My Leak Down Test

(Wayne Mertz) #1

I have finally completed a leak down test on my XJ40. First readings on a cold engine (45-20-40-25-45-25) the 2nd reading (45-20-25-25-40-25) then I tried (not sure if I should have tried this, but I moved the cylinder with the gauge connected and was able to get the following readings (30-20-25-25-25-25). Tweaking the cylinders brought the numbers in line, however, I still start steaming down the road once the engine warms up! As mentioned previously, I have had the head milled and new gasket and bolts, etc. Where am I going wrong???
Again, this test was made with a cold engine, and no bubbles were visible in the water reservoir.



(Wayne Mertz) #2

Also, I just got back from warming the engine and retesting…Pretty much the same results.
Hopefully, Bryan and others will have the answer.

(Robin O'Connor) #3

Are you saying these are your compression test results? After a head gasket replacement?
If so something is seriously wrong with the engine.
Prior to the work being carried out was the engine overheated?
If so did you have the head checked for hardness?
Before the head was bolted down were the bolt threads clean and no debris in the bottom causing the bolts to bottom out before full pressure has been applied to the head?

(Bryan N) #4


What you describe doesn’t sound like a “leak down test” to me - do you mean you were trying to do a “compression test”?

If so, follow the recognised procedure - e.g.

Read through this thread too to see the proceedure and the results you should expect from a good engine -

(Wayne Mertz) #5

No, I bought a leak down tester and tried to follow the users manual. I hand cranked the cylinders to as close as I could to TDC. Then, I put air pressure to the cylinder after setting gauge to zero. The results were pretty close from cylinder to cylinder (25% loss which was in the green zone). According to the users manual, this said the cylinders were within reason. That being said, I had to move (tweak) a few of the cylinders by hand to get the readings down to 20-25% loss of vacuum. Hope that explains things better. Hot or Cold engine didn’t seem to matter.

(Bryan N) #6


My apologies - I mis-read your first post. So the numbers you quoted were the pressure loss over time?

Your reference to ‘tweaking the cylinders’ confused me. Don’t you mean moving the pistons up or down the cylinder from TDC? If so, the difference in pressure loss is associated with leakage past the piston rings.

I’m not sure that a leak test of that type by positively pressurising the cylinders will identify a head gasket fault which is causing ‘steaming’ when the engine is hot. Steam is generated by water entering the combustion chamber under negative pressure via faulty gasket / head-to-block seal.

Quite often, a faulty head gasket sealing problem allows coolant in to the combustion chamber after engine shut down where the static coolant pressure increases in the head due to latent heat after the water pump ceases operation to circulate coolant through the radiator to keep it cool. Since, unlike when the engine is running, there is no longer any pressure in the combustion chamber to counter the pressure outside in the coolant passages any defect in the head-to-block sealing will allow coolant to seep in the the cylinders.
When you next crank the engine that coolant is forced out of the exhaust valve to settle in the exhaust pipe/silencer and when that gets hot you see steam emerging behind the car.

(Wayne Mertz) #7

Yes, When I manually brought the cylinder to TDC, I may not have been in the perfect spot, so I moved the cylinder up/down until the gauge showed the best result.
The user manual on the leak down tester said to check the reservoir for bubbles coming out from pressurizing the cylinders. Engine being cold, I couldn’t detect anything, and when hot, I didn’t want to remove the reservoir cap as it was hot and under pressure. The gasket is new and torqued to specs. The head was milled flat. I purchased and used new head bolts. All this, and I still have a small leak somewhere in the system.


(Wayne Mertz) #8


I should also mention that there is no visible “Steam” coming out of the tail pipe until the engine gets warm/hot.


(Bryan N) #9

I don’t have any more brilliant ideas - except that if the leakage of coolant in to a cylinder only occurs when the engine gets hot (due to expansion of the head creating a gasket leak) thus creating the steam from the exhaust, you should see evidence of that in the condition of one or more of the spark plugs. A plug subject to water ingestion will appear ‘super clean’ with no sign of oil residue visible on the other plugs.

(Wayne Mertz) #10

I was wondering if I would get into serious trouble by trying to turn the head bolts an extra 45 degrees?

Maybe this would create a better seal. I’m running out of options. I didn’t see any evidence of a cracked block between cylinders, however I might have missed something.

The heads were milled by a professional, so I have to trust that. Oh well…

(Paul Wigton) #11

Have you combustion gas-tested the system?

(Wayne Mertz) #12

No, I thought that would be a waste of time, as I already know there is water getting into a combustion chamber.
And if that being the case, combustion gas would have to get into the cooling system. I just don’t know how and were it is getting in. I’ve already taken everything apart twice, rechecking everything and replacing gaskets, bolts, etc. Still, once the engine warms up, I get steam out of the exhaust pipes.

(Paul Wigton) #13

Remove the plugs, then pressurize the cooling system: see if you can localize the leak, at least, by what cylinder its in.

(Wayne Mertz) #14

Thanks Paul, I will give that a try.

(Bryan N) #15

Following on from Paul’s suggestion, I would expect a leak through the cylinder bore to contaminate the engine oil.
Any sign of ‘mayonaise’ on the dipstick or in the oil filler cap?

(Wayne Mertz) #16


No, nothing like that. Looks good and clean.

Of course, the car doesn’t have 20 miles on it since I had the head milled and new head gasket. I’ve just taken it for short spins three or four times to heat the engine and check for “steam” coming out the exhaust pipes.


(Casso) #17

Hi Wayne, you mentioned you needed the head skimmimg following previous trouble with it leaking, Is there any possibility the exhaust boxes have old coolant in them and that’s causing the steam ? Is your coolant level dropping ? Can you see bubbles in the coolant when the engine is idling or does the coolant smell of exhaust gas ?

(dr_gaz) #18

How do you differentiate between steam in the exhaust coming from leaking coolant, and steam being the water created by combustion of petrol?

(Robin O'Connor) #19

If the coolant level is steady and not being overly pressurised I would take the car for a decent run to eliminate the possibility of old coolant in the exhaust system.