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Old 10-17-2009, 02:07 PM   #1
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Default 1991 F-150 losing coolant

Hi all! I thought maybe it would be a good idea to register here since old Ford trucks seem to mysteriously wind up in my driveway all the time. Currently a 1991 F-150 has been sitting there for about six weeks, and a 1998 F-150 has been sitting there since 1998! They run.

Anyway, the 1991 F-150 has a 5.0 liter engine. It is a Lariat model with air conditioning and dual gas tanks. It loses about half a gallon of coolant in between fuel ups, so we're talking half a gallon or so every 400 miles.

Here's what I've checked:
Nothing drips out underneath the truck. I can't see any fluids on the ground at all. This rules out some gasket leaks, but not all.

The oil looks fresh and clear (the clear brown color, that is). This may rule out a leaking head gasket.

The truck smokes white in cold weather (roughly 50 degrees) for the first mile or so it is driven, then the exhaust is clear. This may also rule out a head gasket.

There's no overflow bottle to check, so I've been looking in the radiator a few days after my wife gasses up. I am going to get a bottle at the junk yard this afternoon. From this, I know for a fact that the overflow bottle is not leaking. :|

There's no smell of coolant in the cab, nor does the defroster cause the windshield to fog up. The only time I've ever seen the window fog up was when I sat in the truck for about an hour after shutting the engine off, but both trucks do this and I've only had to add coolant to the newer one once between changes. This could rule out the heater core.

The carpet is wet near the driver's door, but it's bone dry under the glove box. I can think of no way at al that coolant would leak from the heater core, go all the way across the cab, and soak the carpet on the other side. Stranger things have happened... a leaky master cylinder showed up near the left front wheel, and another time what started off as a mess on the windshield in another car turned out to be coolant and led to a pair of blown head gaskets.

Here's what I have not checked but need to:
I haven't checked the t stat housing.

I don't know if any bubbles are coming up in the radiator during normal operation. I don't think so, but I wasn't really paying attention.


When we purchased the truck, there was still coolant all over in the engine compartment. A hose had broken during a recent test drive, and the person we got the truck from showed me where he'd replaced the broken coolant hose going into something near the throttle body with a fuel hose. I have not seen the "fuel hose" leak at all, nor have I seen any coolant leaks inside or outside the engine compartment (not that I've looked very close). I've owned three other vehicles with 5.0 liter engines, and I've never noticed that hose before. I don't know what it's for (besides coolant, that is).

I have not looked real close at all the hoses yet. I had thought that if they were leaking it would drip on the ground.

Yesterday I Fixed the battery on my newer truck, so I'm off to Repair something on this one today. (ok, kidding, I'm a Ford man...)
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Old 10-17-2009, 03:33 PM   #2
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look over the truck real well....what I find is a leak that leaks when cold but not when warm or it's force dried by the engine heat...look for corrosion around hoses and the thermostat housing....

look under the water pump for tracking lines down the pump or lower radiator hose...this would be one that will leak when warm and blow off by the fan...

you may have a hose leaking when the engine is running and maybe drying off the intake and running down the back and blowing off...but the key is to look for green corrosion...if you had a head gasket go there would be green on the spark plugs or lightish moose whipped oil....
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Old 10-17-2009, 05:20 PM   #3
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I just took the truck down to the junkyard for a coolant bottle and some other stuff. Once I got back, I listened to the entire fourth quarter of OU--Texas (Sam needs to get well) on the radio, then got to work finding that leak. There's some dry brown stuff around the thermostat housing and there's a milkshake underneath it. Evidently the engine has leaked oil at some point. It may be leaking oil currently, but if so, it is not enough to show a difference at all on the dipstick between fill ups. In my other truck, I can tell that it's leaked, say, four ounces of oil since the last fill up just by looking at the crosshatch marks on the dipstick. In this one, though, I can't see that the oil level has changed.

I added the coolant bottle but have not filled it yet. I need to check the wiper bottle with clear water to see if the pump is running OK, then I'll get some straight antifreeze, mix it with distilled water, and dump it in.

The truck used about one quart of coolant in the time it used three tanks of gas (1 and 1/2 fill ups).

Gotta take the newer truck to the parts store so this one can cool down some more.


There's also corrosion around the junction where that fuel hose was added in place of a coolant hose. It's bone dry, but it got onto the alternator. I don't like the idea of water having been in the alternator.


I am thinking of changing the stat and its gasket. If that's the leak, it will be stopped by the new gasket. I also considered Bars' Leaks, but I don't know if it's been used before and I wanna stay away from it if I can.

Labor to change the timing chain gasket is $455 from a known expensive shop. I cannot get any more quotes until Monday.
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Old 10-17-2009, 06:49 PM   #4
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Judging by how much you're losing, if you let the truck run and look from the side to see the bottom of the t-stat housing, take a flashlight or something too 'cause it'll make it easier. But you'll be able to see a leak from there easy.

Also, you might wanna wipe off the whole area under the housing to make sure there's no distractions.

Another thing, if the thermostat in the housing is like the one on my '91 I6, then you'll need to set the stat into the slightly recessed groove thing in the housing. The stat likes to slide and if it isn't in that grove, it'll cause a leak. It took my dad and I 3 times before we got it to stay in the groove while we bolted the housing back on.
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Old 10-17-2009, 07:00 PM   #5
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silicon works wonder in holding a t-stat...
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Old 10-17-2009, 10:25 PM   #6
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We never thought of that. We just got really stubborn and made it work! lol
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Old 10-18-2009, 12:13 AM   #7
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I tried silicon on a replacement t stat. It held nothing. I re used the old stat because I got the wrong one from O Reilly's. I have been to four different places for various parts in the last week and every single time I have had to return stuff because it was the wrong item.

I changed the t stat gasket and plumbed in the overflow tank. We will see how much coolant it uses. I did not change the radiator cap but I probably should.

Question: Is there any possible way that the coolant level could drop and not cause the level in the overflow tank to drop as well?
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Old 10-18-2009, 01:58 AM   #8
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If you are refilling the radiator regularly without having a overflow than all the antifreeze added will run out of the hose onto the ground. The overflow catches the antifreeze forced out of the radiator due to normal thermal expansion, without one you need to have a couple inches of air space to accommodate the expansion.
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Old 10-19-2009, 12:15 AM   #9
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So with the overflow bottle on the truck, is the radiator supposed to stay full? I checked the level just a while ago since the truck hasn't been driven all day. The coolant level was perhaps two or three inches below the cap.
I thought that the system would keep the radiator full and draw coolant out of the bottle. On the other hand, I have never run a 5.0 with the radiator more than about 80% filled, with or without the coolant overflow bottle. It doesn't seem to matter how much I fill the radiator on one, they seem to have their own level they want to stay at.

There's more coolant in the radiator now than there ever has been since we got the truck, so it has to have drawn some of the coolant back out of the bottle. I think there's more in the bottle than I put in there, too, but I'm not really sure. Matter of fact, there has to be, since I had the radiator itself completely full while I was trying to force all the air pockets out.
No coolant has ever leaked on the ground since we got the vehicle. The one time I filled it before this, I started the engine with the cap off, let it warm up to normal, then filled the radiator. the next day I looked in there to see where the level was so I'd know where to fill it to. With the bottle installed now, that level should have changed.
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Old 10-19-2009, 03:33 AM   #10
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When the truck is warm, the coolant is pushed into the overflow. When the truck cools, coolant is drawn from the bottle.

You can check your freeze plugs to see if they're seeping. It's usually so slight you don't notice until it rots out enough to leak steady.
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