I have a 95 4.9 with a 5 speed tranny. I've had this truck since April or so, and around May it started to have overheating issues if I would sit at idle, but would return to normal temp once I got going about 30 or so. It wouldnt get too hot but it would get a little on the warm side, so I just brushed it off as a minor issue. Sometime Around late June it overheated pretty good a few times when idling over a period of a week or so, but it would still return to normal temp once moving. I then decided something needed to be done. I opened the coolant resivour only to find it bone dry, I filled the resivour half full and started the engine. It sat idling as I watched it suck up all the coolant, I filled the resivour half full like 2 maybe 3 more times after that before it stopped sucking it up! It didn't over heat anymore but it would still get hot so I did some more looking around the radiator and found a peice of cardboard stuffed between the two radiators. I assume the previous owner put this there to help heat up faster in the harsh MN winter. Once that was out it stayed perfectly cool no matter how long it idled in the 90 degree weather even with the AC on.
However... Just last week it started to get warm again, not overheat but the needle probably gets 1/8" away from dead center. It now gets to around the same position if I spend any more than 20 min on the highway. I'm confused though, as now it happens like I said when I'm on the highway doing 60 or so. If I get home and let it idle it will go back down to normal temp in about 5 minutes. Today I looked in the resivour and it was low again so I topped it off but that has had no effect yet.

It does not leak coolant anywhere I can see so this scares me to think its a head gasket issue, however if I look In the radiator it's perfectly green and I just recently changed my oil and didn't notice any mayonnaise looking stuff. Also it doesn't produce any white clouds like it's burning coolant.

The only thing that could make sense to me is maybe a clog in the cooling system somewhere or maybe it's just taking a while to burp the system and that's why it keeps taking more coolant from the resivour. If anyone has an answer please let me know!

 

You might have a slow coolant leak, that only leaks while under pressure. I've seen it very often on radiators themselves.

Do you still have your fan shroud?

 

Yes I still do have it

 

If I were you I wouldn't be worried. There's a reason that part of the range is labeled "NORMAL".



Tan mayonnaise on the cap or dipstick is probably oil film and condensation that hasn't burnt off. It's perfectly normal for a 20yo truck, especially if you do a lot of short trips. Melted chocolate milkshake oil means coolant is in the oil.


Do you have a 1-row or 2-row rad? The 2-row is about as thick as the radiator cap is wide, the 1-row is closer to half. What about fan shroud? Do you have the big shroud or the small plastic thing that goes on top.


With a less than 5yo 2-core rad and no fan I can go through the McDonalds drive through, have them make fresh fries and deal with a few people in front of me without getting past A in normal in ~30-40 ambient temps. Back when I had an electric fan set to 210deg it wouldn't kick on in the summer if I was at least going 15-20mph. With a 10" fan (that's too small to work efficiently with the shroud) taken from a air compressor I need ambient temps around 90 and DC beltway stop and go to get past A in normal (this is my "winter fan"). I swap on a clutch fan if I know I'm going to be towing/hauling or spending a lot of time in stop and go traffic in the summer. Point is these trucks have plenty of cooling, especially for a 300. If you've got the right fluid in the right places the truck should have no problem staying cool.


I run a 205 thermostat and M in NORMAL is where it rests. A is where the high idle kicks in (to move more air over the rad).


Keep adding coolant (mix your own, a gallon of coolant costs the same as pre-mix and a gallon of distilled water costs $1.50, tops). This isn't a pressing issue since it's not actually overheating and it's gonna be winter in a few months. Write down the milage now and when you go through a gallon or two write down the mileage. If you can't find a residue trail and you're not smelling coolant when you open the engine bay after a long drive then it's probably a small enough leak that it will be a PITA to track down and a ~7psi cap will probably be a worthwhile investment.


When you notice the temp creep up note if it's idling faster (over 1000rpm, normal is 750ish). The computer does that to move more air over the rad.


It's probably not a head gasket issue. Unless you're really unlucky you're going to have to abusively overheat it to pop a head gasket and even then you probably won't. Take a second and consider how many people obliviously drive with a leak and don't notice until it sounds funny, most of them let it cool, refill it and are good to go. Popping head gaskets on a whim is mostly reserved for engines with iron blocks and aluminum heads.

 

Just off of what I remember I have a 1row. And I have the full shroud that goes all the way around.
As far as watching the RPMs while at idle, just off of memeory I haven't noticed any difference between idle speed after starting and after a 40 min drive. I don't have a tac in my truck so I go purely off of ear. But then again it doesn't idle very smoothly anyway so it's hard to tell. I'm thinking the cause of this is maybe a bad O2 sensor, as I looked a few days ago and the whole top half is cracked and only being held there by the wires. I have a replacement sensor already but the thing is so rusted in there I've been trying to get it it for 3 days now. I've just been repeatedly spraying it with PB blaster with no luck...

 

O2 usually won't affect idle too much. An engine will idle rich or lean no problem. It's under varying load conditions that an 'O2 sensor is most useful. It tells the computer if it got it right. Since you have a new one on hand it's worth replacing. Since the current one is already cracked I'd just cut the top off and use a 6pt socket and breaker bar. Don't be surprised if you dent/bend the exhaust pipe a little bit. Depending on how rounded it is you might be better off to use a 12pt wrench and hit the wrench with a hammer to break the rust loose (rust is very brittle compared to steel). Install the new one little more than finger tight with anti-seize.

 

Ok thanks for the advice, what would you assume the rough idle is from then? Some days it will also buck while going down the road when accelerating slowly. It will do the same if I take my foot off the gas then back on it real fast. It's not too violent, it will just kinda hesitate a little then slightly buck once or twice before the rpms start to climb again.