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Discussion Starter #1 (Edited)
Guide to thermostat and ECU sensor modification for the 97 TLS

This has been compiled from various threads in an attempt to produce an easy to follow guide to the best mod you can do to the TL from stock (in my opinion). Thanks go out to all those planeteers who have posted on this subject - you know who you are. Special mention goes to the thread from UncleMeaty, and I musn't forget Octofinger.

Reason for modification:

The stock 97 TLS has a poor cooling system design for several reasons.

  • The ECU temperature sensor is mounted on the radiator. This causes the ECU to monitor temperature of coolant on the radiator circuit side of the thermostat. Therefore this means that the correct engine temperature is never delivered to the ECU, and as the ECU thinks the engine is running below 80 degrees Celcius, the cold map (choke) is continously applied. This aggravates the TL propensity for eating its own spark plugs, and will also cause poor running at small throttle openings, surging and poor fuel consumption.
  • The stock thermostat is on the cold side, and also has a bleed hole to avoid hotspots. Unfortunately, when taken with number 1 this only compounds the problem with incorrect temperature monitoring.
  • 1 & 2 above can also cause oil levels to rise indicitive of the oil becoming contaminated with gasoline.

The Solution:

Suzuki rolled out a revised cooling system from the 98 (SW) model onwards to alleviate these problems. The aim of this mod is to match the changes Suzuki made in either a cheaper fashion, or as a complete replacement of all affected parts. This obviously depends on the depths of your pockets.

Solution 1
  • Replace the thermostat housing with the revised version, keep the existing thermostat and block off the bypass from the new thermostat housing. This is not the preferred solution as the system will still continue to suffer continuous bleeding into the cold part of the radiator circuit thus delaying warm up. The stock thermostat is too cold anyway.

Solution 2
  • Replace the thermostat housing with the revised version, replace the existing thermostat with a hotter one, block off the bypass on the new thermostat housing and drill a 2mm hole for bypass in the new thermostat. This again is not the preferred solution for the reasons stated above. However, you should get a big enough improvement to stop using the cold map all the time.

Solution 3
  • Replace the thermostat housing with the revised version, replace the
    existing thermostat with a hotter one, cut into the radiator return hose (lower hose from water pump), insert a T-Piece to connect to the revised thermostat housing. This again is not the preferred solution as it does not match Suzuki's design. However, due to cost, this is the route that I followed.

Solution 4
  • Replace the thermostat housing with the revised version, use the later model thermostat (or a hotter one), and replace the water pump housing with the later model version. This is the preferred solution as it matches Suzuki's intensions but is the most expensive route.
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2,630 Posts
Discussion Starter #2
My Mod
As noted above, I carried out solution 3 above, and here is the list of parts required:
17662-02F11 - Thermostat Housing
17670-50F00 - 1998 TLs (W) Thermostat (or alternatives, see below)
17663-42E00 - Thermostat Housing Cap (Optional)

1m length of 8mm Internal Bore coolant hose (cut to correct length when fitting)
22mm x 22mm x 8mm T-Piece (Off the shelf, or home made from plumbing fittings)

Full OEM Style Mod
To complete the full suzuki style modification (solution 4 above), the following parts will be required in addition to those listed above:
17854-24F00 - By-pass Hose
17410-02F30 - Water Pump Housing
17435-02F00 - Water Pump Housing O-ring
09401-12409 - Hose Clamps

NB: You will not need the T-Piece or coolant hose from my mod as you will be using the correct Suzuki bypass hose and new Water pump housing.

Alternative Thermostats
UK Market - Quinton Hazell QTH266 (MTO266). Rated 89 Degrees Celcuis. Required modification to thermostat and housing and is available from most motor factors (probably even Halfords!)
US Market - STANT 29209. Rated at 90 Degrees Celcuis. Will require a slight size reduction and is available from Pepboys
International Market - Unknown, but there are rumours that a stat for a Ford Trident van may fit, as well as the 3.8 L Holden Commodore

According to moosesp, the complete unit from an SV1000 fits successfully, so its worth checking for those part numbers. I will try and find out what they are.

2,630 Posts
Discussion Starter #3
Modifying the QH thermostat
Here is the QTH266 thermostat as supplied

To make this fit, the large flat disc on the end must be removed.
To remove this, I first of all cut the majority off with a hacksaw then finished with my linisher (belt sander).
You must be very careful not to grip the body of the stat in a vise as it will distort.

Next, 0.5mm of material must be removed from the outer ring of the thermostat.
Again, to do this, I used the linisher, and rotated the thermostat against the belt to evenly remove the excess.
Once all the metal work has been done, use high pressure air to blow all the swarf away.
After this, the rubber washer fitted beautifully.
Now is a good time to boil some water to make sure the thermostat is working correctly and that you haven't broken it!

Modifying the Thermostat housing.
To make the QTH266 thermostat fit, some easing of the housing was required. This may not be required with other thermostats.
First of all the new thermostat housing with temp sensor fitted

I had to remove some of the internal material with a dremel to allow the new thermostat to fit in correctly.

Again, once finished, use high pressure air to blow out the swarf before fitting the thermostat.

Here is the housing with the bypass line fitted, ready to install

Preparing a T-Piece.
If you can get a T-Piece made or off the shelf to connect the radiator coolant pipe to the bypass line, you are ready to go.
Unfortunately, I was unable to source anything, so I had to make one up

Here is my T-piece, made from a 22mmx22mmx15mm solder ring fitting coupled with a 15mm - 8mm compression fitting and some 8mm pipe.
Note that I have inserted 22mm copper tube into the fitting and soldered in place to avoid crushing by the pipe clips!

Its now time to fit the thermostat housing back into the bike

To fit the bypass line, cut the radiator bottom hose (right hand side of the bike), and insert the T-piece. Allow enough distance from the water pump housing for free movement of the coolant hose.
Insert the T-Piece, lock all your hose clips down

Finally, refill the cooling system and make sure no air is trapped, and there are no leaks.

Go for a ride and enjoy your new bike.

For reference, my bike now holds a steady 91 degrees C in traffic with 30 degrees C air temperature.
On the open road (60-70 mph) it holds 89 degrees.
It also holds 89 degrees up to a speed that I won't mention!

Thanks to UncleMeaty for US Market thermostat details, Octofinger for details on making a T-Piece, and Fish for the rest of the full OEM mod part numbers.
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