Understanding Compression

So I'm new to this diesel thing. Picked up an older tdi with over 400k miles on it. Previous owner kept it up but got rid of it because it's due for a timing belt, and its got coolant mixing with oil. So he called it quits. I was just dumb enough to buy it off him

I want to run a compression test to see what I can't decipher about the overall health of the engine, but I don't own a diesel compression tester, and local parts stores don't seem to have them on hand for rental. I could send it to a shop, but I'd rather do it myself for the learning experience.

This brings me to my question. From what I understand, compression ratio tells you how much the engine will increase the pressure of air during the compression stroke. Googling around says the tdi makes around 19.5:1, and if air at sea level is about 15psi, then wouldn't you expect to see a reading of about 290 psi? Gas compression testers could handle that. But searching around this forum, I see folks posting numbers in the upper 400s. Why is the theoretical pressure so vastly different from what people are actually reading? Thanks in advance for the help.

Harbor freight sells a diesel compression tester kit with an adapter that screws into the glow plug holes to check compression with.

with the same adapter you can also cobble together a leakdown tester with metal plumbing tees and a 100psi pressure gauge to check the health of your valves and rings if you desire or get odd readings from the first test.

if you want to learn to do it yourself, this kit isn't over priced.

ALH compression spec new is 25 to 31 bar. The wear limit is 19 bar, but perhaps just as important the difference between cylinders is not to exceed 5 bar.

While not "exact", you can for the purpose of engine compression values consider 1 bar to equal 1 atmosphere, so to be simple we'll call that 14 psi.

So the engine compresses the air XX times one atmosphere. So a healthy ALH will be usually around 420-450 psi of cranking compression. But don't really split hairs here, look for consistency across all four cylinders. Because there are variations in gauges (especially the cheap Chinese ones) as well as starter cranking speeds, etc.

While the wear limit value of 19 bar is pretty low (you will rarely ever see an ALH in good working order that wheezes out only ~270 psi), you may see some in the 380-400 range. But again, so long as all four are close and the engine starts and runs acceptably, I would not be too concerned.

I rarely ever have need to check an ALH engine's compression, and I still service a boat load of these cars as well as own a few myself.

If you have coolant and oil mixing, the oil cooler is perhaps more likely the culprit than anything else, and is cheap and easy to change. But know that it can take a lot of time to purge any residual oil out of the cooling system.

You may do more harm than good (or may learn some other new things) attempting to remove the glow plugs to do said compression test. Depending on whether they have been removed in the past, what your patience and/or skill level, more than a few have been broken off in the head.
There is plenty of other things to do and learn about your "new" TDI and personally I wouldn't do this just out of curiosity.

Soak the threads of the glow plugs with 1:1 atf and acetone mix every day after you get home when it's hot and cooling down. For a good 4 days and dont use it sparingly. More is better. It is the #1 for depth into threads lube for getting things out like this.

oilhammer said:

ALH compression spec new is 25 to 31 bar. The wear limit is 19 bar.

Click to expand...

Ok, those numbers make sense. Clearly my Google search yielded the wrong numbers. Just curious, where did that info come from/where can I read up on accurate specs? And thanks for the heads up on the oil cooler. I'll look into that first.

Growler, have you used that harbor freight tester much? It gets a lot of BAD reviews, which isn't surprising since it's going for $30 compared to some that go for well over a few hundred bucks.

jettawreck said:

You may do more harm than good (or may learn some other new things) attempting to remove the glow plugs to do said compression test. Depending on whether they have been removed in the past, what your patience and/or skill level, more than a few have been broken off in the head.
There is plenty of other things to do and learn about your "new" TDI and personally I wouldn't do this just out of curiosity.

Click to expand...

This. If the car starts quickly and runs OK, I'd not mess with it. Look at the oil cooler for leaks, but you may also find that you have a failing head gasket. Look for telltale signs of coolant leaking around the head, pink crusty dried coolant. If you're lucky it's only the oil cooler.

My son has an '02 Golf that's approaching 400K, head's never been off. Runs like a top. These engines last.

My information for that came from Alldata.

To take a stab at your thermodynamics question: You are in effect simplifying the Ideal Gas Law a little too much when you say P1V1=P2V2.

You're ignoring the fact that when you do work on the gas in the cylinder by compressing it it heats up and the final pressure is higher. I think P1V1/T1=P2V2/T2 would apply but since P2 and T2 are unknown you can't use that equation.

That's as far as I can go without thinking too hard or digging out the Thermodynamics book.

I've got the HF tester. It works ok after I fiddled with the leetle valve.

I wouldn't care if it's not accurate. For your use you'd be looking more for repeatability. If you're 50 psi off on the reading it doesn't really matter since you're expecting one or two holes to be quite a bit lower than the others. It'll tell you that. If it's so bad that they're all low I'd expect that they would all be really low.

If you do the compression test don't forget to remove all the plugs first and disable fueling. Don't want the cylinder to fire while you're testing it.

The Horrible Freight tester is actually not so bad, if you remember the adapter is made from BRASS and can easily strip. Don't do more than snug it down.

Also, the schrader valve inside the adapter is usually no good. The gauge will leak down and not hold pressure. So, in a country with a Billion Bicycles, you'd think they could get a good schrader valve. We replace it with a good one.

The gauge is a relative measure. So, if you get a good cylinder pressure and it's 10% off, no big deal. I usually find the actual gauge for the diesel compression tester to be fairly accurate and unlike SnapOn, won't eat up your weekly income to buy it.

As for stuck glow plugs, once again I removed a glow plug that was attempted to be removed with an EZ-Out and broke off flush with the hole. And once again, we got the broken EZ-Out removed. DON'T USE EZ-OUTS. Better yet, don't break off the top of the glow plug. Set Torque wrench to 30 lbs and if it clics, reverse direction. Work it out with penetrating oil and be sure seat inside cylinder head is clean and works.

Thanks all, massively helpful responses. Look forward to NOT breaking my glow plugs anytime soon

What I would do first is pressure test the coolant to 30 psi. Might just be a oil cooler issue. Typically oil mixed with coolant is a head gasket but on a tdi it's not as common to mix unless its catastrophic. Usually you get exhaust pressurizing the coolant system.
Drain the coolant system. Pressurize to 30 psi maune 50. Listen for air rushing from the dip stick or the intake making sure you have the intake lobe open for each when testing. If it comes from the intake then you have a head gasket failure. If you have it from the dip stick it s probably the oil cooler. If it's from both you have a headmaster that is gone for sure. Testing the oil system as well by plugging the ccv will also rule out the head gasket or confirm. Same with pressurizing the intake. Just a back door to doing a leak down test/ compression due to issues with glow plug, if they dont come out.

More than likely the oil cooler creating the coolant mixing with oil.

All leaking head gaskets I've encountered all tend to over-pressurize the cooling system, which plays out in inconsistent or nearly no cabin heat, slow leaking coolant out the overflow due to over-pressurization of system, and very hard coolant hoses (again from being over-pressurized).

I don't think I'd go above 18 psi doing a pressure test....... (think about the fun replacing a leaky heater core)

You do the test by the heater core hoses thus bypassing it. Best and easiest place to test at due to small hose being able to take most common fittings at hardware store.