Overheating problems

My TZR engine had been suffering from overheating for a while. After just a few minutes running in the practise field it would overheat and I would have to stop.

TZR radiator

I had started off with the original TZR radiator (shown left) mounted fairly low down on the engine frame, in front of the bottom of the fan. This meant that it was partly shielded by the (hot) engine and the air filter box. I have always used a 12 volt Davis Craig “EBP” water pump.

In my practise field, the long grass meant that I was getting poor lift, so I was running the engine at 10,000rpm but my forward speed was poor, and so the airflow through the radiator would rely mainly on the main fan which was about 30cm behind it. With this arrangement, within a few minutes of 9,000 - 10,000 rpm operation, the temperature gauge needle would move quickly up to just under the red (danger) zone, and the water in the engine would boil, with steam jetting out of the rad-cap overflow pipe.

The engine should have been running at around 75-85 C so why was it boiling over? Was there a blockage? I had already flushed it through several times, to remove swarf and the like, but occasionally I could hear some solids rattling trough the pump, so I flushed it through a few more times. On some occasions after I had filled it I would looki into the radiator filler hole and find that there wasn't much water movement - this turned out to be because of an air lock in the pump (due to the angle it was mounted at), so I remounted it and the air lock probelm was cured. I took it out to the field again, and again it boiled over :-(

I found an unplugged hole in the top of one of the carbs (that was previously used to vacuum-control the petrol flow). Was this causing the engine to run lean and hot ? I plugged the hole. Rather than go out to the practise field and find that it still boilded over I thought I would test it in the garage until I sorted out the problem, so from then on that is what I did. Testing involved running the hovercraft on crates in the garage for a few minutes at low revs to warm it up, and then  blatting it up to 9,000 rpm for a few more minutes. An engine should never be run without some sort of load, so I had the fan coupled on as usual. The neighbours put up with one hell of a racket at this point, and I had to stand outside the garage (to avoid the fumes) holding the lanyard, so tha if anything went wrong I could switch off the engine. Every now and then I would nip in to check the engine temperature. Running the engine inside, with a fan, at near full power was quite scarey I can tell you! It still boiled over though.

I  inspected the spark plugs - they looked fine, just like the pictue in the Haynes manual - but it still boiled over.
I changed the TZR rad for another TZR rad (but kept the same cap as I only had one) - but it still boiled over.
I unbolted the thermostat housing in the top of the cylinder head -  there was no thermostat – it was free flowing and it still boiled over.
I flushed the system through with a (aluminium compatible) chemical flush - but it still boiled over.

Lots of people on the bulletin board provided possible solutions. Some were really bad news - like a cracked engine. I looked at the flow rate from the pump - no problems there.

Honda radiator

I bought a bigger rad from a 500cc Honda (costing £50 !) and fitted that directly  in front of the fan hub, rather than behnd the engine. This rad didn't have a filler hole or cap (just a 20mm inlet pipe and a 20mm outlet pipe) so I attached a 1 ft length of vertical coolant pipe via a T-piece so it would run at atmospheric pressure - but it still boiled over.

Strangely, the guys at the bike breakers didn't seem to understand the real problem either.

( <-- This pic shows the pressure cap I fitted later)

Some people said the cooling system needed to be pressurised so that it would boil at a higher temperature. Yes, some vehicles use this arrangement so that the engine will run at say 110 C without boiling over, but the TZR was supposed to run at 75-85, which is well below the boiling point of water, so why would it need to be pressurised ?

non-contact thermometer

It seemed that I had a lack of information about what was really happening, so I bought a non-contact IR thermometer and measured the temperature of the outside of the barrels, the top of the engine and the outside of the rad at various points. The readings were all around 75-85 C. But I noticed that now the temperature gauge was reading bang in the middle of the normal zone even though water was boiling out of the top of the vent pipe...

 Curiouser and curiouser said Alice…

I went off to Black Ditch with an overheating engine, and a set of gaskets, expecting that I might have to strip the engine. On the Saturday morning there I spoke to Paul FitzPatrick and Tony Broad about the problem. They agreed that it had to be pressurised but why was that exactly?

The answer which I teased out of them is this:

1) Athough the average water temperature in the engine/rad is around 80 C ( as measured by the thermometer) there are a few small hot spots in the engine where the water does boil (probably around the exhaust ports). This boiling water is easily diluted by the bulk of the cooler circulating water and so the water temperature is generally at normal levels. However this boiling action pushes water out of the top of the vertical coolant pipe, making it appear that all the coolant is boiling – even though the ejected water would have been at around 80C.

2) The new 500cc rad does cool the engine sufficiently (which is why the temperature gauge needle stays in the normal position) but it needs to have a pressure cap on itt prevent this localised boiling.

3) The original TZR rad was indeed in a poor location and the engine was simply getting too hot - hence the temp gauge needle going towards the red, and the steam being emitted from the rad cap overflow.

So, pressurising the system doesn't raise the temperature of the coolant, and it doesn’t prevent the whole system from boiling. But it does stop the very localised boiling which causes problems when it blows coolant out of the vent.

Fortunately at Black Ditch, Tony loaned me his pressure cap and housing, which allowed me to run two Novice races with no overheating problems.


pressure cap and housing

I have since bought a housing which takes my original pressure cap, and can be connected to the rest of the cooling system via the T-piece.



Home   Dec2002 Jan2003   Feb Mar   April May   JuneJuly   Aug Sept   Oct Nov   Dec2003 Jan2004   Feb Mar   April