GPZ500 overheating

[Nipedley]

Hey guys,

My friend's GPZ500 runs extremely hot, it will overheat and start to boil over in minutes at anything under 50 miles an hour. Over that speed, the air flow is enough to keep it cool.

The bike was a long(ish) term fixer-upper and when I first took charge of it I flushed the coolant, chucked some radiator flush in (but probably not run for long enough) and changed it for new (the PO had been using horse trough water to top up the coolant, so that was the first thing I wanted to do) and it does seem to run cooler than it did then, but evidently not enough

The fan is broken, I suppose really I am wondering whether this engine is designed to run so hot that it absolutely needs the fan, or whether there is perhaps else something wrong here and the fan shouldn't be a requirement on a 22c British summer day. Things I can think of to do: check that the water pump isn't fucked, use some cooling system flush properly and re-flush it with new coolant afterwards. Any other ideas?

Many thanks for reading and any replies

[Kickstart]

Hi

I would try flushing it through again. When it gets hot is the radiator hot all over?

Neither of the GPZ500s we have owned have had problems keeping cool - certainly not when riding along at low speed.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Nipedley]

Hi Keith,

Thanks for the info, I have a plan I am going to effect over the next couple of weeks, if anyone has any other suggestions please do:

Bought a low usage radiator & fan from eBay (the fan is broken on his and to get a low mileage radiator with it was only an extra £20, so no brainer) which I will fit - if needed blow compressed air through the rear of the fins to make sure they are all clear
Test the thermostat
Check out the water pump to make sure that's working okay
Fit a new radiator pressure cap and while I'm there check out the housing
Put in some coolant flush and run it properly then flush it through and put in some fresh coolant

If it's still overheating after that I guess I can suspect head gasket? It's weird that it's fine and not spewing coolant over 50mph though, you'd think the extra load would pressurise the water even more?

Any other suggestions greatly appreciated

[Kickstart]

Hi

A blown head gasket is certainly a possibility, but I would hope it isn't. Is there are sing of water in the oil or oil in the water?

Checking the water pump is well worthwhile. If you can check where the radiator is getting hot it might help narrow down whether it is pumping coolant around or not.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Nipedley]

Fingers crossed it'll be either a very clogged rotted radiator (given the use of horse trough water to top up the expansion bottle, this is entirely possible) or a broken water pump, and not the head gasket. The new radiator and fan is on the way so I'll get started on the to-do list

The water and oil both looked fine when they were changed as part of the service, so that is a good sign

Thanks for your help Keith

[Kickstart]

Hi

No problem, although I think all I have done is enable you to confirm what you already know .

Probably also worth checking for any minor hidden water leaks. Such as if the O rings for the water pipes from the head are damaged.

I do have a GPZ500 engine floating around if you get stuck (from a badly crashed bike and never heard it run).

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Nipedley]

Hi guys,

The bike is back on the road, new radiator & fan fitted, new thermostat assembly (a bolt broke off in the old one and I found a newer tested whole assembly for £12, so no brainer)

Water pump all checked out, the pipe from the water pump to the engine block was rusted in solid so I had to hacksaw it out and replace it, but that was all fine inside apart from being quite cruddy

The bike still runs 'hot', on a slow road (30mph) getting up to about 3/4 on the temp gauge and the fan kicks in and keeps it there, but it doesn't get too hot now so no more overheating. On anything over 40 mph the temp goes down to half due to the airflow

There's definitely no head gasket issue, at least I believe so, as there is no coolant loss now due to not overheating, and still no oil/water mixing

I'm wondering if that's normal for a GPZ500s for it to be running hot, the only other thing I can think to do is run another coolant flush through it (I did do so but after starting it the bike pulled crud through from the tank so I had to abandon it, clean the carbs again and fit a fuel filter, so it only ran for about 2 minutes with the coolant flush in - about quarter on the temp gauge)

There's nothing else I can think of that would cause it to still be running reasonably hot except some crud build up in the block perhaps that another flush might sort ? Thermostat/Water pump/Radiator & fan all being fine, and head gasket apparently fine too

The bike is usable and rideable anyway but it's just in the back of my mind

Thanks guys! Sorry for the long rambly post

[groovylee]

if it was running lean would that cause it to run hot? just thinking out loud
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Past - 1988 honda revere 600, 1996 kawasaki gpz500s, 1999 Yamaha R6

Current - 2002 Aprilia RSV1000R & 1999 Kawasaki ZX6R

[RhynoCZ]

[Carvel]

I think mine used to run quite hot.

It also used to run hotter when I had the infill panels fitted.

Another thing to consider.

Like that

https://i979.photobucket.com/albums/ae271/RxsOllie/gpz5_zps011fc6b9.jpg~original
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2002 Aprilia RSV1000R, Cbr600Fs, Mk1 B12, Vfr400 NC30...

Matt B: I can deal with crushed nuts to beat Ollie.

[Carvel]

The rads are also completely unprotected on these and get caked in shit. Consider a solution if you use it in shitty weather.
____________________
2002 Aprilia RSV1000R, Cbr600Fs, Mk1 B12, Vfr400 NC30...

Matt B: I can deal with crushed nuts to beat Ollie.

