Congrats! You are really dedicated! Love the old Volvo instrument pods!
The heater... make sure there is full coolant in there, no air pockets. I take it there is no coolant leaks. These cars don't have a special bleeding procedure... But the gauge being low and the heater not very hot, hmm...
Check the gauge sensor, the one toward the front of the head, driver's side, that it's well connected, check the heater valve, make sure you have good circulation. I "burp" the hoses as I pour in the coolant. Sometimes I undo the top rad hose at the rad and wait for the coolant to start spilling from there while I pour into the coolant tank.

Re the fuel filter, in this case, a car sitting for a long time, yeah, I can see that it'd need it.

 

Hey

Car still starting up beautifully. Could use some perspective/help on the temp gauge (and sensor) and yeah I guess on the coolant system in general. (Hijacking the hijacking here?)

Re: the cooling fan, going to set that aside for the moment; I actually don’t think I have good reason to question the thermo-switch, the wiring checks out, and I actually know the relay is ok because at one point in my early explorings I switched it with the RSR and it worked fine (I also ran a jumper between red and grey-red wires tonight and once again on came the fan).

Speaking of my early explorings, they are the reason my gauge temp sensor is currently connected only by those two pigtails (though the contacts are still good). And speaking of jumpers, I read on another thread here that jumping the yellow-black and yellow-white wires is a way to eliminate a possible problem with the wiring to the gauge or with the gauge itself. When I turned the key to ON with those wires jumped the gauge responded, slamming over all the way to the right (much as it is flopped all the way to the left when properly connected up). Wondering how to test the gauge sensor.

All that said, heater still not blowing hot. The engine block didn’t seem to be getting all that hot either, actually, even after idling for 15 minutes (it is pretty cold out though). Coolant at MAX in reservoir, no coolant leaks. Tried popping off top hose on radiator, coolant came gushing out, probably lost about a half a pint onto the driveway (I guess I have an air pocket now, if I didn’t before?). Could use some hand-holding re: checking on/eliminating those. (I don’t even know what/where the “heater valve” is...also, what's this I hear about thermostats getting stuck?...)

 

Air bubble would be in the heater core... The heater valve is what controls the water going into the heater... It's a vacuum controlled device on the IN hose, near the fire wall... just follow the heater hoses... stuck or unplugged valve would give you low heat or no heat... but would not affect the engine temp. Stuck closed thermostat would make the engine run hot, open would make it run cold and taking long time to get warm. I still vote for an air bubble: "burp" the hoses, add coolant slowly, rev the engine to get the water pump to move the coolant around better...

 

i found 240/740/940's cooling to be pretty hard to get airlocked, as long as you crank the heater temp valve on high while you're flushing the system, they always seemed to get the air bubbles out by themselves.

my similar-age mercedes has a breather screw on the thermostat housing you can crack to accelerate getting excess air out of the system. but they also have a little electric booster pump on the coolant going through the heater cores, part of their 'monovalve' assembly, this pump comes on any time the heat is on, and greatly accelerates how fast the car delivers heat to the cabin on a cold start in cold weather.

 

OK so I’m admittedly circling the airport when it comes to the whole burp/flush process (forgive me that I’m still warming up to this new coolant adventure, heh). Meantime doing some poking around, going to throw some observations/symptoms out there:

- Engine getting plenty hot. Top and bottom rad hoses, plus hoses to and from (what I think is) heater valve (pictured?) also hot. But unless I’m revving engine, heat from heater is tepid (though I'm not that familiar with how typically hot this heater runs; I've only lately adopted this car). If valve were stuck (doesn’t look unplugged) how to test/restore it?



- On one of my restarts, I let it idle for about 15 minutes, then turned off and went to start again, and while key was still in ON position the needle on temp gauge rose about halfway to halfway, trembling/wavering as it held position. Soon after starting the car, it flopped over dead again. (Whereas it seemed to respond steadily and robustly to that jumper test last night.) Again, how do I tell whether gauge temp sensor is bad?

