[tamathumper]

At the very least, I'll have fixed a bunch of leaks, and it certainly didn't hold *boost*, so I might have an improvement there too.



But, will it cure the stumble!?

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[Cloud9...68]

Are you taking bets? <img src="/forum/images/smilies/968/wink.png" class="smilie" alt="" />

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[tamathumper]

I am, but you might not like the side I'm taking. <img src="/forum/images/smilies/968/biggrin.png" class="smilie" alt="" />

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[MB968]

I hope I'm wrong, but it just doesn't seem like the vac. leaks you've found would cause the stumble. If the stumble is from a lean situation, I'd expect it to take a larger vac leak to cause it. When you suddenly push down on the throttle, the computer will momentarily give an extra shot of fuel by extending the pulse width to the injector (today's version of the accelerator pump that was on carburetor equipped cars). So, this normally causes the a/f after a sudden change in throttle to be on the rich side. Thus, why I think it would take a much larger vac. leak to be the issue. But, hope I'm wrong.



And, you comments on gauge accuracy caused my to look a bit. I couldn't find anything that would verify accuracy of a mityvac (sp?) gauge. It would be nice to have a way to check calibration of these so you could truly know what vacuum you were getting at idle. But don't know of a way to do that and don't recall how we did it in the metrology lab. My experience is that these gauges arean't that accurate in an absolute sense. Best used to study changes.

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[flash]

a vacuum leak absolutely can cause it to stumble. it can even cause it to stall when you step on it. i went through this myself a couple of times.



yes, there is a program that fattens things up when you step on it. it is load and rpm based though, so when and where it does things depends on those factors. the MAF and TPS determine the load.



that being said, i don't know that he had a real leak. based on the gauge reading issue now discovered, i wonder if it was fine all along, and this was one big wild goose chase.



we'll see though.



once things are buttoned up, if the problem persists, there would be a couple of things to check:



running the Durametric is a must. this is a tool that anybody doing their own work should have. trying to diagnose anything without it (or the hammer) is foolhardy at best.



MAF wiring is suspect. it had to be cut into in order to install the kit. any one of those wires could be a problem, and it might not throw a code.



valve timing could easily be an issue here. the cam overlap that helps the naturally aspirated engine, hurts the supercharged one. if the cams were set with increased overlap (which you do to increase top end power on a naturally aspirated engine) it could easily be losing boost. i'm not sure how that would affect stumbling though.



valve leak will cause boost loss. if there is a severe problem there, it can cause stumbling. it would also explain the oil in the intake. a leakdown test will determine if this is an issue.

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[tamathumper]

I did have air pressure leaks (boost), and I'm sure they attributed to vacuum leaks, so it's a good exercise anyway, no matter how frustrating.



I have the Durametric ready to go, and have already used it to monitor the <acronym title='mass air flow'>MAF</acronym> values, but I have a whole course of tests lined up for when the engine is back together. Still need some gaskets and o-rings.



I also have a leakdown tester on the bench and will perform that test when I get a spare minute.

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[MB968]

Yeah don't be too discouraged. Either way you've improved some things that weren't quite right. I'm hoping when you get it all back together all will be well.

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[tamathumper]

AutoAtlanta is now going to backorder-jerk me around for a week or two, and basically eat up all of the only good driving season we get around here. I knew better...

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[flash]

bummer - been there done that

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[tamathumper]

Leakdown test shows about 18% on all cylinders at 15 psi.

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[flash]

bummer - that's pretty big, but not catastrophic. is it at the rings or the valves?

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[tamathumper]

From everything I've read that's OK, especially if it's consistent across all cylinders? Because it's a low pressure test, I wouldn't expect the valves to be pushed closed and the reading would be a little high anyway compared to the 100 psi testers. This is also a test with the engine cold, whereas some say it should be warmed up first - but I can't do that with the intake manifold off.

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[flash]

it's "ok" but not good. it should be below 10%. the real question is where the leak is. is it the valves or the rings? it's not as bad if it's the rings. if it's the valves, that's pretty significant.

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[tamathumper]

I'll have to do it again and pay attention to where it's leaking - it was so far into the green zone I wasn't all that concerned. And, having been lead astray by gauges recently, I don't know how much stock I'd really put into this thing, either.

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[flash]

yeah - that will be the real determination of condition. if it's mostly rings when cold, then don't even worry about it. if it's valves though, then you need to start looking at things. i am concerned because of the oil in the intake. you may have some leaky guides or seals. again, the numbers aren't so bad that it won't run decently, but you may not get all of your boost, and may still have a vacuum leak.

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[Lear35A]

tell me a little about this leak down test you're doing. What I am familiar with for leak down is a test done on every piston aircraft engine at least once per year. With that leakdown test you set 80psi going in and look at the psi the cylinder will hold. On a real good cylinder that pressure will be in the low to mid 70psi range. Normally the test is done with the engine warmed up.

I have such a tester and will do an "airplane" leakdown test on my pretty well broken in fresh engine rebuild at the first opportunity. To do this you need someone to hold the crank with a wrench. The 80psi in the cylinder gives it a great tendency to rotate!

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[tamathumper]

This test is done at <acronym title='top dead center'>TDC</acronym> to prevent piston movement and to ensure the valves are closed, and I think the tester maxes out at 15 PSI on its output side, even though its operating pressure is between 7-100 PSI on the input side. The instructions don't call for the engine to be at operating temperature, but admittedly they're in "very good Chenglish" so I don't expect them to be complete. 15 PSI isn't enough to force the valves closed, so they are only closed based on their own spring strength. I'll try to re-run the test tomorrow at a higher input PSI just to see what changes - today's test was with 40 PSI on the input side of the gauges.

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[Lear35A]

The test I am familiar with is also done at <acronym title='top dead center'>TDC</acronym> with valves closed. However, even a fraction of a degree off of <acronym title='top dead center'>TDC</acronym> with 80 psi on the top of the cylinder (that's 1005 pounds pushing down on that piston top) is going to make that crank want to spin. You do need someone on a wrench with big breaker bar to stop that rotation potential. I have no idea why 80psi was chosen as the standard for testing leakdown on airplane engines, but it has been that for a looooong time.

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[flash]

yeah - with every tester i have ever used, you have to use a flywheel lock and a lot more pressure than 15. i use 100psi to keep things easy. you have to do it cylinder by cylinder, listening for the leak in various places to determine where it is.



here is a decent link:



http://www.gregsengine.com/cylinder-leak...sting.html

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[JWahlsten]

Good article. Thanks, Flash.

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