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Sudden Increase in Blow-by

Lee_in_KC

Active Member
This one's got me scratching my head. Within the last 100 miles or so the blow-by in my motor has increased dramatically, blowing oil out my breather pretty much constantly. I've checked the PCV valve, hose and connection at the carb base... all are clear. Plenty of vacuum at the carb base port. Motor has a little less than 10,000 miles on it in 9 years of driving. Here are the specs...

SBC .030-over 350, short block built by a race motor shop, 8.5:1 "blower" pistons, Iron Eagle heads, Comp all-roller valve train, "blower" cam with hydraulics, 6-71 blower 1:1 (chart says 5# boost at 5000 rpm), 2x Edelbrock 600s set up for the blower, MSD ignition 34* all in by 2600, 5600 rev limit

I very rarely get on this motor, have never hit the rev limiter and have only seen positive boost on the gauge maybe twice! This thing has run like a Swiss watch since day one.

Besides a compression check, what else can I do to sort this out? Thoughts as to cause(s)?

AB543D2F-13E6-430A-A081-2DB0422593C6_zpsmef0n457.jpg
 
I had a situation where I was baffled by amount of blow by. I checked the good vacuum, hose, pcv, still no solution. When the engine finally came out the screen in the manifold was completely blocked. Long, short, make sure the baffle, screen or port for the pcv can draw from the crank case.
 
Do a cylinder leak down test on all eight and look for a low cylinder.
Could be a broken ring (or two), or a broken ring land or an cracked piston.
Hard to diagnose with so little info.
If you are running a PCV off the manifold or valve cover why the breather? Thought the system is supposed to be a sealed system, giving a negative pressure in the crank case.
You seem to be getting positive pressurization of the crank case, hence, oil being forced out out the breather (on to the valve cover?).
Have you checked the oil in the blower itself?
Which side has the low leak down (when you check it)?
Could also do a compression test first.

Anyway, full compression test first. Then full leak down test next.

Might be time to to do a 10 year "freshening". Get a 350 with a carb to keep driving and take the blower motor apart.

John

Don't mind me, I always go for the worst possible diagnosis.
 
My understanding of Positive Crankcase Ventilation is you have a vacuum line sucking on one side of the motor and fresh air coming in the other side to replace the nasty air sucked out by the vacuum. My PCV tube connects to a nipple in the base of the rear carb. I put a vacuum gauge on it and it is 28 in. at idle. The blower being positive displacement, it sucks pretty hard on the carbs with the throttles closed.

I have this kind of breather cap...
mrg-9810_ml.jpg

Droplets of oil are coming out of the bottom and spraying around the general area, including on the headers and the firewall. There is a baffle in the valve cover to help prevent oil coming through the breather... under normal circumstances. The motor has run almost 10,000 miles without this problem, so something has changed.

I don't have the equipment to do a compression or leakdown test. I may have access to such later this week.

John, I was thinking broken ring or holed piston as well. I park the T over the winter anyway, so yes, maybe time for a refresh... or maybe a 383...?
 
After you run your compression test , run aleakdown test, gives you another way to diagnose..Sorry , this didn't post a1/2 hour ago ...your pcv is correct....pull the plugs , look for 1 that's fouled , give you a plce to start ...HF has cheap comp.& leakdown testers ..... you need a compesser for leakdown


dave
 
I don't know if this is new or I just now noticed it, but there is a considerable amount of steam (not smoke) coming out of the breather. There is no sign off water in the oil, and oil pressure and water temp are normal. I'm waiting for the motor to cool off so I can pull plugs.
 
No, correct me if I am wrong, but even at idle the supercharger will be providing a positive displacement of air into the carbs, however minor. At idle , depending on how worn the blower has gotten in 10,000 miles, all the blower may be doing is supplying air at the normal atmospheric pressure while beating up the air/fuel mixture going thru the blower. Perhaps less than a pound of pressure.
You never told us if the blower was over driven or under driven. And at what percent.
Also Googled PCV and found out that there is open and there is a closed PCV. You appear to have an open PCV system. Also that PCV valves can break or become clogged. Call a blower mfg. and find out what PCV valve you should have. Also whether if you should be running an open or closed system.

Try AA1Cars.com on Google for picture and info, right near the top of the page.

John

An open system was used to help evaporate water in the oil.

Check for steam in the exhaust. And loss of coolant from the radiator.

Be prepared to operate.

Does the oil look kinda like chocolate mike? See the above.

Come on ! Pull the plugs hot. Hot is the way the engine is normally run.

(Just getting ya. Get to work, pull the plugs and do the leak down & compression test. Hot).
 
Last edited:
It's kinda buried in my first post, but the blower drive ratio is 1:1... supposedly will produce 5# of boost at 5,000 rpm on a 350 per BDS's chart.

My blower set-up is not blow-through... carbs are on top of the blower. Blower sucks on the bottom of the carbs and blows into the intake manifold. PCV is a standard open set-up recommended by BDS.

No steam out the exhaust, no loss of coolant. However, as noted above, there IS steam coming out the breather. No sign of water in the oil (I have seen that before, I know what it looks like!!).

Pulling the plugs when they're hot is not the issue... I have to reach through the hot headers to get to them... I don't need any more header tatoos!! :mad:

OK, more mystery. Pulled the plugs... all 8 are perfect!! Same dark tan, light brown color on all, completely dry/no oil, no signs of detonation.

