Hijet Overheating problems? Head gasket or not?

Overheating Hijets are very common, so I thought I would put together a little article that explains many of the symptoms and causes, along with some of the solutions and components that are involved with the system.

This is by no means the exact answer to your own vans problems, every van will be a little different after all, but many of them have a combination of actual faults which cause the symptom you are suffering from being overheating.

This guide covers:-

  • Overall System Overview
  • Radiator Caps
  • Pressure Testing
  • Thermostat issues and fitting
  • Radiator Faults and Design
  • Head Gasket Issues and Replacement


Hijet Coolant System Overview

First the the System overview, as taken from the Daihatsu parts catalogue, we have some coolant system diagrams:-

This is the system from the 1.3 Hijet

Parts of the system which cause overheating

This is the system of the 993cc hijet, which I do not own and cannot offer much advice with:-

Looking for the cause of overheating

Now as you can see the two systems are different although they share some common features. One component which is missing from these diagrams is the heater matrix buried within the dash board. It looks like this:-

head gasket hijet problems daihatsuThe blue arrow shows the air bleed pipe which comes down under the grill at the front of the van. On the 1.3 it is plumbed into the radiator cap neck on the top of the radiator under the grill, while on the 993cc 1 litre model there is a bleed valve for it.

Because the heater matrix is the highest point on the system it is prone to air locking causing the heaters to only blow cold air.

If you suffer from this problem it means you have a coolant system fault, possibly head gasket issues but you need to carry out some tests to help diagnose the problem.



Hijet Radiator Caps

So now I’ll talk a little about radiator caps as we have the diagrams just above which have some circles drawn around the radiator caps for both the 1.3 and 1.0 engine versions. Radiator caps have been highly debated on the forum due to the abnormal cap found on the 1.0 radiator as fitted when manufactured. The common radiator cap is a pressure release valve rated at 0.9 bar on a hijet, with a return valve in the centre to allow for the contraction of coolant so that fluid can be drawn back from the header/overflow tank under the passenger seat. They are a common failure point, sometimes due to being taken on and off when refilling coolant due to another fault, sometimes due to corrosion. Whatever the cause they need to function correctly. I feel that the 1.0 litre can have 2 x standard radiator caps fitted, just like the 1.3 version does. But I’ll let you examine this part number sheet from daihatsu so you can make up your own mind! Green = 1.3 Pink = 1.0 as circled on the parts diagrams above.

Daihatsu Hijet head gasket overheating

I have collected a little hijet radiator cap grave yard along my journey so far. They fail due to perished rubbers, blocked up centre return valve, weak springs, gummed up from rad weld or scum, distorted tin cap lids and a combination of all these faults. See if yours looks like any of these and bin it. They are cheap to replace at around £5 each.

0.9 bar failure overheating

Now there has been a fair amount of hunting to find a decent replacement radiator cap, avoid buying cheap unknown rubbish off ebay as many of the caps are not fit for purpose. The cap many forum members have been having success with is the Firstline FRC74 available from most motorfactors, I buy mine from Partco which is a national chain of motorfactors. They are about £5 each but prices change along with hair styles and underwear.

Daihatsu head gasket over heating

But the problem may not just lie with the cap, but the seats the caps fit to themselves, the pipe underneath the passenger seat which accepts the pressure cap and over flow bottle is prone to corrosion, especially if the coolant system has been filled with just water, or the coolant hasn’t been changed. The coolant acts as many different functions, it acts as an anti corrosion inhibitor, an anti foaming agent, it is a lubricant for the water pump, it stops the coolant from freezing in winter and it raises the boiling point. If it has been neglected and not changed you will soon have water pump issues and corrosion. Anyway back to the radiator cap seat – one of my Hijets had a really nasty case of coolant system corrosion along with over heating, the cap seat looked like this, the arrows point to the important seal points which must be clean and defect free:-Radiator cap seal corrosion

You may be able to clean these up to make the system work again but sooner or later the whole pipe will need changing. I will now go onto pressure testing, where I will show you some pictures of a leaking radiator cap while pressure testing the system.


