'94 S-10 overhearing. I'm grasping at straws.
 

I'm posting this here because I'm running out of ideas.

To try and sum up… the truck started overheating out of the blue. I couldn't figure out why so I changed the thermostat. A week later I found the lower hose had a pin hole so I changed it. The truck would be good for a while, then overheat. This is going on for 4+ months now and I'm out of ideas.

Where I am:

Waterpump is fairly new (year or two?) (belts are right and tight)
Changed upper and lower hoses.
Changed thermostat (3 times actually, 195, 195, 180)
Changed fan clutch.
Changed radiator.
Changed radiator cap.
Heater core is not blocked, plenty of heat.
Engine running same as always, no loss of power or MPG so I don't think it's the cat.
Did exhaust gas test on the cooling system, no signs of head gasket leak.

With no thermostat I can drive for hours on a hot day, engine runs about 150ishº at most, never goes any higher.

Put a thermostat in and she's up 230-245+ in only a couple miles.

I've been trying to purge the system of air for a while, with the nose high cap off hot/cold cycles thing. 5-6 times now? Doesn't seem to have had any effect.
Will do it a few more times tonight after work.

I'm at a loss. :icon_shrug:

Did three more cycles tonight with it up on ramps. No change when I finally took it down and drove it.

Have you checked the coolant temperature sensor? Unplug the connector and check the ohms with the engine off. Check it after the engine sits to completely cold, and once completely warmed up.

177 ohms @ 212 deg. F. or 100 deg. C.
241 ohms @ 194 deg. F. or 90 deg. C.
332 ohms @ 176 deg. F. or 80 deg. C.
467 ohms @ 158 deg. F. or 70 deg. C.
667 ohms @ 140 deg. F. or 60 deg. C.
973 ohms @ 122 deg. F. or 50 deg. C.
1188 ohms @ 113 deg. F. or45 deg. C.
1459 ohms @ 104 deg. F. or 40 deg. C.
1802 ohms @ 95 deg. F. or 35 deg. C.
2238 ohms @ 86 deg. F. or 30 deg. C.
2796 ohms @ 77 deg. F. or 25 deg. C.
3520 ohms @ 68 deg. F. or 20 deg. C.
4450 ohms @ 59 deg. F. or 15 deg. C.
5670 ohms @ 50 deg. F. or 10 deg. C.
7280 ohms @ 41 deg. F. or 5 deg. C.
9420 ohms @ 32 deg. F. or 0 deg. C.

Check the two wire sensor, it sends a signal to the ecm to control air/fuel mixture, and the single wire controls your dashboard gage.

No but I do have a new one on the way. I figured what the heck, I've pissed enough money away at this point. :icon_wink:

Why are we talking about replacing the ECT sensor?
Other than to verify the gauge sensor reading, the ECT is irrelevant
If it's a two sensor system, the one that you're addressing is the gauge sensor.
And the gauge sensor seems to be good because it shows 150 when thermostat is out.
So.. back to the problem.

Do a cooling system pressure test. You may have a head gasket problem... esp if the temp overshoots then settles lower.

internal blockage or sensor

I only ordered the sensor/sender for the gauge, not the other one.

If it were a head gasket leak (exhaust into the water jacket) it wouldn't have passed the exhaust gas test, no?

It shouldnt have. Not only that... if it were bad enough to run that much hotter, there should be other serious symptoms.

You need to replace the two wire sensor, the gauge one doesnt matter. The two wire sensor affects the air/fuel mixture and ignition timing. If this one is not working properly, it will play havoc with your entire engine operation.

I think I might have got it. (so far so good anyway) Mechanic friend suggested I try to purge it by introducing water. I pulled the heater hose from the radiator and capped radiator side. Then I attached the heater hose to the garden hose and turned it on slightly, just a little water coming out. Ran the engine than way for maybe 25 minutes or more.

Let some water out of the radiator to make room for antifreeze, then went on 10-15 mile drive with never going above 200.

That's great... if it stays this way. A couple times in this summer I thought it was fixed then as well. We'll see.