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Low compression in only one cylinder


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kaioken
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Jul 27, 2010, 12:16 PM

Post #1 of 17 (21213 views)
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I have a 98 Infiniti QX4, what could cause low compression in only one cylinder (cyliner 2 has 96 PSI)? How much will it cost to fix if it's the ring or clogged exhaust valve or shoud I just get a new engine? the current engine has 186,500+ miles. because of the low compression it obviously has some starting issues but only when it has been driven and then sits for about a half hour. It then takes holding the starter on for a bit longer and then stepping on the gas to get it started. The dealership told me to put some oil additive like restore in it because it might fix my problem. So far it has. starts just fine everytime. Still can't believe a dealership would suggest that though. I'm sure this is just a bandaid fix and it will soon stop working. any suggestions out there?


(This post was edited by kaioken on Jul 27, 2010, 12:33 PM)


Sidom
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Jul 27, 2010, 11:29 PM

Post #2 of 17 (21191 views)
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If restore is helping the problem then more than likely the problem is in the rings. Even though it's just one cylinder, to fix this problem the whole engine would need to be disassembled. Obviously tests would need to be run to confirm this, at the minimum a leakdown test in addition to the compression test that has already been done.

So basically you are looking at either rebuilding your engine or installing a new one.

It would make no sense to tear a motor down to rering one cyl. I can't imagine a shop willing to do that. Maybe a backyarder might but good luck finding that guy when the job goes South.....


Also I'm as surprised as you are that a dealer suggested that.....It was nice of them to throw out a temporary snake oil fix but it still surprises me they did given the nature of this business these days....






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(This post was edited by Sidom on Jul 27, 2010, 11:32 PM)


Hammer Time
Ultimate Carjunky / Moderator


Jul 28, 2010, 5:37 AM

Post #3 of 17 (21185 views)
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I would suspect that the additive suggestion was done as an alternative to what they really suggested which would have been a complete rebuild.




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Hemi Guy
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Jul 28, 2010, 1:31 PM

Post #4 of 17 (21179 views)
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The most common causes of low compression in single cylinder are a burnt exhaust valve or rounded lobe on the camshaft. Have both (or one or the other) been ruled in or out? You don’t need to replace or rebuild the entire engine. In the worst case scenario, (a burnt exhaust valve and rounded cam lobe) a repair is possible. Note that this sort of repair is not for the faint hearted. You have to get the car to a real old time pro. Figure on spending just under a grand. The possibility of a stuck exhaust valve is also very real. For that try three applications of “Sea Foam” at one thousand mile intervals. Don’t pour the “Sea Foam” into the fuel tank. Instead follow the instructions on the can for introducing the “Sea Foam” into the engine via an engine vacuum source.


Hammer Time
Ultimate Carjunky / Moderator


Jul 28, 2010, 1:41 PM

Post #5 of 17 (21175 views)
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You really should get a more in depth diagnosis before making any rash decisions. This could be as simple as a broken valve spring.




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Loren Champlain Sr
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Jul 28, 2010, 8:31 PM

Post #6 of 17 (21166 views)
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Hemi; Wouldn't a rounded cam lobe keep the valve from opening? If so, compression would be good. A dealership recommending snake oil? What's up with that? LOL.
Loren
SW Washington


Sidom
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Jul 28, 2010, 11:31 PM

Post #7 of 17 (21160 views)
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Wow so many questions......so little time......Sly Cool
Angelic

How would Restore® make and engine with a burnt valve or flat cam run better?

How would a flat cam affect the compression test? (actually there is a good answer for this, I wonder if he can get it)

How on Gods green earth is someone with an Infinity QX4 going to get a quality valve job for under a grand????

Hemi guy.....I'm beggin.......I'm pleadin...........Please.......Put the crack pipe down & step away from the keyboard......

The 1st step is admitting you have a problem..........






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Hammer Time
Ultimate Carjunky / Moderator


Jul 29, 2010, 5:42 AM

Post #8 of 17 (21153 views)
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ROFLMAOOOOOOOOOOOOOOOOOO




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Hemi Guy
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Jul 29, 2010, 5:09 PM

Post #9 of 17 (21139 views)
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No crack for me my friend. You keep it and have fun. Enjoy yourself. I’m sticking with “Jim Beam” J

Just a reminder to all those who responded We’re Talking low compression here not no compression. The poster stated compression test revealed a 96 psi in the second cylinder.