Pressure Testing a Hijet Coolant System

The best way I have found to detect any external leaks to the hijet coolant system is by performing a pressure test. Most garages will tell you that you need an expensive testing kit and magic beans and snake oil but really you have pretty much everything you need at your finger tips for free to perform a superior pressure test on a Hijet. Firstly you need to find an old tyre valve, one from a bicycle or an old wheel will be perfect. Cut away some of the rubber so that you can insert it into a length of rubber pipe like this:-

Pressure testing hijet

Then attach it to the bleed nipple of the hijet, but keep in mind on an LPG hijet there is no bleed nipple so you will have to add in another form of T piece, one from a standard Hijet would be perfect if you have one at hand, otherwise I’m sure you’ll find something in the local plumbers merchants. This next picture shows my test valve in place, attached to an extra T piece as its on an LPG hijet, with a tyre inflater pump attached ready.

Leak testing hijet head gasket overheating

Next you will need to bung up the overflow pipe in the header tank otherwise you will lose all your pressure at the caps blow off point (hopefully 0.9 bar if it doesn’t leak before hand), for this I have used a brake hose clamp, but anything like an bit of metal bar or some other clamp would do just fine.

hijet overflow bung test

So now you are ready for the pressure test! I perform the test with all the coolant drained out in the belief that air leaks easier than water, but it is obviously more dangerous to charge it up with air like this as if a pipe rips open you are more likely to be harmed from the breach! Maybe wear safely goggles or something, either way you perform this test at your own risk like I did. This is not a tutorial!!

Anyway I found a few different leaks, by spraying water with washing up liquid in it from some kind of squirty bottle all over the system components your will see bubbles forming where you have air leaks. I test my systems at around 25psi – make your own choice. The water should never get above 0.9 bar being about 13psi, but the system will be hot too allowing the pipes to stretch more that the cold air pressure test. Its not perfect but it will help find most of the leaks in my opinion! If you have any better ideas I’d love to know.

Here are some pictures of my leaks (insert picture of guy wetting himself):-

LEaking radiator cap daihatsu hijet overheating

LEaking pipe coolant system daihatsu

So by curing the leaks you should be a few steps in the right direction, right? Well your not making it any worse that’s for sure! Another common fault with leaking coolant systems is the bottom radiator hose clamps. They rot really badly due to the weather exposure then crack. This is a picture of a cracked one, look under your front bumper and you will spot them. If in doubt just change any suspected clamps with nice new jubilee clips, they are not expensive on the grand scale of car ownership.


Daihatsu Hijet Thermostat Issues and Failure

The thermostat on an engine maintains running temperature, it accelerates the warm up time by closing the water flow, while opening wider the hotter it gets allowing the water to be cooled though the radiator. A common fault with the Hijet is the fan doesn’t kick in when the engine gets too hot. This is probably caused by the thermostat jamming close like it did on one of my newer vans. You can normally detect this because the temp gauge will be very high, but if you get out of the car and feel the radiator hoses under the bumper they will cool along with the radiator. The blower fans inside the cab will still have hot air though because the heater matrix’s water feed is taken from a different point by passing the thermostat. If you end up with this fault, in order to help get you home I would suggest putting your fans of full speed with the temperature on hot, this might help cool the engine a little bit maybe, hopefully?!

Anyway on a different van I have found this quite odd bit of something stuck in the thermostat housing which can’t be good at all. While we are talking about the thermostat housing, it is made of plastic and can easily crack, the one on my picked had cracked causing a pretty major water leak.

blocked thermostat housing diahatsu hijet

The thermostat is a pretty easy item to change and it will only cost you around £12. You can change it without removing very much from the engine, but you will need to remove both the seats, the centre support bar and a few hoses etc. Its about a 1 hour job, but you’ll have to then refill the coolant system afterwards too. When refitting a new thermostat you need to ensure the wiggle pin is at the highest possible point otherwise you will not be able to get the air out from behind it when rebleeding the system.

Daihatsu hijet thermostat overheating head gasket


Radiator Faults and Design

The Radiator on a Hijet is a funny little thing, quite different from pretty much every other radiator on any other vehicle I have ever seen. It has two bottom hoses and no top hoses, and as such water flows in from the bottom on one side, up the core, over the top cap and back down the other side and out of the other bottom hose. Maybe words don’t explain this best, so yet again have another HighlyJetted scribbled on picture!

Hijet radiator flow diagram diahatsu

As you can see the fan is directly attached to the radiator and can be unbolted via the 3 x 10mm bolts, but anyway, this is a strange radiator setup, and not for the better! The problem with this design of radiator is the water has far less veins to travel though so clogging and blockages become more of an issue than say a normal radiator of the same size. I can’t fully remember but I counted only something like 18 veins that the water gets to push through, I realise there are more but the water has to get through them, over the top cap then down the other side of 18.

