How To Do a Compression Test on ALH

duwem · May 22, 2012

Can someone go through the procedure?

I assume pull all 4 glow plugs, somehow crank the engine over while disabling the fuel pump and measure one at a time down the line?

Does Harbor Freight sell a compression tester that will work with our engines?

KLXD · May 22, 2012

Being from the salt belt you need to be very careful removing the plugs. They are known to break off. Start with penetrating oil a week or two in advance.

You need a gauge intended for diesels with a gauge that reads well over 500 psi. HF has it.

Need a good battery and starter. Some say you need it on a charger too.

Pull the connector on the pump or the wire on the cutoff solenoid. You'll get a CEL during this that'll have to be reset with VCDS.

Maybe a fuse could be pulled. Have to look at the schematic. Would this cause a light???

I test 1-2-3-4-1. If you get the same reading both time on #1 you know the battery held up ok.

duwem · May 22, 2012

PB Blaster to the plug threads right after the engine is shut off, or when its cold or something else?

jettawreck · May 22, 2012

duwem said:
Can someone go through the procedure?

I assume pull all 4 glow plugs, somehow crank the engine over while disabling the fuel pump and measure one at a time down the line?

Does Harbor Freight sell a compression tester that will work with our engines?

I've seen posted that HF compression tester works with the TDIs.
Seems like you have the right procedure. You can unplug the connector on the IP to disable fueling. Have the battery full charged or on a charger, spin until compression no longer builds.
Whether the tester is absolutely accurate isn't a big deal, you are mostly looking for uniform numbers from all four holes.

Edit: Sorry for redundant post. Too slow at typing.

whitedog · May 22, 2012

I have found that penetrating oil works best at exactly 137.5 degrees F, so get a digital contact thermometer and put it on the head near the Glow plugs and when the temperature reaches 137.5 spray the penetrating oil in there. Then time it and wait 43 seconds before trying to remove the glow plugs.

OK, that may be over-thinking a very simple procedure.

You have it down in your first post. Depending on why you are checking compression, the engine should be warm. So why are you checking compression?

FlyTDI Guy · May 22, 2012

Consistency is the key. Most recommend keeping a +50 amp starting charger hooked up to try and keep RPMs up over the whole procedure. And, BTW... it's 138°F.

duwem · May 22, 2012

I was thinking I would want to check compression cold as I'm getting smoking and rough idle on startup. Does that make sense?

Your are correct I'm in the salt belt.

So do I soak the plugs with PB shortly after shutdown and try to remove them a few minutes later, or soak them multiple times over a few day period?

Remove them when the engine is warm, or cold?

whitedog · May 22, 2012

FlyTDI Guy said:
And, BTW... it's 138°F.

For you low-landers maybe but up here in the mountains, the thinner air needs a lower temperature.

RacerTodd · May 22, 2012

When removing the glow plugs, note that the maximum removal torque is 25 ft/lbs. Set your torque wrench to 25 ft/lb and use it to remove the plugs. If the wrench clicks - STOP! Apply more penetrating oil, more heat and give it time. I believe removing them on a warm engine can help as well.

duwem · May 23, 2012

Good tip.

Got the Harbor Freight Diesel compression tester today for $30. Hope to try it out this weekend. Depending on how hard the plugs are, I assume I may need to get them all out, then put them back in and run the engine if it takes to long got get them all loose and things cool off?

FlyTDI Guy · May 23, 2012

Though some have had problems with them, they've always come right out for me. Torque on them is only 11 ft/lbs and too many think they have to tighten them like a lug bolt. That, combined with a little oxidation, can make for a problem. I know I'm going to suffer the wrath for this but... because of the dissimilar metal situation between the block and plug, I use (very sparingly) a bit of hi-temp never-seize. With that in mind, I only go 10 ft/lbs then check for leaks.

sdeck · May 25, 2012

duwem said:
Good tip.

Got the Harbor Freight Diesel compression tester today for $30. Hope to try it out this weekend. Depending on how hard the plugs are, I assume I may need to get them all out, then put them back in and run the engine if it takes to long got get them all loose and things cool off?

if its the same one, the GP hole adapter takes a 7/16 socket but the GPs are 10mm. thread pitch is the same but you need to change sockets from one to the other. make sure you use the right one. Also, I have had issues with the O-rings in the connectors on the tester getting scored or pinched so keep an eye on that. compression readings of 120 psi are kind of a giveaway to that

.

test hot for best readings as the rings need to be expanded.

duwem · May 25, 2012

GOOD NEWS:

560 560 560 565!

Those seem very well, especially for an engine with 240,000 miles correct???

Put new glow plugs in and now the SES light is off (had a dead plug).

PB seemed to do the trick, needed to use swivvel joint on the rachet to get the plugs out as the injector lines ran over some of them, and getting the quick disconnect on the adpators was tricky, but it all worked out.

Thanks for the tips guys.

jettawreck · May 25, 2012

duwem said:
GOOD NEWS:

560 560 560 565!

Those seem very well, especially for an engine with 240,000 miles correct???

Put new glow plugs in and now the SES light is off (had a dead plug).

PB seemed to do the trick, needed to use swivvel joint on the rachet to get the plugs out as the injector lines ran over some of them, and getting the quick disconnect on the adpators was tricky, but it all worked out.

