Luke Kay

Hey all. Having some big issues here on a 2.5 forged engine. Major cooling system issues. Car is a 2006 STi spec D with a built engine including forged rods and pistons, ARP head studs, and cosworth updated gaskets as the main stuff. It’s got some other little supporting bits too, upgraded 3 inch radiator, stuff like that. and it’s been 24k ish miles since it was fresh built.



Since I got the car from my dad march last year, it’s been faultless. One day early January while sat in temporary traffic lights after some light pulls in the Peak District, I glanced down at the temp like I usually do to be immediately panicked by the engine temp nearing the higher of the middle two lines. It had never budged from normal operating temp EVER, even after some hard driving before, never moved. I then watched it rise and rise and rise until about 5 degrees above the higher middle line. As I moved off to the side of the road, immediately the temp shot up to just a hair underneath the red at the top of the gauge, so I immediately shut the car off, got the bonnet up. I called my dad because he knows these cars like crazy while I let it cool down. The engine and underside of bonnet was covered in steamy coolant, and the overflow tank next to the rad was nearly full.



I managed to limp it home, worried that it was just gonna blow up on me (silly I know) as it was still riding in the red. I checked the oil and coolant the day prior and they were bang on, so we ended up buying a new thermostat and replaced it. Seemed fine. So I drove it how I normally would, to and from work, and occasional weekend outings with mates.



About 3 weeks later me and some friends headed out, I did one singular pull on a bypass in 4th and 5th, and again, a few mins after the gauge was fine, but as we were sat at some lights I noticed the temp off the gauge. like, PAST the red. No idea how long it had been there, but I again immediately pulled over, let it cool, limped it home, however this time, no coolant was on the engine or anywhere in the engine bay, and the temp after cooling didn’t go anywhere into the red at all on the way home, despite being very erratic going up a lot and then down at random.



Got home, me and my dad ordered a sniff test, new hoses, did a coolant flush and burp, did the sniff test a few days later, and the fluid didn’t go green like it was meant to to indicate HG/cracked block issues. It went a little off colour, a bit turquoise, but not green like the videos on YouTube. No champagne bubbles, no milky oil, no expansion tank bubbles or residue, nothing.



and it overheated again 2 weeks later. Turns out this time, the old hose was leaking at the bottom of the rad, completely loose. That must be the issue right? Letting air in? So we thought. Again, flushed coolant, changed hoses, seemed bang on, even after normal with some mildly hard driving. For about 3 weeks.



We noticed the radiator cap was actually winged, and then changed that for an oem Subaru one. We read that the header tank should be lower pressure than the radiator cap, and the header tank is the one that needs to be “winged” so we changed that. And it has been fine, up until recently.



For 2 weeks after we did the rad cap and flush, the coolant was sucking back and returning to the bottom line, not raising past the middle in between the min and max line in the overflow tank after driving. The coolant isn’t sucking back anymore, and again went out to take some photos with some friends and the more I drove it the more the coolant rose in the expansion. Kept rising until I got home, where it was about 7 inches from the CAP at the top of the tank.



My dad tells me I’m being paranoid, and that’s fine. But it’s barely sucking back, and taking multiple days to and from work to get it back down again. The engine holds at normal operating temp but I genuinely think it’s headgaskets at this point, there’s nothing else to check. But then I also think, how can it be with 24k miles on this engine, with cosworth headgaskets and ARP head bolts, running practically standard boost on a standard IHI VF48? I don’t even track the car or anything, just every now and again spirited drives and moderate pulls, once every 2 weeks at MOST. Im still not an expert on Ej engines but am I wrong to think A BUILT engine should NOT be having issues doing these relatively tame things?



No smoke from the exhaust 99 percent of the time and very irregularly, no milky oil on the filler caps or anything, car holds temp now, but there’s HG signs there, and I’m thinking they must be just early signs before it goes fully.



The ONLY other thing I can think of is that to my knowledge it’s still an OEM semi closed block, so could that have possibly cracked somehow?