The radiator is prone to a couple of different faults, firstly like all radiators impacts from stones collisions and other impacts can damage the structure one way or another – this can be damage to a vein, a crack in the top or bottom caps, or just leaking from where the caps are pressed on. This is a picture of a hijet radiator which is leaking from the bottom cap:-

leaking hijet radiator new rad replacement recore

The next common fault which I’ve mentioned previously and already shown you a picture of is the bottom hose clips cracking and failing, but as you’ve already read everything above (!) you’ll already know that – the same goes for the radiator cap on the radiator, they fail and start leaking. The next thing to mention regarding radiators is the fan itself.

The radiator fan is controlled entirely by the fan thermostat switch which is in the metal coolant pipe under the passenger side foot well. It is VERY common for these things to get messed with and a manual over ride switch added inside the cab somewhere. If you have bought a hijet with  one of these bodged in switches it means you’ve just bought a van that’s been overheating! Welcome to the club! Don’t worry though your not alone.

The reason some people put a switch on the fan is that they come to the conclusion that the fan didn’t kick in while the van was overheating, they blame the thermostat switch obviously. This switch can be easily replaced, there is a guy on ebay selling new ones for about £15 so there is no need for a switch but anyway…. 99% of the time the switch is working perfectly, what happens is the switch is sitting in cold water because either the thermostat is jammed closed, the system is all clogged up stopping water flow, or the head gasket has gone causing air locking thus stopping the water from getting round the system. Do some investigations!

If you want to put your fan switch back to normal, here is another one of my trusty diagrams taken from the wiring loom but simplified so you can see just what you need:-

Fan switch wiring manual over ride hijet daihatsu overheating


Head Gasket Issues and Replacement

Big subject this is really! Probably deserves a page all of its own but i’ll skim over the surface for the purpose of this Daihatsu Hijet Overheating guide.

What happens is, for one reason or another the gasket fails around the cylinder liner seals, this allows combustion gases to get pushed into the coolant, and likewise, water pulled into the cylinders. As the engine gets hotter, normally due to it overheating, the head gasket leak will get worse to the point where it starts to seriously push water into the header tank. Keep in mind that engines sort of need coolant, that’s what carries the heat away from the cylinder head, the next thing that happens is the head gets even hotter, causing it to warp thus making the head seal worse. So you get the chicken and the egg situation? So maybe you started out with a leaking radiator cap, or a broken hose clip – this causes a load of water loss, which causes the engine to get too hot which causes the cylinder head gasket to fail.

So to prevent yourself from going round in a circle like the old chick and egg saga, make sure that any minor or major leaks are cured before replacing the cylinder head gasket otherwise you will be doing the headgasket again 6 months down the line – or selling that damn van! (to some other poor mug)

Some symptoms of having cylinder head gasket failure are the purging of coolant into the header tank, foaming of the coolant like this:-

Foaming coolant water radiator cap hijet overheating

Along with a load of other headaches like the thing overheats every other time you drive it, water seams to vanish from the system, you can never quite bleed all the air out of it even if you try everything under the sun – there are ways to test for a head gasket failure, sometimes it works, sometimes it doesn’t though. You can buy or use a block test kit which contains a chemical that reacts with the combustion gases soaked into the coolant, it goes from bright blue to yellow if the test is positive for combustion gases thus head gasket etc failure. I have found this test to not work on my red van pictured above, along with a couple of other members on the forum who has similar failure of results even though they had a failed head gasket.

This is a head gasket which has failed belong to a fellow forum member (you should really join the forum if your messing with your van as they will help keep your spirit high and aid you in your journey!), as you can see the gasket has tarnishing on the rings showing where the combustion has escaped, and it has actually caused the other part of the gasket to disintegrate:-

Hijet head gasket failure overheating

So to finish of this little textual adventure of overheating hijets, I will leave you with a few other random diagrams and facts which you’ll need when doing you head gasket, as much as I’d love (!) to keep writing I’m sure your sick of my ramblings. But anyway, good luck, you can fix it, be careful who you trust to fix your van as mechanics and garages are a funny bunch like builders, lawyers and insurance companies.

Cambelt timing marks hijet daihatsu 1300 1.3

Head bolt torque tightening tension hijet daihatsu hijet 1300

Need Hijet Parts?

Join the club!