For what it's worth the only time I overheated in the past was on the freeway, traffic was fine... knackered fan clutch was the cause of that one
You will kill it quickly if you drive in this state Water leaking inside your engine would most almost certainly show up as bubbles in the radiator. The difference between city and highway driving is variable temps vs high sustained temperatures (esp with overdrive ON). Whilst it could be water pump and / or water thermostat related, they normally fail completely and you get a specific instant effect, the fact that you can drive around town fine discounts these even further. The evidence points to your radiator cap or radiator water tank hoses as its typical of a failure in the cap system to allow water to flow out through the cap at high temp/ high operation but not return it correctly. If the water expansion overflow is not returned when the system cools down a vacuum will form in the top of the radiator and it will slowly suck air back in through the faulty cap and / or leaking overflow hoses - then when you fill it up, it will appear empty or less than full but the overflow tank will be full as you described. During servicing the car should be at 30 degrees when filling the cooling system as per the manual. This allows the complete cooling system to be accessed as at normal driving angles, a Z cooling system actually has two chambers being front and back and separated by the thermostat etc. The radiator is the forward chamber and driving around repeatedly at 3,000 rpm will actually suck any air created from a faulty cap in the front radiator cooling chamber into the back engine chamber (it has to pass though the thermostat etc. to get to the engine chamber). The higher the RPM and the longer sustained means more air gets into the back and the problem becomes worse whereas low rpm would not suck the air through due to gravity fighting the air being drawn into the back chamber. The end result is you then have air pockets in the top of the engine that are difficult to get out unless serviced due to the narrow designs of the engine passages. Thus the temperature in the top of the heads spike and your gauge goes to high instantly and is just being saved by the coolant in the lower part of the engine from a catastrophic meltdown. Running an engine in this state of high head temperatures will kill it quickly - the valves/ heads are likely to suffer catastrophic damage sooner or later. (Most likely sooner ) Stop and fix the problem immediately - do not drive until its fixed - focus on diagnosing/ servicing the radiator cap and the overflow hose. Flush the system with straight hose water (ideally at the block drain plug) then park/ chock the vehicle on an angle greater than 30 degrees as per the manual or use the correct ramps so both radiator and engine chambers can be accessed) and then refill with distilled water and the recommended amount of coolant - on a stock radiator system there is a Phillips head plastic screw on top to help drain the air in the radiator chamber - its that simple! Your looking for a 1.1 Bar Nissan small neck radiator cap (unless you drive it hard then 1.3 Bar). Goodluck JC In short - Air in your radiator cooling chamber is not so bad but air your engine cooling chamber is a disaster in the works........ :zlove:
It doesn't by itself (remove cap and your engine will not overheat) However if water is allowed to flow out and it doesn't return on cooling due to the cap being defective (or for some other reason), an air pocket will then form in the first chamber, if allowed to stay like this, high revs can then pull those air pockets/ bubbles from the first radiator chamber into the engine block and this is when the overheating occurs (there is now air pockets in the top of the engine block in place of water) At low revs there will not be enough force to draw the air bubbles against gravity in the radiator through to the back chamber - only higher revs can pull the air bubbles through from the radiator / thermostat into the back chamber (has to be enough flow to overcome the bubbles rising back up) Happy to be corrected - but this is my understanding of how it overheats due to a defective coolant return system - of which the cap is a part of JC :zlove:
Hey E I actually had a similar problem with coolant disappearing and more importantly was wondering how the air kept getting into the back chamber? I had replaced the water pump and thermostat already and hence knew it could not be them. I noticed it did not occur on idle - but once the revs went right up (when driving) - the coolant disappeared every time I then stopped It was actually a pain in the arse diagnosing ! It took me a while to track the problem down to the seal on my radiator cap being old - (it looked good but it was actually failing to seal) Once I replaced my cap with a new one - my problems were over instantly! That was my personal experience anyway........ Cheers JC
So if all you did is change the hose and now it overheats, why not look there? If you can drive around town all day without it overheating, then it starts overheating as soon as you get on the highway? Did you replace it with an oem hose or aftermarket item? Possible the hose is getting pushed back towards the engine at higher speeds due to airflow? Mounted any different to oem?
