34 replies to this topic

[Chris D.]

Hi All,

 

So, I'm planning on removing my engine head and getting a refresh done.   I decided to do a leakdown test and a compression test before pulling it.   My results were kind of shocking:

 

Cylinder 1 = 25% Leakdown, 200 compression

Cylinder 2 = 28% Leakdown, 200 compression

Cylinder 3 = 8%  Leakdown, 200 compression

Cylinder 4 = 39% Leakdown, 200 compression

 

I read up religiously on how to do the leakdown test properly for several days before actually doing it last night.  I bought the Longacre tester, made sure I was at TDC on the compression stroke, made sure the engine was hot, etc.  I each case, I had a major leak coming out of the intake side....major hissing coming out of that side.   I did not detect any noticeable air coming out of the oil dipstick hole or the exhaust side.  

 

These results were shocking to me because this engine makes ok power for being a bone stock 1.6L 105k motor that's never been touched (dyno run @ 117 hp last year). 

 

I am planning to pull the head anyway, so based on where I was hearing the leaks I'm guessing the intake valves are not seating very well.  

 

I have a couple of questions:

 

1) I had all of the plugs out as I did each cylinder.   Should I leave the other plugs in as I do each cylinder?   Not sure if this makes any difference.

 

2) Is it possible to make decent hp with these kind of leakdown numbers?   I would think not.

 

Any insight you guys can share would be great.

 

Thanks!!

[James York]

I would think you may have done something incorrect.  It is almost always the exhaust valve that go first.

 

Having the other plugs in or out makes no difference to a properly performed test.

 

If those numbers are real, it for sure is time to service the head.

[Chris D.]

Thanks for the reply James.   Yes, I would have thought the exhaust valves would see more buildup and go before the intakes.   But, like you said, the results do indicate it's time for a look at the head.   That was my plan for this offseason anyway, so the timing is good.

[Danny Steyn]

Chris - two things

 

1. it is very unusual to have such high leakdown numbers, and yet have such high compression numbers
2. have never ever seen all 4 cylinders ever with identical compression numbers

both of those observations make me think something was measured incorrectly. 

[Caveman-kwebb99]

I have had numbers like 10% go down to 2% just by moving the possition of the piston very slightly...  Did it just cause the rings to seat better?  Was it just off TDC by a tiny bit?  I dont have that answer, but if you have those numbers and they are legit you have Cupped intake valves or burn valves.

[marcusmazza]

The compression numbers are actually really good. The last time I checked compression on my 1.6L, they were at 203, 203, 204, 203. It's a Sunbelt engine, and had 25-30 hrs on it at the time. The leakdown numbers were less than 1% on all four cylinders. The engine made roughly 117hp on a dynojet with SAE correction. Engines do form a bit of a ridge on the cylinder walls at TDC. This could affect the leak down numbers without affecting the compression numbers. The compression test is closer to real life, since the piston is moving across the entire cylinder wall, were the leakdown test keeps it stationary at one spot. If that spot happens to have more wear, it can have a greater effect on the test. I had a BMW M3 with leakdown numbers in the high teens/low 20's, and it ran like a champ. The compression numbers on that same engine were always fine. It did leak between cylinders though. The BMW head gaskets were known to fail between cylinders. If the neighboring cylinder to the one being tested had it's intake valves open, it sounded as if the leak was from the intake valves, when in reality, it was due to a bad head gasket. Not sure if the miata head gaskets fail in a similar way.

 

Marc

[Chris D.]

Thanks for all the input....lots to think about.   I will rerun the test again this weekend once the "Polar Vortex" exits the scene and I can work in the garage without freezing to death.   I have no idea what a polar vortex is, but trust me, it's damn cold!  

[Bench Racer]

 I have no idea what a polar vortex is, but trust me, it's damn cold!  

 The polar vortex as we know it is really the arctic vortex, and the suckers home is much further north than the midwest or northeast. Sort of like Jaws refers the Bristol vortex being created above Bristol speedway.

[SaulSpeedwell]

How did you "make sure" you were at TDC?  The TDC mark on the pulley is often not correct, especially on a built motor.  I'll bet you a $4 First Place trophy that you weren't at true TDC.  With leakdown like that, compression numbers and power would be well below what you are reporting.

 

Use a piston stop from a cam degree kit to find and make a TRUE TDC mark that you can use.

