2008 Hummer H3 Misfire Case Study (Part 4) Leak down test revisited

Hummer series playlist https://www.youtube.com/playlist?list=PLAFYVCyenqcoTvkbvE6avGuLJOQnFcx4Z
How to perform a leak down test.
In this video I revisit the cylinder leak down test. Just to give us all confidence (including myself) in the direction we are going with this misfire case study.
Part 1 http://youtu.be/tZWsOupUzhY
Part 2 http://youtu.be/dpQrIhy2zQc
Part 3 http://youtu.be/bUT9cghOZok
Visit my website, www.scannerdanner.com for more about my eBook and lectures.
Visit our school www.rosedaletech.org, to be part of my class in person.
Visit http://www.youtube.com/channel/UCs2NZ_wQFDFvV0QOhoZgASA if you would like to be part of my on-line class. For more information on this topic, I have written a “field manual” called Engine Performance Diagnostics which is available at www.scannerdanner.com as an eBook or paper book.
Want even more diagnostic training? Whether you are a DIY trying to fix your own car, someone looking to become an auto technician, or a current auto technician that wants to get more into diagnostics, subscribe to ScannerDanner Premium right here on YouTube. There is a 14 day free trial.
On ScannerDanner Premium I will bring you right into my classroom at Rosedale Technical College. You will find page for page lectures taken right from my book as well as exclusive classroom type case studies. What is so special about these classroom case studies? I pull live problem vehicles directly into my classroom and we troubleshoot them in real time, using and applying the theory and testing procedures we learn during the classroom lectures. There is no better on-line training of how to troubleshoot automotive electrical and electronics systems anywhere!


Tom Smith says:

Hi Paul, well I think what ever kind of engine it is the con rod and crank will all be in line at TDC, just one comment, when you do this with a Manual Transmission car you can put it in top gear with parking brake on to stop engine moving, I guess a lower reading at tdc prob means the bore is worn a bit at the top, I don’t think valve overlap would come into it for quite a lot of crank travel, with the old distributor cars you can see what cylinder you are on by checking where the rotor arm is pointing roughly..cheers good video, I think you would only need to put oil down the bore if you were getting pressure out the oil cap – Fred in England

wai2machine823 says:

Dan you one crazy sob.

jarin wilson says:

I wonder what a smoke machine test would reveal….

ozzstar says:

Good information. Are you going to show any video with the cylinder head removed? Just curious

Joel Automotive In action says:

what percentage of leak down will be aceptable, I know comparing cylinder  and highest  will be the bad, but wear off even engine ??

Shawn Tatyrek says:

Took my stepson’s H3 to the dealer today for a check engine light, got a 0300 code and they said cylinder 3 misfire. Paid $200 for troubleshooting and they said the cylinder gets 160 psi (good), and the problem is leaking (down-leak?) into the crankcase, replace the rings. They suggested replacing the engine for over $9k, rather than rebuild the engine . We replaced the plugs with platinums (the installed plugs were roasted), cleaned the MAF and Intake, got a new air filter, got a full syn oil change and added 6 cylinder Restore Engine Restore. Have full system fuel injector cleaner to put in next fill up. Check engine light is still on, though only 3 hours later. Vehicle wasn’t acting up , so no difference in performance since light came on. WHAT ARE YOUR THOUGHTS ON OUR SITUATION AND WHEN SHOULD THE CHECK ENGINE GO OUT, IF WE HAPPENED TO FIX IT? Thanks so much for your great videos and your time. Much appreciated.

Fernando Sanchez says:

my brother you sound like is the firs time you ever done this thing I couldn’t watch the whole video

revontheredline2 says:

Great videos, in my experience doing leak out tests I have found that normal engines that have average wear are always about 10 to 20%….. 25% is a borderline time to overhaul, and 30% is need to repair now and usually a misfire or poor driveability issue has brought the car in for diagnosis.

yelorsirhc says:

A note on bdc vs TDC numbers. When I was in school in we would bore gauge cylinder wall wear… The top of the cylinder always read a larger bore than the bottom. I’m talking specifically about in line 6 engines here. It was only a few ten thousandths but it was noticeable. This also accounts for the common cylinder Ridge that can be felt in the liner at the top of the stroke, I’ve never seen a ridge in the bottom of the liner. I guess the thought is there is some cavetation taking place near TDC as the crank rolls over and shifts the direction of the connecting rod. This also happens at bdc but the piston is closer to the crank and under no pressure at this point. Just a thought as to why you’re getting different % of leakage @ TDC vs bdc.