Thanks for the tips guys.

I wouldn't put a lot of value on the actual numbers (they are pretty high/above normal) but the important thing is they are all very uniform and now you have a baseline to go from in the future-providing you use the same routine and tester.
Nice!

duwem · May 25, 2012

Ya, after looking at some specs, those are a bit high, I read 510-520 for an engine with 100k on it. Rather be consistent and on the high side than all over the map or low I guess.

What have others measured using HF testers?

jettawreck · May 26, 2012

duwem said:
Ya, after looking at some specs, those are a bit high, I read 510-520 for an engine with 100k on it. Rather be consistent and on the high side than all over the map or low I guess.

What have others measured using HF testers?

Don't be too concerned much with the actual numbers. Those are very good and consistant. Accuracy w/$30 tester isn't to be expected much. I think anything around 450# +/- 20# per hole would be acceptable.

TDIJetta99 · May 26, 2012

good numbers although they seem a bit high.. could just be the $30 gauge though..

If the cam to crank timing is off a significant amount, it can also give you higher than normal numbers too..

First thing I would check is your injection timing.. If it's retarded enough it'll give you rough and smokey starts..

whitedog · May 26, 2012

TDIJetta99 said:
good numbers although they seem a bit high.. could just be the $30 gauge though..

If the cam to crank timing is off a significant amount, it can also give you higher than normal numbers too..

I can see it causing lower than normal compression, but I don't understand how it could cause higher compression. Can you learn me something?

Ski in NC · May 26, 2012

I would not read much into a Harbor Freight comp tester results as far as accuracy. I could see that gauge reading +/-50psi easy.

But you probably can be confident that the four readings are close to each other, which is more important.

TDIJetta99 · May 26, 2012

One direction with the cam timing will lower compression readings, the other will raise compression readings.. I can never remember which way does which... I believe advancing the cam timing will raise compression gauge readings.. On a gasser it will also give you easier cold starts and better low end torque at the expense of efficiency.. Not sure if that part holds true with an ALH

whitedog · May 26, 2012

OK, I can see it raising the compression if it's turned so that compression is low, but I'm not seeing how it could raise it above normal compression.

In other words, if the cam was turned so that the valves were open too long at TDC or BDC and excess compression was escaping, turning it so that the valves were properly closed would raise the numbers to "real" compression. But I don't see how it could go higher than that.

Ski in NC · May 26, 2012

I think advancing cam will raise compression, but not by much. Intake valve stays open a little bit into compression stroke. If you advance cam, intake will close sooner and more of the air in the cyl will get compressed. Advance too much and valve closes before cyl gets filled, then comp starts to go down.

Advance even more and valves get whacked by piston.

whitedog · May 26, 2012

Ski, it sounds like it's about valve overlap. I don't have an exact picture in my head, but I have an idea of how it would work.

TDIJetta99 · May 26, 2012

There isn't any valve overlap in an ALH.. It's about the exact intake valve closing event that determines cranking compression.. Since the pistons are moving relatively slow, the inertia of the air doesn't come into play.. I don't have the valve timing event chart handy, but the intake valve closes a bit into the compression stroke.. Advance the cam a bit and the intake valve closes a little earlier, allowing pressure to be built a tiny amount sooner which raises gauge readings..

I noticed a similar phenomenon when putting a very aggressive cam in a smallblock chevy.. Between the large amount of overlap and long open times, we only got about 95psi cranking despite an 11.5:1 static ratio.. This particular smallblock didn't wake up until 3500rpm.. A normal engine with that sort of CR will net 250psi on a gauge..

jettawreck · May 27, 2012

Getting a bit off track aren't we???

It's a $30 HF gauge. Don't put a lot of faith in the actual numbers, but the fact that they are good/even numbers accross the holes makes for a believeable test result.

whitedog · May 27, 2012

Not really off track, we are exploring the causes of higher than normal compression numbers and if those higher numbers could be a contributing factor to the rough, smokey starts or if it's just the gauge.

A higher compression ratio due to build-up inside the cylinder could give higher numbers, but that should give an easier start, correct?

And what about leaking oil rings? Someone mentioned recently that excess oil on the cylinder walls could seal the compression rings more than normal, causing higher numbers. This would give smokey starts, but would they be rough?

TDIJetta99 · May 27, 2012

I want to see a video of it starting cold to get an idea of how rough and how much smoke...

duwem · May 27, 2012

Does what you guys are talking about possibly have something to do with this:

http://forums.tdiclub.com/showthread.php?t=353145

I do agree the gauge is probably off, but thats the same car as this one...

sdeck · May 29, 2012

duwem said:
Ya, after looking at some specs, those are a bit high, I read 510-520 for an engine with 100k on it. Rather be consistent and on the high side than all over the map or low I guess.

What have others measured using HF testers?

I was getting 490 +/-10 with a HF tester, that was with coked up rings due to WVO use. 120 +/40 when the o-ring in the adapter was scored

. Actually haven't checked it since aNut rebuilt it 50K miles ago. I should do that.

duwem · May 29, 2012

So your saying 490 on a not so ideal engine, so on a good engine my numbers are probably typical for the HF gauge?

Slightly different topic, what kind of numbers should I get on a cold compression check?

Thanks.

How To Do a Compression Test on ALH

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