Compression Test Take 2

[turbovair]

Okay,
Is a 6.5 compression test done through the glow plug openings or the injector openings? If done at dealer, how much should this test reasonably cost?

[AndyL]

Yes, through the glow plug holes.

Can't comment on the price.

[turbovair]

Thanx Andy...

[gunner 6165]

I just had the compression check done on the 'burb. Cost me $220. Minimum pressure is 370.

[tom.mcinerney]

Want to have done by experienced outfit which can interpret results meaningfully; some commercial adapters use glow plug hole.

[moondoggie]

Good Day!

gunner 6165 said,

[cruzer]

IIRC Diesel Depot sells used engines that have a min of 350psi. IIRc mine had comp around 400psi.

I think you might have to take in consideration of the volume of the cyldr? I don,t know for sure though.

[gunner 6165]

moondoggie

I wish I could give you a scientific answer to how 370 came about, but unfortunately I can't. Dealer said it's the minimum, checked it on alldata.com, he's right. (wow, did I say that?) :eek:

And I can't argue with your number either, my calculator said the same thing.

Is this a indirect injection system? If so, there would have to be another chamber. If this chamber were smaller, the pressure would increase as the air is pushed into it.

I may be wrong on this, I've been wrong before, just ask my wife,(Love ya honey), but I seem to recall reading somewhere that our engine in indirect injection. I'll have to do some research in the morning to see.

[moondoggie]

Good Day!

I don't know what indirect injection is, but all 6.2's & 6.5's are prechamber engines. I THINK the fuel is injected into the prechamber, not the cylinder.

I've heard these high compression numbers here on the Page previous to this topic. I sure hope someone can clear this up. I was kinda hoping the good Doc would chime in. ;)

I also had wondered if perhaps the velocity of the air during the test might affect the result. In other words, when the piston comes up, the air leaving the cylinder & going into the compression gauge is probably moving pretty fast. Is it possible it could overcharge the compression gauge in this way? If it did, the readings would stay at an elevated reading, as the check valve built into the compression gauge wouldn't let any of this additional air back out, as of course it shouldn't.

Oh, well, of all the things I've lost, I miss my mind the most.

Blessings!

Brian Johnson, #5044

[tom.mcinerney]

I think want see 4-500psi. Usually less than 10-20% variance between cylinders. The spinning RPM will affect. The temperatur of the engine, and ambient temp can make big diff. Diff gages will vary.
So a shop that does regularly, with lots of repetition, hopefully on same/similar engines, will hopefully attempt duplicate test conditions, and hopefully be able to give most valuable interpretation.

[TimK]

Tom's correct. It's not just a matter of pressure and volume. Don't forget temperature. Adiabtic heating can be described with the equation PV=nRT. Where P is Pressure, V is volume of air, n relates to the mol wieght of a gas, R is a gas constant and T is temperature. It can be written as V = nRT/P.

The rise in temperature in the cylinder from the compression will cause the pressure to rise. If it wasn't for this are trucks would never run since there wouldn't be any heat to ignite the fuel. The flash point for #2 diesel is about 52 degrees Celius. #1 diesel is about 38 degrees C minimum. Therefore at starter cranking speeds, which is the same speed a compression test is performed the cylinder would need a rise in temperature high enough to cause the fuel to flash. The indirect injection engines help this along with use of the glow plug. The temperature of the incoming air, the cylinder wall temperature, the amount of pressure lost due to blow by, the reduced density of air due to higher elevations, and the speed the starter cranks the engine all have an effect on how well the truck starts.

TimK

[moondoggie]

Good Day!

I knew you guys could clear this up. Thanks! When you

[cruzer]

FYI comp test on the 6.5td is supposed to be w/ a warm engine.

[turbovair]

Wow, I inspired some deep thinking! But what should the test cost? smile.gif Can the dealer tell where blow by is occuring if present?

[TimK]

I can't tell you how much a dealer would charge. I would assume it would take about 1.5 hours of labor. The mechanic needs to remove the glow plugs to install the adapter for the guage. It all depends on how hard it is to get the plugs out. On my 85 Sub I can do this in an hour. Since I added the turbo, it may take longer. I bought my guage and adapter from Mac-tools online for about $75.00.

The mechanic can't really really tell you with certainty with a compression check if it is blow by, a cracked piston or head. In a gasoline engine a common trick was to squirt a little bit of engine oil in the cylinder which would seal up around the worn rings and tell you if that's where the combustion gases were leaking. you don't ever want to do that with a diesel engine. The diesel engine would most likely ignite the oil and therefore exceed the pressure of the guage and blow it up.

The mechanic would need to do a leak down test. Each cylinder is rotated so it is at it's combustion cycle position. High pressure air is injected into the cylinder and then it's a matter of listening for where it's comming out. Hissing out the tail pipe would indicate an exhaust valve, out the intake is an intake valve, from the oil filler tube would imply worn rings or a cracked piston, bubbles in the radiator would be a bad head gasket.

I have had cracked piston crowns before. A crack all the way through the crown has a dramatic affect on compression. When most of the other cylinders were at 325 psi, the one with the crack was at 175. When the engine was cold it would puff white smoke out of one exhaust pipe, there was excessive blow by (puffing from the oil filler tube) and there was the slight sound of a knock under heavy load.

TimK

[ 11-09-2004, 05:54 AM: Message edited by: TimK ]