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Fluctuating compression

Tiny
PHAREZCATS
  • MEMBER
  • 2004 MITSUBISHI LANCER
  • 1.5L
  • 4 CYL
  • 2WD
  • AUTOMATIC
  • 192,786 MILES
I am compression testing my vehicle, at times the compression goes up to 175 on all cylinder next test, number three will be at 130 and number four would be at 150, then it jumps back up to 175, I am so confuse, the timing marks are on point, the lash adjusters are to factory specs, but if I unscrew the intake on three and four the compression goes up and then goes back down. I am looking at the cam gear at the point. Any help would be so great, thanks.
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Monday, January 15th, 2018 AT 6:00 AM

14 Replies

Tiny
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What do you mean if you unscrew the intake? The intake what?
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Monday, January 15th, 2018 AT 6:25 AM
Tiny
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The intake lash adjuster, so it would close a little earlier.
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Monday, January 15th, 2018 AT 7:28 AM
Tiny
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Okay, why just the intake adjuster, why not the exhaust, too?
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Monday, January 15th, 2018 AT 7:39 AM
Tiny
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Because the psi on the compression stroke only deals with the intake, for the cycle the exhaust is close, so why touch it.
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Monday, January 15th, 2018 AT 7:41 AM
Tiny
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No, both valves should be closed at the end of the compression stroke.
Anyhow, I suppose you were trying to rule out a valve not seating. Perhaps you should just go ahead and back off the adjusters on both intake and exhaust lash adjusters to rule out hopefully a valve not seating. Then squirt a little motor oil into the spark plug hole to help seal the rings some and repeat the tests to see if perhaps you have a ring not sealing. The only of possibility is a blown head gasket but that would not fluctuate if you ask me.
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Monday, January 15th, 2018 AT 8:52 AM
Tiny
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Yes I know, what about the cam gear? If it is out of round.
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Monday, January 15th, 2018 AT 9:37 AM
Tiny
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Because it is kind of beat up.
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Monday, January 15th, 2018 AT 9:38 AM
Tiny
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As long as both valves are closed and fully seated, I cannot see how the camshaft would be affecting your readings at all.
I think you should do a cylinder leak down test on those two cylinders. That way there will be no cranking of the engine with the starter, so no spinning of the camshaft and no worry about early or late seating valves. I suspect that is what you are concerned about. Thus with both valves closed at TDC of the compression stroke while performing a leak-down test, if anything is leaking you will surely know what the problem is.
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Monday, January 15th, 2018 AT 10:37 AM
Tiny
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I will try the leak down test.
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Monday, January 15th, 2018 AT 10:48 AM
Tiny
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I might have to grind valves.
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Monday, January 15th, 2018 AT 10:49 AM
Tiny
KHLOW2008
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If increasing the valve clearance makes a difference to the compression, that means the valve clearance has not been adjusted correctly.
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Monday, January 15th, 2018 AT 8:02 PM
Tiny
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Yes it has, I used a gauge an measured it. I am thinking of getting an adjustable cam gear to see if advancing it a little will work. My camshaft timing gear is kind of beat up.
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Tuesday, January 16th, 2018 AT 6:56 AM
Tiny
STEVE W.
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Moving the cam won't correct the problem but it will set a bunch of codes and likely cause the engine to run worse because of the various items the PCM uses the cam timing sensor to control.
The more likely issue is carbon build up on the valves or seats. It only takes a small piece to let a valve leak and drop the compression, then it falls out and compression returns. I would suggest a full top cylinder cleaning to remove the carbon first. Then run it some and check the valves. If they are good then run it. Your actual running compression numbers will all be lower than the static numbers you are seeing anyway because of the way the engine operates and the actual airflow the engine needs and sees from the sensors.

Now if the compression was say 160 in 3 cylinders but 100 in the last one when tested I would be concerned.
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Wednesday, January 17th, 2018 AT 6:14 PM
Tiny
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Ok, I am new to this and need experts to share there knowledge, I will readjust the lash adjusters to FSM and try and run some sea form through the intake by the pvc hose and see, Thank you all so much :)
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Wednesday, January 17th, 2018 AT 10:56 PM

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