Hi all! This is my first post, so I apologize if I say anything that's already been covered. I just became the proud owner of a 1978 Ford F-150 custom with a 351m and 4 speed. So far, the only issue I have is the defroster won't defrost the windshield. It blows, although I don't know if I would describe it is overly strong, and it doesn't get up to temperature, but both heater core hoses get hot. Any ideas on what to check and how to troubleshoot? Thanks in advance. I'm a little new to this. I have a good experience working on engines, but haven't had to tackle the heating system on a vehicle as of yet.

 

First off, Welcome to the FTE!!!
Your off to a good start, just knowing that both heater hoses are warm. So we know that the coolant is circulating through the cooling system.
If you slide you temperature control side to side, can you hare the control, opening and closing the flaps in the duct work?

Unk Bob

 

Does it have AC. If yes the heater control has vacume lines, and that part will go bad.

 

People sometimes carry lots of their "crap" on the dash, so over the years a lot of small things often fall into the defroster ducts that block or slow down the airflow reaching the windshield. Sometimes you can get that junk out with a small vaccum attachment or needle nose pliers or someone with small fingers. BUT sometimes it may also be internal to the box unit that the heater core is inside of, with flaps that the seals are bad on, or a plugged heater core. Sometimes people use an undersized heater core fuse in the fuse box instead ot the proper 30A fuse, I have found 10A's and 20A's in the past, and putting in the proper 30A fuse allows proper current to turn the fan faster. OR you could have a weak blower motor fan that needs to be replaced. ALSO, where the box unit meets the windhshield vents ducting, the foam between that junction could have broken down and needs to be replaced, so it would be leaking air in-behind the dash, this would be behind and above where the cigarette lighter/power point is. AS WELL, there is a vent in the passenger footwell, that if it is open it reduces pressure of air that is forced through the heater core. AND if someone has replaced the heater core in the past and did not have the proper gaskets to go around it, then there could be air rushing around the heater core instead of being forced through it.

 

Wow guys thank you so much for the responses and the help! I now have a new issue. My solenoid blew up, but replacing it didn't fix the problem. I'm assuming it's the wire leading to the ignition switch that's my problem. I've heard that on old fords this wire has built in resistance and can wear out. Anyone have any thoughts or insight? Thanks again!

 

Which solenoid? The starter solenoid?

 

There are 4 wires on the solenoid.

A big one from the battery and on the opposite side, a big one to the starter.

The 2 little ones are labeled S and I. 12 volts applied to the S terminal will turn the motor over. The I terminal will have 12 volts on it when there is 12 volts applied to the S terminal, this 12 volts is used to bypass the resistance that is in series with the coil. Dents used a resistance wire that is located inside a wiring harness coming from the key and headed to the ignition module or coil if it's a point type ignition.

How did your solenoid blow up?

 

Not really sure what went wrong to be honest with you. I went to the hardwares store and truck drove great, turn key start up, and then when I came back I turned the key and it behaved as if the engine was flooded. Then as I pumped the gas and tried to fire her up, all of my electrics cut out, and the starter motor stopped turning. Truck went dead.

 

that almost sounds more like your battery died. Test it. Modern batteries do not give much, if any, warning when they puke - I have had the identical experience where the vehicle started and ran to the store flawlessly, but when I cam out 5 minutes later the battery was gone.

 

Thanks HoustonDave! That was actually what I thought too, and was really hoping would be the culprit, but I took a multimeter yesterday and checked it, and sadly the battery seems pretty happy.

 

How about a load test?

 

2X^^

A volt meter won't put enough of a load to really test a battery. You could turn on the lights or have an assistant try to start it while you have the meter leads on the posts of the battery. That'll give you a better idea of the batteries condition.

If it's good, you will need to check for voltage drops in the wiring and for open fusable links. Fusable links are just wires designed to melt or burn open when too much current runs through them. They can be hard to identify but usually have a plastic "flag" or bump on one end. DO NOT replace with regular wire, you will run the risk of burning your truck to the ground.

 

Hey guys, so it turns out that I had a fuse and a relay blow out. All fixed now. I cleared out a ton of leaves and junk that fell into the blower motor for the defroster, and that seems to have done the trick. Thanks for all the tips! I was wondering if anyone could give me some insight into my engine. I know that the 351m is part of the 335 series and is essentially a 400 with lower compression and a shorter crank. What's the difference between the 351m and the Cleveland? Anyone have any specs, opinions on the engine, and experience with their reliability/capability? Should I go the route of converting it to a 400, or am I better off just putting on a holley intake, 4 barrel, and dual exhaust (currently has a single pipe)

 

Well if you switch to a Cleveland youll need a different bellhousing. Every one always says what boat anchors the 351m and 400 are. I have a sweet running,strong 400 that takes care of my needs.

 

Good_to_doo- Thanks for the tip! I've heard mixed things about them. Weight wise, I checked, and according to the research i've done the 351m weighs in at 575 lbs, about 145 lbs less than the 429/460. Keep in mind, I haven't personally put one on a scale, so who really knows? Being a bit of a purist, I'd like to keep the truck as close to stock as I can with some minor mods to improve torque and power, but I'm not looking for a muscle truck/ burnout machine. I am looking for something tough and reliable that can haul things from A to B, whatever that may be, and potentially do a bit of off roading; it doesn't have a lift kit, 4 wheel drive, and big tires for nothing, am I right? I called the boys at Holley, and they recommended I go with a Weiand 8010 small block ford intake (makes sense), and a 570 street avenger. Before I move forward with that, if anyone has suggestions on a better set up I'm all ears. Also, I'm wondering about what gas to put in it. Should I put regular 87 octane, or because of the lower compression put in something a little punchier? Lastly, and I'm sorry to just dump a bunch of questions on everyone, I read on another forum that a smart upgrade to see some real power out of a midland is to put in an aftermarket cam like an Elgin, and for protection plum the oil port on the side of the block to the top port in the rear together with a hard line. Anyone have any opinions on that?