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Engine - General, Exhaust & Induction Topics about the general engine items, exhaust system discussed here & Improving the intake tract - air filter to intake valve.

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Old 09-17-2012, 03:47 PM   #1
low compression?????  
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hey everybody! got a strange problem. compression test results are bad
1-60
2-115
3-85
4-115

did a leak down test at 100psi and got only 3 lbs drop on each cylinder. sounds good right?

checked cam/ crank timing next and it is dead on.

just repaired a broken airbox that was sucking air for about 200 miles. im worried that i ran lean and burned up my rings.

why did the leak down go good but my compression is so bad?? it idles rough and has 14-15 vac at idle. i know the timing is right and the tension on the belt appear perfect(newer timing belt,2000 miles tops).

whats going on?!?!?!?!?

thanks everyone as always
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Old 09-17-2012, 05:14 PM   #2
Re: low compression?????  
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update- after reading a post on here, i put a little oil in the cylinder and compression skyrocketed to 180, then dropped every test over the next 10 minutes until it hit 60 again. not sure about the leak down test, but sounds like new rings, right???

also-no smoke, no coolant in oil etc...

how is the sealed power re-ring kit ?
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Old 09-17-2012, 08:36 PM   #3
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i use sealed power rings quite often, never an issue with them.
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Old 12-02-2012, 10:44 PM   #4
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Originally Posted by shelby x's 2 View Post
why did the leak down go good but my compression is so bad?? it idles rough and has 14-15 vac at idle. i know the timing is right and the tension on the belt appear perfect(newer timing belt,2000 miles tops).

whats going on?!?!?!?!?
OK, ready to get to work on her, any other things I can check before I get too far into it?

Leak down? Thinking rings... anyone have any other input?

Also, read in one of Jan's posts that when timed properly, the front of the distributor is roughly parallel with the head. checked the timing with a gun and is perfect to 12 TDC but mine is nowhere near parallel....????

Thanks in advance!!!

Last edited by shelby x's 2; 12-02-2012 at 10:50 PM.. Reason: more info
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Old 12-03-2012, 05:39 AM   #5
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Quote:
Originally Posted by shelby x's 2 View Post
Also, read in one of Jan's posts that when timed properly, the front of the distributor is roughly parallel with the head. checked the timing with a gun and is perfect to 12 TDC but mine is nowhere near parallel....????
DDG- Quick and easy timing belt check
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Old 12-03-2012, 07:13 AM   #6
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Originally Posted by shelby x's 2 View Post
Also, read in one of Jan's posts that when timed properly, the front of the distributor is roughly parallel with the head. checked the timing with a gun and is perfect to 12 TDC but mine is nowhere near parallel....????
this is true only if the intermediate shaft is timed properly to the camshaft,
and the firing order on the dist. cap hasnt been shifted...

anyway, this isnt related to your low compression issue ...
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Old 12-03-2012, 07:16 AM   #7
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When you ran your compression test were/was the...???
1)Battery fully charged
2)All plugs removed
3)Throttle held wide open
4)Engine cranked no more than 5 revolutions(Chrysler says 3)
5)All cylinders were checked identically.

Usually a cam timing issue will cause a a low compression reading on all 4 cylinders but if your distributor is not close to parallel to the block either the intermediate shaft is not properly aligned or the oil pump was removed at one time and not properly aligned when re-installed.

Always start with the known problems and work from there.

Here is another link on Cam Timing
http://www.turbododge.com/forums/f4/...n-84-95-a.html
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Old 12-03-2012, 09:38 AM   #8
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To properly do a leakdown test you have to keep both valves closed on the cylinder you are checking. So you have to get that cylinder at TDC and keep it there. Thats tough to do sometimes on an 8valve. The best way I have found to do it if by yourself is to slide off the timing belt,pull valve cover,and loosen up the cam enough to close all valves. That will give you a correct reading for leakdown numbers.

