Bongo Fury

I think I may have a problem with the head gasket on my 2.5 Diesel.My radiator appeared to be leaking along the top seam .I had lost about a litre of fluid.Hoping there was no serious damage done I replaced the radiator.Being new to Bongos I had to study the video on how to bleed the system on youtube.After 3 attempts mainly because the bottom hose remained cold I was reasonably satisfied with my job but not fully certain.After a short trip of 3 miles or so I noticed that the temp guage was slightly higher than the 11 0 clock norm ( about 3 mins to 12) opened the bonnet and coolant and steam was coming from the rad cap.Someone suggested I try a new radiator cap although the cap was new with the rad.Surprisingly although the temp is too high the leaking from the cap has stopped but there is high pressure in the hoses as they feel hard and the level in the expansion tank is about 2 ins higher.Is this the sign of trouble with a cracked head or gasket failure or am I lucky enough to have just an air lock?

I'd advise you not to drive it again until you work this out. I can just about read your post as everything being ok (ie you've changed the system, it's settling down and you're paying more close attention to it than before), but worth putting in some effort to bottom it out, imo.

The radiator cap should not be of a pressure release type. The header tank cap should be rated 1.1 bar. Can you confirm this?

Is your temperature gauge unmodified, or modified? An unmodified gauge should sit still for pretty much the entire range of operating temperatures as it has been dampened to do so, only moving to the right under a severe overheat. A modified gauge will move left and right as you're going up and down hills, etc... It can be done either by soldering a resistor to the back of the dash, or if you have a Mason alarm fitted. Can you confirm what state yours is in?

Have you got either a Low Coolant Alarm or a secondary digital temperature gauge installed? If not, and you plan on keeping the vehicle, order at least an LCA *now* to get it delivered and installed asap.

At operating temperatures, the hoses should feel nicely hard. As long as your header tank cap is rated at 1.1 bar and not siezed, you shouldn't be OVER pressured.

Regarding header tank level, "2 ins higher" sounds like a lot, but there's maybe not quite enough info in your post to go from. Initial fill level affects running pressure - 1/2 an inch more fluid in the header tank than "normal" to begin with can give you an extra PSI or so. After a bleed you can have the odd bit of air still in there and will get replaced by fluid as you drive. Therefore you can expect level to drop on the first run or two. Coolant will expand when hot so you can expect the "cold" level and the "hot" level to differ by an inch or (possibly) two. Air will also expand when hot and coolant will expand rapidly when turning to steam, so if your coolant level is reaching the header tank cap and you're blowing off coolant you have a problem. An OVER filled header tank to begin with could cause excess coolant to blow off, but you'd have to be silly to overfill that much.

In the very first instance, I would do a cold pressure test on the whole the system and then see if it will bleed up on the level. Get a wooden lolly stick, dip it vertically into the header tank and measure your level. Let it cool, dip and measure it again.

http://mazdabongo.wikispaces.com/Servic ... ant+System

Result from pressure test, bleed and level tests might give more to go on.

Alternatively, get it lifted to a garage or get someone knowledgeable and local to give you a hand.

The Radiator cap and the header tank cap should be rated 1.1 bar.

They are in fact the same type of pressure cap.

(my mistake, but you wouldn't want it blowing before the header tank cap...)

Driver+Passengers wrote:Is your temperature gauge unmodified, or modified? An unmodified gauge should sit still for pretty much the entire range of operating temperatures as it has been dampened to do so, only moving to the right under a severe overheat.

Relaying on ANYTHING a standard non-modified temp gauge tells you is a road to disaster. It only has two readings normal and "you cooked it 2 minutes ago".

has the RADIATOR got a little spout next to the cap, and has it got an unsprung----flat cap fitted, without a large spring on it. or is it marked up 0.9.

Yes the rad cap is marked .9.The rad cap that came with the new rad was likewise.For some reason the one on the new rad which I replaced vented steam and fluid when the engine got hot.The new cap seems sound no leaks.The temp gauge is I believe the standard unmodified version where normal temp is indicated by the needle indicating 11 O'clock.As I have explained, not clearly enough apparently, the gauge now indicates a couple of mins past 11 if we use the clock face analogy.I know these gauges are crap but I am in no doubt that the cooling system is getting hotter.The essence of my question which is in danger of being overlooked perhaps is whether I have a bleeding problem or a head gasket failure?The expansion tank is marked 1.1 and as stated the rad cap is .9 which I believe to be correct.My local garage tell me that a failing head gasket can produce excessive pressure in the cooling system although they are not familiar with Bongos and the strange bleeding ritual and cannot rule out an air lock problem.

you want no venting at the rad so get a 1.1 cap for it

if the rad has a vent as asked it needs blocking as well

Replace rad cap first. In the meantime, you could probably swap them over - running 1.1 on the rad and 0.9 at the header tank is not going to kill the system. You'll just limit the system to 0.9 bar which is ample, imo. You choose. Pressure raises the boiling point of the water/coolant mix, but my system typically runs <0.9 bar anyway.

