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View Full Version : Compression test... odd results



burgurboy
02-21-2013, 05:59 PM
Hey guys, I'm considering throwing a PES G2 kit on my 2000 2.8 ATQ. It's currently got just under 140k miles on it.

I went out this evening for a compression test. The results were:

6- 205
5- 190
4- 186
3- 200
2- 195
1- 95,152,90,120

Yeah, the results on cyl 1 were different every single time. I don't understand it... During one of the tests, it completely gave out after 4 turns. I felt it "pump up" 4 times, then just let go. After I removed and re-installed my compression tester, it read 152. There was no consistency at all on this cylinder.

Any Ideas??

98A4TurboAWD
02-21-2013, 06:11 PM
Pour a little ATF into that cylinder and see if it gives a more consistent reading. If it does, and then goes back to being erratic, then there is a problem with the piston rings.

That is assuming the gauge is functioning properly.

And also assuming that you shut down the fuel supply, took out all plugs and held the pedal to the floor while you performed the test.

GetBoosted84
02-21-2013, 06:51 PM
Pour a little ATF into that cylinder and see if it gives a more consistent reading. If it does, and then goes back to being erratic, then there is a problem with the piston rings.

That is assuming the gauge is functioning properly.

And also assuming that you shut down the fuel supply, took out all plugs and held the pedal to the floor while you performed the test.

Why ATF? I've always used regular engine oil for the same purpose so I'm curious.

Also to the. OP, did you happen to ensure you weren't running out of juice on the battery when testing that cylinder?

audinutt
02-21-2013, 07:04 PM
i had a similar issue on my 4.2 and it was carbon buildup causing the erratic compression results.

98A4TurboAWD
02-21-2013, 07:18 PM
Why ATF? I've always used regular engine oil for the same purpose so I'm curious.

Also to the. OP, did you happen to ensure you weren't running out of juice on the battery when testing that cylinder?

ATF can break up carbon buildup on rings and pistons that can cause funky compression. You can use engine oil if you want to but it does not have this effect. I have always used ATF and not engine oil.

You do raise another good point, in that the starter does need to be functioning properly. I assumed because it was the first cylinder that he did it toward the beginning. If you do not have good juice then you will get an inconsistent reading. The starter has to spin as it should in order to get accurate results.

GetBoosted84
02-22-2013, 04:51 AM
ATF can break up carbon buildup on rings and pistons that can cause funky compression. You can use engine oil if you want to but it does not have this effect. I have always used ATF and not engine oil.

Ahh gotcha. I never worry about breaking up any carbon since my cylinders look squeaky clean from the meth.

burgurboy
02-22-2013, 08:14 AM
Throttle was not wide open for any of the tests... I'll drop in some atf this afternoon and see what that does.

FWIW. #1 was the 4th cylinder I tested. I came back to it several times in between the other cylinders.I also tried different comp testers.

Is there any way to know for sure if its carbon build up? If that'sthe case... supercharging + WMI would help!

Call it wish fun thinking but it seems odd to me that a ring would be bad this early in it's life. These motors seem to have an awesome reputation for ring life.

ddillenger
02-22-2013, 09:23 AM
You can't just put the throttle to the floor on DBW cars, you must remove the intake and hold the TB open.

burgurboy
02-22-2013, 09:59 AM
Well that sucks... It won't open even with the key on?? At what point does the ecu decide its okay to open the throttle?

ddillenger
02-22-2013, 10:01 AM
There's a map for that (lol). Anywho, it won't open until engine load requires it to open.

burgurboy
02-22-2013, 02:24 PM
welldang.

I ran the jones out of the car on the way home from work, let it sit for 30 minutes, took everything apart again, hooked up my comp tester and was greeted with this...


http://i1020.photobucket.com/albums/af321/burgurboy/IMG_20130222_163642_zps0b57e696.jpg


Wooh! I don't know what I did different but, I'm not complaining. On second thought... It may not have been warm enough last night.

Time for BOOST!!!

M-Hood
02-23-2013, 10:07 AM
You can't just put the throttle to the floor on DBW cars, you must remove the intake and hold the TB open.

You do not have to do that, when you do a compression test you take out all of the spark plugs which will allow enough air into the whole motor and intake manifold when the motor is cranked. Why make it harder then it really is?

