Thursday 21 July 2016

TMax Fever

This article is about hot weather starting on the Yamaha XP500. You can ignore it if you're here to read about the stupid and embarrassing things happen to me.

There is a much discussed phenomenon where the TMax will run poorly or not start in very hot weather, or will not start in direct strong sunlight after it has been parked for a few tens of minutes. The received wisdom is that the fuel pump doesn't work at high temperatures and starves the engine.

I have a different idea, and a little evidence to back it up.


TMax fever is simply a symptom of a weak battery. When the coolant gets over temperature the bike runs a large electric cooling fan. The current that the fan draws pulls the battery voltage down and the bike either refuses to run the fuel pump, or the fuel pump runs ineffectively. 

Supporting evidence

My Tmax has failed to start in hot weather on three occassions. The engine turns over but it never fires.

I know for sure that the battery was on the way out, but I was too cheap and lazy to buy a new one.

When the cooling fan cuts in it's loud, on the most recent occasion I could hear the fan labouring and its pitch changed with engine revs - it was getting more power when the engine was turning the alternator.

I'm unconvinced that the fuel pump itself can develop a temperature sensitive fault and recover when the temperature drops, but we know that there is an accurate thermostat in the electrical system - the one that turns the cooling fan on.

But if the battery is borderline-fucked how do you keep starting and riding the bike?

As soon as the bike is running, battery capacity is almost irrelevant, all the power comes from the alternator. TMax's are remarkably easy to start. They only have two cylinders and those have a low friction ceramic lining, there's a heavy contra-piston storing up energy like a flywheel, it's got semi dry-sump lubrication, and the ecu is brilliant. I bet it waits until the engine is up to speed and richens and retards like crazy to get the first spark to spin the engine.

I've been riding to work for months a bike with such a knackered battery that it can't keep the luggage locker light on for more than a couple of hours.

Further experiments

I've now got a TMAX that is susceptible to the fever, but that has a brand new high-quality battery. If the temperature ever gets up to 30 I'll ride it until its hot, park it in the sun, and then see if it starts half an hour later. I'll update this article with any results, but sadly I live in the ancestral home of grey tepid weather.

If anybody gets a TMAX turning over but not starting in really hot weather, please try to get some jump leads on it and let me know what happens (in the comments). You need a phillips #2 screwdriver, a 10mm spanner, and about 20 minutes.

Richard "Fever isn't such a new thing, fever started long ago" B

Updated summer 2017

TMAX Fever is a symptom of a faulty fuel pump. I have seen the ECU put 12v onto the pump when I turn the key, but the pump didn't run and the bike didn't start. Ordinarily when you turn the ignition on you can hear the fuel pump running under the saddle for 2 seconds. If it has overheated then it doesn't run and the bike won't start. You can buy a pump on amazon or ebay. getting the pump assembly out of the bike is time consuming. Prising apart the assembly and changing the motor is challenging.
look here and here for more information about my struggles with TMAX fever.


  1. Could it be vapor locking or did you discount that? Something seems to affect my car when it's been left hot for more than 5 and less than 30 min, making it reluctant to start, and I like to think it's that.

  2. At the time I wrote the article, No I hadn't really discounted it. Now I am absolutely sure that the electric motor in the fuel pump overheats.

  3. Hello
    So what have you now concluded? I have a 2009 TMax that gives me this type of grief periodically. What did you end up doing? How much did it cost and do you have any suggestions?

  4. Hi Gneed.

    Look at this article

    After that I bought a cheep endoscope. I'm now 100% sure that the only problem is the motor in the fuel pump assembly. They're easy to get on ebay or amazon. It's a complete pain to get the fuel pump assembly out of the bike, and very difficult to prise the assembly apart to fit the new motor.

    Depending on how much time and skill you have it's a choice between a £50 motor and a whole weekend working on the bike, or about £600 to a yamaha main dealer to do the whole job.

    Look at the pictures in this article to get an idea of how much work is involved.

  5. I had the Fever on my 2011 Tmax 500 after about 8k miles on hot days.
    I tried everything, nothing worked until I replaced the fuel pump.

    Same fuel pump and same problem on Majesty 400's. So that's the problem.
    You can buy a Yamaha FP for $300+ or buy a Chinese FP built for your scooter for about $25.00
    Labor is another cost. But THAT IS THE FIX!