If all he did was replace a hose - then its possible that the hose clamp is tight enough to prevent water from flowing out but loose enough to let the air in when cooling down as opposed to drawing the water back from the coolant overflow bottle under vacuum It would have a similar effect to a faulty sealing cap A pressure test would show this up using air (with little fluid in the system at the time)- the only thing that won't show up under this test, is a faulty cap.
I've got a slight cough Doctor...and a bit of the sniffles. .....hmm...You've probably got Ebola. :rolleyes2:
Change the hose to the overflow bottle, make sure it's clamped well. Change the radiator cap. If neither of those things do it, a head gasket is the issue. I chased the same issue for a couple of months, turned out to be a head gasket.
If the spring is shot and it's not pressurising then the coolant will expand and dump unto the overflow to early and stuff.
First alarm bell for me is the hose blowing, Hoses don't just blow, There was a problem before this event! I'm going to be slightly rude & not read the entire thread - please no cry babies, I don't have the patience or spare time to read so many posts. Here is the list of things I would do in rough order: Check condition of coolant - you might have to recall what it was like before the hose blew, any signs of rust/mud etc points towards poor coolant system servicing. If the coolant system has been neglected the rad is very likely clogged up with mud/rust. Do NOT piss about getting it flushed etc, just throw it away and replace it but BEFORE you do that run some coolant system flush through the system to flush out the engine as much as possible. Otherwise the crap that is in the engine will clog the brand new radiator almost immediately. Before you replace the rad do a TK test, or a carbon monoxide test to rule out issues with the head gaskets, if it's all good there are still a number of other things to deal with. Fan clutch, just buy a new one OEM, if it hasn't been replaced it is likely shot & not doing it's job 100% DO NOT buy an after market one and DO NOT fit an electric fan kit, I will laugh at you and call you all sorts of names if you do & still have overheating issues. Clean all the dust, leaves, dead rats and small children out of the air conditioning condenser, this WILL need blowing out with compressed air & WILL have corrosion built up on it that reduces the air flow through this condenser - with obvious results for engine cooling. If either splash shield is missing under the front of the car replace them with OEM or after market - just don't waste money on designs that have vents that allow cooling air to exit under the car instead of going through the rad. Those designs are like gumboots with ventilation holes around your toes!!! When you have the rad off (to do the job right you WILL need the rad out no mater what the actual issue is) replace the thermostat and the bypass hoses, they are all cheap and easy to do, use an OEM thermostat NOT a stupid after market pile of junk that wont have the air bleeding nipple on it - I have seen several overheating Zs that were simply air locked due to the poor design of the thermostat & being impossible to bleed sufficiently. New rad cap, easy cheap, again OEM only. Remove and clean out the overflow bottle, it will be full of rust and mud especially if the rad or engine were. the mud blows out of the way to let water into the tank then blocks the path when the water wants to return. I make a tank that fixes that issue & bleeds any air from the system every temperature cycle, there is NO better alternative on the market. Bleeding, there are posts here telling you how to bleed the system, I don't need to look and see, the forum in general has this sussed & doesn't often give bad advice there. Just bleed it up and let it cool then bleed it again to be on the safe side, you can damage the eng in just five minutes drive if not bleed correctly. To shorten that all up as brief as possible: Test for CO in the coolant system, if good flush system, replace rad, fan clutch, thermostat, bypass hoses, rad cap, clean overflow tank and air con condenser, bleed twice to be safe. Should be sweet after all that
If the radiator cap seal is damaged the coolant will be expelled into the overflow tank. Damage to the seal, will also allow air to enter the cooling system as the engine cools, rendering the return system ineffective. https://www.youtube.com/watch?v=0JdDWWoX-70
But that assumes that the top seal (under the cap) is damaged and letting air in. I have rarely seen those gaskets damaged, Its usually the main pressure poppet that splits. E
Ahhh...but not if it's really buggered and only goes one way!! Anyway, what's the verdict here, have we found out why she's running hot?
Just run with no cap, let the system boil occasionally and top it up when needed. Avoid inclines or it may spill. By this point im guessing your head gaskets are leaking so no cap will stop hoses bursting and avoid further damage. Try to stick to high altitude driving also. Or just fix the problem...
