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Re: SVO: Rich running, will it toast the guides or what?
On Dec 18, 17:22, Scott Shidel... wrote:
> Subject: SVO: Rich running, will it toast the guides or what?
> The other day when someone was attributing guide failures to not hooking
> up the VAF meter, I stated that the SHO has a commonish problem of
> running rich on the average owners car, because of neglect. I wasnt sure
> what the reason was(been about a year since I was on SHOtimes), but for
> some reason I was thinking O2 sensors. Last nite on my journey to my
> parents house for the holidays, it hit me that the problem was the
> thermostat goes bad in the SHO often and gets stuck open, causing the car
> to run rich as balls. Mine had that problem, and would only get about
> So whats the point....Dave said the result is broken cat media, that will
> suck up into the cylinder on deceleration, and smoke the engine. Well,
> that is only one of the results. The most common question it seems
> mechanically on the list is why an individual's car uses so much oil.
> Usually these same individuals also say the temp gauge hardly ever gets
> warm. I haven't heard anyone on that list have an engine die when it was
> running rich due to a seized valve though. Most people just have the
> rings slowly wear away.....
> I may be wrong about this, but Gary M is very knowledgeable about these
> cars, and may be able to shed the lite.
Scott pretty much got the whole story here. The Yamamotor in the Taurus SHO is
saddled with a very expensive ($26 each!), and prone to failure thermostat. It
fails open, constantly bypassing coolant thru the radiator. This, coupled with
the SHO's exquisitely efficient cooling system, results in folks driving cars
around for thousands of miles with the coolant temperature never getting over
100F. The engine coolant temp sensor sees this and keeps the air/fuel mix
artificially rich all the time. Base tables for the SHO calibration keep the
A/F at 11.5 or 12 to 1 for moderate to high loads if the coolant stays below
105F. Additionally, below 140F, the strategy will never go to closed loop, so
its virtually always running rich. This causes several problems:
1. Poor fuel economy, as Scott observed.
2. High oil consumption due to the cold oil control rings allowing excess oil
to remain on the cylinder walls.
3. Oil is diluted by raw fuel due to cylinder wall wash down and generally poor
ring seal, again because the engine is cold.
4. The unburned fuel puts quite a load on the catalytics, eventually, the
ceramic monolith breaks down and comes apart. The monolith breaks into chunks,
the chunks vibrate against each other and make ceramic sand. The sand gets
sucked back into the cylinders when the throttle is closed abruptly during high
load situations. The end result is akin to throwing a handful of sand into your
intake; rapid loss of compression and incredible oil consumption, a quart in
200 miles isn't unusual.
5. Very long term, rich mixtures can accelerate valve seat wear, which on the
Yamaha's direct acting valve train will show up as a gradual decrease in the
cam lobe -to- tappet clearance. This is exactly the opposite of what normally
happens in these engines: If everything is normal and happy, all the valve
clearances will increase as the engine ages.
Rich mixtures causing rapid valve guide failures is a new one on me. Very long
term, rich mixtures will accelerate wear in the whole damn engine, but I can't
see how short term rich would ruin valve guides.