Heading to Harbor Freight for compression and leak-down test kits
 
If your tests look good [fingers crossed] I suppose it's possible that because you're running a blower , you could have a bad intake gasket that's allowing [boost] pressure into the valley, just guessing ...BTW , fel-pro offers intake gasktg w/ a steel core , if you're interested
dave
 
You're right, the blower pulls air thru the carbs. My mistake. How much boost does it show at idle?
IIRC you are in Florida. Extra humid?
One other thing mentioned inAA1Car.Com was that to check the PCV valve it should rattle when shaken. But that test will not show if the internal spring is broken.
Any way, tomorrow will tell the tale.
Car still looks good.

John
 
Start your motor, pat the gas pedal really fast....looking at your boost gauge, does it flutter when you do that, like say looks like its hits 1 or maybe 1 1/2 when you pat on it? If so, clean or wipe your oil breathers in your valve covers. When you pat on it, can you see a mist being sprayed, really light?
If so, some of your boost pressure might be getting into your crankcase. This could be from valve stem seals, a ring being unseated, possibly even broken, head gasket, etc. Gotta start playing detective!
BUT, Maybe nothing is at all wrong. Every-once-in-a-while, on a blower motor, your rings will rotate around, and all the ring gaps will line up, and you'll get blowby....see it several times. Compression check will show a little low, borescope will show a nice smooth bore, valve seals good....just the rings lined up. If that is the case, just run it a while, baby it, they'll rotate back out of alignment.
Pull your dipstick and smell it. Does it have a tinge of gas smell to it? If not, pressure isn't getting into your crankcase.
If the motor seems nice and peppy, your not drawing or blowing alot of oil, look for something small.
Run a compression check, if all show within about 5 to 8 PSI, your good there.
On my carbs I will run a vac. hose from the baseplates to the valvecovers, toward the rear, and have the breathers up front. Theres a million diff. ways to vent your crankcase....

Idling, at 1:1, your motor should be pulling a decent vacuum, and right as soon as that motor sees some butterfly, she ought to rev like theres no tomorrow....Boost pressure should hit your gauge almost as soon as the pedal is tapped....

On a blown motor, because your pressurizing your intake, even if your babying it, oil WILL get contaminated. On a blown 350, thats usually at the bottom of your intake ports, where some blowby occurs.
Gaskets really thin there, not alot of clamping pressure. Check that also....
Doublecheck all your intake bolts are tight. Also, check all the torque on your head bolts also....
Remember this, even though you hardly rev it, whenever you do, that 6-71 can blow out a intake gasket very easily! I used to have several on hand because mine used to like to spit the gasket out between the blower case and the top of the blower intake....Rev it and it'd sound like somethings whistling at ya.
 
Well Lee, there you have it. I'd trust what Metal has to say. He's in the biz. Has lots of blower experience.
Maybe it's time for that reseal. If the compression test and leak down test are good, I would take it apart this winter and check everything. Replace parts as needed (not as wanted). You could have the same engine you have known and loved for the past 9 years, resealed & gasketed for under $1000.00. Thats if you d all the work. Fresh paint too. MMMMM....... pretty!

If you go to a 383 might as well go after market block and 400 inches.

If you use everything off the present engine just the new block, crank, pistons, cam, etc. maybe $7500.00.

Ahhh ... the world is your oyster. Do what you want.

John

P.S. Do you run studs on your heads?
 
OK, here's the morning report. Compression test... 120# +/- 2-3# on all cylinders except #1... only 110#. I'm thinking #1 is within tolerance. What do you guys think? All cylinders hit maximum pressure within 3 compression cycles.

Decided to do the compression test before springing for another $40 for the leak-down test kit. I'm thinking leak-down isn't going to tell much more than the compression test did at this point.

No smell of gas in the oil.

I can't get to the intake bolts without removing the blower, so they are not checked at this point.

Motor runs like it always has... very "peppy", no misses, bogs, farts, etc.

My vacuum/boost gauge is tapped from the intake manifold. Idle shows around 18 in. and is steady... no wiggle to the needle. Stabbing the throttle the boost goes positive 3-4# (only winding it up to about 4000).

I'm now thinking what I was pretty sure was steam coming out of the breather is actually smoke or a combination of steam and smoke. It has a smell to it, but it's not burnt oil smell. Kind of a chemical smell, maybe favoring gasoline. Could be antifreeze, I suppose.

John, no studs on the heads... just good ole ARP bolts.

I'm supposed to be going to the Indianapolis T-bucket event tomorrow (trailering to and from) where there will be fairly long cruises every day. Do you guys think it's OK to keep driving my hot rod?

Thanks for the help, guys. I'm fairly knowledgeable about engines, but this one has me stumped. No secondary indications at all... just oil mist and steam/smoke out the breather!! :confused:
 
One last thought, when did you last do an oil&filter change?

Anyway, since you're trailering the car, enjoy the pee out of it. Nothing builds bonding with fellow car enthusiasts than breaking down.

Enjoy, wish I was there.

John

Perhaps a 30 second video of said "smoke"?
 
Addendum to last thought.

Compression checks should be done @ operating temp with the carbs wide open. With IIRC with 5 compression strokes.

Actually, leak down tests are preferred. They will show leaking rings & valves, the true condition of the engine.

John
 
Oil and filter have about 1,000 miles on them. Oil is dark but still translucent, not burnt. Pressure is as good as ever, water temps 180-190 (180 thermostat).

I tried to video the "smoke" earlier on my iPhone... it doesn't show up very well.

Compression check was done at op temp with the throttle wide open. I did crank every cylinder for at least five compression strokes, but they peaked after three strokes. I took that as a good sign. I guess I'll go get the leak-down kit... was hoping to avoid that since it such a PITA to do!! Gonna hafta open up the motor no matter what, but I guess it would be good to know which cylinder(s) have issues going in.
 

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