 

And - just for fun - do the test at greatly reduced pressure - like 10-20 psi or whatever is convenient for the Longacre gauge - and use a breaker bar on the crankshaft nut to stroke the motor and you will "feel" where TDC is and "see" on the gauge where the lowest leakdown is.  Again, I bet where you THINK TDC is ain't where it really is.  (Then - if I'm right about this - crack a beer and think about what this means for your ignition timing measurements and cam timing, etc.).

 

Let us know what you find!

[SaulSpeedwell]

 

The BMW head gaskets were known to fail between cylinders.

 

I have been told repeatedly that BMWs do not simply fail.  They only fail when improperly driven and/or maintained.  I've heard the same about Audis and Mercedes.

 

(See also:  "Hey, sewer rat may taste like pumpkin pie, but I wouldn't know because I wouldn't ... ")

[marcusmazza]

Saul,

 

Then I must be an awful driver or an awful maintainer, because I've owned all three, and they failed all the time.

 

Regarding your post on leakdown testing, I'm trying to figure out how being slightly off of TDC has a major effect on the leakdown numbers. Yes, the rings will be at a slightly different position on the cyclinder wall, but unless that section of the wall is worn significantly more, not sure how the numbers would change much? On most engines, the most significant wear is found closer to the middle of the cylinder, where the side load generated from the rod is the greatest. Other than the possible ridge formed at TDC, I would imagine the cylinder wear would be pretty even towards the top of the cylinder. Unless you are that far off TDC that the engine is trying to rotate from the pressure supplied by the leakdown tester, I don't see how being slightly off TDC has a significant effect.

 

 

Marc

[SaulSpeedwell]

Not the rings so much as the valves ... meaning I'm questioning whether he had the valves slightly open to get those high leakdown numbers. 

[marcusmazza]

Valid point. I suppose this could be possible if you are at TDC after the exhaust stroke. If you are at TDC after the compression stroke, you should have plenty of play before any valves start to open. I suppose Chris may have been testing at the wrong TDC if he was basing it on the pulley mark only. I usually hook up my compression gauge to the cylinder I plan on testing. I rotate the engine until I see the gauge register pressure build up. I then remove the gauge and insert a prope (long socket extension), and continue rotating the engine until I hit TDC. I base this on when the prope stops moving, not on the pulley timing mark. Seems to work pretty well. I also read on some aircraft sights that if you go past TDC and rotate the engine back, it can effect the results since the rings are now seated against the top of the piston groove instead of the bottom. Not sure how much this effects the reading, having never tried it. Good luck Chris.

 

Marc

[Chris D.]

How did you "make sure" you were at TDC?  The TDC mark on the pulley is often not correct, especially on a built motor.  I'll bet you a $4 First Place trophy that you weren't at true TDC.  With leakdown like that, compression numbers and power would be well below what you are reporting.

 

Use a piston stop from a cam degree kit to find and make a TRUE TDC mark that you can use.

 

And - just for fun - do the test at greatly reduced pressure - like 10-20 psi or whatever is convenient for the Longacre gauge - and use a breaker bar on the crankshaft nut to stroke the motor and you will "feel" where TDC is and "see" on the gauge where the lowest leakdown is.  Again, I bet where you THINK TDC is ain't where it really is.  (Then - if I'm right about this - crack a beer and think about what this means for your ignition timing measurements and cam timing, etc.).

 

Let us know what you find!

 

Ah hah!   I think I may have found the problem....I was indeed using the timing mark on the pulley.  I thought that would be more accurate than the watching long extension I had plopped into the spark plug hole stop moving.   I will do as you suggest and report out what I find (certainly the reduced pressue method at least.....I will need to read up a bit about the the piston stop as I'm not familiar with it).   This weekend it's supposed to hit a "balmy" 36 degrees....perfect to come out of hibernation and back into the garage!

[Chris D.]

Valid point. I suppose this could be possible if you are at TDC after the exhaust stroke. If you are at TDC after the compression stroke, you should have plenty of play before any valves start to open. I suppose Chris may have been testing at the wrong TDC if he was basing it on the pulley mark only. I usually hook up my compression gauge to the cylinder I plan on testing. I rotate the engine until I see the gauge register pressure build up. I then remove the gauge and insert a prope (long socket extension), and continue rotating the engine until I hit TDC. I base this on when the prope stops moving, not on the pulley timing mark. Seems to work pretty well. I also read on some aircraft sights that if you go past TDC and rotate the engine back, it can effect the results since the rings are now seated against the top of the piston groove instead of the bottom. Not sure how much this effects the reading, having never tried it. Good luck Chris.

