I have 1985 F150 4.9 that I've been working through a number of issues to get back on the road. Previous owner had sliced and diced wire harness, vacuum tubing and emissions with reckless abandon and left it a non running mess. Good news...the new tires on it were worth more than I paid for it. Bad news it's taking a lot of work to get her on the go.

Truck current condition:

Computer- gone too mangled to fool with.
EGR- gone, inoperative, corroded, tube destroyed, block off plate fabricated between carb and intake.
Wiring harness- minimized all solid connections
FB Carb- replaced with Weber 38 (I know it's smallish but will be adding a dual intake on the 300 I am rebuilding per my long term plan)
EVAP-piecing together, have all but ported vac to control purge
Engine-running, good comp well sealed, around 150k on clock but solid
Distributer-new, vac advance style for 1972 300, points set to .030 initial timing at 10* BDC
Drivability- not there yet. I can get good idle and smooth unloaded rpm transitions but on the road after 2nd gear hesitation, studder, no power. I am @ 90% certain that when the carb jet kit arrives, that will change.

My issue is with the PCV system. With the PCV disconnected, make up air attached to air cleaner, and all vac plugged except brake boost I am getting a rock steady 19 inch manifold vacuum at 800 rpm. Which is outstanding as I live 4000 ft above sea level. As soon as I hook the PCV to the vac tree my manifold vacuum drops to 4 inches, idle pops up, and the drivability goes from as stated above to much worse. It's so bad that I don't even have enough vac to boost my brakes. I suspect that I have a metering issue between the make up air and the PCV percolator valve on the rocker cover. Unfortunately I am at a loss as to how to rectify the issue. I am a diesel mechanic and no stranger to gas motors but this little gem has me scratching my head. I have scoured the forum here but not found anything to set me straight. I appreciate any help you folks can provide.

Thanks

Brew

 

PCV when used is a known (and corrected for) vacuum leak.
You can't jet the carb(s) and then connect a PCV valve.

If you are jetted with the port open -and then connect the PCV- and idle and vacuum fall, you have a lot of blow by or fuel in the crankcase.

 

Jim,

Thanks for the quick response. I understand what the PCV is and does. It's cleaning/scavenging characteristics is one of the reasons I want to keep it and the EVAP system instead of vented fuel caps and a road draft tube. The EGR is not going to be a player as I am putting together a new rotating assembly and it will be dual carbed. Time and money puts the build down the road to next summer at the earliest and I would like to discover and correct any drive train issues this winter. Currently the carb has stock jets. I adjusted it for lean best idle with the PCV connected. I couldn't get her quite smooth and steady but got it close. It was after the initial mixture adjustments that I connected the vacuum gauge and discovered that I only had 4". THEN I pulled the PCV and plugged the port and discovered the 19" vac. I hadn't considered fuel in the oil. That may be an issue as the intake was wet when I pulled the old carb off. I will get on it again after work, change oil and see what that gets me. There is some blow by, I didn't think it was excessive but will post comp test results as I don't remember the exact numbers.

 

You are using manifold vacuum and not ported for PCV, correct?

 

ebbsrig,
I didn't mean to suggest you didn't understand how PCV worked.
There has to be some issue causing the drop.
15"Hg is huge!

Grasping at straws with the rich mixture idea.
But given your altitude it seems plausible.

Welcome to the forum!

 

KsCop,

I am using manifold vacuum off the tree in the center of the intake. My only other VAC connection on the tree is the brake boost. Advance is pulled from the base of the carb.

 

Jim,
Didn't take offense, sorry if it sounded like, I did. Just trying to get my brain wrapped around this caper. And yes... losing 80% of my vac is paramount to cutting a line but everything is tight and squared away that I can think of. I didn't know if unimpeded make up air would register as a vac leak through the PCV. I experimented by plugging the lower (larger) line off the PCV and teeing in the upper (smaller) line to the manifold. I was getting about 8"-10"Hg rolling vac.

 

Is the PCV valve itself good? Does it rattle when you shake it?
Almost sounds to me like it isn't doing its its job and is stuck open.

 

I always thought the check ball inside the PCV was just as a one way valve and did not modulate the amount of vacuum drawn through.
I have other vehicles with reed and poppet valves for the PCV, so maybe I'm misunderstanding the issue.

If the hose is disconnected and left open to the atmosphere does the engine exhibit the same low idle vacuum and running problems?
Because my 460 doesn't.

