Author Topic: Compression test procedure (Read 11623 times)

tazz · « **on:** April 14, 2011, 09:06:05 AM »

I need to know the proper procedure for a compression test and probably a leak down test would be better but I want to test something first.
IIRC I need to remove the fuses for the fuel pump and disconnect the CP's from the plugs correct? I have a older gauge that reads correctly but since our cars have a 1-2 sec crank time my ? is do I take the number off the first crank cycle or 2-3 crank cycles until the needle quits moving any higher?

Critterman · « **Reply #1 on:** April 14, 2011, 09:11:45 AM »

good practice would be to shut down the fuel pump. Removing the fuses would work.

Remove all the plugs so the engine is free wheeling. Install the gauge, let the engine spin over several revs and read the gauge. Do it one cylinder at a time.

Besides the pressure all being within the recommended range there should not be a significant difference between adjoining cylinders.

tazz · « **Reply #2 on:** April 14, 2011, 09:17:05 AM »

Glad I asked I would have kept the plugs on the other cylinders.
Whats the "Normal" range for our LNF?
Thanks Critter.

Critterman · « **Reply #3 on:** April 14, 2011, 09:43:59 AM »

You would have to check the manual. I don't have one here.

tazz · « **Reply #4 on:** April 14, 2011, 09:57:08 AM »

I also assume that the cylinder just behind the cam sensors is cylinder 1 correct?
With that assumption cylinder 1,2 had the same readings 150psi for the first crank then 170psi for the second/third crank.
Cylinder 3 had a reading of 120psi for the first crank then 130psi for a second and third crank.
Clylinder 4 had a reading of 145psi for the first crank then 165psi for the second crank and 170psi for third crank.
Is this a problem that the dealer should look at?
Thanks

Critterman · « **Reply #5 on:** April 14, 2011, 10:21:53 AM »

I would think those are all in range, but without the manual cannot give you a definite answer. 3 while in range looks low to me. Squirt some oil in there and do a leak test. Let it turn over for a while.

TomatoSoup · « **Reply #6 on:** April 14, 2011, 10:37:28 AM »

This is what I found...

Quote

The minimum compression in any 1 cylinder should not be less than 70 percent of the highest cylinder. No cylinder should read less than 690 kPa (100 psi). For example, if the highest pressure in any 1 cylinder is 1 035 kPa (150 psi), the lowest allowable pressure for any other cylinder would be 725 kPa (105 psi). (1 035 x 70% = 725) (150 x 70% = 105).

• Normal -- Compression builds up quickly and evenly to the specified compression for each cylinder.

• Piston Rings Leaking -- Compression is low on the first stroke. Compression builds up with the following strokes, but does not reach normal. Compression improves considerably when you add oil.

• Valves Leaking -- Compression is low on the first stroke. Compression usually does not build up on the following strokes. Compression does not improve much when you add oil.

• If 2 adjacent cylinders have lower than normal compression, and injecting oil into the cylinders does not increase the compression, the cause may be a head gasket leaking between the cylinders.

Sol Asylum · « **Reply #7 on:** April 14, 2011, 01:19:06 PM »

Do you still need directions on doing a leak down check?

tazz · « **Reply #8 on:** April 14, 2011, 01:27:26 PM »

No
I had them replace the CAT last week and it's still just popped a code now for o2 sensor and something else.
I'll mention to them about the Compression check and have them do a leak down test. Funny thing is 2 months ago I did the compression check test and every single cylinder was about 165psi and now #3 is 130psi. Still think that when they replaced the turbo and cleaned out the IC they may have dislodged a piece of turbine blade that may have chewed up part of the ring or chipped a valve.

Sol Asylum · « **Reply #9 on:** April 14, 2011, 01:45:50 PM »

It is possible that you had some FOD go through the engine but compression numbers can vary quite a bit in a short time. A leak down test will help to tell what may be leaking and point you in a direction to look further. Even with leak down tests I always looked at the trend in the results over time before deciding we needed to tear into an engine.

tazz · « **Reply #10 on:** April 14, 2011, 01:51:33 PM »

P2251 P0130 are the codes pending right now but no CEL at the moment.
Still getting light smokey exhaust on WOT but not as much as before the CAT replacement so it probably tied to the O2 sensor code but Im always wondering now which is why i checked compression this morning.

snaponbob · « **Reply #11 on:** April 14, 2011, 05:04:28 PM »

The leak down test is an excellent diagnostic procedure. That said, given the results of the tests (ignore the differences in the two tests, as they are close enough), there IS a problem in #3. If they had a bore scope, they could SEE if there is any damage in #3. But the leak down, down correctly, will tell a lot. If you do not have an air compressor, it is not doable. As for the O2 sensor code, assuming the circuit is good (the plug is seated, etc.) it is probably contaminated. BTW, LNFs should not smoke at all.

tazz · « **Reply #12 on:** April 14, 2011, 06:32:36 PM »

I personally think it's proabably a ring but who knows could be a valve?
Talked to the service Mgr and he said bring it in and they will look at it since it's under warranty.

