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My truck keeps blowing its cap

My husband just bought a 2000 Ford Ranger 4x4. On the way home from work the recovery tank cap kept popping off. When he got home the radiator was cool to the touch, hoses were warm. He removed the radiator cap and no coolant was showing but the over flow tank was full. I looked in the recovery tank and it looked brown which to me indicates a head or head gasket problem. I checked the oil and it was clean and seemed to be slightly overfilled. I was wondering if anyone might have any other ideas except my worst fear.

I looked in the recovery tank and it looked brown which to me indicates a head or head gasket problem.

Brown just means OLD.

Did you add any coolant? If so how much?

If the coolant is a solidly brown color, that is probably just an indication that the previous owner(s) never changed the coolant, which is now…about a decade overdue for changing. Unless you can verify that the coolant was changed w/in the past 3 years, I strongly suggest a flush of the cooling system, followed by a fill with a 50/50 mix of the specified type of coolant.

On the other hand, if the brown color is limited to a line at the top of the overflow/recovery tank, that is an indication of a breached head gasket, and the brown line is the residue of motor oil that leaked into the cooling system.

Can you clarify the nature of the brown coolant for us?

Incidentally, when a head gasket is breached, the symptoms can take different forms in different vehicles.

It can lead to an exterior oil leak, which is not very serious
It can lead to oil being forced into the cooling system, along with excess pressure–which would probably be enough to blow the cap off of the overflow tank.
It can lead to coolant infiltrating the motor oil supply, and this can be…fatal…in short order.

The problem is that either of the first two symptoms can suddenly morph into the third, rapidly fatal symptom, so it is important to find out soon if the head gasket is breached, and then to decide whether to repair it or to ditch the truck.

If the overflow tank was physically full, not at the full or “hot” line, that could cause the cap to blow off.

Faulty thermostat and to a lesser extent, maybe a weak radiator pressure cap which is allowing the overflow into the coolant bottle.

This really needs to be addressed now and the truck should not be driven until it is repaired. Hopefully the brown coolant issue is nothing more than a need for flushing which should be done in conjunction with a new thermostat.

Thank you for your input. we did take it make to the place that sold it to us, which by the way they so far seem very willing to get the problem taken care of at no cost to us since the truck is two days old for us. The brown color was only in the coolant bottle and I couldn’t really tell. To me it wasn’t the type of brown associated with rust but it does look like the truck had a problem at one time and maybe the coolant tank didn’t get cleaned out or it could have been residue. I will let you know what they find out. Thanks again for your input.

Year of truck is good. What motor does it have. How many miles.

I dunno, I think I’m with VDC on this. A pressurized recovery tank is not a good sign but should be easy to diagnose if its that bad.

A blown head gasket can cause this or similar problems. Visble bubbles in the coolant in the radiator which increase w/rpm would be consistent w/a head gasket problem, might try that as an experiment But at this point it could be a half dozen other things too. If it were my truck I’d be inclined to replace the coolant and watch it carefully, go from there.

Sorry to have taken so long to get back with you but I wanted to make sure the problem was fixed. It would seem that it was a faulty water pump. My husband has been driving the vehicle for two weeks now and the problem has not occurred. Thanks again.

Sorry to have taken so long in getting back but I wanted to make sure the repair was the correct diagnosis. It would seem that the problem was a faulty water pump. My husband has been driving it now for two weeks and the problem has not occurred.