Quick ? how to do a compression test on 99 GT and results

TheGrayt1 · May 1, 2008

I am about to do a compression test on the drivers side head and was wondering if I need to disconnect a starter selenoid or just take all the plugs out and crank it over? Also what numbers should I expect for a stock basically 99 GT?

Pearl02 · May 1, 2008

I would think the ignition system would also have to be disabled. I not sure what numbers your looking for, but all of the cylinders should have about the same compression. I'm sure someone else could answer this better than me. Pearl02.

TheGrayt1 · May 1, 2008

Yea I tried doing it real quickly with 2 plugs out and the car fired up briefly and then I cut it off under a second. Can anyone tell me what I need to do to disable the systems? I have all 4 plugs out on the drivers side now, will that be acceptable?

BurningRubber · May 1, 2008

Unplug ALL of the fuel injectors from their electrical harnesses (they clip and un-clip, super easy), drivers and passengers side.

You can also unplug ALL of the coil packs on each side also. It do it this way, so you have no fuel and no spark at all.

You do not need to pull all of the plugs to do it. Just pull the plug for the cylinder you are testing and leave the rest in place.

Jbauer · May 1, 2008

...OR you could just pull the fuel pump relay.

TheGrayt1 · May 1, 2008

where is the fuel pump relay located? That is a good idea.

Jbauer · May 1, 2008

look around or in a haynes manual. You either have a fuse or a relay for the fuel pump, you need to pull it. This is the only certified way to do a compression test. Either pull the fuse or relay for the fuel pump, or cut the spark. On a car such as the 99GT, with COP and stuff- its easier to just pull the fuse or relay for the fuel pump, where as on a 4 banger with a little ignition coil, its easier to just pull the harness for the coil.

TheGrayt1 · May 1, 2008

I do not have a haynes manual can anyone tell me which one is the fuse for the fuel pump. Thanks guys.

Will Smith · May 1, 2008

ok here are the steps u need to do...if you can run the car i would recommend it prior to doing a compression test to clear the fuel from the lines... leave all spark plus and connectors connected.. start the car... located in the trunk on the left side behind the carpeting is a little box with a red button on the top... make sure to have the car running at this point...take a large screw driver or preferably a rubber mallet and knock on this box...when u hit it hard enough the little red button on top will pop up, and your fuel pump will shut off... This will clear all injectors and lines of fuel pressure...your vehicle wills top running by itself when the pressure gets too low...
next take your plugs out and installed a compression guage... crank the ignition and make sure all cyl have roughly the same compression levels...don't forget to push the red button back in on the relay when ur done...

Dragstr05 · May 1, 2008

This thread is way too long with too many opinions for a simple compression test...

If you open your fuse/relay box, USUALLY they are all LABLED as to what each fuse/relay is for. If its not labled, check the owners manual. Alot of times you can find that info in the owners manual.

Pull the fuel pump relay, or if all fails unplugging all the injectors will work. I wouldnt recommend just disconnected spark however...could be quite a nightmare trying to finally start it up with a ton of unburned fuel in your cylinders.

As mentioned above, what you are looking for are any numbers way off from the rest. Example: cyl 1: 180 cyl 2: 180 cyl 3: 90 cyl 4: 175

BurningRubber · May 1, 2008

Right, spark is ok but cut the fuel. You dont want to have raw fuel sitting in the cylinders, not only for startup but you do not want it seeping into the crankcase.

So fuel pump relay, unplugging the injectors, whatever. Just do one of those and make sure you dont have any fuel.

If you cut the spark thats up to you.

General rule of thumb is cylinders should be within 30psi. of eachother...

the98stang · May 2, 2008

Be sure to do it on a warm engine. Remove all spark plugs first. Keep the TB open with your foot on the gas pedal. My car usually has about 170-180psi, but what matters is consistency. Each cylinder should be about 15% within each other. So if you have one cylinder that has 170 psi and another with 145 psi, you most likely need new rings or valves.

TheGrayt1 · May 2, 2008

Update did the compression test on a cold engine and they all the plugs were out at once and here is what I came up with on just the drivers side.

5 150
6 120 but not sure it was screwed ina ll the way
7 150
8 160

Then I added oil to each cylinder at a time and just wanted to see what the readings did and every cylinder jumped by at elast 50 psi, is that normal?

5 200
6 200
7 220
8 near 240

I then put everything back together and started it with a ton of oil smoke for about 20 minutes and then drove it around and it is running fine. Should I redo the test on a warm engine and 1 cylinder at a time? These numbers seem so whacky I am not sure what to read into them. Also how long will it take for all that oil to burn off completely to redo the test? I dont want the oil to aid the numbers.

Stan Weiss · May 2, 2008

Did you have all of the plugs out?
Did you have the throttle open?
How much oil did you add?

TheGrayt1 · May 2, 2008

I did have all the plugs out and the throttle open and added a teaspoon of oil.

I drove the car around to clear out all the oil and while it was still warm with just taking the number 6 plug out it read 120 again. Does that seem too low with the others reading right at 150 psi.

Stan Weiss · May 2, 2008

You can do a leak down test on the engine, or just bring number 6 TDC. Screw a air hose in the spark plug hole and put 100 PSI into the cylinder. Than listen for where the air is leaking out of the cylinder from. Intake valve, exhaust valve, rings, or probably not head gasket.

TheGrayt1 · May 2, 2008

Are these numbers too low? Should I be concerned about that one cylinder?

BurningRubber · May 2, 2008

Thats about the most tolerable difference it should have.

Probaly a little more ring wear, or even a valve seal issue with that cylinder.

Does your car consume a bit of oil? Either way I would not be too too concerned over it... ill post my numbers real quick.

Ok, this is the drivers side... #1 is closest to the radiator.

1: 162psi
2: 172psi
3: 170psi
4: 165psi

Now the passenger side, where my issues are...

1: 150psi
2: 123psi
3: 160psi
4: 120psi

Now, what bothers me is that the whole passengers side is lower. This is with 126k or so on the engine. Uses about 1 quart of oil between changes, no noticable smoke out of the exhaust.

What I am wondering is about the timing of the cams. I have read (and im sure others have) that even from the factory they can be off as much as 6-8* wich would cause one side to have lower compression.

That doesnt help the fact that cylinder "2" and "4" are definatly showing wear, but it may bump the overall numbers up. Kind of dont think that the cam timing being off is an issue though, it put down some strong numbers for just bolt ons.

bhuff30 · May 2, 2008

I'd say yes, you should be concerned and try to start figuring out the reason. It may be something simple like, you aren't seating the tester well and allowing a leak, but you should figure it out.

Does the engine smoke or use any fluids? What prompted you to do a compression check in the first place, and ONLY on that bank? If the test and method is correct, then it could be anything from bad rings to a valve leaking.

When I had a crack in the cylinder wall of my 2.3, it would only read about 8-10psi lower than the other cylinders.

TheGrayt1 said:
Are these numbers too low? Should I be concerned about that one cylinder?

TheGrayt1 · May 2, 2008

I did the test because I got bored and wanted to see what it read. I just bought the car and am not sure if it uses that much oil. It was about a quart low when I got it with 3k on the oil. The engine does not smoke at all from what I have seen. I think my first test numbers are correct. How worried should I really be. This car is super clean and runs well and I would hate to lose her.

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