Excessive Blowby Advice - The 1947 - Present Chevrolet & GMC Truck Message Board Network

qbrew · 01-17-2012, 05:21 PM

I'm trying to resurrect my old 77 Chevy C10 Long bed 350/350. I finally got it to a mechanic to repair the rear-main seal and he said it has "Excessive Blow-by, and the aftermarket blow-by filter was plugged. He advised me that the new seals and gaskets may blow again, so my parts and labor doing so may be wasted.

Should I go ahead and have the Rear main seal replaced knowing there is excessive blow by, or should I just put that money toward a motor? The motor was pulled from a running vehicle and was supposed to be good and running. It's not looking like that was the case.

OleBlue79 · 01-17-2012, 05:51 PM

If you have excessive blow by, you are probably wasting your money on the rear main seal.You need more than just a rear seal. Question: Is your PCV valve working properly? This might be some of your problem.

grs · 01-17-2012, 05:56 PM

What is a blow by filter?

qbrew · 01-17-2012, 06:12 PM

I"ll ask about the PCV Valve. I do think he's going to replace it. The blow-by filter (Maybe I'm using the wrong term) goes in the valve cover gasket. Where it would normally go into the factory air filter, it has an aftermarket filter instead.

**Corts60** · 01-17-2012, 06:44 PM

That is not called a blow by filter. That is just a breather tube that vents the crankcase into the air cleaner. The PCV valve is probably on the other valve cover and that needs to be connected to a vacuum source, usually the bottom of the carb. The mechanic should know about that.

If he is saying there is blow-by then I would ask what the results are from the compression tests he did to verify it is blow-by causing the gaskets to fail. And if he did a compression test, ask if he put oil in the cylinder and re-did the test to see whether or not it is the rings that is causing the blow-by.

Sorry if that seems like a lot but it is necessary to determine what is the proper place your money should be spent.

qbrew · 01-17-2012, 10:36 PM

Thanks for the info Corts. Those questions will be a huge help.

cjlinkster · 01-18-2012, 12:00 AM

A compression test would be the best way to determine.

How long had the engine been sitting? When was the last time that the oil was changed?

qbrew · 01-18-2012, 09:43 AM

Engine was pulled from a running truck about 8 years ago, and has been sitting ever since for the most part. Oil was changed about a month ago, and the truck has only been driven to the shop 5 miles away.

vtwinsport · 01-21-2012, 12:10 AM

Valve seals and valve seats are my first check. Do a compression test, then leak down test if you have the xtra $.

qbrew · 01-21-2012, 11:15 AM

Thanks for all of the info. I went ahead and took the truck away from that shop. He was doing the right thing by letting me know that the blow-by might cause the Seals to pop again. But I don't think he was willing or able to help me figure out what was causing the blow by. Looking for a new shop in the Central Arkansas Area.

silverbullet01 · 02-05-2012, 12:30 AM

if ita got blow by and been sitting for that long i would also think a broke/stuck ring. i had a 74 350 interceptor that had terrible blow by to the point it spewed oil out the breather tube. turns out the top ring on every piston was broke.
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Desert1957 · 02-05-2012, 10:51 AM

Just so you know , a small block Chevy "rear main seal" CANNOT be pushed out by blow by. The purpose of the rear main seal is to keep oil pressure to the rear thrust bearing from leaking externally. that's all.

All the other seal can be pushed out with excess crankcase pressure, oil pan gasket , valve cover gaskets , timing cover gasket.

Desert

01-17-2012, 05:21 PM	#1
qbrew Registered User Join Date: Sep 2011 Location: United States Posts: 19	Excessive Blowby Advice I'm trying to resurrect my old 77 Chevy C10 Long bed 350/350. I finally got it to a mechanic to repair the rear-main seal and he said it has "Excessive Blow-by, and the aftermarket blow-by filter was plugged. He advised me that the new seals and gaskets may blow again, so my parts and labor doing so may be wasted. Should I go ahead and have the Rear main seal replaced knowing there is excessive blow by, or should I just put that money toward a motor? The motor was pulled from a running vehicle and was supposed to be good and running. It's not looking like that was the case.

01-17-2012, 05:51 PM	#2
OleBlue79 Registered User Join Date: Jan 2010 Location: Louisiana Posts: 127	Re: Excessive Blowby Advice If you have excessive blow by, you are probably wasting your money on the rear main seal.You need more than just a rear seal. Question: Is your PCV valve working properly? This might be some of your problem. __________________ 79 C-20 Scottsdale 350/400 3:73 gears 9 leafs and a headache rack (I speak English, thank you)

01-17-2012, 05:56 PM	#3
grs Registered User Join Date: Jan 2010 Location: Roanoke, Tx. Posts: 1,561	Re: Excessive Blowby Advice What is a blow by filter?