Dave Sage says:

BTW. As others have mentioned you should NEVER try to hold the wrench. Simple math (pi x r^2) for a 3″ bore and 100psi will tell you that there is about 700lbs of force on the top of the piston and that’s further multiplied by the stroke of the crank  SO you’re looking at a broken hand (as you almost found out). Putting the a standard xmission car in gear doesn’t always work either. I’ve had the car move. If you must test at 100psi then maybe a long wrench jammed against something would make sense.

flineman says:

you always have some ring blow by   cylinder wear is less at bottom of the cylinder than at tdc   when you tear down a motor you always find more ridge at the top of the cylinder than at the bottom.  leak down sees the extra wear at tdc and adds to the leakage.   best to do test at bdc once then add oil to check for the amount of leakage added to leak down by ring wear.

Sandbag1300 says:

@4:21, does the thermostat have to be open to see air bubbles in the radiator from a leak down test with a bad head gasket?

Steve Guest says:

Mike P that’s exactly what I was thinking,would it be an idea to perhaps remove all spark plugs prior to test ?

swillieboy says:

a coworkers case scenario  …basically followed these instructions on a 4 cyl. Toyota Rav 4 …. also had big leak out of the intake clinder 4…after teardown no problems found with valves…but tech was instructed to replace head and valves…warranty …. after put together … still had a air fuel ratio imbalance on #4 … removed valve cover… redid leakdown visually seeing the cam lobs were off the valves…leakdown did reveal leak out of crank case… NOT intake….  i would definitely recommend leakdown down with valve covers off instead of on… a broken piston was found on complete teardown….  just my opinion.

Smokeasammie says:

Just for giggles when you pull the head fill the runners with water and see how bad they leak. Thanks for the vid.

Themi Xinos says:

Why didn’t any of the valves open at BDC because of valve lead or lag, depending which way the engine rotated? I don’t know cam spec for that engine but I assume the exhaust valve would have opened before BDC. Did you not allow the piston to go all the way down to BDC with the wrench? Great videos by the way.

Larry Frazier says:

I need to review d leak-down procedure. Haven’t done it in awhile. Look like a good tool u got there

Billy Bob says:

Paul, attaching a Vacuum Gauge to the fitting will put you right on by watching the needle.

Steve Hulett says:

Glad you liked my tip! That works on all motors, and the more centered you are on piston dwell at TDC, the safer you are against the crank suddenly rotating on you. I try to estimate the middle of dwell, and that IPA TDC finder helps me to a bit.

Bore wear is usually greater near TDC, so measuring at the top can be important. Head gaskets are above the piston, so there is no concern about blocking the air pressure from escaping out the head gasket at TDC.

The ideal air pressure is 100 psi, because it makes the math easy. The left gage reads how much air pressure goes into the bore. The right gage measures the pressure remaining in the cylinder. So that right gage is only dead accurate if the left gage reads 100%. Otherwise it’s reading a percentage of a percentage!

The OTC leak down tester has two gages that both read psi, and it includes a chart that converts the leak down % when using lower air pressures. That snap on gage is nuts if it thinks a 40% leakage rate is anywhere near acceptable. 15% would definitely indicate a problem!! At 40%, you have major blowby or valve sealing issues.

You might also want to start the test at 0 psi on the regulator, and slowly increase the air pressure to 100. Much safer and less likely to rotate the engine suddenly. Otherwise, you had better be at true TDC in the middle of piston dwell. Engine should be warm as well, so rings seal correctly and cylinder bore is oiled up. FYI, piston dwells for probably 6 to 8 degrees at TDC… but the rod is still swinging side to side except when it’s right near the middle of piston dwell. Visualize the piston, rod and crank as the rod rotates from one side of the bore to the other.

SuperCortes10 says:

Putting a ratchet on the crank would be a lot safer because you can still apply pressure but if it wants to take off on you it wont fling the wrench and hurt you. 

Romeo Duarte says:

a timing chain, guides and tensioner can give you that problem with poor sealing valves, timing correlation, misfire, lean condition are also symptom related to a loose timing chain. great videos, keep it up.

DJDevon3 says:

Part 4?  I understand it’s a great learning experience for your students but compared to all of your past videos it seems like you’re going overboard on your leak down results.  Yeah it’s the company vehicle and you want to ensure you’re diagnosis is correct but you were correct in the first video.  You’re on TDC, timing is good and mechanically sound (no broken cam), you have air in the intake, that’s it, done.  Time for a valve job.

Zeus Carver says:

I dont think a V type engine would be any different. As long as the connecting rod is perpendicular within the cylinder to the crankshaft it should stay.

Discretesignals says:

Just curious as to how much shop air pressure you actually need to make the test accurate?  Would be a lot easier to control the crank with 40 psi than 120 psi for example.  
Here is instruction on a harbor fright compression leak down tester:
“NOTE: Set the air compressor’s regulator 
to 7 to 100 PSI. Never operate this 
tester with air pressure set higher than 
100 PSI, which can damage this tool.”

Not sure if 7 psi is enough to even move the crank.

