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Old 09-09-2009, 04:48 PM   #1
jonnycube
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1997 SL2
Default Smoke from exhaust after extended engine braking

In order to spare my brake pads, I was in fourth gear most of the way down 6 miles of mountain at a 6% grade in Utah. When things started leveling out, I shifted to fifth. As I took my foot off the clutch, it felt like it wasn't quite engaging properly. It felt "funny". I pressed the clutch, revved the engine, and then everything was fine. However, I noticed a large puff of white smoke come out the back of the car. Otherwise, no problems.

Any idea what this was? Should I be concerned? The car is a 97 SL2 with almost 180,000 miles.

My clutch is long overdue for replacement, if that's relevant.
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Old 09-09-2009, 04:59 PM   #2
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1998 SC2
Default Re: Smoke from exhaust after extended engine braking

When you use the engine for a brake you suck oil into the combustion chamber and blow it into the exhaust and cat. You also contaminate the O2 sensor with the oil. The PCM shuts of the injectors when the throttle is full closed and the VSS signal is not zero. The brake pads will stand the abuse, engine braking is not supposed to be a routine evolution.
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Old 09-09-2009, 05:05 PM   #3
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Default Re: Smoke from exhaust after extended engine braking

Braking down mountains soon makes the pads so hot you can smell them. A pad that is saturated by heat loses its ability to brake. I was under the impression that it's better to use the engine and brakes together so you don't fry the pads. Isn't this how trucks do it? Is this a bad thing?
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Old 09-09-2009, 05:45 PM   #4
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Default Re: Smoke from exhaust after extended engine braking

Quote:
Originally Posted by OldNuc View Post
The brake pads will stand the abuse, engine braking is not supposed to be a routine evolution.
Holding 50mph in 4th gear for 2-3miles on a 10%+ grade oughtta be more "engine abuse" then a 6% grade. Never noticed extra/odd puffs of smoke outta my '93, even with the "new" '97 engine in it. Sometimes I have it hold roughly 50 and other times I cruise down the hill at 65-70, then start slowing around 1/8th mile from the stop sign. Dunno which is better for the car, but blasting down a mountain at 70 is much more interesting

One way or another _something_'s going to wear out, though in the long run I don't see engine braking being a detriment to engine life. Or clutch life for that matter, but then I tend to just dump the clutch on downshifts so it's not being "ridden" much.
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Last edited by fetchitfido; 09-09-2009 at 05:50 PM.
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Old 09-09-2009, 07:17 PM   #5
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Default Re: Smoke from exhaust after extended engine braking

There's absolutely nothing wrong with engine braking. If there were any harm done then why are automatics always in gear whether uphill or downhill!? To pass on misinformation is misleading to say the least. Every automatic stays in gear and whether leaving it in Drive or selecting a lower gear to use the engine for supplementing braking has no effect on the engine; selecting a lower gear in an auto simply throws the engine rpm into the next higher rpm band thereby slowing a car somewhat and helping to keep a car from accelerating down a long steep road. Just because the auto is in high gear just means a lower engine rpm unless deliberately selecting a lower gear, 3rd for a 4-speed for instance. So what if the engine rpm rises a little, its not over revving beyond its rating. If this were incorrect then every trucker has been doing it wrong for years when deliberately selecting a lower gear before coasting down steep roads. Its even posted in YELLOW CAUTION WARNING SIGNS to alert truckers to use lower gears. Diesels or gas, engines are engines and nothing will harm running an engine at the next lower gear while not over revving. A worn engine with issues will always exhibit its weaknesses; worn valve stem seals, worn exhaust system, poor maintenance, all exhibited when under slight stress such as coasting with the engine running a slightly higher rpm. Every trucker worth his/her CDL will downshift at the first hint of any down grade that would make a rig over speed. They'd better. I don't want 80,000 lbs of tractor/trailer bearing down on me doing 75mph and getting larger in my rear view mirror!?

Downshifting when done normally usually entails shifting when engine rpm is at or near the low end of the tach so engine over revving doesn't occur. A naturally steep and long road that allows almost every car/truck/van to accelerate without throttle application would still raise engine rpms but careful selection of the right gear will keep the engine at the midrange of the rpm band that allows the engine to act as a brake as well as not over rev it to cause unwanted issues. When done correctly this will always work; truckers do it all the time and are required to since brakes do not last at all in extended braking. The same with cars. Not using a manual transmission to engage engine braking on steep roads by selecting a lower gear to keep speeds down and then braking to halt any over speeding then releasing immediately is not using mechanical advantages to forestall prematurely wearing out brakes, dissipating the kinetic energy built up on the way down. The engine is used to soak up this kinetic energy as well as sharing the excess with the braking system. Used together, nothing harmful will occur to the engine and the brakes cool off in between applications. This isn't rocket science but plain common sense when learned properly.
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