Coolant Temperature Rises Rapidly After Flush, But No Overheat

[VolvoHonda2014]

I recently flushed the coolant in my neighbor's 2015 XC60, and we've been watching it over the past few days to add more coolant as needed. We have noticed a very weird issue; if the engine has been off for a few hours and is cold, the temperature gauge rises to the middle within a few seconds of starting. The strange thing is, it never gets above operating temperature, it holds there.

Not an answer to the draincock but important: The engines with thermostat on block (V8, all 3.2 short 6, 3.0T short 6, P1 5 cylinder, and all P3, and any platform with the Drive-E 4 cylinder) are all designed to use the cooling system vacuum evacuator and fill system. Absent having that tool then you take upper hose, the one NOT going to thermostat housing, and fill it with 50/50 coolant/pure water mix to get the block filled up. The coolant often does not get into block due to the upper hose routing and can't go through closed thermostat. You have HUGE risk of engine damage if you don't get the coolant mix into block. Also, you aren't getting nearly a complete drain if you don't find the block drain on back of engine (two on V8, one each for each bank) so the block drains. The same captured/block issue exists for draining that exists for filling.

I saw this warning from Jimmy57 that the block may not be full. I never drained it (I didn't even know that block drain existed), but my theory is that the block has air in it. Does anyone have a picture of the line that is supposed to be filled? The only upper radiator hoses that I can find that go into the block go into the thermostat (two total; one large, one small).

The engine is the B4204T12, if that helps.

[pbierre]

MVS posted a long article on the "gotchas" involved in replacing the coolant on modern Volvo engines. If the engine is operated while the block is emptied of coolant, block heat will build very quickly (not good for alum. block), and the thermostat won't allow water to rush into the engine until it is hot enough to open it up. This could result in serious engine damage. The engine is designed to be started with the block coolant channels filled, which requires special attention to completing a coolant refill.

Here is the article: https://www.matthewsvolvosite.com/coola ... rn-volvos/

[VolvoHonda2014]

I saw that. I can't find where to fill up the block channels on this car. Do you have a picture of the upper radiator hose referenced in the article?

Is it the one on the far left side of the radiator that looks like it could be a transmission cooler line?

[pbierre]

I found the write up cited above hard to stomach. I have the B2306 "T6" engine, and cannot see any "upper hose" going to the engine block except the one containing the thermostat.

As far as the access on the driver's side of the radiator, yes, he's talking about the hose that enters the radiator adjacent to the air intake conduit. It comes up from the transmission oil cooler and carries coolant from/to that box. But, this fill opening is 3" below the high point in the radiator, so you cannot fill the entire coolant volume this way! I find the whole procedure (as written up) annoying in its opaqueness about flow paths and venting. Whatever happened to the radiator with a cap, and you just poured coolant into an opening at the very top of the cooling system?

OK, so the flow circuit through the engine block is impeded by a cold thermostat being completely closed. If there were a small bypass hole in the thermostat, you could just pour coolant slowly into the reservoir and wait for it to settle. I think I'm going to do that next time a new thermostat goes in...drill an 1/8" bypass hole thru the thermostat's diaphram. So what if the engine takes slightly longer to warm up??....I don't live in Sweden.

[VolvoHonda2014]

Ya, that's where I got lost... I think the real issue is that the thermostat opens at over 100, and from my experience, the coolant never gets that hot during a normal drive..

Thanks. So I'll fill the block through that opening and bleed through the overflow. I wish this radiator had a cap- my Honda radiator does!

[pgill]

If the gauge rises to the middle in a few seconds then you have an electrical problem not a coolant level problem.

Possible causes

1. The sending unit in the Thermostat housing is broken and it needs to be replaced (I looked and the sending unit seems to be molded into the Thermostat so you need to replace the assembly)

2. The cable connection for the sending unit has been contaminate with a conductive fluid (i.e. engine coolant)

There are other possibilities but these are the most likely

To confirm do the following

1. remove the connector for the sending unit and verify that the gauge drops

2. measure the resistance of the sending unit with the cable removed (on a cold engine it should be about 2000 Ohms)

3. Use an electrical cleaner to wash away coolant residue from the cable connector and sending unit (MAF cleaner should work for this or electrical contact cleaner, or even isopropyl alcohol and some compressed air)


Note: the sensor resistance should be about 2000 Ohms cold and about 400 Ohms when the engine is warm. If you remember when you studied electrical circuits the Parallel resistance needed to go from 2000 to 400 is 500 Ohms. Five hundred is a large value and it wont be difficult to achieve this with some coolant contaminating the connector. To put this in perspective a contaminate like this will be difficult to detect, if the resistance was very low (say 1 or 2 ohms) then you should be able to see it.

Good luck

Please post your findings


Paul

[VolvoHonda2014]

Thanks, Paul- that is definitely worth looking into.

When I was doing the initial bleed (funnel in the reservoir), I had my OBDII monitor plugged in and the coolant temperature got to 110°C, so the thermostat was open. The biggest "burp" happened when it was that hot. Is it possible that that was the block filling? Or should I still add more through the radiator?

[VolvoHonda2014]

I discovered that the thermostat in this car is quite buried and I couldn't access it in the time I had.

Do OBDII monitor readings match with the needle? I plugged mine in and a reading of 73°C had the needle in the center.