Grateful for the continuing education.

 

that heater valve looks /very/ corroded. I would replace it with a new of of the updated style...

https://www.fcpeuro.com/products/vol...80-940-1259327

or this style,

https://www.pelicanparts.com/More_In...n=9447896-M212


re; Idle, a fully warmed up engine should idle about 700 rpm. a very cold engine might initially idle 1200-1500, but will drop within a minute or so to the 700 rpm target speed. the idle should be pretty much the same speed in neutral as in gear with the brake on, and with or without the air conditioner running (when you change the load on the engine, it might dip for an instance, but should bounce right back)

 

Hey pierce, thanks for this, I'll get on ordering that part. Thanks also for the idle info. Might my ECU be repeatedly opting for a higher idle, even on the nth test startup/run, because it "thinks it's still cold," e.g., because a junked heater valve isn't letting water get around? (Don't really know what I'm saying here, of course.) Would a junked valve also be resulting in tepid heat from the blower (as I think Lev was suggesting)? Also might gauge be behaving the way it is simply because coolant flow is crappy? Again thanks

 

Heater valve only lets water in the heater. And, yes, if it's not opening it's not letting water in the heater core. The valve should be changed, then flush and refill making sure no air pockets develop. The gauge seems to be trying to come to life, get circulation first. i am not sure about the testing for that. What RPM are you having now?

That second heater valve is a generic one widely used, available at parts stores, you don't have to buy on line and wait...

 

Hey Lev. Idle hovering just under 1000RPMs, and a wavering idle at that (though that might be because I just had the fuel pump out to fashion a tighter contact for the power wire; maybe the ECU is adjusting?)...

Re: valve, I was glad to get your "don't wait, just AutoZone it" take (I'm paraphrasing of course) because that's what I had just done an hour before reading your reply (it'll be in the store for pickup tomorrow).

I have a bucket in place under the lower rad hose. I'm afraid I could use as much of an ELI5 walk-through as you guys can stand. I understand I am to run the heater fan while draining old coolant into the bucket. Then I understand I am to detach top rad hose at both ends, remove the thermostat, then reattach top rad hose to block but without thermostat in place (not sure if I'm supposed to evaluate thermostat and if so how). Then flush into top rad hose... with what? Water from garden hose? Distilled water only? Also flush radiator at same time (how)? Then what new coolant? The online disquisitions on that subject are legion, copious, scholastic; I'm tired already. Do I run the heater while refilling (thermostat now having been reinstalled)? Afterwards? When bleeding do I just run heater fan and watch for bubbles to stop (in coolant reservoir?) or must I burp and what exactly does burping look like? What am I missing/getting wrong here?

Again and again thanks

 

um, if the heater valve isn't opening, running the heater fan will do nothing.

I flush with hose water, then drain as much out as possible, and fill with a 50-50 mix of Zerex G05 and distilled water (or spend more on the zerex g05 ready-mix.

 

Hey pierce, thanks for all this. Guess I'm still getting clear on the function of that valve, or rather, on the nature of the entire coolant circulatory system. Which I could also use an ELI5 on. A diagram like this, but for a non-turbo 1990 740 GL, would be perfect. I don't even know where the heat that the heater fan blows actually comes from or how it gets there, let alone whether the loop for my radiator is different from the loop for my heater core, to say nothing of where to access each of these. It looks to my untrained eye like water to the cabin heater (heater core?) is a tributary off the main coolant path so can't see (yet) how a bad heater valve could be fatal to flow to the engine and the gauge temp sensor? Ignorant, of course; here for the education (and to get this old girl back on the road).

Any critiques of my outline above would also be appreciated. Meantime it looks like I'm waiting for that valve to come in tomorrow before starting in with all this. Thanks again for being there.

