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Vacuum Leak - Can't Find It

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Old 01-27-2017, 10:55 PM   #41
rahaul
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Wow, I missed that engine note. LS6, as in 1970 454 with 450 HP?

Thought you had a 327. The LS6 must be a lot of fun. The big blocks had just as many if not more issues with cams going flat and high lift high load springs exaggerate that issue.
El Correcto Westlotorn. Interesting you mention BBC and flat cams. When this crate engine('71 425 HP) was new (30 yr ago) it was balanced/ blueprinted and the GM cam was replaced with a 292 Comp Cam. Very strong motor. Mid 12s on street tires in the 1/4. High 11s on 8" slicks. After a ton of street racing and 5 yr later, the Comp went flat. In the process of breaking in 2 more 292 Comps, they went flat on different lobes. After talking with everybody I knew that had anything to do with BBC, I came to the conclusion that when the lifter bore wear becomes excessive, and it doesn't take much, the lifter will tend to bind in the bore in a high lift application. When you consider that the higher that lifter is from center, it has a better chance of binding the higher it goes and it only takes one or two revolutions to wipe the oil film and it's gone. So the fix is go with a low lift cam or go roller. Roller is free HP so I went roller.
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Old 01-27-2017, 11:05 PM   #42
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Big blocks have a well known issue with the lifer bores being out of blueprint spec, drilled in the wrong place at the factory. They were close enough to work with factory cams but cause many failures on high lift cams. A couple companies make jigs to drill the holes in the proper place and then sleeve them back to the proper lifter size. I know machine shops with CNC machines programmed to do this job it is, or was so common.
The LS6 is a rare factory beast. Enjoy it.
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Old 01-28-2017, 07:27 AM   #43
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Big blocks have a well known issue with the lifer bores being out of blueprint spec, drilled in the wrong place at the factory. They were close enough to work with factory cams but cause many failures on high lift cams. A couple companies make jigs to drill the holes in the proper place and then sleeve them back to the proper lifter size. I know machine shops with CNC machines programmed to do this job it is, or was so common.
The LS6 is a rare factory beast. Enjoy it.
I didn't mean for this vacuum leak issue to go so far off topic. Sorry. I had never heard that said about the factory lifter bore issue. Extremely interesting. Rick, one of the guys I talked to about the flat cam thing, was into BBC pulling trucks. Years ago when stuff was cheaper they had a cam go flat and decided they would just keep throwing cams in until one seated. He said they stopped at 11. Ended up trying a low lift cam and it seated.

I do want to point out that the those 1/4 mile times were with 4:11s and Muncie 4 sp. The T-56 would never hold up to the punishment that the M21 got. The LS6 is indeed a beast. I have put over 130,000 miles on this engine with the first 20K seeing many RPMs. It was amazing how effortlessly it went to 7 and 8K. It became a fuel mileage engine around 1994 because the wife and I were going cross country driving. I also didn't like all the RPMs at 70 MPH with the Muncie. The first roller was a Chet Herbert after the Comp cam refused to seat. I guess you might say I "accidentally" started making fuel mileage.
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Old 02-11-2017, 07:38 PM   #44
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That is a new one one me!
Where did you put these "openings" and how many did you do?
could you post a picture.
Joe
Joe, if you look at my avatar you will see the openings.

2 sight holes into lifter galley/ 2 holes to look at intake runners
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Old 02-11-2017, 07:43 PM   #45
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Seems as if all the time spent on cam development, valve springs, etc., would have cost much more that any savings obtained through fuel economy. In any case, the important question is how the engine runs now that you have discovered (and presumably fixed) the vacuum leak. If that didn't do it, my money remains on the distributor advance. By the way, we would all like to see some pictures of the inspection holes and their plugs.
My avatar shows the holes.
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Old 02-11-2017, 08:07 PM   #46
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You must have some serious, serious concerns about cam lobe wear is all I can say..
After replacing the intake gasket, eliminating all other possibilities of vacuum leaks, swapping out Qjet for a rebuilt, changing out HEI, new plug wires, plugs, I'm still getting the same scenario no matter what combination of changes.

I'm down to this: Since I have quite a bit of oil in the heads and valve stems (I noticed this when I pulled the intake), is it possible to have enough excessive valve guide wear to be my vacuum leak?
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Old 02-12-2017, 01:40 PM   #47
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I'm down to this: Since I have quite a bit of oil in the heads and valve stems (I noticed this when I pulled the intake), is it possible to have enough excessive valve guide wear to be my vacuum leak?

No, your valve guides are not causing a vacuum leak. Valve stem seals may be one source of your oil but it sure sounds like your manifold is not fitting correctly to the heads. Do you have another manifold you could test with?
If this block has been decked or the heads milled you might have an alignment issue.
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Old 02-12-2017, 02:14 PM   #48
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I went through this with a 56 Cadillac 365, WCFB. Engine wouldn't idle once at operating temp and choke opened. We put a kit in the carb. Checked everything, propane torch, replaced intake with a another, new gaskets and ruled out mechanical problems and ignition. We had a classic vacuum leak. In desperation I sourced a similar WCFB on the shelf of a rebuilder. (2333S). He freshened it and on it went, problem solved. Still have the troublemaker in a box.

