So I've had my S40 for about a year and six months, I had a Ford Ranger before this so I don't really know to much about Volvos, but I've learned a bunch as I've had a few issues with it. Recently I've been having problems with losing coolant. I just replaced the coolant tank because it had a big crack in it, and I also replaced the cap. I had this done on 1/22, and over the weekend it was still losing coolant so I kept refilling it and checking it. On monday the coolant level stayed normal and it was fine. That wednesday I ht the interstate after about 3/4 of a mile it said it was overheating, but the coolant was normal, when I got off the gas the needle would go down, and up when I pressed the gas. I took it in, but they found no leaks or anything. I took it on the interstate again tonight, as soon as I accelerated I heard a pop, when I got home I popped the hood and that pop was the coolant tank cap blowing off. So I'm just very confused on what is going on, if anyone has any idea I would greatly appreciate it!

 

Check engine light?

Coolant is going somewhere. Do you smell it in the cabin? My 2005 had the leaky heater core o-rings and the coolant was leaking under the carpet. You could just get a faint whiff at times and the interior would fog up occasionally. I finally noticed the rear pax side floor mats were wet.

How about the oil? Is the oil level fine? Is it the normal oil color, not milky grayish/yellowish? Asking to see if coolant is leaking into the oil from a head gasket.

How about the thermostat? Have you replaced it lately?

EDIT: How many miles does the car have? Has the timing belt been changed? If yes, did they also change the water pump? It doesn't take too long to get the bottom cover removed so you can check the pump for leaking. Probably the last thing I would check after replacing the t-stat.

 

The check engine light did come on when it overheated on the interstate and the computer showed me a message that it was too hot, but I'm not sure if they pulled the codes for it...

I don't smell anything in the cabin, I'll take a look at the carpets, but I don't think that's where it's going I had something similar happen where my rear passenger floor board was wet, but that was cause by a clogged AC drain and a broken sunroof drain.

I checked the oil the other day and it look completely normal to me and was a normal level.

The thermostat hasn't been replaced since I have the car, unfortunately I don't have any maintenance records from the previous owner, but that was one of the things recommended to me.

My car has about 137,000 and the timing belt hasn't been changed as far as I know, so I'm pretty sure neither has the water pump.

I took a look at the new tank tonight and saw a little opening in the ring of the opening of the tank, but I wasn't sure if that's sealed off once you put the cap on or not. One of the main things I'm wondering at the moment is why did the cap to the tank literally shoot off when I accelerated on the interstate? Also I have a 2.4i, if that makes any difference

 

You may be living on borrowed time. Most 2005 S40 engines are due a timing belt change at 120,000 miles. There is an exception if the car is from certain states: Models identified by engine VIN codes 39 & 64 sold in California, Massachusetts, Vermont, New York and Maine are classified as PZEV vehicles. The timing belt and tensioner replacement interval for these vehicles only is 15 years or 150,000 miles / 240,000 kilometers.

ALL 2005 VOLVO MODELS

The cap screws onto the plastic tank. It operates at about 20psi of pressure. This is normal. I don't see it being forced off without damaging the threads to the tank. I can only guess it was not installed properly. The system needs the pressure to raise the boiling point of the coolant. This may have been your problem all along. Have you checked the coolant for it's boiling point? They make simple little testers for this.
https://www.fcpeuro.com/products/ant...FQoAvAod02kAPg

The holes are to let air back inside the tank as it cools. You would not want the tank to collapse as the fluid shrinks when cooling off.

 

definitely time to do the thermostat. I had a boil over on my 850 once - popped the overflow cap as well. Replaced the thermostat, new cap, fresh coolant and no problems in 5+ years since.

 

Got the thermostat replaced, they did some type of test for exhaust gas in the coolant system and it came back positive. So they said that means the cylinder head is either warped or cracked. So of course now they have to take the whole engine apart, get the cylinder head off, and send it off to see how bad it is and whether it's repairable or needs a new one. So it sucks pretty bad

 

I think this is a good time to get a "second opinion". Usually there will be other symptoms for a blown head gasket or a cracked cylinder head - ie do you see any white smoke or does the exhaust smell sweet? do you see any foaming in your oil? is there any oil in your antifreeze? Are you losing any anti-freeze over time? You can also have the shop do a coolant system pressure test and a cylinder leak down test as well. If you want to try redoing the exhaust gas test (ie not sure if the overheat can create a false positive) see if you can rent a tester from Autozone or see if they have a test kit (they run $25-50 on Amazon/Ebay). If it turns out you have a leak, then your head needs to be magnafluxed (a dye used to highlight cracks) or you may just want to shop for a used part, try car-part.com - you should be able to find a used cylinder head in the $400-500 range. These other tests can clue in whether its a gasket or a head problem so hope for the best, plan for the worst.

 

A blown head gasket or cracked head won't matter much- the repair is almost the same. The head has to be pulled and sent to be checked. Obviously if it comes back good, then you just need a new gasket. Even if you buy a used head, I'd have it checked out before installing it.

 

Though my first instinct is not to trust the "shop" I think in this case it's pretty clear they got it right. OP's cooling system is being overpressurized - first the cracked coolant tank and then the coolant tank cap blown off. The only way I know to do that is combustion gases entering the coolant circuit.

Also, I think OP must have omitted part of this story.

 

I agree. This is very good advice. You may be lucky enough to have caught it before doing any major damage to the head or block. I have noticed that it is common for the head gasket to develop a small leak between the steel layers of the gasket. This is a sneaky type of leak that can go unnoticed for months before being detected or diagnosed, letting owners drive around unaware of the ticking time bomb hiding in their engine. Once the leak gets big enough it will completely blow, causing the engine to overheat, damaging much more than the head gasket. This has killed many Volvo engines by cracking or warping cylinder heads and blocks. The 2004-20?? S40 doesn't have a low coolant sensor to give the owner or driver warning. Add this to a temperature gauge that doesn't show the actual real time engine coolant temperature (it is designed to notify the driver when the engine reaches operating temperature, once reached the gauge remains in the same position until the temperature has changed for an extended amount of time), and you don't have any early warning to prevent an overheating engine.
This happened to me just 5 weeks after buying a 2004.5 S40 T5 with 112,000 miles. The dealer gave me a 90 day/ 3,000 mile warranty that covers 50% parts and 50% labor. The warranty is void if serviced by anyone other than the dealer.
If I would have known how the system was designed to work I would have installed a secondary coolant temperature gauge and low coolant sensor.

 

Welcome to VF LinkFL

 

You said that you replaced the expansion tank cap. Did you replace it with an aftermarket cap or genuine Volvo? I ask because aftermarket caps are known to not hold pressure on these cars and they boil over at a lower temperature. I had this happen with a Stant cap and replaced it with a Volvo cap. All was good with the Volvo cap.