 

ebbsrig,


Welcome to the forum!


This would not affect the low vacuum condition, but is the .030" point gap normal? It has been years since I fooled with points, but for some reason .016" come to mind.


The guys who have already started trying to help you are A-OK!

 

Get a new PCV valve for your truck and see what happens. They do make different models, and they have different amounts of flow. You will be out there a little bit since your are modifying your engine, but I would start with a new one for your engine and see what happens.

I have also found that putting the PCV on a manifold port away from the carb can make the engine not run correctly. It introduces too much air and fumes to particular cylinders, which can cause them to miss-fire. If you notice the factory always introduced the PCV right under the carb or very close to it. I found this works better, since it will mix the air and fumes and distribute them to all the cylinders.

 

All righty then. Tonight's festivities included Oil and Filter change. Jim (ArdWrknTrk) gets a gold star. The sump did indeed have a large amount of fuel. The oil change netted me 6"Hg bringing the total, with PCV attached, to 10"Hg. Great call on the fuel, I hadn't even considered that. 82F100SWB, PCV is new, all basics checked shake , rattle , and roll. For reference, the pcv has either a ball/spring or a plate/spring set up. At idle (high VAC) the VAC defeats the spring and you have low flow. As throttle increases VAC decreases and the spring begins to defeat the VAC increasing scavenging (flow). That is what is cramping my brain. 19"Hg is great and I only expected a couple inched of VAC drop with the VAC connected. 1983F150six, you would be spot on at 0.016 points if this was a V8, your memory serves you well. I went off memory for my settings and was off. Points on a L6 300 should be @0.025 with a dwell of 37*-42*. Your comment made me dig into the books. I have adjusted to spec, thank you. Did dry compression check pressure ranged from 95pai to 100 psi. Don't panic. Remember I am at 4000 ft. A mean average of 97 psi equates to about 109 psi at sea level. Basically a serviceable engine, not that tired yet, but heading that way. Now the weirdness. While warmed and running pulled the PCV valve out of the rocker cover grommet. Definite pressure equalization sound. Then VAC jumped to about 16 PSI, right where I expected it. Replaced the valve reved engine BOOM back to 10"Hg. I think Franklin@ might be on the right track. I will try a few different PCV valves and keep trying to wrangle this beast. I will post my progress.

Brew
1985 F150 4.9 (The Toy)
2007 F350 SuperDuty 6.0 (The Horse Hauler)
52 Dodge M37 232 Flathead (The Tank)
1932 Tudor body and frame (The Dream)

 

Franklin makes an excellent point. Diluting the intake charge of a couple cylinders with blowy should definitely have an effect on the cumbustion in those two cylinders and unbalance the system.

I agree that the PCV belongs at the base of the carb for even distribution of the blowby gasses. But in my younger years I believe I had an old pickup with the PCV routed to a nipple on the underside of the air cleaner housing. Thing ran great for as long as I had it.

 

Lots of EFI 5.0's had it routed to the #8 runner.
This would cause misfire and detonation in that cylinder.
stuck rings and holed pistons were not uncommon

Ford issued a TSB

So it is possible to have this happen if not routed to the plenum.

 

FIXED, Well, sort of. It is definitely a PCV sizing issue and the mass of fuel in the oil sump exacerbated the problem. Adding to my pain was the inconstancy of over the counter PCV valves. Two identical part number valves had wildly different operating characteristics. Different flow, different closing vac, all around mess. There is a fully adjustable PCV valve made by M/E Wagner Performance. $130 bills is NOT an option at this time. When I get the dual carb intake and the other engine together maybe, but for now, not happening. For the interim I opted for a fixed orifice PCV elbow. It basically provides a constant flow and is thus not as efficient as the standard dual flow PCV valve. My vac is steady at 16"hg at idle and repeats after hard acceleration. 16" works, I lose 3" over a closed engine but still have plenty to work advance, brakes, and eventually the EVAP system. I'm down to jetting the carb and this little gem will be on the road. Weber carbs are awesomely adjustable, BUT you got to fiddle with them to get them right. For the winter I will have basically stock 300 with weber 38 on the factory log. Set of points in the dizzy, and set of points in the glove box. Thanks to everyone for their input.

Brew
1985 F150 4.9 (The Toy)
2007 F350 SuperDuty 6.0 (The Horse Hauler)
52 Dodge M37 232 Flathead (The Tank)
1932 Tudor body and frame (The Dream)