Gregory · « **Reply #13 on:** April 16, 2011, 02:53:37 PM »

I ran a test at my LNF recently and all 4 of them gave 125 psi.

tazz · « **Reply #14 on:** April 16, 2011, 03:54:03 PM »

I ran mine on a cold motor and did three crank starts to hit around 170psi. Not sure if that the proper way but with just one crank start they all hit right around 125-130psi. What had me a bit worried is that while the other cylinders hit a max of 165-170psi cylinder three could not get higher than 130psi.

Sol Asylum · « **Reply #15 on:** April 16, 2011, 05:42:15 PM »

Typically compression checks are done on a warm engine because all the clearance items in an engine are designed to be at a minimum when the engine is up to normal operating temps. Doing the test cold can give false low readings.

It is not uncommon to have variances between cylinders and sometimes you can take a reading and then go run the engine hard and take another reading and find the second set of numbers to be quite a bit different from the first. However over time you should see a trend in those numbers and that trend will help tell you when you need to be concerned about something in your engine.

In the aircraft maintenance world we only do leak down tests as they help to tell what's going on with the engine, all you have to do is listen to where the air is leaking. I personally don't find much value in the other type of compression test and when I have had to check compression on car engines I borrow the tools from work and do the tests. I have seen several automotive leak down testers and have not been impressed with them. A proper set should have two gauges one to tell how much pressure you are putting in the cylinder and one to tell how much it is holding. On aircraft we do the tests at every annual and 100 hour inspection and then record the results in the log books so that we can see what the trend is like.

tazz · « **Reply #16 on:** April 30, 2011, 11:08:37 PM »

A bit of an interesting twist.
Well it's smoking again at startup and under light throttle as well as rough at idle and can see and smell oil burning now but no codes so I assume that the ring has been trashed. Looked at the plugs and funny it's #4 not #3 that is oil fouled and compression in #4 is around 100psi and I didnt check the rest but the other plugs are NOT oil fouled yet.
?
Since it's under warranty what should I ask for in terms of the engine and what will they do at the very least?
My guess is that some of those tips of the turbine finally made it through and down one or more of the cylinders to wreck havoc on the walls and or rings maybe both on one or more cylinders and the reason #4 compression is low and plug is oil fouled. I would think that they would inspect the other cylinders for damage and if they find more damage replace the engine is that reasonable or will they just rebuild the cylinder/s that are damaged? At what point will they say lets just put in a new engine and call it good.
Thanks

1LILNDN · « **Reply #17 on:** April 30, 2011, 11:26:22 PM »

tazz Mine wasblowing oil in 3 out of 4 Pistons cracked and rings . The dealer told me they were putting in for a whole new Motor. Got it back couple weeks later. they said GM central told them to rebuild it . Good Luck. Mine only has 21,385 miles on it.

tazz · « **Reply #18 on:** April 30, 2011, 11:28:14 PM »

Mine has 32,800 on it.

Arabas · « **Reply #19 on:** May 01, 2011, 05:45:08 AM »

tazz, as you say they will probably try to see what's the problem and see if they can fix it.
i would propose to push them a lot to give u a new engine since u are under warranty.
show th.em u are fed up with this problem and u want radical actions asap

tazz · « **Reply #20 on:** May 03, 2011, 10:28:11 PM »

Talked with the dealership the other day and basically said it's TAC's call. He said they look at it as a cost analysis and if it cost more than 75% of NEW to rebuild then they will give me a NEW LNF if it's less than 75% of NEW then they will rebuild. He said it has to be real bad before they replace since the engines now are relatively easy to rebuild as long as the damage is not HUGE. The cylinders are sleeved so they just pop out and re sleeve and check pistons and replace rings and check the rest of the engine for damage and if good put her back together as long as it's just the rings and cylinder walls.

Gentleman Jack · « **Reply #21 on:** May 04, 2011, 01:45:39 AM »

Quote from: tazz on May 03, 2011, 10:28:11 PM

Talked with the dealership the other day and basically said it's TAC's call. He said they look at it as a cost analysis and if it cost more than 75% of NEW to rebuild then they will give me a NEW LNF if it's less than 75% of NEW then they will rebuild. He said it has to be real bad before they replace since the engines now are relatively easy to rebuild as long as the damage is not HUGE. The cylinders are sleeved so they just pop out and re sleeve and check pistons and replace rings and check the rest of the engine for damage and if good put her back together as long as it's just the rings and cylinder walls.

Pretty smart of them really... Make repairs easier! What a concept. kappas - so state of the art

GJ
Tongue in cheek

tazz · « **Reply #22 on:** May 04, 2011, 05:46:27 PM »

Well the dealership called and said piston #4 was cracked so it looks like there replacing #4 psiton and all the rings in all the cylinders and gaskets head bolts and I'm not sure what else. They said the cylinder walls all looked good but would they need to re-hone the walls so that the rings will seat properly?
They will be re-honing all the cylinder walls and replacing the cylinder rings.
Thanks

Sol Asylum · « **Reply #23 on:** May 04, 2011, 07:03:28 PM »

It looks like you already answered your own question, but yes if you replace rings you hone the cylinder so that the rings wear in and seat and seal properly.

Arabas · « **Reply #24 on:** May 05, 2011, 02:39:54 AM »

tazz, have you considered buying aftermarket pistons and rods and supply them the to the dealer? you still won't pay for the job done, but you will have upgraded internals

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Author Topic: Compression test procedure (Read 11623 times)