I had excessive coolant pressure which I resolved by replacing the head gasket. The problem with mine was that it would not show up revving it on the drive but only under load. I was using an electronic pressure sensor and over time determined pressure increase correlated with heavy use of my right foot. So in that sense, it was a "marginal" head gasket failure. It drove >2000miles in this state. Even with a failing head gasket, you can run with increased temperature, exhaust can mix with the coolant and degas in the header tank and periodically vent out through the cap and you'll be none the wiser - failing head gasket doesn't always mean air lock (exhaust lock, really), and boil over, steaming coolant and overheat. I only had two "overheats" in all my time with a (margially) failing gasket. And none since - but it's the monitoring of the pressure, not the absence of boil overs, that gives me the confidence in my system.

A typical way to determine gasket failure is to use a gas analyser of colour-changing fluid at the header tank to detect combustion gas. But in my case, parked up in a garage, I don't think it would have shown up as the engine needed to be under load to lift the head sufficiently to pressurise the coolant with exhaust.

So getting a sniff test is one for later, perhaps...

Given that you've just change the rad and refilled and bled, it's possible that there's air in there. Hence you should bleed it up again. If you keep getting bubbles and keep getting bubbles, etc... then it may well be gasket. If it bleeds up ok, then fine. The reason to do the cold pressure test first is that "air/gas in the system" might be down to two or three things - 1) you left some in there when you bled, 2) the gasket is blowing exhaust gas, 3) you have a small leak somewhere in the system that sucks in air when the system cools. In the case of the 3), it generally doesn't manifest in an airlock and overheat until the system has heat cycled a few times, so as it's only been a few miles since the work you did, this may not be the cause. But worth cold pressure testing anyway to know from the outset that you have an air/water tight system.

Seeing whether it bleeds up again is next, sniff test on the cards after that. But a negative sniff test may not indicate absence of gasket failure, as bleeding and sniff test is done under no-load conditions.

Regarding temperature - having an unmodified gauge sit a couple of angular degrees further to the right is difficult to derive anything from. I know it's part of the info you've given us, but I'm tempted to ignore it myself. Doesn't mean you should, so do consider either modifying your gauge or fitting an after-market digital gauge somewhere. My understanding is that temperature will rise for one of a limited number of reasons - engine is actually under greater load, some air (perhaps not a full air lock) in the head reduces coolant contact around the exhaust ports, possibly coolant strength, or coolant flow is reduced due to air elsewhere, a full air lock, a failing pump, etc... A full overheat with boilover is generally down to poor/lost flow and coolant turning to steam in the head. In your case, the low rating on the rad ap may have played a part in coolant/steam appearing under the bonnet without actually boiling in the head. Whatever the cause/extent of your apparent temperature rise, my focus would be on determining any likely causes for "air/gas in coolant" first.

See what other folk think...

For a start swap the caps over....1.1 on the rad and 0.9 on the tank....you want the tank cap to release first.

However if you have the correct type of rad fitted....( NO OUTLET STUB ) then it really down,t matter what you fit on there co,s it cannot release pressure without a stub.

If it has a stub...BLOCK IT OFF

THERE ARE 2 seals in a cap....a sprung one which releases pressure via the stub....and a second one under the cap which seals off the filler hole....this cannot release pressure unless the rubber seal has failed.

Once you have bleed it thoroughly, if you are still getting excess pressure, then this does indicate issue with head gasket / cracked cylinder head or in very rare cases, a cracked block. Next step is to get a 'sniffer test' done to check for presence of exhaust gas in the coolant (plenty of info via search facility). Gas in the coolant does confirm there is a problem, but no gas does not mean you are in the clear.

In the majority of cases, head gasket failure then leads to a cracked head, but it is not a given & there are probably a number of heads replaced unnecessarily every year. The coolant becomes pressurised & then gets forced out of the system which allows the head to overheat & then it cracks. A leak in the system will allow air in which also start the same chain of events.

I have bled the system again and fitted a new 1.1 rad cap same as expansion tank.Decided to take a short trip of 6 miles or so to see if there is any improvement.There was one short whiff of steam from under the bonnet but stopping to investigate could not find the source.Hopefully this may have been from water spilt when topping up the tank before starting out.Anyway I kept the bonnet up and revved the engine to see if there was any leaks of any kind.The two rad fans kicked in but everything remained bone dry.On returning home I checked the level in the tank and it appeared to have risen by an inch or so though it is difficult to check the level exactly.I really need to make sure that I am not losing water but the lolly stick method is a bit uncertain.I am still of the opinion that the engine is running hotter than normal but I am a bit more optimistic regarding the head gasket though of course it may be reserving it's worst for a rainy night down some distant motorway.....

Sounds good so far... keep the heid!

Can you bear to part with cash for an LCA and digital temp gauge? LCA will probably save your head on the motorway if that's the way it goes for you , and temp gauge will let you start putting some temperature numbers against your driving conditions.

If you can get all under the bonnet nicely dried off, then wrap some kitchen roll or bog roll round the caps/overflow and rubber band in place. You'll soon see pink leakage if there is any.

Handy tip: extend the overflow pipe by about 18" so you can poke it out the top of the bonnet and pop the loose end under a wiper blade (if not raining). You will then see if the coolant overfloweth.

banjoboy1 wrote:Decided to take a short trip of 6 miles or so to see if there is any improvement.Anyway I kept the bonnet up and revved the engine to see if there was any leaks of any kind.The two rad fans kicked in but everything remained bone .....

6 miles from cold and fans on, I have alarm bells ringing.
Ask yourself if you can afford a new head or the alarms that others have suggested. IMHO you still have air trapped in your system that could kill your Bongo.