Throttle body water pipes

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Throttle body water pipes

Postby George Mercer » Mon Nov 26, 2018 23:53

I have just bought my first Cappuccino. The radiator needed a recore so I have taken the opportunity to go through the cooling system. Two small pipes go from the thermostat housing to the throttle body. Should air pass freely from the inlet to outlet? When I blow air in one end I get nothing at the other? I know there's a valve in there but in the manual it doesn't make it clear if this blocks the flow until the water is hot. Any advice welcome?
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Re: Throttle body water pipes

Postby Ian Linden » Tue Nov 27, 2018 10:27

Hi George,

This is a new one on me. The manual is reticent about the function of the "engine cooling water pipes" to the throttle body.

I can think of two possible functions:

Vitara throttle bodies have a thermowax valve which closes when the coolant is hot, and is open when it is cold. The function is to supply extra air to a cold engine to supplement the IAC valve, as that valve lacks the ability to control over the full temperature range. It is well documented in the Vitara manual, so unlikely to be relevant to the Cappo. However, as you say there is a valve in there, maybe that's it? Clearly, it would need to flow water continually to work, so if this is what it is, it must be blocked.

The other is to prevent throttle tract icing in very cold, damp weather, by circulating hot water through passages inside the casing, in which case the valve may only open when very cold temperatures are experienced, because heating the throttle body when not required would reduce the volumetric efficiency of the engine (like dispensing with the intercooler).

My thoughts are that, if it is the first function, but the engine starts and idles well, don't worry about it - perhaps it was fitted but disabled during engine development (but why retain the pipes?). If it's the second, you could try blowing very cold air through the throttle body to see if it opens.

Anyone else any ideas?
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Re: Throttle body water pipes

Postby George Mercer » Tue Nov 27, 2018 17:52

Thanks Ian. Looking more carefully in the manual I think it's the former type of Vitara valve. I can only guess it's blocked. I'll definitely try the cold air on it.
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Re: Throttle body water pipes

Postby Murray Betts » Mon Dec 03, 2018 14:39

Must admit it's not something I'd really taken notice of before. I'd assumed it was a simple heated throttle body for icing, as Ian described. However the parts diagram clearly suggests a valve housing of some sort so probably either the auxiliary air theory or a shut off thermostatic valve for limiting the hot coolant circulation.
I'll be interested to learn what it turns out to be.
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Re: Throttle body water pipes

Postby George Mercer » Tue Jan 22, 2019 22:23

Hi Murray.

It's definitively a temperature operated valve. After I had replaced the radiator, I started the car. It idled around 1500 rpm and the pipes to the throttle body remained cold. After a few minutes of engine warming, the idle changed and it started hunting badly between 1000 and 1500 rpm.

With the intercooler pipe off the intake manifold, I put my finger over the hole in front of the throttle flap that feeds the idle bypass valve. The idle immediately settled down to a smooth 1000 rpm. I drained the radiator and found a piece of 4mm nylon pipe that shoved it into the throttle body water pipe and managed to get a lot of gunk out ( the manual tells you not to do this by the way ). I blew some air through it and it came out the exit pipe. I fitted the pipes and refilled the radiator.

After starting, the engine idled at 1500 rpm again. However this time, the throttle body pipes started to get warm. After several minutes the idle changed and it settled down to a smooth 1000 rpm. Very satisfying.

So, if your engine is hunting after a 10 minutes idling from cold, check that the throttle heating circuit isn't blocked.
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Re: Throttle body water pipes

Postby Ian Linden » Wed Jan 23, 2019 00:04

Well done, George! It seems safe to assume it is an extra air control valve, like the Vitara.
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Re: Throttle body water pipes

Postby Murray Betts » Thu Feb 07, 2019 13:24

Many thanks for letting us know, George, extremely useful to have this sort of knowledge/experience passed on.
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