Diagnosing a cooling system problem. - Ranger-Forums - The Ultimate Ford Ranger Resource


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Old 04-19-2016
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Diagnosing a cooling system problem.

Hello,
I have '91 3.0L six cylinder. Recently, when it was still cold, I had trouble with my engine running cold and my heater not working well or at all. Then when the weather started to warm up, I was on a longer drive, and the coolant came out of the cap and my engine overheated. I replaced the cap and flushed the radiator and this seemed to fix the problem, except that the engine was still running cold. Last week I was on a trip that involved a two hour drive (and the weather has really started to get warm now), I began to smell coolant and it looked like it had come out of the overflow tank. The engine also heated up a bit, but it did not over heat. Since then, and with the weather being warmed, I've noticed that it is running a bit warmer and now, after it has warmed up, it stays around 2,000 rpm when idling. It also doesn't always run smoothly. I can't really seem to figure out the problem, but from what I read it may be the thermostat. I just wanted to get some other's ideas on what it could be.
Thanks.
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Old 04-19-2016
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Welcome to the forum


You will want to do the Glove Test to take head gasket issue off the table before investing any time in other possible problems, it is a free test

Cold engine
Remove rad cap, if coolant is at the top drain it down 1/2 to 1 inch
Remove overflow hose from rad, plug this hole, vacuum cap, gum, putty or ??
Unplug coil pack wire, you want a No Start

Put latex glove over rad cap opening and seal it to rad with rubber band, you can also use a balloon or even a condom, lol, instead of the glove

Crank engine and watch the glove
If it pulses up and down you have a head gasket or cracked head issue

If it just lays there no pulsing then you are good to check other issues.

If it pulses you can find the leaking cylinder by removing 1 spark plug at a time and cranking engine, glove will stop pulsing when that cylinders spark plug is removed.
Reinstall spark plug to confirm.

Cooling system pressure is 14-16psi, rad cap pressure rating.
This pressure comes from the engine heating up the coolant and the coolant expands in volume.

Cylinder pressure is 160+psi when starter motor is turning engine, that's the pulsing you see if there is a leak, after starting, cylinder is 900+psi when firing.
Cylinders are surrounded by coolant, that's the point of the cooling system, so any head gasket issue will always show up in cooling system first.
Coolant in the oil can happen but it was more common on pre-1980's engines.

Your full overflow tanks points to extra pressure in the cooling system.
So best to eliminate that possiblity before spending money on things that won't fix the problem.

Last edited by RonD; 04-19-2016 at 12:20 PM.
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Old 04-20-2016
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Thank you, I'm definitely going to run this test this weekend. If the head gasket is eliminated as the problem, do you think the next thing to test would be the thermostat?
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Old 04-20-2016
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Running to cold usually means thermostat has failed, if they do fail they are suppose to fail fully open for safety reasons, i.e. no overheating.

So yes, replace with stock 192degF model, be sure to get one with a "jiggle valve", yes that is the real name for it, it allows air out while refilling the system.
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Old 04-20-2016
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Alright, thanks for all the help.
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Old 04-20-2016
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I had an 02 3.0 liter that had a similar issue. Check the lower radiator hose. On the 02 it had a branch off that ran to the motor and radiator On mine the small branch collapsed some causing cooling issues.
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