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Old 04-08-2014, 07:23 PM
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Blowby from valve cover - rings testing good

I've got a newly built 347 Ford stroker motor that I've been having some issues with.... but this is probably the most confusing one so far for me.

I've got what appears to be copious amounts of exhaust gasses escaping from the passenger-side Valve cover breather hole where my PCV normally hooks up. It looks and smells like exhaust fumes. So I assume this is blowby, and figure there's got to be something wrong... but both compression and leakdown tests appear to be checking out okay. I'm getting right around 180 PSI on all cylinders with the compression tester. A wet compression test showed the EXACT same results. Leakdown numbers were between 10-20%, which might seem high, but it's this cheapo Harbor Freight tester that has been compared to other low mileage, very nice running engines and it's actually getting better numbers on the 347 than it is on them.

Oh, and to top it off... completely contrary to what I'd expect, it ONLY seems to be happening at idle. I'll rev the engine up and the smoke seems to dissipate very quickly.

Any ideas what is happening here?

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Old 04-08-2014, 07:29 PM
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Describe your fuel system and fuel pressure at the carb bowl inlet.
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Old 04-08-2014, 07:41 PM
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Quote:
Originally Posted by srry View Post
I've got a newly built 347 Ford stroker motor that I've been having some issues with.... but this is probably the most confusing one so far for me.

I've got what appears to be copious amounts of exhaust gasses escaping from the passenger-side Valve cover breather hole where my PCV normally hooks up. It looks and smells like exhaust fumes. So I assume this is blowby, and figure there's got to be something wrong... but both compression and leakdown tests appear to be checking out okay. I'm getting right around 180 PSI on all cylinders with the compression tester. A wet compression test showed the EXACT same results. Leakdown numbers were between 10-20%, which might seem high, but it's this cheapo Harbor Freight tester that has been compared to other low mileage, very nice running engines and it's actually getting better numbers on the 347 than it is on them.

Oh, and to top it off... completely contrary to what I'd expect, it ONLY seems to be happening at idle. I'll rev the engine up and the smoke seems to dissipate very quickly.

Any ideas what is happening here?
20% is a lot on a new build, Just think what the blowby is at combustion pressure!!!

You should check your leak down at about 25 degrees before TDC.

Was the block torque plate honed ?
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Old 04-08-2014, 08:00 PM
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Oh I agree it seems like a lot, but I've been reading quite a bit about leakdown testers and how unreliable their numbers are across all different models, especially cheap ones like mine. That's why I compared it to a good running, low-mileage engine with no problems at all, and found that the numbers were actually lower. (The other one was an Escort engine that got about 25% with a warm engine.) I'm going to test it once more on an even newer motor, but I think that this particular gauge just reads high, as some apparently do.

And no, unfortunately the block was not honed with a plate. I'd kind of forgotten about it at the time, and my machinist claimed it was not necessary, though I'm now wishing I'd had it done.
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Old 04-08-2014, 08:05 PM
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Originally Posted by techinspector1 View Post
Describe your fuel system and fuel pressure at the carb bowl inlet.
Fuel pressure is at 5.5psi run off of a regulator w/mechanical pump. I'm running a Summit 750 vacuum secondary carb.
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Old 04-08-2014, 08:07 PM
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Like Carl (CNC Blocks NE) said, leak it with the piston down the bore a bit, instead of TDC.

Another question, does the intake and heads have an exhaust crossover/or EGR provision??
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Old 04-08-2014, 08:11 PM
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Quote:
Originally Posted by srry View Post
I've got a newly built 347 Ford stroker motor that I've been having some issues with.... but this is probably the most confusing one so far for me.

I've got what appears to be copious amounts of exhaust gasses escaping from the passenger-side Valve cover breather hole where my PCV normally hooks up. It looks and smells like exhaust fumes. So I assume this is blowby, and figure there's got to be something wrong... but both compression and leakdown tests appear to be checking out okay. I'm getting right around 180 PSI on all cylinders with the compression tester. A wet compression test showed the EXACT same results. Leakdown numbers were between 10-20%, which might seem high, but it's this cheapo Harbor Freight tester that has been compared to other low mileage, very nice running engines and it's actually getting better numbers on the 347 than it is on them.

Oh, and to top it off... completely contrary to what I'd expect, it ONLY seems to be happening at idle. I'll rev the engine up and the smoke seems to dissipate very quickly.

Any ideas what is happening here?
I think you need to run it awhile to let the rings and walls work in with each other. A sub 10% leakdown on a fresh engine takes an extremely good combination of machine work and high quality parts found in high end shops. For the rest of us mortals often it's a little like the good old days where the parts need some run together time. Break it in for around 500 miles than look at it.

Bogie
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Old 04-08-2014, 08:12 PM
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Quote:
Originally Posted by ericnova72 View Post
Like Carl (CNC Blocks NE) said, leak it with the piston down the bore a bit, instead of TDC.

Another question, does the intake and heads have an exhaust crossover/or EGR provision??
Nope, no EGR. Air Gap intake and Twisted Wedge heads.
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Old 04-08-2014, 09:06 PM
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I have seen newly built engines with a 800 miles or so on them still have poor leak down numbers.