01-17-2012, 06:12 PM	#4
qbrew Registered User Join Date: Sep 2011 Location: United States Posts: 19	Re: Excessive Blowby Advice I"ll ask about the PCV Valve. I do think he's going to replace it. The blow-by filter (Maybe I'm using the wrong term) goes in the valve cover gasket. Where it would normally go into the factory air filter, it has an aftermarket filter instead.

01-17-2012, 06:44 PM	#5
Corts60 Just here to tinker Join Date: Jun 2008 Location: Holland, MI Posts: 3,697	Re: Excessive Blowby Advice That is not called a blow by filter. That is just a breather tube that vents the crankcase into the air cleaner. The PCV valve is probably on the other valve cover and that needs to be connected to a vacuum source, usually the bottom of the carb. The mechanic should know about that. If he is saying there is blow-by then I would ask what the results are from the compression tests he did to verify it is blow-by causing the gaskets to fail. And if he did a compression test, ask if he put oil in the cylinder and re-did the test to see whether or not it is the rings that is causing the blow-by. Sorry if that seems like a lot but it is necessary to determine what is the proper place your money should be spent. __________________ 78 C10 SWB Diesel #1 (wrecked)http://67-72chevytrucks.com/vboard/s...1978+silverado 78 C10 SWB Diesel #2 (sold)http://67-72chevytrucks.com/vboard/s...d.php?t=650751 60 GMC Suburban (sold)http://67-72chevytrucks.com/vboard/s...d.php?t=445526 60 GMC 1000 (sold)http://67-72chevytrucks.com/vboard/showthread.php?t=298235 67 GMC 1500 (sold) http://67-72chevytrucks.com/vboard/s...d.php?t=803695 73 Chevy C20(Daily)http://67-72chevytrucks.com/vboard/s...d.php?t=852767 86 Suburban(summer unit) http://67-72chevytrucks.com/vboard/s...d.php?t=810697 88 Suburban 4x4(sold)

01-17-2012, 10:36 PM	#6
qbrew Registered User Join Date: Sep 2011 Location: United States Posts: 19	Re: Excessive Blowby Advice Thanks for the info Corts. Those questions will be a huge help.

01-18-2012, 12:00 AM	#7
cjlinkster Registered User Join Date: Jun 2011 Location: Atlanta ga Posts: 150	Re: Excessive Blowby Advice A compression test would be the best way to determine. How long had the engine been sitting? When was the last time that the oil was changed? __________________ 1984 C20, 454, 4 speed named "Gold Member"

01-18-2012, 09:43 AM	#8
qbrew Registered User Join Date: Sep 2011 Location: United States Posts: 19	Re: Excessive Blowby Advice Engine was pulled from a running truck about 8 years ago, and has been sitting ever since for the most part. Oil was changed about a month ago, and the truck has only been driven to the shop 5 miles away.

01-21-2012, 12:10 AM	#9
vtwinsport Registered User Join Date: Nov 2008 Location: edmonton, alberta Posts: 293	Re: Excessive Blowby Advice Valve seals and valve seats are my first check. Do a compression test, then leak down test if you have the xtra $. __________________ 76 Silverado, 2wd, long box, 350/th350, 3.42 posi, edelbrock performer 64cc heads, edelbrock 1405, 2101 intake, Hedman Headers, Comp Cam XE262 http://beerbreadbike.wordpress.com/

01-21-2012, 11:15 AM	#10
qbrew Registered User Join Date: Sep 2011 Location: United States Posts: 19	Re: Excessive Blowby Advice Thanks for all of the info. I went ahead and took the truck away from that shop. He was doing the right thing by letting me know that the blow-by might cause the Seals to pop again. But I don't think he was willing or able to help me figure out what was causing the blow by. Looking for a new shop in the Central Arkansas Area.

02-05-2012, 12:30 AM	#11
silverbullet01 Registered User Join Date: Dec 2009 Location: Annapolis Royal Nova Scotia Posts: 56	Re: Excessive Blowby Advice if ita got blow by and been sitting for that long i would also think a broke/stuck ring. i had a 74 350 interceptor that had terrible blow by to the point it spewed oil out the breather tube. turns out the top ring on every piston was broke. Posted via Mobile Device

02-05-2012, 10:51 AM	#12
Desert1957 Registered User Join Date: Apr 2009 Location: New Freedom Pa. Posts: 1,335	Re: Excessive Blowby Advice Just so you know , a small block Chevy "rear main seal" CANNOT be pushed out by blow by. The purpose of the rear main seal is to keep oil pressure to the rear thrust bearing from leaking externally. that's all. All the other seal can be pushed out with excess crankcase pressure, oil pan gasket , valve cover gaskets , timing cover gasket. Desert



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