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68340 | I am going to post what I think is the way to do these test. Tell me where I am wrong. Doing a Chevy 327 in a 1966 car.
How expensive or good does my leak down tester need to be? New Snap On is over $400. Used Snap On on Ebay about $200. ATD for $88. Hmmmm.
Compression test.
Remove all spark plugs, (allows the engine to turn over easier by hand, if desired)
Cold engine or hot engine. Should not make any difference, right ??
Hand tighten hose into first spark plug hole. Turn over engine with starter 6-8 times. Write down max. compression reached for each cylinder.
Leak down test.
Turn engine over to get TDC with valves closed for piston I want to test. Not sure of the easiest and best way to get an exact TDC. Do not think there is enough room to get a TDC indicator screwed into the spark plug hole on this car. Do I want to remove the valve covers so I can observe the valves to get TDC on compression stroke ? How do I determine when piston is TDC with closed valves if I do not remove the valve cover to watch the valves ? Is it critical to have the piston exactly a TDC ? Is a TDC whistle a good way and accurate enough to get TDC compression position ?
My thinking was to remove the valve covers, use a small rod or wire to watch piston travel, while watching the valve to get TDC on compression stroke. Now I am ready to do the leak down test. Yes ?
OK. You all have told me how to get TDC with closed valves. I now use the tester, with Schrader valve removed, use air compressor incoming air to flow into the cylinder. If I have 100 psi coming into the cylinder what is acceptable leak reading 90 psi, 80 psi, ect.? Engine is not new.
It was suggested that if I have a bad leak testing, say 50 psi. I am losing pressure in one of these areas or maybe several ways I am loosing pressure.
(1) Leaking past rings. If I pull dipstick tube I may be able to feel air coming out of the tube hole. Yes? Or is there some place else on the motor the air will be escaping from other than the dipstick tube and the dipstick tube for air flow will not be a correct measure of gas escaping ?
(2) Leaking past intake valve. I should be able to hear the air coming back out of the carb ??? Or how do I know the intake valve could be the possible loss of air ?
(3) Head gasket leak. I can remove the radiator cap and see bubbles come up in the coolant. Yes ?
(4) Ex valve leaking. Hopefully my ex pipes are tight and if I put a rubber glove over the ex pipe end it will inflate. Yes ? Or maybe go around all the ex connections feeling or listen for air. Yes ?
How did I do ?
Edited by billybob 12/12/2024 11:11
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| The throttle should be wide open for the compression test
6% leak down or less is acceptable with an up to operating temperature
Are you looking for something or just a general health test? |
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Roseglen, North Dakota | With your knowledge and questions you have surpassed 99% of the people on here.
But I think you're way overthinking it unless you're doing government work.
Just take all the plugs out, screw the tester in, and crank it over until the guage doesn't go any higher. |
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Milford, IL | As noted above, have throttle blades wide open for compression test. Ideally, all cylinders will be within +/- 10%.
Leak down test, lock engine at TDC for that cylinder. Can find TDC by rolling engine by hand and watching piston approach, or use a small straight tool (wooden dowl or screwdriver with great care) and stop when piston is no longer travelling upwards. For high performance small engines (dirt bike, atv, utv) I liked to see 95%+ for a fresh engine. Much under 90% and it was getting time to replace piston or inspect cylinder head for worn valves/seats.
Leak down test is time consuming vs compression test. |
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Mid Michigan | You don't have to open the throttle at all to do a compression test. If you don't believe me try it both ways. I've got a compression and a leakdown tester. The leakdown tester is kind of worthless. I rarely used it over the years. All it does is allow you to add pressure to the cylinders with your air compressor and see how it holds it. If you have a leak you can sometimes hear it. The compression tester should tell you pretty much everything you need to know if you test all 8 cylinders. If they all test at about 130-140 lbs you probably have a pretty decent engine. BUT, just because all the cylinders test ok an old engine could still have quite a bit of blowby and use some oil. |
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68340 | General health check. Not quite sure what is the problem. Runs good at idle. Runs good up through the 4 gears till about 3500 or 4000. Then I get some misfire or backfiring. It might be in the distributor also. Just trying to do some general checking before taking to a shop.
So your saying doing a hot leak down test on a cylinder should be 94 psi if the incoming is 100 psi ? Do not understand the 6%. |
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68340 | Yes, the compression test is quite easy to do and I believe I understand and can do that one. The leak down test I have not done before. |
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| I doubt you have a compression problem if it runs good up to 3500.
I agree, it is probably an ignition of even a fuel / vacuum problem.
Does it have an electronic ignition? It sounds kind of like a condenser problem I once had. |
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Faunsdale, AL | I’ll second everyone else that says you’re worrying too much about the leak down test. Compression is going to tell the story on the effective cylinder condition. What you’re looking for is mainly the balance/difference between cylinders. Go ahead and block throttle open if you want to or leave it at idle…….just do all 8 cylinders under the same conditions.
