Question regarding vacuum at PCV port on intake manifold--4.0 SOHC - Ranger-Forums - The Ultimate Ford Ranger Resource

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Old 02-05-2016
Thread Starter
Join Date: Sep 2015
Location: Lansing, Illinois
Posts: 3
Question regarding vacuum at PCV port on intake manifold--4.0 SOHC

Hey Folks,

The Vehicle: 2003 Ford Ranger Edge, 4.0 SOHC

The Question
: There is a port on the forward right side (passenger side) of the intake plenum (manifold) where the PCV elbow and hose is attached (the PCV valve is on the rear of the left valve cover). Should I get full engine vacuum (18-20") at this port?

The Background
and Analysis: I rebuilt this engine last year. It ran fine for about 1200 miles (no Check Engine Lights) then suddenly, early on New Year's morning, the lower O ring on one of the injectors failed, causing the injector to drop lower into the port in the head, causing a loss of fuel pressure and fuel spillage at the top where the injector is attached into the fuel rail. The engine shut down. a fair bit of fuel spilled, but I was able to get it started and limp off the expressway into a parking lot. Two days later, I went out to the vehicle, removed the intake plenum to access the fuel rails and injectors, and I replaced all O rings on the injectors. Given my less-than-ideal working environment (a parking lot in 30 degree weather), rather than completely remove the plenum, I kept the PCV hose and the coolant lines that attach to this port attached and just swung the plenum out of the way. While it was up and out of the way, a couple of ounces of oil spilled out of the plenum ports for the #1 and #2 cylinder. When I was finished replacing the O rings, I reinstalled the injectors and rails, reinstalled the plenum, started it up, and got it home. About a day later, the Check Engine Light (CEL) came on, and it showed a P0172 code (Bank 2 system too lean). Then, a couple of days later it showed both P0172 and P0171 (Bank 1 system too lean). The vacuum gauge showed 18" of manifold vacuum.

While in my troubleshooting, I pulled the PCV hose off of the port where it attaches at the manifold. As soon as I pulled it off, I noticed that there seemed to be full vacuum at that port, and the engine died instantly. Hence my question for this forum, since I am not sure if it is supposed to have full vacuum there.

A week later, while the truck was in my garage, I checked the PCV hose and elbows, and as a precaution I replaced both the hoses, the elbows, and the PCV valve. Prior to replacing the hoses and elbows, I removed the intake again to check for loose or missing gaskets, but they seemed to be OK. I reinstalled everything again, but that damned CEL came on again, this time only showing a P0171 code (Bank 1 system too lean). The engine runs fine.

Since the weather is supposed to be relatively warm this weekend, I plan on removing the plenum again, this time completely removing it, so I can check for possible cracks in the plenum. or a gasket that is not seated.

Although the P0171 code can have several causes, I am suspecting a vacuum leak because, when I first ran the engine after rebuild last summer, I had a vacuum tester on it and it read 20". Now it is only reading 18.

The reason I asked the question about the port for the PCV valve is because I am wondering whether there is supposed to be a restrictor in that port (or in the plenum where the port connects) to prevent full engine vacuum from developing in the PCV hose assembly. If there is supposed to be something of that sort, then it is possible that it got knocked out of the way or broken when I pulled the intake while fixing the truck in the parking lot.

Thanks in advance for your advice.
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Old 02-06-2016
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I would just do a smoke test before pulling things apart again.

You can rent these machines or just get a cigar and good bottle of booze and do that way.

Remove air tube from intake, put food wrap or ?? over intake hole and tape it up to seal it.

Remove vacuum hose from PCV valve for adding the smoke to intake.
You use PCV Valve hose because that valve will be closed when there is vacuum but wide open if there is pressure, blowing smoke into intake is pressure, and most of the smoke would go to that hose.

Light cigar have a drink and start blowing smoke into the PCV hose and intake.

The intake is sealed pretty well, most air flow will go to open intake valves and passed the rings, but if there is a leak that will allow air to exit as well and with it some smoke, so keep an eye out.

Sometimes takes 1/2 to 3/4 of a bottle to find a leak, these things take time, lol.

I think you asked about the PCV valve on another forum, have a look here: Positive Crankcase Ventilation (PCV)

PCV Valve is pulled closed by high vacuum, like at idle or deceleration, so restricts vacuum going to valve cover.
As you accelerate engine vacuum goes down and PCV valve open more.

So there is always some vacuum in the valve cover/crankcase areas but never 18", always lower vacuum, just enough to maintain positive pressure that will pull oil vapors into the engine to be burned.

PCV = Positive crankcase ventilation.
positive in this case means there will always be a little negative pressure in the crankcase......when engine is running.

Marginal air leaks can show up on only 1 bank.
Lean code is not what most think it is.
Lean code comes from the length of time the fuel injectors are open.
If the computer thinks it is having to open the fuel injectors on one bank longer that it should then it will set the Lean code.
It is quite normal for fuel injectors to drip a bit when they get dirty, so spray but a little drips, this would cause computer to open these injector longer, not enough for lean code, just a little longer.
Now say you have a small vacuum leak, not enough to effect performance but extra air is coming in, by passing the MAF sensor, so computer has to open injectors a little longer.
When the two things are combined..................Lean code on 1 bank, maybe other banks injectors are cleaner, or may be leak is bias to one bank more that the other.
Point is there is a line drawn as far as injectors open time, if the line is crossed then Lean code, if other bank is at the line but hasn't crossed it then no Lean code.

Last edited by RonD; 02-06-2016 at 07:39 PM.
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