 

Let's not overthink this.
Drain it via the radiator plug OR lower hose.
Fill it with water and run the motor to operating temp, use some radiator flush chemical if you wish, I don't.
Drain again once it cools a little (scalding hot otherwise), repeat if coolant is really dirty.
Open the petcock on the engine's passenger side if you want to do a thorough job, but not a must.
Drain again, make sure the heater valve is OK and heater is fully ON.
Refill SLOWLY with 50% coolant 50% distilled water.
Try to park front end elevated (not crucial but I hear it helps) to help with air pockets, burp (squeeze) the hoses as you are filling the system.
I leave the top house at the radiator off until I see coolant seeping out then put it on and tighten it.
If you really struggle, take out the thermostat when filling the cooling system. Once full, all closed up, run the motor, rev it until steady good heat is present inside. Then replace the tstat once the engine cools, top off next day as needed.
(And prey that your heater core is OK!)

 

First of all... very grateful for the step-by-step here.

My gauge isn't working (reason I opened this can of worms), not sure how I'll know when I get to that temp? Shall I just run it for 10 minutes or something?

Glad it's not a must lol, because I'm a little unclear what a "petcock" is (a drain plug?) or where it is on my car; is it something in the attached pic (you said it's on the engine, passenger side)? When I do find it, any best practices/tools for removing it? (Am I overthinking yet? )



So having the heater on isn't necessary for the first drain?

Ignorant people like me love definitions! Thank you!

I found a definition of "heater core" that identifies it as a radiator-like device (i.e., a heat-sink?) that hot coolant flows through and that the cabin fan blows hot air off of. I've also learned that a "bad" heater core is a clogged heater core. What I haven't found an answer to yet is how a side-show like this mini-radiator could ever f*ck sh*t up for the actual radiator, or how it could affect the flow past the temp gauge sensor. I'm here for the flush in any event (I want a working heater!), but it would be cool to know that I'm directly (or even indirectly) attacking (or even diagnosing) the original problem of my non-working gauge.

Grateful as always. Looking forward to tomorrow.

 

Run it till hot, feel the hose, the idea is to get circulation, some washing action going...
Yes, a drain plug basically, it's under the exhaust manifold toward the back of the motor but leave it alone for now, address the basics first. Not sure about the size but it's not that hard to get to on Non turbo cars.
Keep the heat ON throughout this whole procedure.
The heater core is just a small radiator, fills with coolant, fan blows thru it-- you get heat. It could be plugged up or leak inside the car, and in these cars it is the hardest part to get to, they say the car was built around the heater core!
Yes, let's get good heat, meaning good circulation, then look at the gauge...

 

OK cool. Again thanks. More soon.

 

https://www.diyauto.com/images/impor...13/01/1266.jpg
this is the block drain plug, the one on the left...

 

Hey forgot to ask... since I'll need to have heater fan running from the start of the flush, the heater valve change-out will have to happen first... how much old coolant will fall out when I pull the old valve? Do I need to catch it as it falls out so it doesn't touch any other car parts? Thanks

 

Don't run the FAN, just have the heater **** full ON HOT so that the heater core is getting circulation, the heater valve OPEN! Coolant will leak out but not that much and no harm if it touches other parts, it's brake fluid that damages paints fast...

 

Got it! Thank you!

 

So first thing was to replace the heater valve. Vacuum felt a lot stronger. Tried the heater. Still not hot (unless revving). Went back under the hood, felt the hoses. Noticed top rad hose much hotter than lower rad hose. Remember another thread in which someone (pierce?) said that if that's the case, your thermostat is busted. Pulled it. Yes it was. The breakage looks like it kept the (temperature-responsive?) valve closed so there was my circulation problem? (Not bailing on the flush, though.) Ordered OEM replacement, coming in tomorrow at AutoZone. Also remembered another poster on another thread (Lev?) recommending running without thermostat to see if that was the problem. Ran it without thermostat (was sure to burp as I gradually topped off); top and bottom hoses became evenly hot. Idle also seemed to improve. Gauge still dead, but that's measuring coolant temp of course, and coolant can't heat up without thermostat restricting flow, right? Educate away. And again thanks.