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Leakdown tests and what they can tell you?


wvumtnbkr
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Sooo, there is a chance that we may have over revved the crap out of our engine at the last race (I seriously don't know but the driver wasn't 100% sure)...

 

I was thinking a leakdown test would tell me if we have a bent valve.

 

What can a leakdown test tell you?

 

What can't it tell you?

 

Is it really better than a compression test?

 

How do you do it? 

 

Any help is appreciated!

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4 minutes ago, wvumtnbkr said:

Sooo, there is a chance that we may have over revved the crap out of our engine at the last race (I seriously don't know but the driver wasn't 100% sure)...

 

I was thinking a leakdown test would tell me if we have a bent valve.

 

What can a leakdown test tell you?

 

What can't it tell you?

 

Is it really better than a compression test?

 

How do you do it? 

 

Any help is appreciated!

 

If you have bent valves, they'll show up as low compression (<50% of spec) and very high (60%+) leakdown.

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3 minutes ago, Team Infiniti said:

Still running quiet /idling smoothly?

I don't know.  Didn't run it after that last stint at Gingerman.

 

I guess I should do that first...

3 minutes ago, red0 said:

What does your ass dyno say when doing street pulls with it? When we had bent valves, car was way down on power. 

No brakes at the moment.  My ass dyno says nothing.

 

Which is strange, ya know, my ass normally has lots to say.

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2 minutes ago, wvumtnbkr said:

I don't know.  Didn't run it after that last stint at Gingerman.

 

I guess I should do that first...

Yup, I know others may say otherwise but this being of American origin I would say rods/rod bearings are more a worry then tipping a valve like a bimmer would do.

 

As a tech, the ONLY valve issue personally seen on a 3.4 (older malibu) was when the camshaft broke(high mile, oil water mixed 2x times from intake gasket fail)

Edited by Team Infiniti
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Just now, Team Infiniti said:

Yup, I know others may say otherwise but this being of American origin I would say rods/rod bearings are more a worry then tipping a valve like a bimmer would do.

ohhhh!

 

Are you saying that if it is running quietly, and idling all right that it is probably okay?  WWTID?  Would you pull the pan and check bearings?  drop the oil and check the drain plug?  Send oil off to blackstone labs?  Cut open the oil filter and look for metal?  All of the above?

 

I did notice it sounded like it had some valve noise for a few minutes on startup on race morning.  Went away after about 2 minutes.

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1 minute ago, wvumtnbkr said:

WWTID?

Fire it up with a critical ear, warm motor then give it a good flogging.... inspect for glitter and new noises, if neither is found, flog it more, sign up for the next event.;)

 

Done this many times with 100% success in determining suitability to continue, not to mention those engines are cheap, if it knocks its now a parts unit.

Edited by Team Infiniti
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Just now, Team Infiniti said:

Fire it up with a critical ear, warm motor then give it a good flogging....

 

Soo, if it doesn't explode or get worse with the flogging.....  Im good. 

 

If noise is present and gets worse, I should report back....

 

If it explodes, I should be prepared with video and popcorn... 

 

Does that sound right? 

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I love leakdown tests. I think the others have helped your particular situation, but I wanted to chime in on the leakdown test specifically.  In case others read this.

1 hour ago, wvumtnbkr said:

I was thinking a leakdown test would tell me if we have a bent valve.

 

What can a leakdown test tell you?

 

What can't it tell you?

 

Is it really better than a compression test?

 

How do you do it?

1. it will, or at least tell you if you're leaking past a closed valve.

2. if you are leaking it tells you where (int/exh valves, coolant, crankcase) so you'll know if bent valve, blown HG or bad rings.

3. Can't tell you how much compression each cylinder is producing (only indicates % leaking)

4. I think so.  Its more descriptive and can be done with the engine on the stand if needed (can't do that with a compression tester, at least not easily)

5. Move piston to TDC on compression stroke, screw in plug adapter, turn on compressor and hook up the gauge.  If it reads low, check intake, exhaust, coolant and dipstick for air sounds.

 

If you're leaking past the rings, you can put a couple drops of oil in the cylinder and re-test to see if it improves.

