How to compression check VR6?

[Dilated 0]

I have the bentley manual, but it doesn't cover my specific engine (only engine code AAA) it mentions about removing the main lead to the distributor, trouble is, mines coilpack.

 

What do I need to do to do this without chucking fuel into the cat?

Failry URGENT cos i just took one of my plugs out to give it a clean (started to misfire) and the plug was DRIPPING oil all over the place. Need to check it now really. Bit worried. =[

[Dilated 0]

I took out Relay number 167 as per Phat's advice in another post, but what about stopping it sparking?

Yea i know it's nearly 10pm on a school night, but need to check this out now! =[

[Dilated 0]

Okay, I managed it anyway. Here's my results if anyone can shed any light on these?

 

1 - 187

2 - 144

3 - 175

4 - 161

5 - 175

6 - 171

 

I find it a little odd that 1 and 6 are good and those are the plugs which oil up, 1 especially badly. (1's even the highest!)

I'm assuming the oil loss/burning is more than likely the head then? Probably the guides/stem seals?

 

Gonna have a go at these when i get back from hols i think. I've simply put off doing it til i get less busy!

[Scott 0]

No.2 seems very low.... anything above 160 is considered to be ok but its more important the figures are similar...

 

I assume you did the compression test accurately?

[Dilated 0]

I would hope I did. Fuse No.18 out and relay #167.

Took all the plugs out to make it less effort for the engine to turn (and use less power).

Stuck the gauge in 1, then 3, then 5, then 2 etc

Then turned the engine over until the pressure wasn't going up any more in each one.

[Skimask 0]

If the plugs are dripping oil, then that may cause higher cylinder pressures. Because the oil will seal the compression rings, giving a higher than normal reading.

[Dilated 0]

Good point, that could explain why number 1 is higher than all the others. Is this where a "leak down" test would be beneficial? Not had any experience of a leak down test to be honest.

[Scott 0]

You shouldnt crank the throttle either until the guage stops going up - that gives false readings...

 

With fuel fuse and relay out:

 

- crank engine over with throttle WIDE open so that it turns over four times - take reading

 

do same for all cylinders and record values.

 

You can then compare the figures.

 

But yes a leakdown test woulld be better although trying to find a garage to do it is a joke

[Dilated 0]

Hmmm, My pops told me that too - crank each one the same amount of times (say, 4 or 5), but it does say in the Bentley Manual to keep cranking until the pressure stops going up.

And yea, I had the throttle fully open whilst doing it too.

[dr_mat 0]

Doesn't that feel weird, cranking a car with your foot hard down on the throttle but no spark? In carburettor days (yes I'm that old) that was a bad idea! :)

But yes, it sounds like you could do with a leak-down test doing.

[Dilated 0]

Heh, yea I know exactly what you mean - My Golf Cab has a weber carb on it and it would DEFINATELY be a bad idea on that thing!

 

So this leak-down test. I have an airline and can make up tools if need be, but would it be worth getting a proper garage to do this test (if I can find one to do it like Scott mentioned!) ? Whats it going to tell me?

[dr_mat 0]

The leak-down test tells you whether the cylinder can *hold* pressure. If it's being maintained by the oil on the rings, it typically won't hold that pressure for any length of time, though it may show a high pressure over the very short term. Note that since the air leak (probably) can't go through the (closed) valves, chances are any loss of pressure over time is down to the bores or head gasket... Or at least that's how I've always understood it.

I have no idea about the technicalities of doing the test or setting up for it...

[Scott 0]

Yeah thats basically it Matt...

 

A skilled mechanic can tell from doing this test if the pressure is leaking through the rings / valve guides or gasket...

[Kevin Bacon 5]

With fuel fuse and relay out:

 

Or even easier, disconnect the crank speed sensor. No fuel or ignition then :wink: The plug is on the front engine mount and easy to get to.

 

But yes a leakdown test woulld be better although trying to find a garage to do it is a joke

 

That's because it's a very involved process. Each cylinder needs to be at TDC and x 6 in the case of the VR! Then they blast air into the cylinders and "listen" to where the air is leaking from, either through the valve guides or through the rings - or both, or even the head gasket in some cases. They also look at the pressure drop per cylinder. No piston engine cylinder is truly gas tight, but the pressure drop should fall within certain tolerances.

 

Lifting the rocker cover and dropping the sump makes the process so much easier but that is time consuming and expensive labour wise, but it is far more accurate than normal compression testing.