PDA

View Full Version : Compression Test


KHall
11-24-2009, 09:16 PM
Finally did this and here are the results:

2003 X2
Indmar 5.7L Predator TBI 733 Hours

1 2 3 4 5 6 7 8
200 200 205 202 210 190 211 200
202.3 Average
211.0 Max
190.0 Min
21.0 Max/Min Difference
10.383% % Difference from average

I believe the numbers are more than good enough to run well.
But, should I be a little worried about number 6?

KHall
11-26-2009, 10:17 AM
Anyone have any input?

1redTA
11-26-2009, 10:43 AM
If they are within 10% you are good

JimN
11-26-2009, 12:07 PM
Finally did this and here are the results:

2003 X2
Indmar 5.7L Predator TBI 733 Hours

1 2 3 4 5 6 7 8
200 200 205 202 210 190 211 200
202.3 Average
211.0 Max
190.0 Min
21.0 Max/Min Difference
10.383% % Difference from average

I believe the numbers are more than good enough to run well.
But, should I be a little worried about number 6?

Was this with the motor cold or at normal operating temperature? Also, it looks like the injectors were firing and you need to check this dry, with all spark plugs removed, at normal operating temperature.

Most GM 5.7L motors show closer to 150-190 lb, which is the reason I commented. If you're not having any performance issues, I wouldn't worry much about the compression. My truck is close to 300K miles and has never been torn down. Last time I checked the compression, the range was 155-163 lb and most were the same- 158 lb.

JLeuck64
11-26-2009, 02:56 PM
Next time you do this compression test... add a tablespoon of engine oil to the cylinder that reads lower compression then re-test that cylinder. If the compression readings come up it would indicate poor sealing of the rings. If the compression readings don't change at all it would indicate poor sealing of the valves.

KHall
11-26-2009, 04:05 PM
Thanks Guys!
Jim - The test was performed cold, with the plugs in. How do i stop the injectors from firing? How did you know? I had the throttle wide open with kill switch removed. Had no fuel pressure but believe the residual fuel did come out of the two TBI injectors.

The 200 psi seems high to me, but my last predator motor also had 200 psi.
The #6 cylinder seemed just a little low, but only in comparison to 200 psi.

As i look at the numbers the 190 is only 5% less than the common 200 psi readings. So i am not worried, but was wondering if i should "watch" this cylinder.

cbryan70
11-26-2009, 10:05 PM
not an accurate reading if you still had the plugs in.

EricB
11-26-2009, 11:30 PM
Yes, a compression test is still "accurate" if all the plugs are in, and even if it is cold. The reality is that part of the reason for doing a compression test is to compare one cylinder to the next. So if your testing parameters are the same across all cylinders when you are testing, it still compares one to another under the same given set of circumstances. A bad cylinder will show up warm, cold, plugs or no plugs.
Now, if you really want to be accurate, you need to ensure the cranking rpms are the same between cylinders. Therefore, you should always ensure your power source(battery) is at the same level before each cylinder test, ensure they are cranked for an exact time count and watch the rate the gauge climbs for each compression stroke. You had better read the cranking amps/cylinder test also. This will give you more valid data. In addition, add a small amount of oil to any cylinder when testing and your readings will change.
To me, the best "compression test" is a dynamic one. There are 2 ways to do this. One is with a compression tester in the cylinder while the engine is running, then snap the throttle open and see how high the reading climbs. Compare each cylinder to one another (this is how you find bad camshafts in LT1's). the second way is to put the boat in the water and warm it up. Put in gear and from low idle..... punch it. If there is variance between cylinder output, this is where you'll feel it.

KHall
11-27-2009, 08:53 AM
Plugs in or out; always seems to get debated. Some even say it spins faster with the plugs in, i don't believe it. I did have a charger on the battery during this test. Still think Jim is on to something because the injectors did pass some gas during this test. Gas could change the readings in a given cylinder. I hope to retest with it "dry". Soon as i figure out how to keep the injectors closed.

JimN
11-27-2009, 09:57 AM
Thanks Guys!
Jim - The test was performed cold, with the plugs in. How do i stop the injectors from firing? How did you know? I had the throttle wide open with kill switch removed. Had no fuel pressure but believe the residual fuel did come out of the two TBI injectors.

The 200 psi seems high to me, but my last predator motor also had 200 psi.
The #6 cylinder seemed just a little low, but only in comparison to 200 psi.

As i look at the numbers the 190 is only 5% less than the common 200 psi readings. So i am not worried, but was wondering if i should "watch" this cylinder.

The compression numbers were higher than normal, so I assumed fuel was being delivered, and the fuel provides a better seal/takes up more space in the cylinder than a dry test. The kill switch kills spark but not the injectors. Look near the ECM for three fuses- one will be for the fuel pump. Wide open throttle should have kept the injectors from firing- that wasn't included in the first post but it may be possible that the throttle wasn't quite at 100%. If you do this again, you might want to verify this by removing the flame arrestor and watching for fuel- if it stops, you're OK to proceed but it still needs to be done at normal operating temperature.

I would re-test it dry soon and again in spring after using it for a month or two. As long as you don't see a significant change, you should be OK. What you indicated doesn't make me think it has a problem and since it's only 6 years old, it shouldn't have wear issues unless it has been abused badly, which I don't suspect.

JimN
11-27-2009, 10:06 AM
Plugs in or out; always seems to get debated. Some even say it spins faster with the plugs in, i don't believe it. I did have a charger on the battery during this test. Still think Jim is on to something because the injectors did pass some gas during this test. Gas could change the readings in a given cylinder. I hope to retest with it "dry". Soon as i figure out how to keep the injectors closed.

It does spin faster- there's no compression to crank against but unless the charger is putting out a lot of current, it didn't do much. These starters draw well over 150A when loaded by the motor with the plugs in place and can draw even more than that. You can verify this with a clamp-on ammeter. Also, with the plugs in place, the cylinders can draw fuel in because they still have vacuum when the piston moves down and that;s why it should be done dry. With TBI, the intake may get the fuel with no plugs and that will lead to a higher reading on the rear cylinders because gravity will gradually cause fuel to go to those cylinders but not the higher ones. When each cylinder has its own injector, there's nothing to keep fuel from going in and that will flood the motor, causing fuel to bypass the rings and dilute the oil.

If you want to stop the injectors without pulling the fuse, remove the wires at the top of the injectors- they can sometimes become brittle, but this will work. I would recommend pulling the fuse.

Ideally, a compression test will be done in conjunction with a vacuum test and cylinder leakdown test. Those three will indicate the general health of the motor.

KHall
11-27-2009, 11:34 AM
Thanks Jim. Yes, i agree with you, it spins faster without plugs, will try this again soon. Either way going wakeboarding one last time on Sunday. Sadly, then the inevitable winterization will occur.