compression test????

steve1989gt · 10-02-2005, 09:19 AM

Hi i have a 1989 mustang gt with a 302 it has 104,000mi on the engine and i was wondering what the compression is supposed to be... thanks, steve

Capri306 · 10-02-2005, 03:33 PM

Please read the Sticky at the very top of the Blue Oval Lounge.

Moved to the Windsor Power Forum.

Jeff Chambers · 10-02-2005, 04:03 PM

About 125 to 135 psi cold after about 10 cranks with the TB open.

steve1989gt · 10-02-2005, 09:14 PM

How about 120???

Gearhead999 · 10-03-2005, 06:05 AM

120 is OK. But what is the variance between cylinders. That is more important. 10-15 % is about all you should have.

steve1989gt · 10-03-2005, 04:02 PM

???? how do you tell

Falcons Talon · 10-03-2005, 04:05 PM

Measure each cylinder. The high and low readings are what you compare.

Capri306 · 10-04-2005, 02:57 PM

Remember that this test should be performed with ALL of the spark plugs removed from the heads and the ignition coil primary harness disconnected (I usually stick a small light bulb into the socket to act as a load for the ignition module). It may also be advisable to pull the fuel pump fuse so that your cylinders aren't getting drenched in gasoline.

Also, remember that after performing this test "dry," you might want to add two tablespoons of motor oil to each cylinder, right before taking another compression reading. If the pressure raises dramatically, it means that your rings are worn.

I second Gearhead's notion that the balance between the cylinders is what is important. Typically more than 10% variance, and it's time for a rebuild. Perhaps you should purchase a service manual for your vehicle, as most of this information is contained therein.

FWIW, 120psi seems awfully damn low. Are you sure your gasket on the gauge's hose is sealing properly, and that you're opening the throttle fully?

eddielugnutz · 10-06-2005, 01:01 AM

When I did my compression test on my 200,000 mile 5.0 I didn't know about opening the TB, or cranking the motor ten times, I only did it about 2-3times with TB closed, but i did remember to unplug the fuel pump, and I still got an average reading of 125psi, all I can say is God bless Mobil 1.

10-02-2005, 09:19 AM	#1
steve1989gt Registered Member Join Date: Oct 2005 Posts: 5	compression test???? Hi i have a 1989 mustang gt with a 302 it has 104,000mi on the engine and i was wondering what the compression is supposed to be... thanks, steve

10-02-2005, 03:33 PM	#2
Capri306 Moderator Join Date: Sep 1998 Location: Grand Rapids, Michigan, USA Posts: 1,001	Re: compression test???? Please read the Sticky at the very top of the Blue Oval Lounge. Moved to the Windsor Power Forum. __________________ Capri306, Moderator The Mustang Works Online 1979 Mercury Capri 1987 5.0L Mustang LX Notchback 1993 5.8L Eddie Bauer Bronco

10-02-2005, 04:03 PM	#3
Jeff Chambers Moderator Join Date: Aug 2000 Location: Milan, OH Posts: 2,699	Re: compression test???? About 125 to 135 psi cold after about 10 cranks with the TB open. __________________ Jeff Chambers 1990 Mustang GT 10.032 Seconds / 137.5 MPH 14-time Street Warrior World Record Setter CRT Performance 2001 Tropic Green Mustang GT - 12.181 / 113.2 MPH 2002 Ford F-250 Crew Cab 7.3l Power Stroke - 17.41@77.2 "There's nothing boring about a small block automatic shifting gears at 9400 rpm!"

10-02-2005, 09:14 PM	#4
steve1989gt Registered Member Join Date: Oct 2005 Posts: 5	Re: compression test???? How about 120???

10-03-2005, 06:05 AM	#5
Gearhead999 Registered Member Join Date: Sep 1998 Location: Oviedo, FL Posts: 992	Re: compression test???? 120 is OK. But what is the variance between cylinders. That is more important. 10-15 % is about all you should have. __________________ "Support our Troops" Dave 1968 Cougar 2004 Thunderbird 2007 F150 Harley-Davidson, SuperCrew 1986 LTD 1997 Ranger 1992 Honda Civic

10-03-2005, 04:02 PM	#6
steve1989gt Registered Member Join Date: Oct 2005 Posts: 5	Re: compression test???? ???? how do you tell

10-03-2005, 04:05 PM	#7
Falcons Talon T.P.M. President Join Date: May 2002 Location: Brownsville Posts: 265	Re: compression test???? Measure each cylinder. The high and low readings are what you compare. __________________

10-04-2005, 02:57 PM	#8
Capri306 Moderator Join Date: Sep 1998 Location: Grand Rapids, Michigan, USA Posts: 1,001	Re: compression test???? Remember that this test should be performed with ALL of the spark plugs removed from the heads and the ignition coil primary harness disconnected (I usually stick a small light bulb into the socket to act as a load for the ignition module). It may also be advisable to pull the fuel pump fuse so that your cylinders aren't getting drenched in gasoline. Also, remember that after performing this test "dry," you might want to add two tablespoons of motor oil to each cylinder, right before taking another compression reading. If the pressure raises dramatically, it means that your rings are worn. I second Gearhead's notion that the balance between the cylinders is what is important. Typically more than 10% variance, and it's time for a rebuild. Perhaps you should purchase a service manual for your vehicle, as most of this information is contained therein. FWIW, 120psi seems awfully damn low. Are you sure your gasket on the gauge's hose is sealing properly, and that you're opening the throttle fully? __________________ Capri306, Moderator The Mustang Works Online 1979 Mercury Capri 1987 5.0L Mustang LX Notchback 1993 5.8L Eddie Bauer Bronco

10-06-2005, 01:01 AM	#9
eddielugnutz Registered Member Join Date: Sep 2005 Posts: 41	Re: compression test???? When I did my compression test on my 200,000 mile 5.0 I didn't know about opening the TB, or cranking the motor ten times, I only did it about 2-3times with TB closed, but i did remember to unplug the fuel pump, and I still got an average reading of 125psi, all I can say is God bless Mobil 1. __________________ 88 Lx project. 96 Z28 daily driver.

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