Don't drive yourself crazy. Put a known good carb on it and see if it idles.

Dan
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Old 02-12-2017, 04:23 PM   #49
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I'm down to this: Since I have quite a bit of oil in the heads and valve stems (I noticed this when I pulled the intake), is it possible to have enough excessive valve guide wear to be my vacuum leak?

No, your valve guides are not causing a vacuum leak. Valve stem seals may be one source of your oil but it sure sounds like your manifold is not fitting correctly to the heads. Do you have another manifold you could test with?
If this block has been decked or the heads milled you might have an alignment issue.
Same block, same heads, same intake for the last 30 yrs. I've pulled and installed the intake at least 10 times in those 30 yrs. If anything the alum is wearing thin from off and on, but not really.

I was really convinced that the intake gaskets were the problem when I pulled the intake 2 weeks ago. Always in the past the gasket stuck to either the heads or the intake but this time they fell off and there was a film of oil on the gaskets. On top of that when I flipped the intake over maybe 1/2 qt of oil came out of the reservoir below the plenum. I don't ever remember that happening before.
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Old 02-12-2017, 04:31 PM   #50
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I went through this with a 56 Cadillac 365, WCFB. Engine wouldn't idle once at operating temp and choke opened. We put a kit in the carb. Checked everything, propane torch, replaced intake with a another, new gaskets and ruled out mechanical problems and ignition. We had a classic vacuum leak. In desperation I sourced a similar WCFB on the shelf of a rebuilder. (2333S). He freshened it and on it went, problem solved. Still have the troublemaker in a box.

Don't drive yourself crazy. Put a known good carb on it and see if it idles.

Dan
Thanks but I have tried 4 different carbs with all the same result and not junkers. The guy that rebuilt one of the Qjets keeps saying vacuum leak. He's an old timer and knows his stuff. Only works on pre-computer stuff.

I'm not crazy yet but I'm close to the edge.
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Old 02-13-2017, 08:03 PM   #51
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I was talking to the guy that does my head work and he said you might take a look at the intake rocker arm studs. He said on my heads the threads go all the way to the intake runners and if those studs are not sealed it could be at the very least my oil problem and possibly a vacuum leak.

I checked manifold vacuum and have 11" at 1100 RPM. Doesn't seem to want to idle much under that w/o dying, maybe 9" at 900 RPM.
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Old 02-14-2017, 02:27 AM   #52
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You have to have vacuum to maintain an idle quality. I don't know how aggressive your camshaft is. The comment about the rocker arm studs is an easy fix. Buy some thread sealant. Perma Tex will have some at most parts stores, pull the studs and seal them with the thread sealant compound and re install. I have never heard of this complaint concerning the studs and I worked with engine warranty for most of 20 years. Late 80's to late 90's. But I have found if you keep your ears open you will learn something every day!
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Old 02-14-2017, 07:12 AM   #53
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You have to have vacuum to maintain an idle quality. I don't know how aggressive your camshaft is. The comment about the rocker arm studs is an easy fix. Buy some thread sealant. Perma Tex will have some at most parts stores, pull the studs and seal them with the thread sealant compound and re install. I have never heard of this complaint concerning the studs and I worked with engine warranty for most of 20 years. Late 80's to late 90's. But I have found if you keep your ears open you will learn something every day!
I must give you a big thanks for that last line but it also extends to everyone who has given suggestion. I'm a thankful person. I just don't hit "thanks" on every post. I'm 68 and still learning. This dilemma has really got to me. I vowed I would never take this car to somebody else to work on it, and in 30 yrs I haven't, but I'm getting real close to eating those words.

Cracked head was also mentioned in the mix of possibilities. I'll be doing a crankcase pressure/ vacuum test to see where that leads.
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Old 02-14-2017, 07:53 AM   #54
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To check for a vacuum leak under the manifold, cap all the openings to the crankcase, PCV, oil filler and any other openings that can allow air into the crankcase.
Remove the dip stick and attach a vacuum gauge to the tube.
Start the engine and bring up the RPM, if the gauge goes into a vacuum then you have an internal vacuum leak, it should go into a slight pressure.
I have found lower intake gasket leaks this way.
Joe
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Old 02-15-2017, 06:33 AM   #55
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To check for a vacuum leak under the manifold, cap all the openings to the crankcase, PCV, oil filler and any other openings that can allow air into the crankcase.
Remove the dip stick and attach a vacuum gauge to the tube.
Start the engine and bring up the RPM, if the gauge goes into a vacuum then you have an internal vacuum leak, it should go into a slight pressure.
I have found lower intake gasket leaks this way.
Joe
Initial indication is that I have 0 crankcase pressure at start up but it begins to build pressure as the engine warms up to a high of 1.5 lb. I like your idea of using the dipstick tube and probably would have never thought of loosing pressure there.

