Got a yota tech question

[Legacy777]

Got a question I want to bounce off some people.

My mom's 90 yota pickup 4cyl overheated and she blew the headgasket. We pulled the engine, replaced the head and put everything back together. Everything is working fine, no leaks that we could see, etc.

However there's a strange behavior with coolant gauge. Whenever you turn heat selector knob up the coolant gauge moves up a little. If you turn the heater down, the coolant gauge goes back down a tad. Her coolant system is setup different then the subaru's. There is a valve in the coolant line that goes to the heater core. This valve is connected to the heater knob. So as you turn the heat up, it opens the valve allowing coolant to flow through the heater core.

My only thought is the water pump. Reasoning is that as you open the valve you allow coolant to flow through another artery, and if the pump is not providing enough flow, pressure will drop, total flow through the system will decrease, and temp will increase.

The motor was rebuilt a while back, and the water pump replaced then. So the pump "should" be good, however I don't know if it wore prematurely when it overheated due to low coolant.

Any thoughts?

[mikec]

That's a good hypothesis Josh.

The only other thing I could think would be along the same lines, and would involve a partial blockage somewhere. So with the heater core being bypassed with the heat on cold, the pressure's high enough to provide enough flow through the blockage.

But what are the chances of a partial blockage in a cooling system? I'm not totally sure, but probably not very likely.

Because normally, wouldn't using the heater help lower the temperature of the coolant, and would possibly show up as a lowering of the gauge, if only a fraction of a needle width?

[evolutionmovement]

Stupid question, but is there enough reserve coolant? With the extra volume to fill when the heat goes on, it could be that the coolant gets a little low.

Steve

[Legacy777]

I should note that when the heater is on "cold" and the coolant temp gauge is slightly lower. It is at the "normal" position that the needle was at before any of this happened.

Blockage might be an issue.....but I sort of don't think it would cause those issues.

Coolant level is good, the system has been purged and overflow is good too.

[Legacy777]

any more thoughts?

[entirelyturbo]

How would a water pump "wear out" though? I don't think it's possible that the fins wore out or something. The only way I've ever heard of a water pump failing is for the bearing to go bad, either from mere age or from coolant leaking into it, which would be identified by coolant leaking from the weep hole.

That's the only hypothesis I can offer.

[vrg3]

Water pumps do wear out when they get old enough. The fins actually slowly erode away. That's how my old Benz's water pump failed. It ended up overheating whenever engine speed was low for too long.

[Lunatech]

Perhaps it has nothing to do with the coolant temperature, but is rather some strange quirk in the electrical system. If you had some other way of measuring the temperature of the coolant you could confirm this or rule it out.

Just a thought, I hope this helps.

[-K-]

I'd have to say it would seem to be electrical, unless it's normal and you never watched the gauge that close before it overheated?

[professor]

I suspect the temp sensor is located in a foolish position, so the change in flow pattern brings water by differently when the valve is opened on the circuit for the heater.

I wouldn't worry at all if it is within a normal range.

You could hook up an ohm meter to the sensor, and see if the resistance changes when you open the valve. That would tell you if it is an electrical ghost, or an accurate read of changing sensor resistance.

[Legacy777]

It might be an electical ghost, or something related to the temp sensor. I'm not exactly sure where it's located.

I can tell you with 100% confidence that the needle did not exhibit this behavior before when everything was working correctly. Yes it's still in range and probably fine, but I don't want another repeat of the last debacle.

I'll take a look at the haynes manual I have to see if it has any info in it on the wiring.

Thanks guys.

[NemesisEJ22t]

Hmmm.....Those 22-RE's are pretty durable motors, i wouldn't worry too much. Does the gauge stay up, or go back down after a while? Also, how quickly does the gauge go up after the knob is turned, instantly or does it take time. My guess would be something to do with the headgasket replacment, but i don't think that anything about that could cause this. When was the thermostat replaced last on the engine? How about the rad cap? I'll poke around a little after my final tonight to see if i can find someone with a similar problem.

[Legacy777]

Yeah....that motor in my mom's yota has a 1/4 million plus miles on it.

The thing is I don't know if the temp gauge exhibited the same behavior before the head gasket issue or not. Also, I'm not sure what components could have been affected by the extreme heat & running it low on coolant.

The thermostat is OEM and brand new, same with the rad cap. I replaced the clutch fan as well.

From what I remember, the gauge will stay at the same spot as long as the heat lever is kept in the same spot. If you move it to "cold" the temp gauge will decrease slightly.

It's very bizzare....it'd be a nice little troubleshooting project if I actually had it near me.

[NemesisEJ22t]

Hmmm....It really is a weird thing happening, logic would say that the exact opposite of what's happening should be what's happening. I would see if turning the blower fan on and off has any effect on it (ie. when the fan is turned off, the needle goes back down, that would be the only electrical connection to the car that has anything to do with the HVAC system, and should rule out an electrical problem. The valve is controlled by a wire that is operated by the heat lever so that couldn't be affecting the electrical system.