Will this work? (Leakdown tester)

[HaroRider]

So I have a HF leak down tester.. yea not the best.. I know.

My question is..the tank pressure gauge has a reading of up to 100 PSI (Air going into cylinder)

The one on the left has pretty colors with % amounts.. which I believe equal up to ~15PSI MAX..making this tool rather useless for a good leak down test.

If I was to change that second gauge to a 100PSI gauge and simply subtract the numbers from the two ..100 PSI in 90 PSI at second gauge.. 10% leak down..would that be correct or is there more to it than that?

Thanks, Tom

 

[sonoronos]

I don't think you understand how a leakdown tester works.

A leakdown tester has a calibrated orifice and the downstream gauge is usually calibrated based on that orifice.

I have no idea why your HF leakdown tester has the gauge that it does, but you may want to consider verifying that the gauge is indeed incorrect before doing odd things to it.

There's no evidence that anything is wrong with the gauge as is, at least based on the evidence you have provided.

 

[HaroRider]

Makes sense.. your right I really wasn't 100% sure how it did work. I was thinking some sort of check valve or something.

I suppose this thing is junk then as getting another gauge identical as the HF % one will be impossible to find..

 

[fordnut85]

I replaced both of mine with 150 psi guages and you are correct. Adjust the regulator until 1st gauge reads 100 psi. 90 psi reading on the end gauge would be 10% leakdown

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[T45]

otc version is decent an not much more than diy-ing all th parts from scratch

 

[sonoronos]

Adjust the regulator until 1st gauge reads 100 psi. 90 psi reading on the end gauge would be 10% leakdown

How exactly do you know this? I don't doubt you necessarily, but your numbers seem to assume an awful lot about the design of the leakdown tester.

 

[mbshop]

You could mess with the hf one but first you would need to make sure the gauges are reading accurately. I just got a mac unit and didn't need to mess around.
at the mb dealer in the 70s we had an odd unit.

 

[isr2kba]

In my opinion a leakdown tester is one of the most misunderstood tools found in a shop or maybe second finisher to the oscilloscope.

Unless you're building new engines or have knowledge of 'passing' readings of the engine under test (in which case you should probably be giving ME advice) with the EXACT MODEL of tester you are using, the specific reading on the right-side gauge is not particularly informative and of course it's relative to the input pressure.

What is informative is the relative reading of all the cylinders. One example of how to use a leakdown tester is:

1. Compression test all cylinders. Record the results. Assuming you have some low reading(s), this informs you there IS a leak 'somewhere'.

2. Use the "best" cylinder (highest compression) to calibrate the leakdown tester. Set it up so that both gauges read the same, even if there's a little loss.

3. Now you can test the other cylinders starting with the second best and ending with the worst. For any cylinder you see a marked drop in pressure on the right side gauge, you can listen at the exhaust, intake, oil cap, cylinder mating surface, coolant hoses to try and figure out WHERE the leak is going.

 

[fordnut85]

How exactly do you know this? I don't doubt you necessarily, but your numbers seem to assume an awful lot about the design of the leakdown tester.

By testing a cylinder before I removed the gauge and putting the standard gauge on. The only reason I went that route is more graduations on the standard gauge that gives a more accurate reading that looks good on a repair order. It's all based on the metering orafice between the two gauges.

With a leak down tester you can find where the leak is I.e. valves, head gasket etc by listening to where the air is going.

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[yhprum]

Aircraft piston engine use 80 PSI input for this test. If you get a low reading, put your ear to the exhaust pipe, air intake or crankcase to determine where the air is escaping.

 

[HaroRider]

I understand HOW to use the gauge. My question is will changing the gauge make the unit unaccurate, or does it not matter?

 

[sonoronos]

1. Changing the gauge doesn't "break" anything.

2. The pressure number you see on the new gauge you install will be less meaningful than the old gauge.

3. If you install a new gauge, and do the test, and the new gauge reads 10psi less than the supply gauge, it does NOT mean 10psi pressure drop. It just means the needle moved that much. That's because the tool is not calibrated at all for the new gauge you put in.

