Vacuum leak through the crankcase (1 Viewer)

[JohnJL]

Hi all,

I have been trying to get the motor tuned up and after running down a bunch of issues I seem to have found another one. In this de-smogged 1F, the crankcase via PCV valve was plumbed into the intake spacer underneath the carb. I believe this is how it was before the rebuild, though I could be mistaken. Thats how I have it now. The motor starts up, runs up and down the street to seat the rings (and backfires explosively off-throttle decelerating) and idles OK cold and warm, though something isnt right. I have a wideband O2 sensor and can see AFRs in 15-16 range when idling warm. Backing out the idle mixture screw only helps to a point but never gets warm idle AFRs into the 13-14s. The old lean-drop idle tuning just wasnt working properly, the idle mix screw wasnt really effecting mixtures, and it was rebuilt by me and then by TLC. Also vacuum never got above 7" at 700 rpm idle...

I suspect a vacuum leak somewhere and even made a smoke machine to try and chase it down...no dice. Checked compression...150+ on all cylinders...I had pinched off the various vacuum hoses without any changes before, but today tried again and voila...the hose from the PCV to the carb spacer, when pinched off, would eventually make the motor die. I have seen a bunch of threads on the PCV and getting a billet OEM one, which I will try and chase down in Canada...

Someone please school me on pcv valves

PCV valve, 70-87 F, 2F: 12204-61010

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FJ62 PCV valve

It's not in the same location as the FJ60's, apparently, and I can't find it. I thought it was on top of the valve cover, in the rear, but this piece doesn't seem to be the correct size. Can anyone give a dummy some help? :bounce: Thanks

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PCV valve

I'm trying to chase down vacuum leaks and I'm new to the 2F world. My PCV valve is loose fitting coming out of the metal elbow, when I spray carb cleaner on it engine speeds up, is that normal to be loose? Sent from my iPhone using IH8MUD

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I have a couple of questions...

1. Should I plug up the intake spacer and route the PCV to the intake before the throttle, like to the airbox? Does it need such a direct vacuum into the manifold or can I relocate the PCV to the airbox?
2. I will order a new OEM PCV and grommet (both these on there are new from SOR)
3. Is there something else going on I am not catching onto?

Thanks!

 

[Dizzy]

Have you tried to clean your existing PCV with gasoline, brake cleaner, or kerosene, and compressed air? Reroute shouldn't be necessary, but, I don't know what pinching it should do?

 

[Splangy]

If your idle mixtures screws weren’t doing much, than your carb probably isn’t running on the idle circuit. The idle screw is likely turned in too much to compensate for another issue. Does your idle solenoid work?

Was the choke on when you pinched the pvc hose?

 

[JohnJL]

Have you tried to clean your existing PCV with gasoline, brake cleaner, or kerosene, and compressed air? Reroute shouldn't be necessary, but, I don't know what pinching it should do?

Hi, and thanks for responding. No, I can do that. Its brand new out of the box from Spectre, and I dont have any oil spillage or foam. Is there a reason the PCV needs such a strong vacuum, or can it just vent to the airbox?

Thanks!

 

[JohnJL]

If your idle mixtures screws weren’t doing much, than your carb probably isn’t running on the idle circuit. The idle screw is likely turned in too much to compensate for another issue. Does your idle solenoid work?

Was the choke on when you pinched the pvc hose?

Hi, thanks for the reply. When I turn the idle screw in all the way, it will get even leaner, like to 18 AFRs. I can wind it out all the way and AFRs still only go down to 15-16s. So yes, I agree, its not running off the idle circuit only and the low vacuum reading supports there being a leak, which I have found to be the PCV which is plumbed directly into the intake. Yes, my idle solenoid works.

I might have other issues, but for sure I have a big vacuum leak in through the PCV. When I pinch it off, the truck dies. For sure there is a vacuum leak there hence my questions above?

Thanks!

 

[Dizzy]

I'd check the throttle linkage and cable for slack. Don't worry if the idle mix screw doesn't do much, it isn't a major player on these carbs when vacuum is low and the throttle plate is at a steep idle angle.

The manifold vacuum pulls air from the crankcase, it is displaced by air from the airbox via the valve cover. That makes a one-way flush of sludge-forming blow-by.

7" is way to low, and it would be important to look for other reasons, like valve lash, etc. I've never had to pinch a PCV line to see how it affects performance.

 

[JohnJL]

I'd check the throttle linkage and cable for slack. Don't worry if the idle mix screw doesn't do much, it isn't a major player on these carbs when vacuum is low and the throttle plate is at a steep idle angle.

The manifold vacuum pulls air from the crankcase, it is displaced by air from the airbox via the valve cover. That makes a one-way flush of sludge-forming blow-by.

7" is way to low, and it would be important to look for other reasons, like valve lash, etc. I've never had to pinch a PCV line to see how it affects performance.

The throttle plate is almost closed, less than 1 turn in on the idle speed screw now. It will idle down to 500 rpm with the idle screw completely backed out. The throttle linkage has a touch of slack and isnt coming into play. I just re-checked the valves again. I agree with your comment on air movement here...air is very likely coming in through the valve cover vent, down through the pushrod passages and along with crankcase gasses and any blowby, getting sucked into the intake manifold.



To my original question...there is enough air coming into the intake spacer while idling to act as a huge vacuum leak, making the idle mix adjustmnts ineffective...is there a reason the crankcase needs full manifold vacuum, or can the PCV and the valve cover both be vented to the airbox?

Thanks!

 

[Dizzy]

To answer your question, I don't know.

However, the PCV seems to work on my 2F. The very early F had no provision for the PCV, IIRC. At my altitude, with an engine that has lower vacuum, the idle mix screw does little to influence the running of the motor - it is because the throttle plate is opened too much to get the necessary air to idle on, and the fuel just comes in thru the next progression hole up the throttle bore. That is why I suggest that you figure out the 7" reading on the manifold - either your gauge is off, or the engine needs something addressed directly. I have had as little as 13.5" (ish) pull, and it isn't enough for the stock carb to be its best.

 

[Splangy]

The throttle plate is almost closed, less than 1 turn in on the idle speed screw now. It will idle down to 500 rpm with the idle screw completely backed out. The throttle linkage has a touch of slack and isnt coming into play. I just re-checked the valves again. I agree with your comment on air movement here...air is very likely coming in through the valve cover vent, down through the pushrod passages and along with crankcase gasses and any blowby, getting sucked into the intake manifold.



To my original question...there is enough air coming into the intake spacer while idling to act as a huge vacuum leak, making the idle mix adjustmnts ineffective...is there a reason the crankcase needs full manifold vacuum, or can the PCV and the valve cover both be vented to the airbox?

Thanks!

They way you described your PCV system working is exactly how it’s supposed to work. You won’t have positive crankcase ventilation without manifold vacuum pulling gasses from the crankcase.

However, when idling and manifold vacuum is high, the PCV valve should restrict flow and it sounds like yours might not be.