I’m at a loss, and I’m scared to drive the car hard or on long distances for fear of it going again. Which isn’t fun because it’s one of the reasons I wanted one… Should I just start driving the car gently off boost and start saving for a rebuild with new gaskets? Possibly head work too if the heat from overheating has warped them?



Any info would be lovely, ask any questions and I’ll try and answer them the best I can. If you want photos I’ll try and find some, cheers

Danjo

Have you performed a compression / leak down test? That will give you answers. Not uncommon for forged engines to still do HG’s particularly on a SCD block that has less surface area to clamp down on compared to a CD block. It’s also not uncommon to see cracked 2.5’s from cylinder walk, so really could be either. What sort of power level has it been running?

JdmSti2006

2nd, 3rd etc build is common on 2.5l -needs ££ to do it right
than 2.35 usually throw a rod
see no issues with 2.1 closed deck forged engines

Luke Kay

is that the one where you plug a gauge into the sparkplug hole in each piston and it checks the pressure right? No we haven’t done that, is it difficult to do on our cars? That’s probably the only thing we have left to check

Luke Kay

yeah I’m tempted to end up swapping a 2.0 into it at some point and building that but I mean, 330bhp and 475ft lbs of torque is quite addictive lol I’m worried I’d lose most of that torque unless I go for a 2.1 / 2.35

Danjo

Thats correct. It’s no more difficult than changing a set of spark plugs. It’s what you really need to be doing to give yourself an idea of what is going on.

Luke Kay

Awesome, I’ll have a look at either doing it at home or getting a scoob specialist to check for me. Sorry I also didn’t see the end of your first reply, the car is running about 15psi of boost making roughly 330hp and 475 ft-lbs of torque

Danjo

That torque seems extremely high for relatively low horsepower. What turbo is that on. With torque like that your cylinder pressures will be quite high and that can put pressure on both cylinder walls and head gaskets.

Luke Kay

yeah it’s the standard vf48, it peaks around 18psi, so not too much more than standard pressure. no idea how it makes so much torque though. When it was mapped they were surprised at how much torque it made, but it has barely any power or torque past 5500rpm, all the torque is down low

Danjo

That does seem a little on the high side for a VF48, I’m only making 125lbft more than that with 560bhp on a GT30 and fully forged/sleeved 2.5. But still, there’s a good chance running high cylinder pressures has contributed towards the issues you are having.

Luke Kay

Wow, I bet that’s a right load of fun at that power! yeah I know the turbo is running near its max capabilities, which obviously can’t be good for it. I wonder if that’s what’s causing it, it won’t be helping with heat seen as though I’m still OEM TMIC too. I’m definitely gonna get a compression test done, and if that comes back negative and all seems good then I’m gonna keep driving it how I used to and wait until I can afford to have it built further. I do want 450ish hp at some point, but I want a bulletproof 450. that obviously is gonna have to wait until I’m a little older and I get some money saved lol

but yeah I didn’t even think about a compression test somehow, cheers for putting that in my head

4evascoob

I’ve recently had mine built and because they had to take more than expected off the heads when machining we used a 1.1mm rcm head gasket.

John 37

Have you checked the fans? Do they cut in at the right temperature?

Luke Kay

yeah both fans kick on when it gets up to temp, only for a bit when idling cause the temp drops and it shuts them off but they cycle properly. Just takes some time cause I’ve got an 3 inch aftermarket rad, typically does a good job keeping it cool but I’ll see what happens these next few summer months. I’ll keep checking as most of the times it’s overheated it’s been less than like 12 degrees Celsius outside so this summer could also show some signs

polest

When it comes to selecting head gaskets for a forged 2.5-liter engine, there are several factors to consider, such as the engine's specific requirements, power output, and intended use. Here are a few popular options to consider:

1. OEM Head Gaskets: Original Equipment Manufacturer (OEM) head gaskets are designed by the manufacturer for the specific engine model. They generally provide good performance and reliability, especially for stock or mildly modified engines. If you're looking for a cost-effective and reliable option, OEM head gaskets can be a suitable choice.