Jump to content

Compression Test Interpretation


Recommended Posts

Hi All,

I've just carried out my first (ever) compression test on my 4A and here are the results:

Engine cold, closed throttle:

1) 160;   2) 160;   3) 160;   4) 145

Engine cold, open throttle:

1) 162;   2) 160;   3) 160;   4) 143

Engine cold, open throttle, a little oil down the plug hole:

1) 176;   2) 174;   3) 176;   4) 155

I suppose my slight concern is that cylinder 4 is about 10% different to the others - which are very consistent. Is this anything to worry about and how may it be explained?

Just for info, I got my tester from a chinese company on ebay (although shipped from Manchester) - it cost £11.50 including postage and looks identical to one at Machinemart for £33.59!

Obviously I don't know how accurate it is, but so far I'm impressed by the consistency in readings.

Thanks for any help.

Alistair.

 

Link to post
Share on other sites

Hello Alistair, I checked mine, or had them checked for me by a chap who owns a garage so he used a top end piece of kit, I got 180,180,180,190, I have a 4A, 86mm pistons, standard head, it sounds like your rings are worn, much blue smoke on start up?  Cheers, Andrew

Link to post
Share on other sites

your data shows that when you add oil the difference between No4 and the others increases... it also shows all compressions are higher with a bit of oil in the bores.

My conclusion: engine is a little worn, but nothing to worry about. No4 has a valve sealing issue.. if the car has stood for a while then also no issue as it likely cleans-up with use, if its in regular use this could be early warning of a burnt valve...suggest drive it & moitor every few 1000 miles with your new gauge.

 

Link to post
Share on other sites

Hi Alistair,

Have you checked valve clearances?

Readings are not bad, to me the compression test is more a comparative analysis than an absolute (value) measurement.

Regards,

Waldi

 

Link to post
Share on other sites

Thanks guys.

Not much smoke on startup.

So I think my plan of action is to check valve clearances in the not too distant future and recheck compression periodically.

I'll report back if I find it's the clearances.

Alistair.

Link to post
Share on other sites

The exact pressure is not that important (- affected by many things such as compression ratio and the calibration of the guage) It is important that the tappets have been checked before the comp test.

Should really be done with the engine warm and with the trottles wide open. Crank over a about 6 times until the guage has peaked.

Less than 10% between the cylinders is generally considered acceptable. If a drop of oil down the bores doesn't change things much it is possible you have a valve issue although a head gasket failure could do it there are usually other signs such as overheating or exhaust gassing in the coolant.

Link to post
Share on other sites

I had 1 cylinder (No.3) a fair bit down on compression (TR3) oil did not make much difference, a leak test (compressed air into the spark plug hole with valves shut) revealed air leaking past the exhaust valve. This promted me to remove the head, & refurbish it while lockdown was here. There was no visible sign of any seat erosion, but had hard seats put in anyway, along with new valves & guides.

Bob.

1666382418_Headleaktester.jpg.4036f089a8389f126f83e2d384c4a6c7.jpg  leak tester (home made)

Edited by Lebro
Link to post
Share on other sites

For consistency, the test should be done with the engine warmed up.    And always with the throttle wide open.

Your oil test shows a rise in pressure,which excludes a valve leak as the cause, which leaves ring leaks, but do it again warmed up to be sure.

'Accuracy' is not an issue, as compression gauges aren't precision instruments, doesn't matter if they are Snap-On or So-Cheep, they cannot be compared, unless you use one on two engines on The same day, same p!ace!    So the absolute pressure is unknown.

 What they let you  do is compare the bore pressures across the block, as you have done.   

Link to post
Share on other sites

You've established there is a problem with no4 because you have a disparity of over 10% and this is only on No4.

Possibilities - blown head gasket - usually accompanied by overheating and gassing in the coolant.

Valve seats worn or burnt.

Piston ring problem - usually excessive breathing or oil consumption accompanies this.

What prompted the compression test? If the car was running OK and not guzzling oil or blowing smoke it's probably he valve seats.