[RhynoCZ]

[evoboy]

Mine also ran hot.

Headgasket was fine. Tried everything to sort the problem. Always ran hot.
____________________
Suzuki GT250 x7------- Fazer 600------CB250RS------Aprilia Rally 70----- Bandit 600

APT Motorcycles

[Nipedley]

Thanks guys, this one doesn't have infill panels (or a bellypan), so I can't try that unfortunately

Given that it isn't getting *too* hot anymore, I think I'll just not worry about it, she's back on the road and that's what matters

Thanks for all your suggestions and comments

[dn38416]

apols for resurrecting this, but i have a similar issue.

Is the thermostat for switching the fan separate from the radiator thermostat?

My fan kicks in pretty much 5 mins into riding, and like the OP i have to keep it over 50mph for the airflow to keep the temp gauge at about half way. (Coolant level looks fine and although they haven't been changed in about 6k miles, this problem has just started from it running fine so i would have thought build up of coolant crud would be gradual.)

Someone suggested my thermostat is gone (not responding to the high engine temp), but that made me wonder why the fan kicks in earlier than usual - so they must be controlled separately?

Cheers

[RhynoCZ]

Yes, there is a temperature sensor in the radiator, that turns the fan on, when certain temepareture is exceeded. Then there is a thermostat, that opens the 2nd (bigger) cooling circuit once the coolant reaches certain temperature.

I would definitely be checking the thermostat. It's a very cheap part, not repairable (at most of the cases), easy to change. Your engine would must run very, very lean to produce this much heat. You can check the colour of your spark plugs just in case, but I think you have got a stuck thermostat.

SIDE NOTE:
Are you sure there's no trapped air in the cooling system?
Is the coolant pump working?
Clogged radiator?
-------
If all works, then it's just a feature of the motorcycle you own. Some run hot. If your coolant never boiled, then it's alright, I guess.
____________________
'87 Honda XBR 500, '96 Kawasaki ZX7R P1, '90 Honda CB-1, '88 Kawasaki GPz550, MZ 150 ETZ
'95 Mercedes-Benz w202 C200 CGI, '98 Mercedes-Benz w210 E200 Kompressor

[dn38416]

OK that makes sense why the fan goes on early. I'll give the whole system a flush in the process of cleaning the carbs and then change the thermostat.

Cheers mate

[RhynoCZ]

The temp sensor is a plug screwed into the radiator.

This kind of thing:
https://g02.a.alicdn.com/kf/HTB1DO2uJFXXXXcdXpXXq6xXFXXXU/Motorcycle-parts-Water-font-b-Temperature-b-font-Sensor-For-Honda-CB-400-CB-1-CBR.jpg
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'87 Honda XBR 500, '96 Kawasaki ZX7R P1, '90 Honda CB-1, '88 Kawasaki GPz550, MZ 150 ETZ
'95 Mercedes-Benz w202 C200 CGI, '98 Mercedes-Benz w210 E200 Kompressor

[stinkwheel]

How do you know the coolant levels are fine? You looking at the expansion bottle? Check the radiator itself. The expansion bottle can be brim full with the radiator empty.

Fan on but temp gauge in cool is a sign of an empty radiator. Temperature sensor is at the top of the radiator in the thermostat housing. Fan switch is in the middle of the radiator.

Oddly, those 500 twin engines seem to work respectably well as an aircooled engine as long as they aren't thrashed and are carrying a reasonable road speed but they are bound to be having problems.

My suspicion would be head gasket. When the head gasket fails on these engines, you get a small jet of high pressure exhaust gas between the cylinder and the water jacket. The usual water in oil or oil in coolant doesn't happen.

What happens is once the revs hit a certain point, the gas pressurises the coolant, pops the radiator cap and pumps all the coolant into the expansion bottle which overflows onto the road. THEN the bike overheats. You check the coolant and the bottle is full.

When it's happened to me before, it only happened at higher revs (say over 7k).

Diagnostically, fill the radiator and fill the expansion bottle to the lower mark. Go for a short ride and keep an eye on the expansion bottle level. If the head gasket is gone, it'll start filling up pretty quickly as the engine is revved higher. This happens before the fan comes on or the temperature sensor comes up.

Sometimes with a head gasket failure, you can feel the rubber hoses getting firmer while revving the bike up on the stand.

Incidentally, the temperature gauge on the second gen. GPZ500 overreads. The fan only comes on as it hits the red area. I fitted a first gen clockset to a second gen bike and the fan kicked in 2/3 of the way up.
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[dn38416]

Thanks Stink, yes i was just looking at the expansion bottle, the level is just below the full line and doesn't budge. The fan goes on when the temp gauge is about half way up i think. Got the thermostat on the way so will try and switch that over then do the diagnostics.


Cheers

[TheManWithThe...]

To really know how much coolant it has, you need to lift the tank off and check the level in the top of the coolant system.
You should be able to see coolant when you pull the cap off.

If not, fill it up, run the bike for a bit to get it flowing, then check it again. Repeat if necessary.
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MBK Nitro 50, Sold. Kawasaki ER-5, Honda ST1100
"Anyone can jump a motorcycle! The trouble begins when you try to land it..."