Do you have a dual 100 psi gauge tester or one that has 100 psi for the one and a percentage gauge for the other?
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Old 12-03-2012, 10:40 AM   #9
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Cracked piston ring lands maybee
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Old 12-03-2012, 06:13 PM   #10
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ok,lotsashelbys(nice name) my leak down tester is a homemade pos, i think my orifice in the line is too big, but thats irrelevant now- i found the problem.

turns out the side of #1 piston was cracked in 2 spots!!!! the only thing holding it together was the rings! so a couple more questions-
1-do i need to replace all 4 pistons or just the damaged one ?
2-the cylinder walls look like new, do i need to hone them still before i replace the pistons?
3-any other advice is always welcome

thanks for everyones help !!!! you guys always bail me out of trouble
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Old 12-03-2012, 08:37 PM   #11
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yes, hone the cylinders. also be sure to carefully check the other pistons for damage as you said you had #3 cylinder with low compression as well. if all the others are ok, then you can reuse them but at your own risk.
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Old 12-03-2012, 08:50 PM   #12
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wow I had the same problem an all I got was a derogatory comment that my motor was trashed. didn't even get near close to this kinda help.
I called Jackson, atleast he told me what the problem was, thanks bud, I made my catchcan, an I'm watching my oil level till I can get it fixed. Thanks for talking to me on the phone. As for the info I didn't get here from the rest of you all, except NAJ an DRAGBARON, I have no comment.

Last edited by GhostDaytona; 12-03-2012 at 08:53 PM.. Reason: sometimes I wonder
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Old 12-03-2012, 09:34 PM   #13
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Originally Posted by GhostDaytona View Post
wow I had the same problem an all I got was a derogatory comment that my motor was trashed. didn't even get near close to this kinda help.
I called Jackson, atleast he told me what the problem was, thanks bud, I made my catchcan, an I'm watching my oil level till I can get it fixed. Thanks for talking to me on the phone. As for the info I didn't get here from the rest of you all, except NAJ an DRAGBARON, I have no comment.
ahhh yes, i remember your thread, you clearly stated in your thread that it couldnt be the pistons because they were real close to being new, seemed that you didnt want help,but rather wanted everyone to agree with what you were saying is wrong with the car, sorry i dont and wont do that, you also stated that you werent a mechanic and didnt have much of an idea as to how to do some of the tests that were required to find out the real problem with it.i offered help, you didnt want my help it seemed or anyone else's fo rthat matter unless they were willing to agree with you with thinking it was something other then bottom end issues (busted ring lands)
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Old 12-04-2012, 01:03 AM   #14
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I am doing the old piston swap tomorrow in fact. Adding another piston and rod from another engine lol. Sound scary? Yeah but it works if factory standard size.

One thing about this little trip is you need to have a clean piston, no crud. Then change the rod bearings, tape the rod bolts for install. Then when you hone, just barely do it long enough to take off the glaze. This goes by pretty fast. Also squeeze the hone together and get it in the cylinder and release it. Don't push the thing in making long ways scratches. Pushing in the hone and honing the crap out of the hole is how you nuke the job. Over doing the hone makes the cylinder not round. There is other issues with needing a hone plate like with a new engine. Do a real quick spin removing the glaze and stop and the job goes well. Better to have a little glaze left than a oval cylinder.

One of the members on here got 3 pistons and rods out of an old 85 engine of mine and did this 3 time lol. The engine I had had one bad piston, his had TBI pistons for some reason. So he blew them out one at a time lol. I didn't see that it had TBI pistons till later, he never blew out a piston he got from me lol. Also have done this with trucks. Not the best plan but us TD folks don't always have the money...
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Old 12-04-2012, 08:05 AM   #15
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I am doing the old piston swap tomorrow in fact. Adding another piston and rod from another engine lol. Sound scary? Yeah but it works if factory standard size.

Doesnt sound scary to me lol. A few years back all I had were Shelby Zs with TIIs. I was pretty hard on them everyday pushing the limits and I cracked many pistons from detonation or going lean. I did alot of the in car "rebuilds" as I didnt have much money to spend on them. I actually just replaced pistons and nothing else. I had a supply of good used TII pistons and all I would do is clean the ring lands and make sure I was using one that was the same size as the one that came out. I would put the old rings on the good used piston and index them the same as they came out. I know I will get some crap for this but it worked for me a few times and I would get the compression back in the 140-150 range believe it or not and the engine would run very well. Total money spent was less than 100 with a new HG and oil pan gasket and a used piston.

Bash now please......


shelby x's 2 make sure you look over that #3 piston as that is the one that usually went first for me in most cases.
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