My 94 F150 4.9L has been running great since I bought it. And I'm getting to the point where I want to start tackling some of the minor problems before they get worse, and I've noticed what I *think* is an issue.

1. Coolant seems to flow FROM the overflow tank if I add coolant there, but almost never to it, even after running for long periods of time. The tube from the radiator to the tank is clear, but the whole thing seems inoperable. ( I suspect the Radiator Cap). The coolant level in the radiator is BARELY low, but if I add fluid to either the overflow or directly to the radiator, it disappears.

2. When I went to do a coolant flush after driving back from the Auto parts store (not but 5 miles total), I noticed that the upper radiator hose was incredibly hot, to the point that I couldn't hold my hand on it for more than a second. Thinking thermostat, I went under and checked the lower hose, which was warm, but not certainly not hot.

The water temp gauge (dummy indicator) is almost always just above the C. My initial thought was a bad sensor for the gauge, but now I'm thinking it could be a bigger problem. So I'm thinking I have a bad radiator cap, preventing the fluid from flowing to the tank, and a stuck thermostat. What do y'all think?

 

In my '94 300, the temp gauge moves all around as the engine heats up and such. The oil pressure gauge is more of a dummy gauge.

A new radiator cap won't hurt, and is probably the easiest thing to start with. Also, make sure the cooling system has all the air bled out. Might take a bit of adding coolant to make that happen.

 

I have issues with getting a radiator cap to fit my truck. The little metal 'ears' on the cap don't like to fit the mating surface on the radiator neck very well.

I had this problem with both the old rad that was in my truck (which was a replacement) and the new replacement rad I just put in. I've tried a couple different new ones, the stant with the locking lever seems to fit the worst but none of them fit very well.

I actually have an older one from the 3 or 4 I have tossed in my toolbox somewhere in the past on there right now. I put it in a vice and very slightly compressed the ears in so they grip better, and I still have to pay attention when I put it back on.

I have to look closely at the cap after I put it on to see if the little metal tabs are fully engaged into their slots. The cap feels like it goes on fine, and it looks like its on fine, but if the tabs aren't where they are supposed to be, the cap will sit slightly crooked and leak coolant out.

If you look below the cap you can see where coolant has run down the side of the radiator if this is happening, but it dries up fairly quickly so it may not be obvious.

Also, directly below the overflow nipple cast into the radiator is another nipple that isn't used - at least not on the 4.9's I've looked at. When you buy a new radiator they give you a cheap little rubber plug and a hose clamp to block it off.

I used the provided setup because I was in a hurry to get my engine in, but it lasted less than a week before it cracked and started to leak. I replaced it with a short piece of hose (5/16ths I think) with a bolt in the end and a couple good hose clamps.

My truck is an '88 and I'm not sure if the radiator in a '94 is the same, but easy enough to check the cap fit, and to see if it has the extra nipple - and look for evidence of leaking coolant down the cap side of the radiator.


If the upper hose is hot, you probably don't have a stuck tstat, or if you do its stuck open. The way to check is to start the truck when its cold and feel the upper hose. It should be cool and stay fairly cool until the tstat opens, then it should get very hot fast. If the temp rises slowly and gradually until its hot, then the tstat is stuck open or missing.

Your upper and lower rad hoses being at different temps when you are in operating range would suggest a clogged, or partially clogged radiator.

Another way to check this is to wait until the vehicle has reached operating temp and shut it off. Then reach in and feel the radiator. You are trying to find places where the radiator is noticeably cooler than the rest of the radiator. This is also an excellent way to burn yourself if you aren't careful, especially if you still have the shroud on your radiator. But if you are careful and turn the fan so you can reach through the wider gaps in the blade it can be done without injury. If you can find cool spots, the radiator is clogged or partially clogged.

I haven't had good experiences with radiator flush. I work in a shop and we don't do flushes with the stuff in a can. Either we take off the hoses and run water through the system to clean it out a bit if it seems to be working ok - as a preventative measure or when replacing a component, or we replace the radiator.

Usually if there is a problem its because somebody has put stop leak or block sealer in the system, or if the system has been leaking and somebody has been running straight water instead of coolant mix for a long time (like years). A well maintained cooling system generally doesn't plug up.

The flush just doesn't work, at best it does nothing, at worst the water pump dies a week later. In my opinion its much better to spend the $100 or so and replace the radiator and flush out the rest of the system with clean water and fill with the proper mix of coolant.

I'm not sure about the letters on the face of your temp guage, mine says NORMAL and runs between the O and the R when it is up to temp. Or just under halfway, say 3/8th - with max ac, hot and sitting in traffic. I have the 195 degree tstat, new rad, new hoses, new lines, new heater core, block was flushed out with a pressure washer on an engine stand, new fan clutch, water pump, and a factory shroud.

During the engine break in period I ran it without a tstat and it ran around 1/8 of the gauge travel, under the N, and came up a little with the ac on and/or traffic, but rarely ever hit the N.

This is about where it ran before I went through the engine, but didn't have ac then, no shroud, and no tstat.

With no tstat in the truck it liked to form an air pocket whenever I had to fill it, and would read hotter than normal until the pocket worked its way out, if I just filled the coolant like usual, ran it up to temp and let it cool down it would bleed itself. With the tstat in the truck it seems to bleed itself right away.

You might also look around at hoses and hose connections to look for leaks, also check the carpet on the passenger side footwell - a leaking heater core often shows itself there. Another place to look is at the freeze/expansion plugs on the passenger side of the block.

 

bdelmar2 - THANKS for your excellent post!

You contributed several things that made me think and I really appreciate it!