GetBoosted84
02-23-2013, 10:29 AM
Mike is right, I've never had to do that to get a solid reading. Just need to make sure the car doesn't have a crappy charge on the battery or it won't crank over.

U_know_me
02-27-2013, 08:02 AM
Just my 2 cents, I have tested compression on many cars and have seen a lot of different things. Driving it hard should not change the reading of compression, and to me would indicate that something that is wrong. Best case scenario you just have bad carbon build up and some seafoam (be very carful with that stuff) or the ATF process described above should fix the issue.

Worst case scenario with the erratic reading cyl you more than likely have a bad oil ring and a compression ring on its way out. Do another test and compare how fast the reading comes the that cyl compared to the others. If oil is in the cyl (passing the compression ring) the dial will increase faster than the others but because the oil is in the cyl it will give a close to normal reading. What I described is just something I experienced and is possible in this application, I hope I am wrong for your sake. Good luck.

burgurboy
02-27-2013, 08:54 AM
Well, I attributed the erratic readings to the engine being cold. The car sat 4 or 5 hours before I began the initial test.

When I talked about running the car hard I was just trying to explain that the car was warmed up.

Maybe I'm just hopeful Lol...

Wouldn't a bad ring burn more oil 1qt in 6-7k miles?

If it were carbon buildup, water/meth injection would clean it up, right?

U_know_me
02-27-2013, 09:20 AM
Once the car was warm, did you get the same reading more than once on that cyl? I see now from your thread of the paint and everything (looks great BTW) that it sat for a while and didnt have much maintenance. I would try some different carbon/engine cleaning processes and see if that helps first. Seafoam has worked best for me in the past, but be very carful with that stuff. If not used correctly it can cause very much harm to an engine, so consider yourself warned. I dont know much about the 2.8 so if I was you I would do a lot of research and look to see if there is a DIY for the 2.8 to seafoam. Remember when using seafoam, less is more! Personally I seafoam every vehicle I own about every 30k.

To answer your question, oil consumption is different for every car IMO, some cars naturally burn a little oil, others dont burn a lick. That does not seem excessive for the age of the engine, but is still possible that a ring is on its way out. Also I have never heard of water/meth as being used to clear carbon, but I am sure it would assist in the process, but like I said above seafoam is the best way and only costs $6ish a bottle.

burgurboy
02-27-2013, 04:13 PM
Thanks for that tip mike, I had never heard of the "throttle open" thing either and it didn't seem to have an effect on the other 5 cylinders

Okay, here's the deal. I came home from work today and tested cyl 1 and 4 again (my other lowest cyl(186)). Both tested a teensy bit above 190, several times. I tested cyl 1 twice, moved to cyl 4 and tested twice, then moved back to cyl 1 for two more tests. They tested consistently each time. 190+*

So, at this point my only variable was engine temperature. I've been doing some reading and severely inconsistent results seem to be "normal" when testing a cold engine. This, coupled with the low oil consumption (with known -albeit small- oil leaks)... I've gotta assume that my rings are just fine. All readings were within 7% of the highest (205).


and on the subject of water injection, It's primary purpose is not to clean vales but, it's a definitely a side effect.

burgurboy
03-17-2013, 09:11 PM
Okay... I'm gonna have to bump this.

I let the car sit for about a week. When I cranked it up, the starter did not sound "even' when cranking over the motor (it look a couple of turns after sitting).

When it first started, it had a bad misfire.

I popped the hood and started to pull plugs to identify the faulty cylinder but, by the time I got around to it, the misfire was gone.

Now I'm confused. Is it possible for a ring to produce nearly low compression when cold but, perfectly in-spec compression when warm?

Please keep in mind that the car only burned 1qt in about 7k miles. That's with short trips in between work sites. The car hardly ever sees highway. I would think a leaky rings (when cold) would burn a lot of oil in a car that does a lot of "cold" running.

I may try a leak down test later this week....

Any other ideas? This whole thing sounds odd to me.

contrldsub
03-18-2013, 09:27 AM
misfire while cold sounds like fuel delivery to me.

as for rings, you have seperate rings for seperate jobs, the compression ring does just that while the oil scraper ring does just that. burning oil is normally a oil scraper ring, blow-by is compression ring.