In pretty much all eventualities you are going to need to remove the cylinder head to check the valves are sealing. Upturn the head and fill with paraffin. If No 4 drains quicker than the other 3 dissasemble the head and check seats and for cracks.

If the head checks out Ok then drop the the sump and remove No4 piston to check the rings & bores.

Whilst the head is off check the cylinder liners haven't dropped.

Link to post
Share on other sites

Not sure I agree. The disparity remained after the oil was squirted into the bore.

A little oil may help the rings seal better but it is unlikely that the oil will significanlty seal a burned valve/seat.

There may also be a ring issue as the compression went up a bit on all cylinders but 4 was still significantly lower.

Here's a link from Popular mechanics explaining compression testing. https://www.popularmechanics.com/cars/how-to/a8520/cars-101-how-to-do-a-compression-test-14912158/

 

Link to post
Share on other sites

Thanks again all.

Andy M: A couple of things prompted the test: It had been intermittently firing on 3 cylinders and as the car was new to me, and as I could get a compression tester very cheap, I thought I would give it a go. As it turned out the misfire was on number 1 and seemed to be due to the (right angle) ht cap touching the heater pipe, which I solved by re-routing the ht cable. However, as all the ht caps seem to be very close to the pipe I think I may swap all the ht cables with a new set which have a straight ht cap and will therefore hopefully clear the pipe.

Taking onboard all the advice - I'll check the tappet clearances, drive for a bit, do another test when the engine is a bit warm. If not any better I'll might have a go at the leak down test, then if necessary head off.

Alistair.

Link to post
Share on other sites
16 hours ago, Chris Hubball said:

Hi Bob

I am interested in your home made leak tester any chance of a brief description of how you made it, I tried to make one myself but could not find a way of sealing the tube to the old spark body.

Cheers Chris

Easy if you have a lathe. I took a spark plug, removed all the ceramic insides (brute force etc) leaving a hollow metal shell with a parallel inside entrance at the top. I then took a plumbing fitting of a suitable size, & reduced the out side diameter to be a tight fit in the old plug, & sealed it in with locktite. Then selected a compressed air fitting to screw & seal into the fitting. Finally removed the metal sealing ring from the plug, & replaced with an 'O' ring.

Bob

Edited by Lebro
Link to post
Share on other sites
2 minutes ago, harlequin said:

I picked up one of these https://www.amazon.co.uk/gp/product/B078KJPJJT/ref=ppx_yo_dt_b_asin_title_o04_s00?ie=UTF8&psc=1 which works ok, there are lots of them out there from lots of suppliers but you will need some sort of compressor 

George 

That's the same as the one I have George but mine was £15 on Ebay.  For what I paid for it, I'm impressed.  I was expecting a load of rubbish.

Rgds Ian

Link to post
Share on other sites
1 hour ago, harlequin said:

I picked up one of these https://www.amazon.co.uk/gp/product/B078KJPJJT/ref=ppx_yo_dt_b_asin_title_o04_s00?ie=UTF8&psc=1 which works ok, there are lots of them out there from lots of suppliers but you will need some sort of compressor 

George 

Aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaargh!   Not the Red Box!   Usually an indicator of cheap Chinese ****.    Not in this case (!)?

Link to post
Share on other sites
2 hours ago, Ian Vincent said:

That's the same as the one I have George but mine was £15 on Ebay.  For what I paid for it, I'm impressed.  I was expecting a load of rubbish.

Rgds Ian

I'm afraid that's the story of my life Ian:mellow:

1 hour ago, john.r.davies said:

Aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaargh!   Not the Red Box!   Usually an indicator of cheap Chinese ****.    Not in this case (!)?

Not cheap enough as it turns out John, it has worked OK so far so maybe I have the deluxe GTI version:)

George 

Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Restore formatting

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
×
×
  • Create New...

Important Information

Please familiarise yourself with our Terms and Conditions. By using this site, you agree to the following: Terms of Use.