Edited by Bremsen
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1 hour ago, wvumtnbkr said:

I did notice it sounded like it had some valve noise for a few minutes on startup on race morning.  Went away after about 2 minutes.

 

My Dad's Chevy Venture does that, so that might be normal-ish on these motors. 

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1 hour ago, wvumtnbkr said:

Sooo, there is a chance that we may have over revved the crap out of our engine at the last race (I seriously don't know but the driver wasn't 100% sure)...

 

I was thinking a leakdown test would tell me if we have a bent valve.

 

What can a leakdown test tell you?

 

What can't it tell you?

 

Is it really better than a compression test?

 

How do you do it? 

 

Any help is appreciated!

How many revs we talking?

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Do a leakdown..  Compression check will only show you a really bent valve.

 

If you ran the race on the motor, its not really bent, just barely tapped if at all.  Leakdown will tell you that for sure.

 

Put a glove over the throttle body and the exhaust tip. If your exhaust doesnt leak, it will balloon the glove if you have an exhaust valve leak.

 

My money-shift saw 9k rpm and still ran normal, but sure enough after a leakdown it showed bent exhaust valves.

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Its just the car telling its new engine that proper RX-7 operation is to lose compression....   I kid, I kid!   'Merica problems....

 

1/2 - yup, leakdown can tell you where you are losing compression.  stethoscope (even shade-tree type, aka, extra fuel line or vac hose) in intake, exhaust, dip stick/valve cover and radiator can give you a general idea of what area is the issue.

3 - compression and if it is not a compression related issue (like bearings going)

4 - both are useful, compression is quick and easy to give a general expected bill of health, leak-down for details and confirmation

5 - Bremsen has it.  Plus make sure that the piston is at TDC, with the crank straight inline with piston/rod.  Make sure whatever tool you are using to turn the crank to set TDC is off before you pressurize anything as the air pressure can (and will) turn the crank if piston is not at TDC.

 

If you buy/borrow a quality tool, the instructions are generally pretty good on the how-to.

 

 

 

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As @NigelStu said above, be sure the cylinder you are pressuring is truly at its' TDC. (Tools on the crank spinning around as the piston goes from TDC to BDC aren't a lot of fun to your extremities, the radiator, etc...)

With hydraulic tappet engines, I've sometimes had to remove the rocker arms to ensure the valves were seated, and not being partially held open by the hydraulic lifters.

in performing a leak down test, the pressure is either going to stay in the chamber, or escape past piston rings, valve seats, or the head gasket. 

Good idea to put a thin rubber glove over the carb(or intake opening), and over the exhaust pipe. Head gasket leaks can be found by uncapping your radiator and seeing if pressure from the cylinder is escaping into the cooling system. 

Edited by mcoppola
Many fat finger spelling errors
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1 hour ago, NigelStu said:

 Make sure whatever tool you are using to turn the crank to set TDC is off before you pressurize anything as the air pressure can (and will) turn the crank if piston is not at TDC.

 

 

 

Put car in 5th/6th gear and roll (forward direction only).

 

After too many times messing up, this is the method I use now.

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Cut open the oil filter and look for metal. I do it after every weekend.

 

At Laguna, one of my rental drivers didn't show when he was supposed to on his fifth lap. He got towed in and we got the story: spun off into the gravel and burnt up the clutch trying to get out. Not much said about why he went into the gravel, but when we got back from the weekend I decided to check the tach memory to see if my rod-stretcher driver was calming down on his downshifts.

 

Imagine my surprise when it swung all the way around to 7600 rpm! Ah, that makes a little more sense: downshift for the corner and get 1st instead of 3rd, loop the car into the gravel while running the engine up 1300 rpm past the fuel cut-off! For the record, the shift light is set at 5500 rpm and the fuel-cut-off is 6300 rpm.

 

Leakdown was still around 5-6% so no valve contact. Cut apart the filter expecting to see glitter, found none. Maybe those ARP rod bolts were worth it ...

 

I've put another 20 hours or so on the engine, timing chain is getting a little sloppy after 80+ hours of racing but otherwise just fine. The GM V6s are pretty tough.

Edited by mender
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