This is kind of a late night post so you’ll have to excuse me for not having pictures at the moment. I spent about half the day putting a replacement dash into my 1996 F-150. The existing dash had been cut (presumably to remove the radio?) and it was something I had been dreading. I also decided to get cracking on the A/C, the Texas heat (although it has been more pleasant lately) and the commute on 610 with horrible traffic has finally forced me to cough up the money for an A/C overhaul. I did all new everything; compressor, drier, evaporator, lines, o-rings, condenser. The works. And the A/C blows moderately cold (although not nearly as cold as the R-12 system in my 1990 F-350 when it was working). I have yet to check the temps but probably low 50s high 40s. One thing I noticed is the air doesn’t really come out of the vents. Some quick google searching confirmed some kind of vacuum related blend door actuation defaulting to defrost for safety (presumably). I would really like to have the vents blow and have max A/C work and I was wondering where the best place to begin is?

Another thing I wanted to ask about is fuel economy. I have been getting about 10 mpg average (city/highway) in this truck since I got it. On a 19 gallon rear tank I go just about 200 miles and now with the A/C running I am worried that already low number is going to drop even further. Now I wouldn’t be driving this if I were that concerned about fuel economy, but I do feel like it’s a bit excessive for the fuel economy to be that low and I wanted to what others are getting with a similar combo. My truck is a 4.9L (300) E4OD w/ 3.08s I believe. The cats are gutted and I have retained the factory muffler. Nothing fancy. If I can get a slight mpg bump I would be open to ideas. Thanks for your patience in this long-winded post.

 

40's from vents is about rite for 134 systems. they will never be "meat locker" temps like a R12 system.
for the gas mileage, 3.08 gears and traffic are mileage killers. the only way to make it better is to find a different route where you can actually drive instead of sit in traffic.
and the rolling along with different scenery will greatly reduce stress levels and your blood pressure too.. i will o 20 miles extra without a thought if it means i am moving instead of sitting.
after we moved the big truck shop 9 miles south, i found the "long route" as one guy called it, which is all secondary one lane each direction roads, is 5 miles longer, but 20 minutes quicker than taking the main highway.

 

Defrost is the default. Did it work before the dash and a/c repairs? You replaced the evaporator so I would check the vacuum lines around that area.

1996 with a 4.9L should be OBD-II so how was the post cat oxygen sensor bypassed? What else might the PO have messed up? Is the check engine light on? What are the fault codes?

 

Yeah I definitely agree with that I’d much rather be moving along even if the route is that much longer. I’ll have to try some other roads, I have tried other routes but the main routes are all clogged up usually due to construction or just purely traffic from stoplights. I wasn’t expecting frigid temps but I think on max A/C (if I can get the blend doors to respond) ought to make it a little bit cooler. It would also definitely
be more noticeable if it were blowing at me through the vents.

I was careful with the vacuum lines behind the dash, I recall it only ever really blowing from defrost but of course I never really had the blower running because all it would do is blow warm air and heat is never used here. I noticed the vacuum line missing from the spot on the black plastic section that was removed to access the evaporator so I put a new line and ran it straight to the vacuum tree, also have that vacuum line sensor that plugs right on top of the evaporator/blower. I’m not sure what that does exactly and it definitely could be setup incorrectly. I would say overall my vacuum lines are pretty poorly setup, partly POs fault but also partly mine for not really fixing it. I’ll send some pictures of that so maybe y’all can help me sort what goes where and do a good job of it.

As for other stuff, yes it is OBD-II, and yes CEL is on and has been. I forgot to mention that. I’ll read codes and report back. Admittedly the CEL has basically always been on since I bought the truck for varying reasons, right now I believe there was only one code and it was cat related but it has been ages since I scanned it.

Finally getting around to a lot of things upkeep/maintenance/quality of life wise and just biting the bullet which is good.

 

Also, what is the tire size?

 

That’s a good question, totally forgot to include that, I can’t recall the exact size and I’m not right by the truck but I think whatever it is roughly translates to a 30” tire.

 

Without any pictures I’m guessing “the black plastic section” that was removed to access the evaporator should be the vacuum reservoir. According to your vacuum diagram, is that where the line is upposed to go? Where does the other line from the reservoir go? Never heard of a vacuum line sensor. Is the fresh air/ recirculate door actuator by the pass hood hinge hooked up to vacuum? That white vacuum line is known to rot away.

 

I believe the “vacuum line sensor” of the air box reservoir would be the check valve. T shaped with a disc in the middle?

 

I realize I left a lot unclear, I apologize as I’m still learning these different piece parts as well. Until now the A/C system had been a complete mystery in the majority of aspects. The black plastic piece I am referring to is the black molding that is removed when replacing the evaporator, not the vacuum can, but the actual plastic on the evaporator closest to the intake manifold that keeps the evaporator covered from the elements.

The vacuum sensor I believe is some kind of regulator for the fuel vapor venting from the tank? It is mounted on top of the evaporator black plastic box and sits against the firewall. Some of the lines leaving it go back into the throttle body, hence the guess that it has to do with fuel vapor venting.

As for the A/C, I got it figured out, originally I thought the blue vacuum line coming from the climate control **** was faulty (broken), but then I remembered there was no difference in any A/C setting at all from DEF to MAX. Using the EVTM for 1996, page 54-1 shows that the black line is the vacuum supply for the climate control system. Tracing the black vacuum line back under the dash and through the firewall, I found it dead-ended wide open in the loom. I tossed a vacuum line on it and T-ed it into an existing line and bam all A/C functions seem to be great. One thing I haven’t decided on is if I smell exhaust with the A/C running, I’d be curious to know if that’s due to something A/C related (say a blend door not sealing? Or maybe a cabin air filter (which I can’t seem to find) or rather like it’s an exhaust leak. Needless to say some further inspecting is an order.

 

I retract my statement then. I would believe the vacuum sensor to be the MAP sensor, probably. The black box is the side cover to the air box.

 

you have no cabin air filter, so that is not the issue. by bet is a blend door not working properly, allowing outside air into the system.

 

If you do indeed smell exhaust fumes though, do look for an exhaust leak. Your exhaust runs all the way to the back of the truck?

As someone who recently redid as many foam seals as I could get to, they were almost non existent. That will help get the air where it’s supposed to be, out the vents.

 

Good to know, I was looking around for a cabin air filter and figured there couldn’t be one because it wouldn’t be so hard to find. It’s a shame I just do all the dash I should have messed with the blend door seals, I figured that sort of stuff just wasn’t really fixable/worth fixing. I’m learning a lot. The truck is a MAF truck being a 96 and it definitely has a MAF on the intake boot near the air filter. I think it’s the vapor canister purge valve or something like that for recycling fuel vapor into the intake.


I need to work backwards on the exhaust, might get one of those cheap small smoke machines and run it into the back of the exhaust. I know for a fact there is a pinhole right before/after the muffler that I need to fix, I just don’t know if there are other leaks. I replaced the manifolds, gaskets, and O2 sensors a couple of years ago. I also did quite a bit messing with the secondary air injection system, although I believe everything for that is buttoned up. I could not get my EGR tube undone one time so I had no choice but to cut it and then came to find it unobtanium so ended up rewelding it back. Lots of potential for leaks it sounds like.

 

Vapor Management Valve (VMV) is correct.

 

Thank you, I knew it was something like that. The top of the VMV spins almost like it would be adjustable? I’ll try and pull codes at work today on their scanner as I always seem to misplace mine and then I’ll report back. I seem to recall just have a code for cat/O2 sensor stuff but I can’t remember.