Saab 9-5 Turbo Estate says:

Hi. Just wonder if you can tell me what the problem is with my engine. cylinder 3 has only around 25 psi. And the rest 160 psi. The engine is tuned up from 150 hp to 280 hp. And the spear plugs is dry and werry white. Hope you can respond. Thanks from Norway.https://www.youtube.com/watch?v=YlJduTNkIRQ

WD Hewson says:

Scanner and Students:

I hope we get to see the dis-assembly and repair of this Hummer engine.

oldskool funk says:

Always great content Scanner Danner appreciate the time and energy you put out. If you guys want to save money and learn how to build a leak down tester watch my “MAKING A CYLINDER LEAK DOWN TESTER AND CHECKING ENGINE INTEGRITY, PISTON RINGS, VALVE & SEAT SEALING” its simple.

Dave Sage says:

I noticed in several shots that the input side pressure dropped off the initial 100psi set point to about 90 or less.  Wouldn’t it be wise to set that back to 100psi. That pressure affects the leak down gauge side measurement as well. It might make the difference between the gauge showing in the good or marginal range and change your opinion of the result. This is why I prefer the type with two regular gauges and look for the difference between them.Sage

wai2machine823 says:

Never ever put air in with the damn wrench on. Especially don’t hold it. Good way to brake your dang arm. Low pressure applyed slow saved u.

Tech Tune Automotive Repairs says:

I have been watching all your videos including your premium channel and I have never heard you say anything stupid, until now 🙂 Sorry, but I have to correct. Regardless of the engine type, TDC is TDC and the crankshaft centre line is straight up and down regardless of the engines design. V type or inline. V type just happens to be on an angle but the centreline is still straight up and down.

Can I also say that doing a leak down test at various points of piston position in the cylinder especially if you are chasing a blow by problem is always a good idea.  A scour in the cylinder does not always run the full length of the cylinder and a crack in the cylinder may be horizontal. I have seen this in race engines that have suffered serious detonation where it would appear the engine is trying to pull itself apart and a leak down did not show up with the piston at TDC. Wind the engine down a inch and bingo.

Love ya work Scanner Man

Gone Tomorrow says:

Pretty ballsy touching that wrench on the crank with compressed air in the cylinder.

wai2machine823 says:

I like to turn into tdc by hand and hold it while noting force… Built in torque wrench on arm.. bionic mechanic man…

Larry Frazier says:

I need to review d leak-down procedure. Haven’t done it in awhile. Look like a good tool u got there

Da Beginner says:

what was the fix ? with this one

spelunkerd says:

I’ve seen leakdown testing done with different incoming pressures (usually lower). Is there an optimum intake pressure for the most accurate test? Presumably doing it with lower pressures might be a little safer….

John Marks says:

More good stuff that TDC trick works on boxer style engines also. I’m a old vw guy. Porsche and Subaru

Mike P says:

what happens when you have a blown head gasket to an adjacent cylinder and that cylinder has either the intake or exhaust valve open? would this not throw off the test? of course you would see this ounce you remote the head anyway.

HybridNz says:

Would it be true to say that running the compression test at TDC and then lower in the bore at different points be a good indication of bore taper wear ?

philbfree1 says:

how long does your class last as in weeks?

juergen scholl says:

I’d like to see a secondary ignition wave form and look for hash on the spark line which would indicate valve sealing issues. Would this be possible?

Thomas EXOVCDS says:

That leakdown tester is wack… on the one that I use, I can connect the hose to the cylinder and then add air to whatever level I want without the worry of sudden crank rotation. Keeping the tranny in gear also helps prevent crank rotation… obviously doesn’t work with automatics. Would love to see a cranking map waveform with one of the spark plugs removed (as someone else already mentioned in one of the prior videos).

MyMatthewdavid says:

hello sir..may I know what is TDC..please?..thanx…

Jeffrey Wilson says:

Good stuff as always man.  I was actually surprised at how much controversy there was about a leak down test during this series lol.  Back when I was a second year apprentice at trade school (here in BC, Canada, second year is focused on engines, rebuilding, diag, maintenance etc etc) we were taught to always do a leak down test at TDC for more accurate results and safety, but to do it at BDC after just as a secondary test to reveal any issues with a head gasket that might have been missed due to the piston covering the sealing surface at TDC.  I always thought this was a pretty “textbook” test lol 🙂 For better or worse, we learn new stuff everyday thanks to the internet 🙂

Autostop says:

didnt quite heard,you said with ring problem in engine,we dont see any smoke from tailpipe?

Allan Ramsay says:

I usually have the leak down tester set at around 25psi input and around the same on the reg so when I turn the crank to find TDC when the hold down gauge begins to rise it’s coming to the top.
The lower pressure makes it easier to turn the crank,TDC is easy to judge,if it’s not then it spins the crank.
Then dial up 100  psi on the input,
Saves all the screwdriver down the plug hole process. 

Ian Cunningham says:

Latex or rubber glove over intake.

 Write a comment


Do you like our videos?
Do you want to see more like that?

Please click below to support us on Facebook!