pmassey31545

I'm sorry, but this is probably going to be a long post. So here goes....

About 2 years ago I started having issues with my AC defaulting to the defrost vents. Did some searching and found that the o-rings in the digital control manifold were an issue. Replaced em and it worked fine. More recently though I'm having the same issue but only when my engine is under load. Once I get up to speed and set the cruise control it never does it again unless I start going up a hill and the engine starts pulling a load.

Also I will say though it may not be related about two years ago I had issues with my rear brakes hanging up real bad. I change the master cylinder the brake lines pretty much everything on the brake system and really there was no big difference. The only fix when it happens is to bleed the brake lines of all the old fluid...about a quart through the system and it's good for several months. But, on a trip the other day my AC started doing its thing AND the brakes got crazy too. Kinda made me think they were linked possibly in some way. Maybe via the brake booster?

Also, my truck constantly has a 0420 code which could indicate a vacuum leak. I replaced one oxygen sensor and still having the issue...hoping it's not my Catalytic converter.

So to make a long long story short, my issue is that it seems like I have a vacuum leak somewhere but I cannot figure out where. Ive spoken with A fellow worker who has experience with some mechanicing and he suggested that my rings in my vehicle may be starting to wear and I'm losing vacuum that way. My only issue with that is my truck has not seen any kind of performance decrease. I'm not losing a lot of oil between oil changes and it still runs pretty darn well for being A few miles shy of 300,000.

So I'm open for suggestions here. I have used two cans of starter fluid Hutton leaks. I've looked and searched for a vacuum leak. Replaced a couple hoses under the hood coming from the intake to main components. Tomorrow I will probably trace out the AC lines in a little bit more detail and see if that's an issue. In the meantime I would appreciate any input.

Yes, I've searched YouTube and yes I have searched the forums but nothing seems to mirror the exact issue that I am facing.

ncranchero

Definitely a vacuum leak affecting the HVAC. Check the typically brittle plastic lines. There’s also a reservoir under the right front fender that you might want to examine (mine was cracked)
Are you losing brake fluid, master cylinder reservoir emptying? Having to change brake fluid makes no sense UNLESS the inner lining of the rear brake hose has defected and blocking pressure to the cylinders. Most of the time that issue results in the brake being not released though. Perhaps a sticking ABS valve? Vacuum issues and braking issues seemingly would point to a booster problem but changing fluid would have no connection. I’d check that rear hose, or just replace it then focus on the rear ABS valve if the hose doesn’t cure it.

pmassey31545

Good info. As for the brakes, it's common to both back wheels when it happens. Which tells me it's something they share. That leaves me with the master cylinder OR the rubber line that drops from the main line to the split block that's on the rear axle. I've replaced that hose and it has happened since.

Sticking ABS valve...haven't checked that. Probably worth a look.

Just not sure why I only lose vacuum under a higher load. Seems like more RPMs would create more vacuum.

I'ma keep hunting though.

mbb

Nope

Exactly the opposite
Manifold vacuum is deepest (lowest pressure) at idle, because the butterfly in throttle body is closed.
the engine wants to suck in air/fuel to run faster..but you dont let it.
as throttle opens, and more air flows in, the manifold vacuum is less (higher pressure ).

So vacuum related issues often present at higher throttle settings, when theres less manifold vacuum

pmassey31545

Ah. So... Could rings be the issue? Seems a little extreme. Especially since my truck runs fine. The more I read here the more I'm leaning towards a simple vacuum leak somewhere that I just haven't found yet.

raysorenson

No, couldn't be rings. Also, P0420 is always a cat, no exceptions.

If you had a general loss of vacuum you would be posting about how your motor barely runs. Your vacuum leak is almost certainly on the HVAC side of the check valve. The check valve is on the passenger rear of the engine bay. When the throttle body opens and floods the intake manifold with air, vacuum is gone but since there is a check valve you're okay because it didn't let air from the intake go back into the hvac system. If you have a leak on the hvac side of the valve, then air slowly or quickly bleeds into the system once the manifold is no longer providing vacuum. Since the vacuum lines involved are so small in diameter, they make little to no difference in how the motor runs when they leak and I wouldn't expect them to set any DTC's.
The vacuum reservoir is attached to the battery tray and rests between the tray and the RH fender. There's a few vacuum lines in that area right by the battery. They get brittle w/age and touching them can cause them to break. Or you could have a vacuum leak under the dash. You need a vacuum pump to diagnose this.

This is what the check valve looks like. It may or may not have a "tee" as shown on the downstream end.

Here is a vacuum pump. Good ones come with all the stuff you'll need to hook up to the hvac lines

mbb

Yes, do not buy the chinese $25 vac pump, it is crap. It pulls very little vacuum 15" compared to 25" by the $45 mityvac above. Its trash.

Vac pump is handy for bleeding brakes by self too.

pmassey31545

Sorry it took so long to respond. I will check this tomorrow with my vacuum pump and I'm gonna go ahead and order a check valve just because.

Thanks again.

ncranchero

I may have missed the OP stating the year of his truck but on both my 97 & 98 (4.6L) the reservoir is behind the right fender. IDK when they were moved to behind the battery but that was a good decision. Both of mine were cracked/broken when I got the trucks.

KB Cool

Mines doing the same thing. Found the line broke off at the grommet on the firewall where it enters into the cab. I have no clue what Ford had there originally or they just shoved a larger vacuum line into the grommet and gorilla taped it and it somehow fell out and was lost. IDK But thats what i plan on doing unless someone can give a part number to a special adapter fitting or whatever ford used. I thought maybe i could find the line inside the cab behind the glove box, But, i didn't see it. It looks like you have to dig deeper or go a easier route with the gorilla tape and hose.