I still don't understand why you want to change the gauge. Is it broken or do you need some help understanding how to use it?

The "0%" position on the gauge means no leaks - it's equivalent to the upstream gauge pressure and should be the reading when you apply pressure to a closed system.

The "100%" position or full-red or whatever position is the opposite of "0%" is the pressure at the outlet of the orifice when subject to an open system (or a free leak). This can be solved using Bernoulli's equations for downstream pressure using simplifications of the mass-flow constants and the actual orifice size. It will change depending on the input pressure.

Or you can perform it experimentally by noting the pressure of the downstream gauge when you apply pressure to a free leak.

The amount of leakage in an engine cylinder should be less than the free flow of air through the orifice in the leakdown tester. If your leakdown tester is pegged when performing the test, then either there is an issue with the engine itself, or you haven't set the piston to TDC. You also may have carbon fouling the valves, which causes a leak. You can usually fix this by gently rapping the valves with a hammer and drift (Careful!)

Remember to also fix your crankshaft pulley so it does not move at all during the test.

 

[HaroRider]

Thanks Sono. Yes the second gauge stopped working. The first one I bought the first gauge stopped working..my luck is not so good. I am a home DIYer and honestly dont use it too often and figured HF was good enough.

Im trying to leak down an engine on a BMW and it would be nice to know how much leak down there is in each cylinder before ripping it apart.

I would like to change the gauges and make the unit "better" but if this will render the percentages useless I will just buy another quality set.

I imagine I couldnt really test the new gauge based upon some known value since I have no way of doing that besides 100% or 0% leakdown.

 

[sonoronos]

Ahh, gotcha.

Changing the gauge is definitely cheaper than buying a new leakdown tester!

Just do it. It doesn't make your tester useless, you'll just have to create your own calibration on the new gauge to figure out the flow pressures. You'll have vague values, but they will be consistent between cylinders. You'll also see the difference between large and small leaks.

 

[2oolhound]

You should post a photo of the 2nd gauge. It sounds like it shows you what percentage of 100 is leaking.

Make sure you are at TDC on the power stroke (with both valves fully closed).

 

[sonoronos]

You should post a photo of the 2nd gauge. It sounds like it shows you what percentage of 100 is leaking.

The HF gauge looks like this. It's a very standard leakdown tester gauge.

 

[T45]

Before you throw another $20 at the HF unit, just consider a decent unit of another $20.

Differential Cylinder Pressure Tester
Otc Stanley / Proto Jfp210m
$46.99 from eBay - tradervar

(from the hot deals thread a couple weeks ago)

 

[HaroRider]

Thanks T45, I may consider that one. Interestingly enough that one is basically the way I want to set mine up..granted - it may POSSIBLY be off because of the internal orifice..haha

 

[pi_guy]

They work great if the engine builder did the test prior to shipping out the motor.
With a race motor it is a good tool to see if you have a loss or an imbalance.
With an aircraft motor it is worth the time.

Often on street car motors with high mileage it is a waste of time exercise. Great if your billing the customer and they want details. But if your compression test is not close your on a learning exercise. It really will not tell you that much outside of confirming that the motor needs to come apart.

 

[HaroRider]

I understand what you are saying, but wouldnt there be a big difference if its leaking past the rings vs the valves?

 

[pi_guy]

I understand what you are saying, but wouldnt there be a big difference if its leaking past the rings vs the valves?

You can find that out with compression test, do a wet test after you find a low cylinder.
I would only go for a leak down if all the compression tests were good or close.
First is plugs that tells you a lot.
leakdown tests are only good if your starting with a base line or you want to run the customers bill and justify the tool being in the toolbox


But outside of that using a HF unit to test is suspect already --find most HF products only work once and not repeatable results from most measuring things.