If they have not been plate honed rings will take a lot of miles to seat if they even seat.

Does anyone have a guess how many more miles this engine needed before the rings would be broke in. This is a GM 502 that was bought brand new, Have seen this many times in the marine engines.
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Old 04-08-2014, 09:46 PM
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Question: How could there be a problem with my rings if a wet compression test (oil in cylinder) came back with perfect results?

Granted, there must be exhaust gasses escaping from somewhere, (and that is indeed most likely the rings), but I'm having trouble correlating my test results with this.

Also, any guesses why it would seem to stop doing this at higher RPMs / off-idle? Is the faster movement helping it seal better or something?

I guess the PCV will recirculate it out of there for now. The plugs don't appear to be getting oil fouled or anything. I certainly don't have the funds to tear it down at this point, so I guess the best I can do is give it time... the thing is. it's running surprisingly well despite the amount of blowby I'm seeing.

Last edited by srry; 04-08-2014 at 10:04 PM.
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Old 04-09-2014, 04:37 AM
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describe your whole pcv system. does the other valve cover have a breather??? where are you hooking the pcv to??? the best I've found, is pcv to direct manifold vacuum port and a breather on the other valve cover. clean the pcv, hose, port, and breather every oil change with carb cleaner and replace the pcv and breather once a year. put 500 more miles on it and check it again, should be getting better.
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Old 04-09-2014, 09:47 AM
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Quote:
Originally Posted by srry View Post
Question: How could there be a problem with my rings if a wet compression test (oil in cylinder) came back with perfect results?

Granted, there must be exhaust gasses escaping from somewhere, (and that is indeed most likely the rings), but I'm having trouble correlating my test results with this.

Also, any guesses why it would seem to stop doing this at higher RPMs / off-idle? Is the faster movement helping it seal better or something?

I guess the PCV will recirculate it out of there for now. The plugs don't appear to be getting oil fouled or anything. I certainly don't have the funds to tear it down at this point, so I guess the best I can do is give it time... the thing is. it's running surprisingly well despite the amount of blowby I'm seeing.
You originally said it seems worse at idle than at higher RPMs. This is normal and is the result of combustion pressure or lack of it at idle, this pressure forces the ring seal. Positive Crankcase Venting (PCV) system helps by pulling a vacuum on the crankcase which has the effect of increasing the difference between the combustion pressure and that of the crankcase. In a similar vein the pistons could be using thin and or light tension rings, either or both would be blowby leaky when there is not much difference between combustion and crankcase pressure. These type rings need positive crankcase venting of some sort whether a PCV valve to intake, a suction bleed valve in the exhaust, or an engine driven vacuum pump in order to pull a seal on the rings.

From a potential assembly error stand point; one can consider that one or more rings were assembled upside down or broken on piston insertion. Or a piece of dirt got into a ring land somewhere preventing a seal with the piston. The rings have to make two (2) seals one with the ring land (groove) of the piston and the other with the cylinder wall.

So many things can mess this up. I'd still suggest you run it a while before going into panic mode and dissassembling the thing.

Bogie
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Old 04-09-2014, 01:28 PM
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. I agree with the other guys, engine prolly perfectly normal...

. On Spike.com (HP TV) videos I seem to recall them building a 347" and using like 4 or 6 breathers in the valve covers because they were expecting a lot of blowby on the road course...

. Stock modern engines use intake manifold vacuum to suck the idle and cruising blowby out of the crankcase and back into the engine for reburning for emissions cleanliness... the PCV valve is closed in the other valve cover and that maintains the vacuum in the crankcase... at WOT, the PCV valve opens and blowby leaves both valve covers, the PCV valve hose taking it to the air cleaner filter for reburning...

. At power, heat expansion closes the piston ring gap up more and they seal better... but still allow some blowby... at much higher price, ZeroGap rings are available that seal better than gapped rings at all times... and give a few HP more... but some blowby still gets by...
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Old 04-09-2014, 03:40 PM
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Thanks for all the great answers guys, you've been really helpful.

Quote:
Originally Posted by jax_pap View Post
describe your whole pcv system. does the other valve cover have a breather??? where are you hooking the pcv to??? the best I've found, is pcv to direct manifold vacuum port and a breather on the other valve cover.
That is exactly how I have it set up. Not entirely sure on the rating of the PCV valve or its original application though, kinda just picked it off the shelf 'cause it fit.


Quote:
Originally Posted by oldbogie
the pistons could be using thin and or light tension rings, either or both would be blowby leaky when there is not much difference between combustion and crankcase pressure.
You are dead on about that, they are indeed low-tension rings. I believe these are just the standard low-tension style that they moved to with the newer 5.0 motors, though.

Perhaps some more extreme form of crankcase ventilation is in this engine's future.
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Old 04-09-2014, 04:48 PM
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((I've got a newly built 347 Ford stroker motor))

You must buy a PCV valve that is correct for the amount of vacuum that you have. Stock PCV valve will not work on one that has a over stock cam, which changes vacuum at idle. The spring inside is calibrated for the amount of vacuum produced. Look up one that is listed for the old hi performance 302. AND, make sure you use PCV hose, as the stock hose will collapse. You may know this, but, I just had to mention it.
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