New head, head gasket now bad compression?? - JeepForum.com

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post #1 of 19 Old 01-01-2014, 05:03 PM Thread Starter
quartermil89
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New head, head gasket now bad compression??

Basically what the title says. Finally got my motor back together today and fired it up only to find it still runs poorly. I have a big vacuum leak but that's fixable without tearing down the motor again (italics).

While searching for the cause of the low vacuum I ran a compression check on the easiest to access cylinders, 1-4, and got a miserable 75, 120, 140, 100 psi respectively. Before I took the motor apart I tested the compression at between 150-160 psi across all six cylinders.

Question is: have I managed to ruin my lower end taking the motor apart or is this a symptom of something else?

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post #2 of 19 Old 01-01-2014, 07:13 PM
frostysquirrel
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It might be worth squirting a little oil into each cylinder through the spark plug hole to wet the cylinder walls and retest. If for nothing else than to help isolate where it is leaking.
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post #3 of 19 Old 01-02-2014, 06:15 AM
Weebur
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You might consider getting some new head bolts and blowing out the holes with compressed air and then torqueing them to factory specs and in proper sequence. It's possible that your new head is warped (not likely). Did you put a straight edge across it before the install?
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post #4 of 19 Old 01-02-2014, 08:08 AM Thread Starter
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The head was fully refurbished before being being put on. Decked, full valve job new intake valves etc
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post #5 of 19 Old 01-02-2014, 08:17 AM Thread Starter
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Oh and besides reusing the bolts I cleaned the bolt holes out with a tap and compressed air
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post #6 of 19 Old 01-02-2014, 10:13 AM
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Originally Posted by quartermil89 View Post
The head was fully refurbished before being being put on. Decked, full valve job new intake valves etc
How many miles on the short block when you installed the rebuilt head?

Why did you replace the head?

Have you tried a compression loss check? Apply around 90 psi to each cylinder while thay are at TDC Compression and see how fast the pressure drops when the air supply is shut off to the cylinder.

Also, with 90 psi applied to the cylinder, listen for air escapng out the TB (bad or not seated intake valve), air escaping out the exhaust pipe (bad or not seated exhaust valve, and air escaping from the oil filler cap or oil dipstick tube (bad rings or blown piston).
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post #7 of 19 Old 01-02-2014, 02:28 PM Thread Starter
quartermil89
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About 253k miles.

Vacuum tests indicated that that my intake valves were bent and sealing poorly which was confirmed by my machinist. He decked the head, replaced all the intake valves, and ground and re-seated the exhaust valves. Additionally, he shimmed the springs to bring the seat pressure back up to spec

I'll try the diy leakdown test when it's not raining constantly
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post #8 of 19 Old 01-03-2014, 08:49 AM
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About 253k miles.

Vacuum tests indicated that that my intake valves were bent and sealing poorly which was confirmed by my machinist. He decked the head, replaced all the intake valves, and ground and re-seated the exhaust valves. Additionally, he shimmed the springs to bring the seat pressure back up to spec

I'll try the diy leakdown test when it's not raining constantly
Did you check lifter preload when you installed the reworked head? Milling the head will affect preload. Should be .020"-.060" preload. A valve being held off its seat will show up on those tests I outlind above.

Hope you didn't blow out some piston rings. I blew the rings out of a piston on a high time '67 Olds I had after I installed reworked heads.
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post #9 of 19 Old 01-03-2014, 01:16 PM Thread Starter
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Ooh great point! Didn't even think about that with non adjustable lash on the hydraulic lifters.

Would I have to get shorter pushrods if the preload is excessive?

Thanks
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post #10 of 19 Old 01-03-2014, 02:12 PM
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Ooh great point! Didn't even think about that with non adjustable lash on the hydraulic lifters.

Would I have to get shorter pushrods if the preload is excessive?

Thanks
Many people overlook lifter preload checking when assembling a rebuilt head to their engine, then wonder why their engine runs rough.

Go to Crane Cams website. They have 'Lifter Installation & Adjustment Tips' that gives one way of checking preload. In the instructions it talks about 1 turn and 1/2 turn on the rocker capscrews. The capscrews are 5/16-18 TPI, so 1/2 turn = .028" and 1 turn = .056" preload.

Crane offers shims to adjust if the final torque wrench movement is more than 1 turn. Not sure if you can buy shorter push rods.
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post #11 of 19 Old 01-03-2014, 03:42 PM
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Originally Posted by CCKen View Post
Many people overlook lifter preload checking when assembling a rebuilt head to their engine, then wonder why their engine runs rough.

Go to Crane Cams website. They have 'Lifter Installation & Adjustment Tips' that gives one way of checking preload. In the instructions it talks about 1 turn and 1/2 turn on the rocker capscrews. The capscrews are 5/16-18 TPI, so 1/2 turn = .028" and 1 turn = .056" preload.

Crane offers shims to adjust if the final torque wrench movement is more than 1 turn. Not sure if you can buy shorter push rods.
I am unaware of OEM pushrods being available in any length other than standard. Shimming the rockers sounds like the solution IF the valve train is overly preloaded.

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I don't own an XJ any longer, but I still think they are the best of all the Jeep products ever made. My XJ was my favorite vehicle in my 50+ years of driving.
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post #12 of 19 Old 01-03-2014, 03:53 PM
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I am unaware of OEM pushrods being available in any length other than standard. Shimming the rockers sounds like the solution IF the valve train is overly preloaded.
Various length pushrods are available from aftermarket vendors, not the OEM.

An adjustable pushrod length checker is required to determine the exact length required.
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post #13 of 19 Old 01-03-2014, 05:14 PM
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When checking preload, the lifters must not be full of oil (pumped up) to test/establish lifter preload.

To check if the lifter is unloaded you can use a pushrod and a flashlight, looking down at the lifter, see if the lifter pushrod seat moves freely down by pushing on it with a pushrod. If not, allow the lifter to bleed down.

When checking preload, slowly turn the capscrew down while turning the pushrod between your fingertips. When resistance is felt, stop. The lifter must be at "zero Lash" to begin the test ( 1 turn/1/2 turn deal).

If it looks like there is a difference between the intake and exhaust rocker preload, remove one rocker and its pivot and check the remaining lifter.
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post #14 of 19 Old 01-20-2014, 01:09 PM Thread Starter
quartermil89
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Finally an update.

Measured preload at #1 and #5 according to the instructions on crane cams website.

I measured .115" and .071" respectively.

Does this sound reasonable given my problem?
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post #15 of 19 Old 01-21-2014, 07:29 AM
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Originally Posted by quartermil89 View Post
Finally an update.

Measured preload at #1 and #5 according to the instructions on crane cams website.

I measured .115" and .071" respectively.

Does this sound reasonable given my problem?
How did you arrive at such precise measurements? Seems too precise for using the Crane 'bolt turns' method. Please explain.

I'm wondering if the machinist adjusted (ground) the ends of the valve stems for the proper height so they all match. You said he replaced all the intake valves, and ground and re-seated the exhaust valves is the reason I ask. If he didn't adjust the stem heights the valves on 1 and 4 may be seated too deep and could be your problem. You may want to call him and ask if he did this (explain your problem with preload).

Get back about how you arrived at your measurements.

Thanks.
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