Weird: Coolant Temp Drops when Revving

[BoostedE21]

Hey Folks,

After replacing the heads on my 91 4x4, I noticed that the coolant temperature indicated on the sensor fluctuates quite a bit, and drops when I rev the engine....which is odd. The heater is also noticeably hotter when driving than when idling.

I replaced the thermostat....is it possible I put it in backwards?

I'm not sure why this is happening....the thermostat is new. Any ideas?

 

[Rick]

My guess is that there is air trapped in the system. I don't think the heater issue is related. The heater issue could be due to the heater control valve or possibly a vacuum line that got knocked off while doing the head replacement.

 

[BoostedE21]

My guess is that there is air trapped in the system. I don't think the heater issue is related. The heater issue could be due to the heater control valve or possibly a vacuum line that got knocked off while doing the head replacement.

Regarding air in the system - certainly possible. I guess I need to get one of those burping reservoir things and run it.

 

[Rick]

Scratch that, I forgot that the 1st gen Explorer doesn't have a heater control valve.
The heater is controlled by the heater control cable on the dash which operates a blend door.

 

[BoostedE21]

Scratch that, I forgot that the 1st gen Explorer doesn't have a heater control valve.
The heater is controlled by the heater control cable on the dash which operates a blend door.

I don't think I fiddled with any cables - I think I can feel the blend door operating when I move the lever.

I'm curious if I could have possibly gotten a bunch of air stuck in the heater core when I did the head replacement....is that possible?

 

[ChildIdenticant]

Air in the system does/can prevent the heater from working, and does cause erratic behavior on guage, though can't say I've seen it that responsive to just revving. Still, burp the baby.

My first gen (1994) does have a heater control valve. Though it was manufactured in october 1994, when parts meant for the next gen apparently were being installed. My center console matches '95 center console. The rear interior panels have the molding inset for the cargo privacy cover, instead of the screw-in mounts. Little things.

 

[BoostedE21]

I watched a youtube video and noticed that the orientation seems to matter - I don't recall setting it in a specific orientation when I installed it. Might that cause my issue? I've noticed that the heater is hotter when I'm driving (load on the engine) and not as hot at idle...and the sensor position lines up accordingly.

 

[RangerX]

Yes, the thermostat has two orientations, there is a hole or valve that goes in the 12 o’clock position. Also the ‘bridge’ side of the thermostat points out toward the front of vehicle.

 

[ChildIdenticant]

I watched a youtube video and noticed that the orientation seems to matter - I don't recall setting it in a specific orientation when I installed it. Might that cause my issue? I've noticed that the heater is hotter when I'm driving (load on the engine) and not as hot at idle...and the sensor position lines up accordingly.

Sensor position means dash temperature guage, right? Temp Sender is the thermistor with the single wire, on the passenger side of the coolant outlet, and is what changes the display on the dash Temp Guage. The Temp Sensor (ECT) has 2 wires, is on drivers side of coolant outlet, and sends signal to the EEC/computer. The sensor doesn't appear to be relevant here.

If by orientation you mean relative to clock face, it isn't crucial. A tiny bypass (Jiggle Pin) being at 12oclock does make passive air purge easier. Put it in backwards, that matters and should be corrected but your temp gauge should end up climbing higher than normal, not the behavior you are seeing. The thermostat should still eventually open if put in backwards anyway btw.
If you aren't confident you installed the thermostat correctly, redo it.
But I would still assume this is a matter of trapped air. There could be weird things happening if for instance your Sender isn't responding quite like it should AND you installed low temp thermostat.
You can purge air by loosening the Sender a bit while running. You may want to re wrap threads with new teflon tape to ensure proper grounding.

 

[BoostedE21]

You can purge air by loosening the Sender a bit while running. You may want to re wrap threads with new teflon tape to ensure proper grounding.

Good idea. I'll give that a shot tomorrow!

 

[Kesp4.0]

If you end up pulling the thermostat housing to reorient the thermostat be sure to stick an aspirin pill in between the thermostat opening and the outer ring. That will allow coolant to fill the engine reducing the amount of time (or possibly the need) to burp

 

[dejen3303]

Over the years I've done a fair amount of cooling system work. The only time I've seen what you describe is if a thermostat is stuck open or missing. On most modern engines it hard to impossible to put them in backwards. If put in backwards the engine will overheat.
I also work on engines that the cooling system needs to be 'burped'. These will always overheat if air is stuck in them.

 

[94EddieBauerRock]

Scratch that, I forgot that the 1st gen Explorer doesn't have a heater control valve.
The heater is controlled by the heater control cable on the dash which operates a blend door.

I am pretty sure it does. It is vacuum operated just in front of the firewall. The only reason I know this is mine disintegrated when I was in high school (like 2003) on my 1994 Explorer.

 

[ChildIdenticant]

I am pretty sure it does....on my 1994 Explorer.

Posts on the forum suggest it was only on the '94 in first gen.

 

[BoostedE21]

Over the years I've done a fair amount of cooling system work. The only time I've seen what you describe is if a thermostat is stuck open or missing.

I'll take a look and see if it may be stuck open. It's brand new, but it's possible it was damaged or not properly functional.

 

[sd_iconoclast]

I have had both heads crack on my 91 EB. The large swing of the temperature gauge that you describe is one of the symptoms. The stock heads tend to crack right between the valves. Are you seeing brown stuff in the radiator?

 

[BoostedE21]

I have had both heads crack on my 91 EB. The large swing of the temperature gauge that you describe is one of the symptoms. The stock heads tend to crack right between the valves. Are you seeing brown stuff in the radiator?

I've drained the coolant multiple times for different parts of this project, and it was perfectly clean and green. The engine runs smoothly and has good compression...I'd imagine the compression would be pretty poor with a crack in the head, right?

I don't have the kit to burp the system, but I've driven it around a bit and would expect the air to get pushed around/out in a pressurized system, no?

 

[BKennedy]

I have never had the air in the system issue, but it is very common with the 91-94 Explorer engine. You can burp the system by squeezing and releasing the upper radiator hose several times while the engine is running. You will see air being pushed out into the reservoir. I also just find a big rock to drive up onto, maybe you could find a steep grade and drive up and down it a few times.

The 1994 does have a heater control valve. Its in the heater hoses and near the firewall. They used it up until 2010 in the Rangers as well. It pulls a vacuum closing the flow to the heater core to keep temps down inside the passenger compartment and opens when the heater is in use. I know several people have retrofitted them into 91-93 Explorers. The only problem with them is they fail without any notice. My daughter's 2010 Ranger just had one come apart a few days ago, dumped all the coolant in a few minutes but luckily she pulled when she saw the steam. I had mine fall apart in the garage on my 1994. Since then I have had two short pieces of 3/8" OD metal tubing in the center console so if it fails I can bypass it until I can replace it, and still have a heater.

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[dcgerard]

I had a bad oil sending unit, that would make my my oil pressure needle take a nose dive when I was accelerating. This didn't happen just once, either. My last first gen explorer did the same thing. I swapped out the sending unit and it's normal now. It may not be what your problem is, but a temp sending unit is pretty damn cheep and worth a try. It's easy as hell and it's worth it to rule that out. I'm not saying that is what your problem is, but it's a possibility.