman85 said:
Not to sound dumb but do you mean two revolutions from TDC or one....I had the head at the machine shop and be said he went through it and all was well. It was milled/decked but that's it. Thank you very much for your help
Basically trying to have you see what's happening through all four strokes which 'seems' correct as in correct order.

You say it was milled/decked. This makes pushrods effectively longer...

Did you ever do the compression and leakdown?

The original post asked about bent valves. What made you wonder if your valves were bent? IIRC you didn't remove cam chain, so contact with the piston seems unlikely.

The adjustable pushrod as another poster mentioned will help you determine correct length- then you measure all the pushrods and record those numbers. If the pushrods all measure longer that's your answer. Pushrods in custom motor building often have the tubes cut to a determined length, then the hard 'ball' ends pressed into the rods.

It sounds like you have a mechanical issue and with your subjective information it doesn't seem likely that cam chain jumped time. So I'd chase the pushrod rabbit...eliminate the variables and the answer will become apparent.
 
#30 ·
Ok I had a little bit of time tonight before the family got home so here's what I did I got cylinder #2 at tdc as close as I could and pulled the rockers neither valve mover bother were closed I did this to cylinder #3 and #4 also then the little guy got home and I played with him insted of the jeep. I just ran out and put cylinder #1 at tdc and pulled the rockers and the exhaust was open. Then I rolled it over to the other stroke and same thing exhaust was open a little. So is my problem a valve. The motor ran great before this mess and has 125,000 miles. Also once I checked the valves on 2-4 I put the rockers back on and when I tightened the it compressed the valves is that because the lifter pumped up. Because the didn't move when I took them off.
 
#32 ·
Not a compression test, but sounds like time for a leakdown test. Some compression testers also will do leakdown, hopefully you have/can find one. You find tdc on #1 and loosen up both rocker arms. Then apply shop air to the tester and then shut off the valve on the tester and the record the pressure. The pressure should hold steady. If the pressure drops quickly, then you need to listen for air escaping. Either through the carb (intake valve) or the tail pipe (exhaust valve). You will probably want to check all 6 cylinders to be on the safe side. The tester will have instruction as to how much shop air pressure to use. You wont find out as much from a compression test if the exhaust valve is not closing.
 
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