Compression test results

[rallydiesel]

Well, I did my first compression test on our 1991 Jetta TD. Can anyone help interpret these results?

Cylinder 1: pressure would only rise on the first stroke, eventually got it to 400 PSI after many, many cranks

Cylinder 2: 140 PSI  :cry:

Cylinder 3: 420 PSI

Cylinder 4: 420 PSI

Why would cylinder 1 only compress on the first crank? Would this point to a valve failure? I have the hydraulic 1.6.

I guess the next step would be to overhaul the engine. I am thinking this may be out of my scope. Anybody have any tips?

There is no oil in the coolant (green and clear) and there is no coolant in the oil (not milkshake-y looking).

[Powjetta]

Did you do the test warm or cold?  You may have a hydraulic lifter that won't pump up except when running with more oil pressure.

[rallydiesel]

Well, I tried to test warm, but by the time I got the injectors out, the engine was pretty cold.

The car runs pretty well so I was surprised by the results.

[jtanguay]

hmmm those numbers just dont add up... i would suspect leaky valves as the culprit for #2 and possibly #1...

#2 & #3 should get the most heat the way that the exhaust manifold is made up... did the 'warm' make any difference???

[rallydiesel]

I wasn't able to get "warm" results.

How do you do the cylinder bore/journal/etc. measuring? Do you need to bring the parts to a machine shop or can you use a caliper? What do you use to measure the bores? How do you install the thrust bearings?

[jtanguay]

just use the caliper to measure bore size... you need two readings, side to side and front & rear... mainly because of the pressure exerted on the walls during the power stroke, and the compression stroke even...

[rallydiesel]

So I can just use the back of the caliper? Doesn't it have a square edge? Don't I need a pointed edge to get an accurate measurement?

[jtanguay]

So I can just use the back of the caliper? Doesn't it have a square edge? Don't I need a pointed edge to get an accurate measurement?

hehe now you're thinking REALLY accurate...  yes there is a slight slope on the cyl wall that develops... very very very minor.  the place where it should have more wear will be when the rod is at a 45 degree angle (or so), which is in the uppermost portion.  this is how the cyl turns oval, because the force of the piston is exerted not only on the rod, but the opposing portion of the cyl wall.

you could always buy a caliper that will measure that, and check it out! i think you would need a micrometer to find any major difference in wall thickness, unless the motor is really worn.

in the meantime, just use the square caliper.. it should be good enough!  I've been farting around with my TDI checking wear etc.  pretty interesting stuff!  it doesn't seem to have much wear at all... i'd like to use a digital gauge to get those really precise measurements.

[burn_your_money]

Low compresion on 2 adjoining cyinders is usually a head gasket issue

[jtanguay]

Low compresion on 2 adjoining cyinders is usually a head gasket issue

especially with such good compression on the two cyls...

[Doug]

To measure the bores front to back and top to bottom with only a set of feeler guages, take a measurement at the bottom of the bore with your piston ring squarely in the bore using the feeler quage that slips firmly through the end gap of the piston ring across the thrust surface of the bore. This is your reference number. Rotate the end gap 90 degrees - check the end gap. Note all readings of different guage thicknesses. Now subtract each one from the reference guage thickness and divide by pi individually. That will yield the difference in diameter at each station that you have measured. Do this up and down the bore. If you have a really good set of calipers and know how to use them your first measurement can also be referenced to a full diameter measurement. Do not use the top of the bore as it will be carbon fouled and unworn from original assembly.

[rallydiesel]

Can I use a 1.9 metal HG? Do these come in different sizes like the 1.6 HG's?

[rallydiesel]

How important is it to overhaul the bottom end? Considering this engine has 370 000km and I have no idea if it has ever been done. How much does a complete piston and ring set run and should I replace the oil pump if I do this?

Sorry for all the questions but I am feeling a bit overwhelmed with how much work, time, and money this could cost. At the same time I want my son to have a reliable car and if I have to do the rings I may as well do the bottom end, right?

[Vincent Waldon]

Rings are going to be cheap... pistons are going to be expensive... once you've measured the bores you will be in a position to know if you should rebore the block... if not and the pistons are in good shape then deglazing and re-ringing will work nicely.

Jack at www.vwdieselparts.com is one source of pistons and rings... haven't found a reasonable Canadian supplier yet... perhaps someone else here knows ??

Bearings are cheap as well... if the pistons are out might as well replace the con rod bearings, and a little plastigauge will tell you if the crank bearings are ok... if not,  you can, with a bit of effort, slide them in as well.

Like any project scope creep can be expensive... but I personally prefer to go bigger if need be and and then drive it for another 300K carefree.


Vince

[Doug]

At 370 km, I would look at the head first. Those bronze guides are probably beat. So new guides, new valves and a head gasket. That will probably run you about $600 if a machine shop does the head work and you do the disassemble and reassemble. If it ran okay before it will run way better now. It is tempting to do rings but if your oil consumption is not too bad then leave them alone.