Michael Benno

Hey all, I need some help and guidance on how to properly perform a cylinder leak-down test on my GTS. @PorKen came over today to help me with the project. Neither of us has done it before, so it was a bit of a learning experience for us. We had limited success and a lot of laughs about how unprepared we were other than having a leak-down tool. We took too long and the engine cooled off. So we figured we'd get dinner and get some answers from the list.

Do we have our pre-conditions correct?

• engine warm - Is there a minimum block temp we should be concerned about maintaining for quality results?
• throttle plate is fully open
• all 8 plugs removed
• dipstick removed
• EZK/LH disconnected
• coolant cap removed
• car raised to hand turn crank

How do you keep the engine from turning from the compressed air?

• I was counter-holding the engine at the crank bolt as Ken conducted the test. But this method was imprecise and we found the crank would turn as a result of me holding too much or not enough. We had previously removed all the plugs for a compression test (see below for more info on that)

Is there a crank position where all the valves are closed?

• - Ken thought it was at 45 degrees, but we could not verify

What are the respective crank degrees for each of the pistons at Tdc compression ?

• My car has the ATI super damper, which has a marking for 360 degrees.
• We tried to calculate the position but got confused with the firing order and crank rotation. Our math didn't pan out. It would be super helpful if someone could help provide the crank degrees for each cylinder at TDC on the compression stroke(valves closed)

what else should we be considering in the test?


BTW, we did do a compression test before the leak-down. The numbers were not great. We saw 12% difference between the highest and lowest measurements. Here is the data
Pre-conditions:

• engine hot
• all 8 plugs removed
• dipstick and coolant cap removed
• throttle wide open
• EZK/LH disconnected
• 10amp charger connected to the battery to ensure consistent cranking RPM
• 4-5 cranks per cylinder

FredR

Michael,

The leakdown test should be carried out with the cylinder being tested set at TDC on its firing stroke. All things being equal this will ensure the cylinder does not go for a walk. At this position both the inlet valves and the exhaust valves will be closed and the system will be as gas tight as it is ever going to be. At this position you can also easily hold the crank bolt if there are two of you. In my case I put chalk marks on the damper at 90 degree intervals, start the tests at No1 cylinder and after each test turn the crank 90 degrees and then go to the cylinder next on the firing sequence. If you have a damper with the full 360 degrees mapped on it even better.

Given the spark plug has to be out I then check for TDC on the specific cylinder [I have the sstock damper] using a wooden barbeque skewer inserted through the plug hole - a dial gauge setup of some kind would be ideal I suppose but fiddely and just not needed in my experience. Ideally the engine should be fully warmed up as that ensures clearances are as tight as they are going to be but again that is not strictly essential given the time it takes to cool off.

The test rig is in fact a means of measuring air flow or more specifically, relative flow as it can be carried out at any nominal pressure but I work on the principle that higher pressures will give more meaningful/consistent results and I try to set the inlet gauge at 100 psig given my air compressor shuts off at about 110 psig. The rig itself is nothing more than a restriction orifice and as the air starts flowing it will create a pressure drop across the orifice. By measuring the number on the downstream gauge the pressure differential is a measure of the instantaneous air flow rate otherwise known as "leak down". If the inlet pressure is 100psig then a good leakdown situation will give a read out in the 90+ psig range. As I recall my engine showed a downstream pressure of 95 psig plus on all cylinders so very good as I was concerned. With such numbers like that no need to go trouble shooting looking for leaks. However if you find numbers below 90psig or whatever suspect threshold one chooses [some sources suggest 80 psig or 20%] one then starts looking for signs of where the leakage is. If the noises come from the crankcase then the rings are suspect. If it is from the inlet tract then it is the inlet valves, and if it from the exhaust pipe it is the exhaust valves that are suspect.

The cooler the engine the greater the leakdown rate is to be expected but not by large amounts- a few percent maybe. I usually take about 5 minutes per cylinder working on my own or 40 minutes start to finish. If i had a bigger air compressor it would be quicker as i would not have to wait for the thing to recycle.

Trust the above helpful.

FredR

Your compression test numbers are fine. Values can vary a bit depending on cam timing. With stock timing I expect to see 185 psig give or take a few psi on each cylinder. If the cam timing is retarded [for top end bias] the compression test will yield lower results and if the timing is advanced [to favour low end torque] the values will be a tad higher.

Taking 185 psig as the expected datum point your deviation is only circa +/- 5%.

Mrmerlin

Fred has some good methodology,
I would put a bottle of techron in the gas and 6 oz of MMO and do short runs so it has time to work.
this will clean the valves and injectors and intake ports.

NOTE is their any reason you have decided to do this testing?
IE are their performance issues you have noticed?
or are you just checking this engine to see what it is doing?

