A new (to me) Ford truck problem

[mgdavis]

Hi all, I'm newish here, you might recognize me from TFL or a couple others.
I've got a 1991 F-150. The I-6 has about 160,000mi on it. Yestersay morning when I got in and started it up I heard a distinct "gurgling" sound that seemed to be coming from the engine bay. It sounded like water sloshing around. I checked my coolant level and added a bit. It looks like I've got a slow leak somewhere I'll have to take care of. I started it back up, got to work, about a 10 mile drive. I heard occasional gurgling on the drive in. After work I repeated the drill, topped off the overflow reservoir. Got the same noises, seemed to be somewhat lessened. Same drill this morning, only added a few ounces of water. Total water added was just over a quart.
Does anybody have a clue what this could be? The only thing I can think of is heater core. I can't smell anti-freeze, and it doesn't seem to matter if the heater is on or off. I also haven't seen any fluid on the ground or my floorboards. I'm really puzzled. Could it just be that the fluid level had dropped enough to let me hear it moving around?

[280plus]

If you can't smell anti freeze it ain't the heater core. Is the truck running hot/cold? Maybe the thermostat is stuck?

[charby]

Worse case could be a blown/leaking head gasket allowing exhaust gases to escape into the coolant system causing the bubbling sound.

With the engine cold remove the radiator cap and smell for exhaust.

Also with the engine running (cap back on) check for bubbling in the plastic coolant overflow reservoir. Smokey bubbles are what your looking for.

Head gaskets are pretty easy to change on a I-6 if you got some mechanical aptitude.

[280plus]

Won't that also get water into the crancase? Pull oil fill cap and dipstick, look for "chocolate whipped cream" that will indicate head gaskets leaking to the crankcase. Same thing for an auto tranny except the cream is strawberry instead of chocolate. So check that dipstick too. DON'T taste it.

[charby]

I had a leaky head gasket one time that didn't escape water into the crank case, but exhaust got in to the coolant

C

[mfree]

+1 Charby. I've had that happen too.

[Brad Johnson]

Could be the popoff valve in the rad cap has failed somehow and is allowing steam to escape into the overflow tank (thus the gurgling sound).

Brad

[mgdavis]

The theremostat has been malfunctioning for at least the last year, it was in that condition when I bought the truck. One of those things I've been meaning to get to. There is no exhaust smell or or bubbling in the coolant. It really doesn't sound like bubbles, it brings to mind water sloshing around in a container, rather loud for something on the other side of the firewall. I pulled the dipstick when the truck was cold and saw a couple bubbles in the oil on the stick, not enough to set off any alarms. I'll check again tonight when it's hot. Shouldn't a bad head gasket cause the engine to run rough? I had it tuned up a month ago, and it's never run better. Tomorrow's my day off, I'll dig into it more then. Thanks for the response guys, anybody got anything else? (not head gasket...not head gasket...)

[Tallpine]

"Shouldn't a bad head gasket cause the engine to run rough? "

No, but it could cause the engine (the coolant, actually) to heat up suddenly, and maybe boil over.  I once had an old chevy six that the head gasket went bad.  It would run fine for about ten minutes and then the heat gauge pegged and it boiled over.  It was about a 4 hr job to change the gasket.

How come we keep hearing about FORD truck problems ...?

[mfree]

"How come we keep hearing about FORD truck problems ...?"

Because the chevy owners are all out in the garage tinkering instead of posting :p

Anyhoo, have the radiator cap checked. Could be that the sound is the rush of coolant into/out of the tank, or underpressurized coolant boiling in the heads.

[Parker Dean]

Could it just be that the fluid level had dropped enough to let me hear it moving around?

That's it in a nutshell.

For some reason the coolant level has dropped and there's air in the system now. When you stop the engine the remaining coolant flows to the lowest point, leaving the highest (heater core) with a big pocket of air. You start the engine and the coolant starts flowing, mixing with the air and making the gurgling noises.

The trick now is to find where the coolant is going. You'll have to get the system full and under pressure while someone does a thorough visual inspection. Every nook and cranny. Twice. Then you should do it again because the vast majority of coolant loss is due to an external leak IME, and they can be sneaky little buggers.

Failure to find an external leak would tend to suggest either a heater core, head gasket, or cracked head. Leakage into a cylinder can frequently be seen by pulling  the plugs. Moisture is an obvious giveaway. Rust on the part that's in the cylinder is also a pretty good indicator. I bit more subtle sign is little white flecks on the carbon deposited on the outer ring.

[gunsmith]

if you can get to them, see if the hoses for the radiator are tight and not leaking or anything, I am not mechanically inclined and was told my head gasket was leaking (300 $ to fix) on my 1990 ford ranger with 160 k on the odometer, a friend of a friend looked at it and pointed to a hose clamp that had gotten lose...
but any way two months later I started leaking alot and had heard of bar's stop leak so I tried it, the bottle sid use one but my leak kept happening, so I used three...that worked.  Bar's pellets...

[280plus]

Watch that Bars. Only use a little at a time till the leak stops. Some want to dump the whole jar and that will clog up the radiator.

There is a device (litle hand pump) for pressurizing the cooling system cold through the radiator fill to see if any pressure is lost.

[mgdavis]

Update: Yesterday I went out, checked fluids, found everything at acceptable levels. I started the truck, and heard zero unusual noise. I came to the same conclusion that Parker opined, air in the heater core. It looks like I'll be checking the coolant level more frequently until I get some more time and track down the problem. Thanks for the input gents.