Overheating Unless Backseat Heater is Running (1 Viewer)

[undermind]

Hey All!

I've browsed through the many overheating threads, and I haven't seen this discussed (forgive me if I've missed anything).

My 77 FJ40 (2F motor, 77k miles) likes to overheat when I start driving it in the spring/summer. It stays garaged in the winter. The magical key seems to be the "Warm Pull" knob on the dashboard. I found that if I keep that pulled out, that my engine runs nice and cool. Pulling that knob essentially causes my backseat heater radiator to steam.

My problem now though is that the knob doesn't seem to have the same effect it used to. There is some warmth coming from backseat heater, but not as much as normal, and the temp gauge gets hot before the backseat heater gets very warm. I would be worried that I've got a clog in my lines back to the backseat heater, except for the fact that it's actually leaking a little coolant.

My reason for posting is to see if anyone has been through this type of thing, and can offer any suggestions based on experience. My 40 spent some time in the shop and they couldn't find any issue but it was on that drive that I discovered the warm pull switch fix. Not looking for a diagnosis, but I'm hoping to get steered in the right direction before I go replacing things like radiators. I've seen a common theme among threads where people replace a lot of things and the problems remain.

My radiator overflow is topped off, but perhaps I should do a radiator flush or try to have the coolant system back to the backseat heater serviced.

Thanks so much!

 

[rkymtnflyfisher]

How's the thermostat?

Have you tried burping the system?

Is coolant flowing properly?

How's the coolant look?

Are you basing your temperature off of the factory gauge or something more accurate?



An overheated engine led me down the rabbit hole.

 

[cruiserkev]

Been there too, it's frustrating to diagnose overheating. I had several issues that contributed to the problem. Flushing the cooling system, replacing the heater valve and taking the radiator out to be power flushed solved all my overheating problems. I run cool even on hot days now. Make sure you have the correct thermostat with both gaskets installed properly and use coolant not tap water. Good luck!

 

[Steamer]

A thorough flush would be a good first step. But even before that you could get a good an idea of what it is like inside the cooling system by draining some coolant out of the radiator and the block drain into a clean container for a look see. The block drain is at a low point and sediment likes to settle there. It may be clogged, and some probing may be required to loosen any crud.

Removing the thermostat to check it out can sometimes be difficult. The steel bolts like to react with the aluminum housing and can seize up. There are methods for dealing with it should that be the case but just be aware of it.

And be prepared for the rabbit hole rkymtnflyfisher mentioned.

 

[rkymtnflyfisher]

The block drain is at a low point and sediment likes to settle there. It may be clogged, and some probing may be required to loosen any crud.

This can also get you access behind the #6 cylinder where all the sediment settles. I spent days with a coat hanger and a shop vac cleaning pounds of stuff from my block, granted it was on an engine stand.

 

[RWBeringer4x4]

the rear heater pulls hot coolant from the engine and circulates it through the finned heater coil, then sends it back to the engine - same with the front heater. Essentially, they’re both tiny radiators. I’m guessing that the engine’s radiator has lost efficiency (ie: clogged) or the thermostat isn’t opening properly and running your heaters has been allowing enough coolant to bypass the clog, circulate through the heaters (bleeding heat) and back to the engine.

If it isn’t working as well as it used to, the crud may finally have made its way into the heater system or blocked the hoses leading into it from the engine.

As others have said I’d start by popping the radiator cap (once cool) and taking a look at the coolant. If it’s green and not sludgy that’s a good sign. Then pull the block drain and see if it’s clogged or full of rust and sediment. If it is, at that point I’d flush the system and maybe think about replacing the thermostat - sediment could be building up there as well.

You can also take a look at the hoses when the engine is hot and see if any of them are swelling. ANY radiator will build pressure when hot but if any of the hoses look particularly “ballooned” it could indicate a clog.

You could also check your trucks fan clutch, if it has one.

You want to be careful driving it until this gets addressed. I’m admittedly not as familiar with the 2F as I should be (my truck has a 350) but if, for example, the recirculating coolant from your heater is exits right by your temperature sender - you may have an acceptable “cool” reading right at that spot while the rest of the engine is, in fact, too hot.

An IR thermometer is super useful for checking block/head temperatures at different points. Best $20 I’ve spent!

 

[erics]

An IR thermometer is super useful for checking block/head temperatures at different points. Best $20 I’ve spent!

Let me be the third to recommend working from known accurate info... the OEM guage/sender is useful only for general indication. IR thermometer is easy and relatively inexpensive.

 

[RWBeringer4x4]

For what it’s worth - these screenshots from the user manual (stolen from another thread a long time ago) reflect what the various gauge hash marks actually indicate in terms of real temperature. I’ve found this to be actually very quite accurate as confirmed by the aforementioned IR thermometer.

 

[undermind]

Thank you so much for all the replies. Really excellent information.

From the sound of things, I think a cooling system flush will be the right first step. Just looking at the small amount of coolant I can see at the top of the radiator, it does not look good. Pretty brown/rusty looking.

The rear heater is also leaking a little bit of coolant, which is a new symptom. Perhaps there is a clog on the way back to the engine and it's getting forced out there.

Thanks again for the advice!

 

[rkymtnflyfisher]

I think a cooling system flush will be the right first step.

Pretty brown/rusty looking.