Michael Benno

The primary reason I am doing this test is to gain insights into the sealing performance of the position rings, valves/seats, and valve guides/seals. This GTS consumes a lot of oil (1 quart per 400 miles). I can see blue smoke under accelerations over 3500 RPM. The car is stinky; I can smell oil being consumed (The fuel mixture is good). For reference, this car has a vacuum pump on the crankcase and does not return evacuated crankcase air to the intake (see graphic).

The motor seems to be running well and has strong power delivery.

Tom. M

Being a GTS, I'd scope the piston tops to look for oil residue buildup. May be a reason for the higher compression numbers from various cylinders.
Also, might get the flywheel locking tool to hold the crank from turning. Tedious but it won't move

If the valve guides or seals are leaking that is typically seen at high vacuum (idle) with blue smoke coming out. Also you'd see some blue smoke on startup as the oil drips past the seal into the chamber after shutdown.

Edit: When doing a compression test, run the sequence from 1 to 8 and the 8 to 1 to see if they change. Also, crank as long as it takes for the PSI to stop going up. Should be pretty obvious when it stalls.

uraniummetallurgist

Hi Mike,
I have the same vacuum setup (Colin) on my '93 GTS and see similar oil consumption ~ 1 - 2 litres/1,000 km. I had not driven the car for a while (3 months) and added MMO 3 days ago to the crankcase and will let you know if that helped. There seems to be significantly less "noise" in the engine bay after driving for an hour at high rpms. There is no sigh of blue smoke now under hard acceleration. Hopefully the MMO is dissolving the encrusted/coked oil on the rings and will help with the vacuum pump to minimize oil entering the combustion chamber.

Might be worth checking the vacuum level as the drive belt may need adjustment.

All the best,

Joe

Mrmerlin

FWIW I was talking about adding the fluids to the fuel not the oil.
HOWEVER adding MMO to the oil might be a viable option.

See cylinders 4 and 8 for having more wear/oil consumption,
due to them running hotter.

Michael Benno

Thanks all for the useful guidance and suggestions for both the compression results and the leakdown testing. Next time I have a helper to crank the engine, I'll redo the leak-down tests. If I do the tests with the engine cold, the driver's side cam timing is set to Zero. However, the Passenger side cam timing is set to 2 degrees of retard (which moves to zero as the block expands from heat). So If I am doing the leak-down test cold and using TDC, I would expect more leakdown on cylinders 1-4 vs cylinders 5-8, correct? Should I reset the cam timing to zero on the passenger side for the purposes of the leak down tests?


@Tom. M I snapped some photos of the piston heads but they are not very good because some of the cylinders were too close to the top for good resolution. I will get better photos with the pistons at the bottom, But this will give you some insight on the condition:

Cylinder 1

Cylinder 2

Cylinder 3

Cylinder 4

Cylinder 5

Cylinder 7

Cylinder 8

FredR

The impact of potential cam timing changes on compression test results is not relevant when it comes to the leak down testing procedure. Both the inlet valves and the exhaust valves are closed for approx 180 crank degrees -i.e. 90 degrees either side of TDC on the firing stroke.

Andrew Schauer

Michael, you can rig up a remote start button using some alligator clips on the correct starter terminals. This lets you crank the engine without needing a second person.

StratfordShark

You’re always such a good engineer Fred! What is your technique for applying those marks? Just a straight edge across max diameter once you’re at no 1 TDC or am I over- thinking it?

Thanks

FredR

Hi Adrian,

No great science here- I just eyeballed the approximate position and marked it - I then use that as a visual guide where to position the crank for the next cylinder in the firing order. One must always forward the crank in the direction of motion and to do that I use an old torque wrench wound fully in to provide a tool with a ratchet as it were. Getting the piston to spot on TDC is needed so as to put the compressive forces directly through the axis of the rod- if this does not happen the out of balance force will easily push the piston downwards. Once the piston is in this position I stick the wooden skewer thorugh the spark plug hole and using a rigid lever on the crank socket rock it back and forth slightly until I detect TDC has been attained. When the piston is at TDC holding it there is usally not needed but holding it with the rigid lever arm is easy. One could also fit the crank locking tool but that is a PITA. A damper upgrade using a unit that is degree marked through the full 360 degrees is even better.

StratfordShark

Thanks Fred for thorough instructions and great point about the force needed to be on axis.

i don’t have any reason to suspect valve/ring issues, but do have a leak down kit so would be fun to try it out.

FredR

Compression test results and leak down test results together build a reasonably comprehensive picture of the state of health of one's motor and together form a sound reference package for future comparison- a must for enthusiasts I would think. These tests are the kind of thing one does once a blue moon and using the same kit to do the tests is preferable for purposes of consistency.

I did quite a lot of leak down testing during my first professional assignment back in 1974 when we were testing flow rates through the pyrolysis tubes in a number of gas reformer furnaces- the intent being to ascertain whether the gas flowrate through the tubes was indicative of the catalyst inside the tubes being in good working order. The test rig was just like that used in engine leakdown testing but on a much bigger scale.