5.0 technical question, I Know someone knows the answer!

crowbar73 · July 31st, 2007, 06:19 PM

I recently purchased an 89 gt automatic and wanted to to do a compression check on the cylinders to make sure they were no dead cylinders, etc. I can't find anywhere that will tell me what the compression should be in p.s.i. I only tested one so far and it was around 125 psi. is that good or bad?? Would appreciate any help.
P.S. the engine seems to run fine but would like to check anyway
Thanks in advance

theninja · July 31st, 2007, 07:53 PM

From everything I've found, the compression should be somewhere between 110-120psi. I don't, however, have the FSM for your car so I couldn't tell you the exact numbers to shoot for.

With you getting 125psi on the gauge I can reasonably assume there's nothing wrong (on that cylinder). If you get something like, 20 or 50psi then you'll KNOW something's messed up, or the tool isn't making a good seal in the spark plug hole...

Iownastang · July 31st, 2007, 07:54 PM

There is nothin wrong with that cylinder,check the others and see what you get

woody1 · July 31st, 2007, 08:01 PM

forget individual readings, look for wide variances from cylinder to cylinder. Do it hot as well, then you can squirt a few shots in the cylinder and see how high it goes up.

88dragstang · July 31st, 2007, 11:04 PM

Woody is right. Depending on the miles on the engine the compression will decrease a little. The important thing is that they are all very similar in compression. If you have 125 on all cylinders, you are doing well.

crowbar73 · August 1st, 2007, 06:40 AM

Thanks so much for the help, i'll check the rest of the cylinders and keep my fingers crossed. I think it's probably ok because it runs good but i just want the peace of mind. You know how it is when you buy something used and have no idea what it's been through?

hopper · August 1st, 2007, 08:49 AM

when doing the tests, do multiple tests on each cylender, (eg 3-4 tests runs) this will give you a more accurate average across the board. 125 is a good start that is usually a sign of proper ring seating followed by good compression. If one cylender is way off through a few ounces of oil down the cylender, then run the tests again, if the reading is still low, you know there is a valve or other head problem, if the pressure raised then slowely seeps back down, you know there is either a ring or scored piston.
hope this help

crowbar73 · August 1st, 2007, 10:25 AM

great advice, thanks hopper, and everyone else too

July 31st, 2007, 06:19 PM	#1
crowbar73 Regular 1989 Mustang GT Join Date: Jul 2007 Location: Waynesboro, VA Posts: 132 iTrader: 0 reviews	5.0 technical question, I Know someone knows the answer! I recently purchased an 89 gt automatic and wanted to to do a compression check on the cylinders to make sure they were no dead cylinders, etc. I can't find anywhere that will tell me what the compression should be in p.s.i. I only tested one so far and it was around 125 psi. is that good or bad?? Would appreciate any help. P.S. the engine seems to run fine but would like to check anyway Thanks in advance

July 31st, 2007, 07:53 PM	#2
theninja Regular Join Date: Jul 2007 Posts: 100 iTrader: 0 reviews	From everything I've found, the compression should be somewhere between 110-120psi. I don't, however, have the FSM for your car so I couldn't tell you the exact numbers to shoot for. With you getting 125psi on the gauge I can reasonably assume there's nothing wrong (on that cylinder). If you get something like, 20 or 50psi then you'll KNOW something's messed up, or the tool isn't making a good seal in the spark plug hole... __________________ 1986 Mazda RX7 (302 cubic inches soon) 2000 Subaru Impreza 2.5RS

July 31st, 2007, 07:54 PM	#3
Iownastang I Post Entirely Way Too Much 1987 ford mustang gt well see when shes done Join Date: May 2007 Location: mascot, tn Posts: 5,020 iTrader: 0 reviews	There is nothin wrong with that cylinder,check the others and see what you get __________________ 2004 tahoe z71 Daily 2004 BBM Gto ls1 6 speed 57,000 miles 1987 mustang hatch,347,world heads,systemax, kooks lts,nitrous, 6 point cage, team z front and rear suspension,8.8, 4.10s, 31 splines strange axles,wilwood brakes, tko600

August 1st, 2007, 06:40 AM	#6
crowbar73 Regular 1989 Mustang GT Join Date: Jul 2007 Location: Waynesboro, VA Posts: 132 iTrader: 0 reviews	Thanks so much for the help, i'll check the rest of the cylinders and keep my fingers crossed. I think it's probably ok because it runs good but i just want the peace of mind. You know how it is when you buy something used and have no idea what it's been through? __________________ He is no fool who gives what he cannot keep to gain what he cannot lose." Jim Elliot Garage: http://www.moddedmustangs.com/forums...-member-10915/

August 1st, 2007, 10:25 AM	#8
crowbar73 Regular 1989 Mustang GT Join Date: Jul 2007 Location: Waynesboro, VA Posts: 132 iTrader: 0 reviews	great advice, thanks hopper, and everyone else too __________________ He is no fool who gives what he cannot keep to gain what he cannot lose." Jim Elliot Garage: http://www.moddedmustangs.com/forums...-member-10915/

July 31st, 2007, 08:01 PM	#4
woody1 Enthusiast Join Date: Nov 2006 Posts: 559 iTrader: 0 reviews	forget individual readings, look for wide variances from cylinder to cylinder. Do it hot as well, then you can squirt a few shots in the cylinder and see how high it goes up.

July 31st, 2007, 11:04 PM	#5
88dragstang Regular Join Date: Apr 2007 Posts: 83 iTrader: 0 reviews	Woody is right. Depending on the miles on the engine the compression will decrease a little. The important thing is that they are all very similar in compression. If you have 125 on all cylinders, you are doing well.

August 1st, 2007, 08:49 AM	#7
hopper Regular Join Date: Apr 2007 Posts: 390 iTrader: 0 reviews	when doing the tests, do multiple tests on each cylender, (eg 3-4 tests runs) this will give you a more accurate average across the board. 125 is a good start that is usually a sign of proper ring seating followed by good compression. If one cylender is way off through a few ounces of oil down the cylender, then run the tests again, if the reading is still low, you know there is a valve or other head problem, if the pressure raised then slowely seeps back down, you know there is either a ring or scored piston. hope this help

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