Worn camshaft lobe

Mild wear of a camshaft lobe will cause reduced height of the lobe or a flat spot on top of the lobe. Often this condition is called a "Square Cam". Even with the right valve lash adjustment the valve will open and close at the correct time, but it will not open as far as it is supposed to. Like this the engine will run pretty well at idle and up through midrange speeds. However at higher speeds the engine will lose torque because the valve doesn't open far enough causing restriction of high speed flow. At higher speeds the engine may lose so much torque that it seems like its misfiring. A tapping sound is also possible at any speed, but the tapping sound would sound greater at a lower engine speed. That’s because the engine makes less noise at a lower speed, and it makes it easier to hear the tapping noise. A badly worn cam lobe is called a "Round Cam". With this the cam lobe is worn down so far that it looks like a circle. With this condition the valve could open late and close early, and could also have a small amount of lift. With just a small amount of lift the engine could run nearly normally. At idle the engine could shake a little. On any speed higher than a slow idle the engine will seem to be completely misfiring on that one cylinder. To check for a worn camshaft you have to measure the valve travel at the top of the valve spring while the engine is turned slowly by hand. When the camshaft is removed from the engine you have to measure the cam lobes directly for base circle and cam apex height. Now you subtract the base circle diameter from the total lobe height to get cam lift. Now multiply the cam lift by the rocker arm ratio to get the valve lift

Now after writing all this I need a drink. J







(This post was edited by Hammer Time on Jul 29, 2010, 6:03 PM)


re-tired
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Jul 30, 2010, 12:29 PM

Post #10 of 17 (21125 views)
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I dont know what manual your quoteing but it has some errors. A worn cam lobe cannot have correct valve timing it will open late and close early .Compression will be near normal as the cyl stiill takes in air from the good valve . however it will show up as a weak cyl as the fuel air mixture is not being Drawn in. Valve lift can be determined by measuring lift at the push rod and multipling the ratio of rocker on a overhead valve eng or the thickness of the shim on a overhead cam eng. cam wear is asymertical and will always result in sympathic lobes .An exception to this of course is a broken cam where as the undriven half of the cam will always be late equally regardlessof of absense of wear.


LIFE'S SHORT GO FISH


Hammer Time
Ultimate Carjunky / Moderator


Jul 30, 2010, 12:34 PM

Post #11 of 17 (21122 views)
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RT, your just making his head hurt now..............LOL




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Hemi Guy
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Jul 30, 2010, 3:52 PM

Post #12 of 17 (21115 views)
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(Quote) RT, your just making his head hurt now..............LOL

Not even close Hammer. Granted its 3:10 here in Ridgewood Queens New York, and granted I’m about half way through a fifth of “Jim Beam” LOL, (as already stated I don’t do crack) but understand something….I’m a big strong man and I can take it. Here’s a little background about myself. I started spinnin wrenches when I was 14. I put down the wrenches long enough to enlist in the Marines. I couldn’t wait to get to Nam, and once there I quickly earned a purple heart killin the enemy. After Nam I picked up the wrenches and started them spinnin again, and haven’t put them down since. I’m always willin to learn something new and different from a better man. I know a lot, but don’t claim to know it all. Now all you gotta do is convince me that you’re that better man. You have a right to disagree. I fought for and gave you that right. Now just post the facts buddy. If I’m wrong, tell me why? Facts only guy.


Retired,

Granted you may know a little something about engine rebuilds. Here’s a question for you. Straight up and with no tricks. Usin simple arithmetic and the information posted in my last response, tell us why why the industry standard rocker arm ratio is 1.42:1?


re-tired
Veteran / Moderator


Jul 30, 2010, 5:47 PM

Post #13 of 17 (21106 views)
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The ratio you have quoted depends on who and where you get your info from.....Im a student of the great Smoky Yunick . I have the ratio at 1.5 to 1. But if you want an extra 50+ ponies out of a stroker put 1.6 'ers on the coners with a shoebox single plane .I'm retired military ,lost a buddy in TET.What was your unit and dates of tour and where did you get heart ? I'm sota a nam history buff . I may have some pics of your unit .


LIFE'S SHORT GO FISH

(This post was edited by re-tired on Jul 30, 2010, 5:52 PM)


kaioken
New User

Jul 30, 2010, 6:33 PM

Post #14 of 17 (21094 views)
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Thank you everyone for responding. I have already started saving for a new engine...halfway there Woo Hoo! In the mean time I'll try some sea foam as one responder suggested. One note though My gas mileage is at 19.4mi per gal hwy and I have recently had the suv up to 105mph so I don't think the engine is misfiring. Hopefully I can get an engine from 2003 to fit in it. I would love a jump form 168hp to 240hp.


Hammer Time
Ultimate Carjunky / Moderator


Jul 30, 2010, 6:44 PM

Post #15 of 17 (21093 views)
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Smokey Yunick, I know that guy.

He's the one that made that famous quote "Basa ball ben a berry, berry good to me"




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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.



Hammer Time
Ultimate Carjunky / Moderator


Jul 31, 2010, 12:23 PM

Post #16 of 17 (21078 views)
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Quote
I quickly earned a purple heart killin the enemy.


That's interesting........... How many do you have to kill to get that award?





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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.



Loren Champlain Sr
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Aug 13, 2010, 8:18 PM

Post #17 of 17 (21031 views)
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Your quote:
"The most common causes of low compression in single cylinder are a burnt exhaust valve or rounded lobe on the camshaft. " Any more questions?
Your quote:
" Now after writing all this I need a drink"
Me, too.

Oh, and by the way, The Purple Heart is awarded to one that is wounded or killed in battle. I'm not aware of an award that is given for 'killin' the enemy.
Loren
SW Washington

(This post was edited by Loren Champlain Sr on Aug 13, 2010, 8:22 PM)




Low compression in only one cylinder




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