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Battling coolant heat with new engine.

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Hi there!

I dont know if this is the right location for this post considering it not directly related to tuning. Maybe more engine building related or racing in general.

I have a twin turbo setup for a Nissan 350Z. This was running on a build engine but was not build by me. The engine itsself was burning a lot of oil and i know the engine was not build correctly. So i decided to build a new engine. With all the bell's and whistles. Darton sleeves, billet parts and a coolant modification known as 'the pathfinder mod' which should lower coolant temps and cool the back cylinders a bit more. This together with stronger rods and pistons. 262 cam. Bigger valves. The whole deal.

The engine is running for some time now on the car and performs great but i have been battling coolant heat. During summer when driving the engine was pushing 96C with the fans on. This with an outside temp of 25C. And this is just cruise and not pushing anything. This time of the year its near 0C outside and im still hitting 96C while driving.

From the change to the new engine i changed the radiator to a bigger one(52MM). The old one already was an upgraded one. Also running a thermostat that opens sooner. As coolant im running water with water wetter and a 1.3bar cap.

So the question here is: Is it normal to have such a change in coolant temps when changing from a opendeck engine to a sleeved engine? What else can i do to optimise the coolant temps?

I hope you guy's have some insight. Thanks in advance!

Kind regards,


Attached Files

I'm not so familiar with the engine, but it looks like in OEM state it uses a recirculation cooling system when it's "cold", and as the thermostat opens it closes off the recirculation and directs it all to the radiator. The coolant passing through the block to the rear where it's drawn out of the rear cylinder heads.

From what I was able to find, it looks like that conversion taps into the rear block cross-over to draw some coolant directly from the rear block, in theory helping cool the rear cylinders? The kits I looked at used a thermostat to, I expect, just open the cross-over tap when it got to higher temperatures?

You said you changed the thermostat for a lower temperature one, that should certainly be expected to lower the coolant temperature, but which one did you change, what was the original rating, and what was the replacement?

If applicable, did you change the water pump dive speed witha pulley change?

Is your problem with running with the fans (i.e. low vehicle speed), or when running on the highway / track?

Radiators work with air pressure differential. What are you doing to provide lower pressure on the back side of the radiator? Is the engine floor sealed, or do you have an air-dam below the radiator to create a low-pressure zone? Is the air-going into the radiator completely sealed (high pressure air will choose to go around a radiator rather then through it).

If the problem is when running on fans, this is a similar problem is the area around the fan sealed to the radiator? This is important so that the fan(s) pulls air through the radiator, and hot just from around the fan(s).

Finally, are you sure the system is bled completely. There are both vacuum systems (create a vacuum in the coolant passages, then introduce coolant), and tall-funnel systems that do a better job of "burping" air. I have found these low cost solutions to be effective. Here is an example:



Where is the temp sensor located?

Do you have all the pipe work in places as per the diagram? Subaru motors can have an issue with cooling where by if you remove/block off the heater and/or throttle body coolant pipes and do not maintain the correct system flow through them, the Bank 2 coolant passages do not have any flow so that bank heats up quickly and boils the coolant.

Hi there!

Thanks for all reply's! I will try to add as much information here as posible.

The engine has a few circulation flow's blocked because im not using them. one was in the diagram i added but i'll add one more to this post. these are blocked because i dont have the same intake any more.

The pathfinder mod does have a thermostat. but i dont know at which temp this one will open. The one in the oem location will open at about 72C. I do see a big drop in temp at around this temp. The water pump is inside the block. This is still oem. I dont know if there is something else availible.

The ECT sensor is located at the back of the block, on the same tube where the flow from the heads and the pathfinder mod come from. I will attach a picture. This picture also shows one of the blocked of ports.

With placing the darton sleeves the cylinder cooling is not a open sleeve any more. its just some holes to feed the coolant instead of complete open sleeves. I was thinking this could also rise the heat? i will attach a picture of an oem cylinder block for comparison.

Im running a 1.3bar cap and have tested this while i pressurize the system until 1.6 bar. it showed all the realy small leaks and i fixed those. the overflow tank is also doing what i expect(low at cold, full at warm). with this coolant the car ran over 500km. so i assume the system is bled completely. if there is advise on this i would love to hear it :)

Most of the time when the heat is high im driving at low speed. Somewhere until 60kmh. On the highway the car will hit 95C and stay there even with the fans off. In this cold weather now the temp will drop to 88C on the highway. But its 0C outside now.

Currently i dont have a duckt for the airflow in the radiator. There is an intercooler in front of it. and also a gap where the crashbar is located. its about 15cm wide. I was thinking of closing this gap in the hope the air does not flow around the radiator. i do not have any flooring in the engine bay. with the idea of the heat has an easier escape.

The fans are fit onto the radiator no gap there. there is even some sealing tape. but no duct from the fans to somewhere else to get rid of the heat. i do not realy have any place for some form of a duct. i will add a picture for reference of the space between the redator and the engine. of you have some recommendations im all ears.

Thanks in advance!

Kind regards,


Attached Files

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