 

Marc

 

I did make sure I was on the right TDC on the compression stroke.  Before I hooked up the Longacre gauge, I screwed the hose that came with the gauge into the plug hole and felt with my thumb the other end of the hose as I rotated the engine.   A little low tech, but I was certainly able to feel when pressure was building up.   But, from there, I decided to use the pulley mark instead of the probe method.   Again....I thought it would be more accurate.  

[Qik Nip]

"Saul" and his $4 trophy (first place) is almost certainly on the money. I recently had a similar episode with a squirly leak down test. Turns out the crank pulley / harmonic balancer had slipped. (They are two piece assemblies with a rubber coupling between the outer and inner segments). In my case we only found the issue after I'd pulled the motor and had in at the builders shop where we used a dial indicator to establish TDC. It turned out that the timing mark was retarded four degrees from TDC!  

Rick

P.S. Let's hope that correction explains the lack of power last season!

[SaulSpeedwell]

I don't have any idea how those dampers slip, but I've had them be off by 15-30 degrees!  Moreso on the 99 than the 1.6.  One thing is for sure - put NO confidence in the factory TDC mark on your damper pulley.  It is worse than meaningless, it is MISLEADING!

 

I made my own leakdown tester - I made sure the 120 psi gauge was calibrated, I used a gutted NGK spark plug brazed to a 1/8" NPT 4 inch long (or however long it needs to be to traverse the full depth of the head and valve cover) pipe nipple to make the "sealing" entity that screwed down into the head, and I made sure that nipple/plug assembly could take the same torque as a "real" spark plug.  Basically braze fittings together with a gutted NGK until you have a "flow through" NGK that you can torque from above the valve cover.  The connect air hose to it.  I guess I should take a picture,

 

I don't know how the Longacre gauge is built, I've never used one, but I know/knew my gauge wasn't BS-ing me.  I had motors that were rockets with 8-12% leakdown (Builder "A" with "loose is fast" philosophy), and I had motors that were rockets with <3% leakdown (Builder "B" with "good sealing is fast" philosophy). 

 

 

Something isn't adding up about the original poster's numbers,.  Forget the piston stop for a minute, just put a long 3/8" extension down the #1 hole, and convince yourself where TRUE TDC is.  Make a mark on the pulley halfway between where the extension starts to go down on either side of TDC.  I don't have cam timing numbers in front of me, but you have to be more than a few degrees off of TDC for the valves to actually be starting to OPEN.  But no way you make 117HP with valves leaking that bad.  Plus your intake mani would be all sooted up.

 

Its possible your cam belt is off a tooth, but again - you shouldn't be making 117HP, assuming that is legit Dynojet HP without fudging or weird environmental conditions or hot fuel.

[Pat Ross]

Speed,

In response to your comment about BMWs I felt I must comment. My wife has a 335ci convertible with about 80k miles on it. It has had the fuel pump replaced twice, the vvt controller replaced, the camshaft replaced, water pump replaced. I have a Lexus ISF that has 140k miles on it and it has never spent an hour in the dealer's shop. I have been to multiple track days in the Lexus and it just keeps on ticking.

When we bought the BMW Mike told us to be sure and get the extended warrantee. We did, and it has paid for itself

Pat

[Tom Sager]

My longacre gauge was not properly calibrated right out of the box.  With 110+ PSI in the compressor and the input gauge adjusted to 100PSI, the right hand gauge (before connecting the gauge to the engine) did not read zero.  It was off by a few PSI.  I sent it to Longacre and they calibrated it and returned it to me.  Nowhere near enough to get the kind of numbers posted, but could be a contributor.  

 

On an old SM engine (very old) a couple years ago we were getting about 15% - 20% leakdown on 2 cylinders through the rings.  The guy doing the work suggested putting in some super high detergent oil and an additive and running the engine for a couple hours.  I was thinking this old wives tale of stuck rings couldn't be true but he went and did it.  After that and putting in fresh oil the darn thing was giving very respectable numbers in those 2 cylinders.  

[Bench Racer]

 

I made my own leakdown tester - I made sure the 120 psi gauge was calibrated, I used a gutted NGK spark plug brazed to a 1/8" NPT 4 inch long (or however long it needs to be to traverse the full depth of the head and valve cover) pipe nipple to make the "sealing" entity that screwed down into the head, and I made sure that nipple/plug assembly could take the same torque as a "real" spark plug.  Basically braze fittings together with a gutted NGK until you have a "flow through" NGK that you can torque from above the valve cover.  The connect air hose to it.  I guess I should take a picture,

Man must believe in tool before believing the numbers. I believe the plug washer ^ sealing better than the Longacre o-ring razzel dazzel.