PCV Check Valve Question
Thread Starter
Staging Lane
Joined: May 2013
Posts: 67
Likes: 2
PCV Check Valve Question Hey Guys,
Question for all the PCV Gurus on the forum. I have an LQ4 with TBSS Intake, Holley EFI Throttle Body, and Holley HP EFI Controlling. I have stock truck valve covers and a mishimoto catch can. This is a purely race only application. I would like to run the Driver's Side Rear Outlet to the catch can, and then from there to the intake manifold would provide clean PCV vapour back into the system. My question is on the fresh air input side, I'd like to connect the Throttle Body "Fresh Air" supply to the front pipe on the Passenger Side Valve Cover, like it should be. However, I've seen other people find under high engine load and speed, oil can be forced back up this line and into the throttle body, contaminating the intake.
If/has anyone installed a one-way check valve in this line from TB to Valve Cover, to prevent oil from travelling back up, but allow the engine to pull as much fresh air the other direction. If you did, what kind of inline valve did you use? I see some designed for vacuum brakes that are around 2lbs, just wondering if the engine would draw fresh air still or if these inline valves are too much of an obstruction.
Application is a pulling truck, spends 25 seconds at 7,500RPM straight, so worried about excessive blowby at the end of the run.
Thanks in advance
Question for all the PCV Gurus on the forum. I have an LQ4 with TBSS Intake, Holley EFI Throttle Body, and Holley HP EFI Controlling. I have stock truck valve covers and a mishimoto catch can. This is a purely race only application. I would like to run the Driver's Side Rear Outlet to the catch can, and then from there to the intake manifold would provide clean PCV vapour back into the system. My question is on the fresh air input side, I'd like to connect the Throttle Body "Fresh Air" supply to the front pipe on the Passenger Side Valve Cover, like it should be. However, I've seen other people find under high engine load and speed, oil can be forced back up this line and into the throttle body, contaminating the intake.
If/has anyone installed a one-way check valve in this line from TB to Valve Cover, to prevent oil from travelling back up, but allow the engine to pull as much fresh air the other direction. If you did, what kind of inline valve did you use? I see some designed for vacuum brakes that are around 2lbs, just wondering if the engine would draw fresh air still or if these inline valves are too much of an obstruction.
Application is a pulling truck, spends 25 seconds at 7,500RPM straight, so worried about excessive blowby at the end of the run.
Thanks in advance
TECH Fanatic
Joined: Oct 2014
Posts: 1,220
Likes: 179
From: Virginia
I don't see why it would be an issue. The crankcase is going to get air regardless, because of blow by. If for whatever reason it can't get some fresh air for a moment, I don't see how it could be dangerous.
I went with a vacuum pump on my engine. I don't have any of these issues, but in checking vacuum and other small maintenance things, I have ran it with the oil cap (vacuum pump hook up) off, and it just lets the pressure out normally. Normally just at idle, or revving a bit, looking for leaks etc...
You can also consider a larger diameter pcv line from the back of the driverside head to the catch can. That will give it less of a chance to build pressure on the fresh air side.
I went with a vacuum pump on my engine. I don't have any of these issues, but in checking vacuum and other small maintenance things, I have ran it with the oil cap (vacuum pump hook up) off, and it just lets the pressure out normally. Normally just at idle, or revving a bit, looking for leaks etc...
You can also consider a larger diameter pcv line from the back of the driverside head to the catch can. That will give it less of a chance to build pressure on the fresh air side.
TECH Fanatic
Joined: Oct 2014
Posts: 1,220
Likes: 179
From: Virginia
I really like losingits idea there. That's pretty creative too. That looks like a vacuum pump puke tank. If oil ends up in it, it won't go anywhere, and it acts like a filter. Pretty brilliant solution there.
The old school way was just a vented cap, no tank. I have a modified version of one that I used to make my vacuum pump cap.
The old school way was just a vented cap, no tank. I have a modified version of one that I used to make my vacuum pump cap.
Thread Starter
Staging Lane
Joined: May 2013
Posts: 67
Likes: 2
Do you worry about unmetered air through a vented Catch Can? I realize I’m not running MAF, and only MAF and WBO2, but wondering if it’s still better to pull from Intake charge.
Dont want oil blasting back into the Intake tract
Dont want oil blasting back into the Intake tract
TECH Fanatic
Joined: Oct 2014
Posts: 1,220
Likes: 179
From: Virginia
As I think about it, you might actually need to have more positive pressure than what the tank would offer. The valve covers, and lower block are under a positive pressure, and not vacuum.
Maybe the check valve is indeed the answer. Or a second catch can in the fresh air line perhaps. You could run that tank in line.
Maybe the check valve is indeed the answer. Or a second catch can in the fresh air line perhaps. You could run that tank in line.
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You run your regular PCV system and catch can as usual, The can I posted is added to that system on the fresh air side so no oil can be forced backwards out of the fresh air side and sucked into the inlet of your throttle body.
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Some people have a line going from the fresh air inlet port on the passenger side valve cover to the catch can. Then another line coming from the dirty air driver's side valve cover port to the catch can as well.
With the catch cans that have the little air filters on top, that air would be completely isolated from the inlet and exhaust ports as it would be inside the crank case via the cylinder heads and could only seem like an issue if the piston rings sealed very poorly.
At least that's how I see the unmetered air debate, but if the intake manifold or the throttle body behind the blade was tapped into, or I suppose anywhere after the maf, that air would be unmetered air if using a maf.
SD (no maf) would calculate incoming air primarily via the map sensor based on what it senses in the intake manifold...so in that case it shouldn't affect it much.
The only issue I see is if behind the throttle blade or intake manifold are tapped into for a catch can vacuum source, and the catch can is vented with the mini air filter on top, it would be like making a vacuum leak. While in front of the throttle blade would forsure be fine as the throttle blade would be regulating the air coming in no matter what's going on in front of it.
With the catch cans that have the little air filters on top, that air would be completely isolated from the inlet and exhaust ports as it would be inside the crank case via the cylinder heads and could only seem like an issue if the piston rings sealed very poorly.
At least that's how I see the unmetered air debate, but if the intake manifold or the throttle body behind the blade was tapped into, or I suppose anywhere after the maf, that air would be unmetered air if using a maf.
SD (no maf) would calculate incoming air primarily via the map sensor based on what it senses in the intake manifold...so in that case it shouldn't affect it much.
The only issue I see is if behind the throttle blade or intake manifold are tapped into for a catch can vacuum source, and the catch can is vented with the mini air filter on top, it would be like making a vacuum leak. While in front of the throttle blade would forsure be fine as the throttle blade would be regulating the air coming in no matter what's going on in front of it.
Last edited by foxsl; Feb 21, 2018 at 03:44 AM.
Also, about the passenger side fresh air inlet and issues under load. For a whole season I had a clear hose coming from a port in front of the throttle body blade to the fresh air inlet on the passenger side valve cover. In between I had a little clear air compressor filter. I'm naturally aspirated around 380 rwhp unlocked but I drag raced it very hard and the clear hose and the filter were spotless. I've taken the mini air compressor filter off since then because the line stays clean so a filter isn't needed there. Under boost though? That could be the issue people were talking about with high crank case pressure possibly backing oil up the lines.
Last edited by foxsl; Feb 21, 2018 at 03:53 AM.
I have a check valve on my clean air hose (from TB to valve cover). I got it online, a "boomba" check valve. Its either unsprung or very lightly sprung, and made of aluminum.