1998 V70 Coolant Cap? Overheating? Something? Topic is solved

[Eddystone]

CORRECTION TO SOLUTION!!!

The coolant cap was bad, BUT the real problem was that the coolant temperature sensor next to the thermostat was bad and reading low.

NOTE #1: If you notice that your engine temp gauge usually reads exactly halfway up and suddenly stabilizes and indicates somewhat below halfway, your temp sensor is probably bad.

Two functions of the temp sensor are to feed the temp gauge on your dash and IMPORTANTLY it tells your radiator fans when to turn on. If it's reading low (for example, the coolant is at 217 F and the sensor reports that it is at only 180 F), your fan will not come on when it should, and your engine will overheat. A sensor costs about $20 and is a bear to get at. If you remove the thermostat housing, it's possible to get it loose with a clawfoot wrench. Or buy the specialized tool.

NOTE #2: If you have the Torque app on your mobile phone and a Bluetooth OBDII device, you can read your engine temperature in degrees AS REPORTED BY THE SENSOR. With the defective sensor, the OBDII was reporting only 185 F when the car was boiling over. The new sensor read much higher AND it activated the the radiator fan. On MY CAR (and your mileage may vary) the radiator fan switches on at 217 F degrees and runs until the temp drops to about 208 F within a minute and switches off.

Note that just a little bit of air through the radiator makes a big difference. My original problem was that the temperature stayed stable below the boil-over point whenever the car was moving, but it would then boil over if I had to stop and let the car run at idle. The temp sensor was incorrectly measuring the temp and reading low, and at idle the radiator fan never came on, so the car boiled over.

Original POST follows:

First of all, I think all three of my Volvos have substandard Brand X coolant recovery tank caps that I bought when I was young and foolish. Tomorrow, I will be receiving three correct Gates caps that appear to be a completely different design.

Questions:

Temp gauge on dash shows normal operating temperature.

1. When the car is idling and at running temp, my radiator fan doesn't seem to come on. Normal? (Seems odd)

2. What should the running temp be when hot, thermostat open and pressurized? (I can't measure the pressure.)

1998 NON-turbo auto with recently replaced radiator and water pump (last six months). Has a replacement Miehle coolant tank that holds pressure and seals well.

I think I just have a faulty cap....

SPOILER ALERT: It was a faulty cap. Installed a Gates 31329 Coolant Reservoir Cap and can recommend it.

[BlackBart]

Does the fan come on at all? Ever? I'd check that. My 850 overheated when the module / relay piece on the back of the radiator failed, and the fans never came on.

[MoVolvos]

.

Turn on the AC and see if the fans work? If you've gone through the trouble of changing the radiator and water pump the thermostat should also be replaced.

I had an odd problem with the temp gauge with the 230K 98 S70 T5 going to the max then returning to normal. Turned out someone didn't installed the thermostat after a repair. Changed the tank and cap I believe and heater core although it needed one but shouldn't have anything to do with it. Just new to Volvo repairs at the time so didn't know much.

.

[454cid]

I'm not clear what you're seeing that's telling you the car is overheating. You say the dash gauge reads normal. The fan doesn't run all the time.

The temp should be very close to the rated thermostat temp.... I think it's 190F, which is about where my car runs according to the 850obd2 app, I use.

[scot850]

Cap is the first thing to check, followed by the thermostat sticking. Some of the thermostats have a bypass should they jam close, many of the aftermarket ones do not. The foreman at the local dealer recommended changing the thermostat every 3-4 years or when the coolant was changed (3-4 years). You already have the system drained down so swapping the thermostat is not a big deal (unless the 2 x T30? bolts seize). Usually, the thermostat makes the engine take a long time to come to temperature, but as one person mentioned theirs was slow to open causing the high short-term reading.

Lastly the fan relay or ECT/connector issues can be issues as you know.

Neil.

[Sveedy]

On Sveedy the fan doesn't come on till the engine temp hits 198*. On a hot day the temp will get up to 206* before dropping back down.
I've noticed that if I leave the car at idle when it's hot, that the fan will drop the temp apx 2* every few seconds till it gets down to 197*, at which point the fan turns off. In cool weather the temp stays at 188*, which makes some sense since I'm running a very cold thermostat.

[Eddystone]

I'm not clear what you're seeing that's telling you the car is overheating. You say the dash gauge reads normal. The fan doesn't run all the time.

The temp should be very close to the rated thermostat temp.... I think it's 190F, which is about where my car runs according to the 850obd2 app, I use.

Just that I am getting spray out of the coolant reservoir cap. I would guess that the temperature is above the thermostat figure since it is a pressurized system. The point of the pressurization is to allow temps above boiling point without actually boiling.

Tomorrow, I get some quality caps, and I'll see what happens. As I said, temp gauge reads normal, thermostat is opening, but steam is coming out of the coolant bottle at the cap. I think perhaps the cap is releasing at a pressure that's lower than its rated value.

[Eddystone]

Cap is the first thing to check, followed by the thermostat sticking. Some of the thermostats have a bypass should they jam close, many of the aftermarket ones do not. The foreman at the local dealer recommended changing the thermostat every 3-4 years or when the coolant was changed (3-4 years). You already have the system drained down so swapping the thermostat is not a big deal (unless the 2 x T30? bolts seize). Usually, the thermostat makes the engine take a long time to come to temperature, but as one person mentioned theirs was slow to open causing the high short-term reading.

Lastly the fan relay or ECT/connector issues can be issues as you know.

Neil.

Thermostat was replaced with a quality unit when rad was replaced. It's closed and then opens. I think it's just that cap, but it gets you wondering about head gaskets and all. I actually have the stuff to check for exhaust gases in the coolant. Not there yet. Thanks, Neil.

[Eddystone]

On Sveedy the fan doesn't come on till the engine temp hits 198*. On a hot day the temp will get up to 206* before dropping back down.
I've noticed that if I leave the car at idle when it's hot, that the fan will drop the temp apx 2* every few seconds till it gets down to 197*, at which point the fan turns off. In cool weather the temp stays at 188*, which makes some sense since I'm running a very cold thermostat.

Thanks very much for a specific temperature reading. I have a tool that can remotely read the temp and see if I am in the ball park. The coolant bottle is good. No leaks anywhere else. Steam at the cap. System won't work with a bad cap.

[abscate]

Snip cut paste from recent 1999 thermostat replacement and testing

“Swapped out thermostat with UBG Caldor? Highway T climbed from 160 to 180F, idle cycle 205-220”