The ones that show up low, hook up the leak down test to each one (with ignition OFF) turn on air flow and rotate engine by hand with ratchet on crankshaft pulley bolt until the pressure on the cylinder you’re hooked to starts to rise. That will be when the valves are closed and piston is coming up to TDC. When you get close to TDC, the engine will stop trying to turn backwards and you should be able to leave it stopped on or about on TDC and check for leaks back into intake, out exhaust or into crankcase. If you rotate too far and pass TDC, the air pressure will push piston down and if you’re using a ratchet it’ll spin it without hurting anything. If you’re using a wrench or a bar on the socket, watch out it doesn’t pinch you as the engine rotates! If you go past TDC and engine rotates, just turn it on around and come back up to TDC again.
The leak down tester has been used to diagnose and condemn aircraft engine cylinders for many years but with the cost of replacement cylinders and the lead time to get replacements and cost to have them installed, there is more interest these days in avoiding replacing them if at all possible. Another reason to avoid unnecessary cylinder work is there’s worrying statistical evidence of higher than normal chance of engine failure following cylinder replacement! In an airplane, it doesn’t just leave you stranded on the side of the road!
There’s pretty good evidence that aircraft engine cylinders that fail the leak down text spec from wear, not burned valve or broken piston etc are capable of providing very close to full power of a new one. One of the manufacturers actually tested an engine with the piston rings left off and showed that it was capable of around 90% of expected power. So I think the lesson for automotive folks is don’t embrace leak down testing to the point of having to meet an exact spec. Instead use it as a diagnostic tool to support a diagnosis of bad valves, blown head gasket, broken piston etc
Compression testing is not too much different. If you’re hung up on exact numbers,something like a weak starter or battery can result in condemning an engine and unnecessary expense. If the valves have been ground or the head has been milled or block decked, cylinders bored oversize etc, the compression test results will be slightly different not to mention the gauge might not read perfectly accurately.
It’s the balance between cylinders that’s most important, and even if you show up some low ones, you need to confirm the problem by spark plug analysis, putting some oil in to seal up rings, leak down test etc. It’s only a diagnostic tool.
I’ll say I really think you’re more likely looking at the wrong thing for the problem you describe. There’s replacement HEI distributors available for your engine (and many others too) that are a plug and play solution to ignition problems.
Edited by ccjersey 12/12/2024 13:50
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Milford, IL | Throttle wide open for compression test always. Except diesel engines because air flow is unrestricted. Anything with a throttle butterfly, needs to be open.
Leakdown test matters more on high performance engines with 20 hour piston replacement interval, engines that turn 8k rpm sustained every time they run. |
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NE Neb | If you have vacuum advance remove the hose see if backfire or misfire goes away. |
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| If it starts and idles good I would not worry real much about compression. Look first at the ignition, then valve springs for possible valve float. Valve springs are easy to change out with the heads on. |
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| billybob - 12/12/2024 12:20
General health check. Not quite sure what is the problem. Runs good at idle. Runs good up through the 4 gears till about 3500 or 4000. Then I get some misfire or backfiring. It might be in the distributor also. Just trying to do some general checking before taking to a shop.
So your saying doing a hot leak down test on a cylinder should be 94 psi if the incoming is 100 psi ? Do not understand the 6%. |
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| When you put air in the cylinder it won't stay at top dead center. This is how I do it. Install the leak down tester then bar the engine over until you obtain the highest pressure reading (least amount of leakage) .Use a long half inch ratchet, it takes a fair amount of effort to hold it at TDC. Also OTC tools are pretty good and usually not snappy priced. |
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| Before you take it to a shop I’d update the ignition system. New plugs , wires etc. A good shop should be able to tell if the distributor is worn out. summit racing is your friend! |
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Midwest | You can buy whichever compression tester you want (or use the one you have already)
Don't worry about getting to an exact number. Just be happy if they're all within the same little range. I'd rather have them all ready 110-ish vs have them read 130 and one read 100 for example |
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SC Iowa | I would guess floating the points or running out of fuel pressure. |
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Mid Michigan | Doesn't matter at all. Give it a try and post the results each way. Won't make any psi difference. |
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| I agree, not needed to go WOT. Just have to do it the same for all cylinders.
INDY |
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68340 | Points. |
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68340 | I hear you all. Will do compression test. Prob. not the leak down test. Prob. will have the distributor sent off for a check up and put in NAPA CS7860 points. Good to 7,000 rpm. I have read to may bad stories of electronic ign failures leaving people stranded.
I was surprised at the price difference on distributors. New elec with tach drive for $160. Others for over $600. What ? Must be for racing or something.
Very possibly could be the cam going bad too. Time will tell. Just trying to make a reliable street drive out of this car. |
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Milford, IL | I do it every day that Im not operating a piece of farm equipment.
Throttle open, seven or eight revolutions and done. Compression test with the throttle blades closed might (and might not) eventually get to the same PSI after a crapload of cranking. It makes a difference, I guarantee it. |
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ECMN | You should check the advance bushing in the distributor too. Its made of rubber or plastic and normally only lasted 25 years
and can mess up how much total advance you get from your distributor. |
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| My favorite engine . Take valve
Covers off and crank it over and make sure all rocker arms move . Take plugs out and stick your finger in hole . Crank it . If it blows your finger out go race ! Look up the first ford . |
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How to do a compression test and leak down test on Chevy small block quesions.
