Have you ever heard the saying “buy cheap, buy twice”? Well…keep reminding myself that every time where quality really matters, but this time I really messed it up… 

After all those troubles with AJP PR7, I was thinking about something that would show me the temperature of the coolant as that would be a good indication if the engine is overheating or not… I couldn’t find a decent and “plug&play” solution for a while, till I was reminded about this little clever radiator cap with a temperature gauge! 

What a brilliant and simple idea I thought, and after looking for a while I couldn’t find a decent thing from a decent manufacturer. At the beginning I was aiming for a Tusk radiator cap with a temp gauge, however all of them were in America and price + postage to UK + tax would make it a very expensive option. So I closed my eyes and bought a cheap chinese knock off…

Few days later, I decided to have a first decent ride in a while, and as usual that involved at least an hour of riding on the motorway. While going at a steady speed of 65mhp, I had a glance at the coolant bottle, and what do I see? Coolant level right at the top, a few moments later had a look again – coolant level right at the bottom!? 

Started checking it every few seconds and nearly got a heart attack! Coolant in the expansion bottle would go up filling the tank maybe up to 80 or 90 % of its volume, stay there for a few seconds, then it would go down and stay at some 30 or 40 %, and then back up, and down, up and down…

What causes engine coolant level to rise and drop rapidly?

Well It is normal for engine coolant level to rise and drop with engine temperatures as when coolant heats up –  it expands, and all the excess volume of the coolant gets pushed out from all the coolant channels around the cylinder, radiator and hoses into the coolant expansion bottle – hence the name “coolant Expansion bottle”.  However it is not normal for the coolant level to go up and down rapidly like in my case!

One of the reasons – blown head gasket. Meaning that when fuel is ignited in the cylinder all the gases that are expanding there not only pushing the piston down, but also pushing sideways and possibly past the blown head gasket into the coolant channels. And that was my biggest fear, as I had just replaced the cylinder, piston and gasket!

Second – faulty radiator cap, but I refused to think it might be faulty, as I have just replaced it too!

What’s the purpose of the radiator cap?

To make it short and simple – a radiator cap is not just a cap, it plays a very important role in the cooling system. Radiator caps have a pressure rating on them (1.1 On AJP PR7) as by holding the pressure in the system, it raises the boiling temperature of the coolant. No steam bubbles in the system – a more efficient cooling process is, less chances of engine overheat. 

So what did I do after seeing that my coolant level started to go up and down?

Well… I stopped! I stopped because I was afraid ill overheat the engine and I’ll blow the piston again! I took the coolant bottle cap off (bottle, not radiator – radiator cap should never be touched while coolant is hot!), and looked down through the cap into the bottle where the hose from the radiator is connected, revved a few times, and I did see some bubbles coming up, but the amount of the bubbles didn’t change with rev range, so I switched off the engine and let it cool down. 

Now to my understanding, if that would be the case of the blow head gasket, there would be a change in the amount of bubbles coming up with a rev range, but there was no difference, so i assumed it was the case of the bad radiator cap – not holding the pressure and allowing coolant to form steam bubbles! Luckily, before the trip I packed my old radiator cap with me, you know – “Just in Case”, and after the coolant was down to 20C, I swapped them!

There was still a chance that there was something else out of order, so while waiting for the engine to cool down, I found the nearest motorcycle repair shop and set my sat nav ready, however after a few nerve wracking miles down the motorway, coolant level was back to normal, level not changing, so when I calmed down I hit the trails like I planned, 2 hours later though!

Explore more!

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• Important things to know about your AJP PR7 – Torque Vales / Bearing / Seal Sizes / Manuals and Stuff.
• How I stored All my Trail Tools and Spares on my AJP PR7.
• AJP PR7 Tool Sizes.
• Cheap ways to make your motorcycles clutch lighter! (Part 2/3).
• How to make a Hydraulic clutch lighter on your motorcycle! (Part 3/3).
• Is it possible to fit the New Athena ECU on the old AJP PR7?

All AJP PR7 MODS:

OR

AJP PR7 Faults, Fails and Fixes

• AJP PR7 Wheel Spacers.
• AJP PR7 Wiring Problem and how to fix it (random stalling problem).
• Not AJP’s Fault – Radiator cap that nearly gave me a heart attack!