I'm going to retest today. The duct tape was not reliable on the valve cover openings after the valve covers heated up. Using plan B.
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Old 02-15-2017, 07:02 AM   #56
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Initial indication is that I have 0 crankcase pressure at start up but it begins to build pressure as the engine warms up to a high of 1.5 lb. I like your idea of using the dipstick tube and probably would have never thought of loosing pressure there.

I'm going to retest today. The duct tape was not reliable on the valve cover openings after the valve covers heated up. Using plan B.
I think that if you were building pressure, it rules out a vacuum leak into the block, no?


Gerry
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Old 02-15-2017, 06:10 PM   #57
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I think that if you were building pressure, it rules out a vacuum leak into the block, no?


Gerry
In perfect world scenario...I would agree.

But...I have had numerous Corvettes that when they pull up for an inspection and what the owner wants is a tune-up...actually end up being something entirely different...A NEW ENGINE. Putting money into a tune-up when the engine is tired and needing help...in my opinion is a waste of money.

I have had engines that when I pull out fresh air tube for the crankcase in the valve cover and the PCV is still installed and sucking vacuum like crazy...that when I place a dollar bill over the fresh air grommet...it gets blown away. If the rings were not allowing so much blow-by pressure to get past them....then the crankcase pressure would be not so great that the PCV could actually provide suction like it is supposed to do and cause the dollar bill to now get sucked down to the grommet and stay there.

I feel that the ginseng in question is good and this is not the issue but if the vacuum leak is slight but the blow-by is greater....this test may mask the problem.

And the test I use with the dollar bill also is not 100% accurate...because...if the valve cover gaskets are missing sections or leaking and allowing air to get pulled in from there...and the dipstick grommet is not sealing it...and the sealer at the front and rear 'China' walls of the engine block are leaking...then air is being drawn in from numerous places. But...when I feel air blowing out of the grommet in the valve cover and the ginseng is idling and I see blue smoke rings coming out of the grommet in the valve cover....that is usually a good indication that the engine is toast and needing to be rebuilt. And if I can effortlessly spin the engine by using the alternator belt when the engine is not running is also a good indication that there is little to no ring drag...and it is needing help.

DUB
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Old 02-16-2017, 07:20 AM   #58
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In perfect world scenario...I would agree.

But...I have had numerous Corvettes that when they pull up for an inspection and what the owner wants is a tune-up...actually end up being something entirely different...A NEW ENGINE. Putting money into a tune-up when the engine is tired and needing help...in my opinion is a waste of money.

I have had engines that when I pull out fresh air tube for the crankcase in the valve cover and the PCV is still installed and sucking vacuum like crazy...that when I place a dollar bill over the fresh air grommet...it gets blown away. If the rings were not allowing so much blow-by pressure to get past them....then the crankcase pressure would be not so great that the PCV could actually provide suction like it is supposed to do and cause the dollar bill to now get sucked down to the grommet and stay there.

I feel that the ginseng in question is good and this is not the issue but if the vacuum leak is slight but the blow-by is greater....this test may mask the problem.

And the test I use with the dollar bill also is not 100% accurate...because...if the valve cover gaskets are missing sections or leaking and allowing air to get pulled in from there...and the dipstick grommet is not sealing it...and the sealer at the front and rear 'China' walls of the engine block are leaking...then air is being drawn in from numerous places. But...when I feel air blowing out of the grommet in the valve cover and the ginseng is idling and I see blue smoke rings coming out of the grommet in the valve cover....that is usually a good indication that the engine is toast and needing to be rebuilt. And if I can effortlessly spin the engine by using the alternator belt when the engine is not running is also a good indication that there is little to no ring drag...and it is needing help.

DUB
This theory I will have to print off and read it several times for it to soak in. I think I have the jest of what you are saying. There are several variables and I can already eliminate the ones that have to do with gasket leaks. I should say I think I can. At one point yesterday while retesting for crankcase pressure the engine stalled and the gauge held some pressure maybe 2 seconds after engine stalled.

My plan B to seal the valve cover openings allowed me to see 2 lbs pressure but I'm not 100% satisfied because I was not at full engine temp when my silicone seal let go. Seams like the oil vapors in the engine reacted negatively with the silicone causing it not to completely set up.

You've given me more material to process. Thanks. Vic in Arkansas
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Old 02-16-2017, 07:56 AM   #59
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I have had engines that when I pull out fresh air tube for the crankcase in the valve cover and the PCV is still installed and sucking vacuum like crazy...that when I place a dollar bill over the fresh air grommet...it gets blown away. If the rings were not allowing so much blow-by pressure to get past them....then the crankcase pressure would be not so great that the PCV could actually provide suction like it is supposed to do and cause the dollar bill to now get sucked down to the grommet and stay there.
DUB
I should have mentioned when I posted my test is that the PCV valve is removed and the hole is capped so either the crankcase will go into a vacuum, indicating a vacuum leak, go toward a pressure, indicating no vacuum leak.
Joe
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Old 02-16-2017, 10:33 AM   #60
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Has a basic compression test been done on this engine? It would tell you if the rings are still healthy so you can rule that out.
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