Warning about TDi brakes.

Guys, I just got back from a trip thru Sequoia Natl forest. (BTW, fuel in Pixley, CA is $1.389/gallon US).

Anyways, I took hiway 155 from the 99 to Lake Isabella, and the pass is at 6100 ft. Coming down the other side, the grade is 11 to 13% down, and my brakes were SMOOOOOKIN!!!! Basically the TDI's have zero engine braking, as I'm sure you already know... but this situation was bad.

Coming down the hills I had it in 3rd and no accelerator, it was reving at 3500. I'd slow down and start over in 1st and eventually end up in 3rd or 4th again, coasting, or riding the brakes. I stopped twice and clouds of smoke came puffing from both front wells. -- And I have Red Mintex pads too.

So be forewarned. Your brakes suck.

I disagree. I drove across the entire continental divide and never encountered these problems. I was very impressed with the engines braking ability. Never had hotter than normal brakes.

Me too. I have come down some serious crap at forgive-me-Father-for-I-have-sinned high speeds and never had a problem. Have your brakes checked. And, BTW, I didn't do too much downshifting. As an ex-racer, I managed to have learned that brakes are cheaper than clutches and transmissions.


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Ted White, Bisbee,AZ
'99.5 Black Jetta TDI
'00 Suzuki SV650

Clatterman, why didn't you throw open your doors and see if they'd work as air brakes? We did coming down out of Oregon on I-5 when I was back in school (too many years ago), and the speed on the gawd-awful station wagon we were in actually dropped about 5mph.
It was kind of a windy ride while the doors were open though..

So... did you have upgraded Mintex pads, or OEM pads?


Gee Norm, next time try sticking yer head out the window and opening your mouth.

I have a 96 TDI and 95 PASSAT GLX with plus suspension (bigger brakes + ABS). I like the TDI braking feel, it just stops nicer than the GLX.

Now I gotta admit that I haven't done any 15% grades, but I would rather do it in my TDI vs the GLX! The smoke must be associated with the racing aspect of the mintex, if you can smoke out the guy behind you, you might blind him!

I have OEM brakes on both.

I dont think the problem is with the breaks, it seems like the lack of engine braking. Is the computer feeding fuel to the engine to keep it from breaking. I remember reading somthing about this somewere (it is supposed to help during slipery conditions).

Nobody knows more about driving TDIs in the mountains than a Utahn. I just got done blasting over a very steep "loop road" over the weekend at the TDI GTG. I was breathing the brake smoke of some crappy Mitsubishi all the way down because the guy didn't know how to drive.

Basically, you employ the same methods you would with any other car. Gear down. Look to maintain an engine speed of 3000 to 3500 or so. If you're descending a steep hill and your revs are lower than that you need to gear down! Do not ride your brakes. Do not attempt to maintain a constant speed. Instead, allow the speed to build up between curves and do all your braking in big "chunks" just before the curves, allowing the brakes to cool during the "speed up" phases. I've been driving in mountains all my life, and I've never yet "cooked" a set of brakes while descending hills. It's true that diesels have little engine braking, but neither do gassers with automatic transmissions unless you gear them down. You can make the most of what engine braking you DO have and avoid cooked brakes if you know what you're doing.

-mickey

By the way, the fuel injection is NOT feeding tiny bits of fuel during deceleration, unlike the FI in a gas engine. When decelerating in gear your injectors are completely shut off. No fuel is flowing at all, until the engine speed drops down to just above idle.

The reason that diesel engines have poor engine braking is that they have no throttle valves. With the throttle closed in a gas engine during a descent you've got a powerful vacuum built up inside the intake manifold. Atmospheric pressure, coming through the exhaust system, is trying to "slow" the pistons as they travel upward during the exhaust stroke. Diesel engines have no such vacuum. There is a huge resistance as the piston travels upward during the compression stroke, thanks to the extremely high compression ratio, but that is countered by an opposite and nearly equal downward force during the power stroke. (I say "nearly" because a small amount of heat energy from the compressed air is absorbed by the metal in the engine, so the downward force is slightly reduced. Internal friction also contributes to what little engine braking we have.) The result is that the upward resistance to piston travel is almost canceled out by the downward force of the compressed air.

Many heavy duty diesels employ what are know as "exhaust brakes" or "Jake brakes", or any one of several other names. When activated the exhaust valves are forced open early, during the power stroke. The remaining compressed air escapes with a loud "pop". That changes the balanced "equation" and you have more force compressing the air than you have force pushing the pistons down. A very simply idea that works very well, though it does require very heavy duty valve train components and causes a lot of noise. (You'll see signs in residential neighborhoods that forbid the use of engine brakes.)

-mickey



[This message has been edited by mickey (edited June 12, 2000).]

Clatterman, do you have an automatic? I mean, how the hell would you "eventually end up in 3rd or 4th again" if you were trying to keep your speed down? Even in an automatic I could keep my speed down using the engine on my Passat 2.0 and Jetta 2.0 just by shifting down. I think my 97 Jetta 2.0 automatic had more engine braking then any other automatic I've driven.

and why do our brakes suck? did they slow you down? it seemed so... because they smoked after you rode them for extended time? what did you expect? try water cooling your rotors, like the big rigs do when they race. oh, and i'll bet you it was your aftermarket pads smoking, not our VW brakes.

jayb79 - the computer will throw fuel at the engine only if your front wheels lose traction. try it in snow some time... if it's slippery enough: 3rd gear, 2K rpm, shift to 2nd, dump clutch w/o gas. There will be a slight jerk as wheels break free and immediately the rpm's will jump up and the car will just coast smoothly, barely slowing down... well, at least my 99.5 Jetta TDI did.

ed d.

Clatterman,

On the "flatland" here in FL I can coast for miles coming to a stop from 55 mph. However, there is a lot more "engine braking" when the A/C is on (A/C braking? )

You might try cranking the A/C on your way down, you will feel a definite difference. I don't know if it will be enough to help your brakes from smoking, but it will help.



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ANTARTI
99.5 Jetta TDI GL Silver Arrow/Grey Studio

Maybe you could put a moveable flap over the snorkel, duct tape the trap door and get "snow screen brakeing"?

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Gregor Switca

Torque is a beautiful thing.

Clatterman,

It really isn't clear whether you ride your brakes or not. If you do, and you're making a long decent, I wouldn't be surprised by the smoke. It was just the pads anyway, right?

Mickey's suggestions are in line with what I would have suggested. You gotta give 'em a chance to cool.

You might want to experiment and get back to us. Just curious.

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BeetleGo, went that-a-way

"You said this was a DIESEL?!"

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Clatterman:
clouds of smoke came puffing from both front wells. -- And I have Red Mintex pads too.
<HR></BLOCKQUOTE>

Perhaps those "Red Mintex pads are inferior to the stock pads???



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Ric Woodruff

Braumeister von Sehr Gutem Bier
Since the Last Millennium

1998 Jetta TDI Sport

"I'm ready to throw in the towel,
but I don't know what to throw it into...
so I guess I'll keep on keeping on..."

If your car has the anti-shudder "changeover flap" in the intake manifold you can give yourself poor man's "jake brakes" by rigging a switch to short the valve's controller and close it during a mountain descent. Presto! Instant engine braking. I haven't actually done this, mind you. Your engine might explode. But I doubt it.

-mickey

Richard: Scroll up to my last post. I explained the lack of engine braking in detail.

Switca: You laugh, but your joke is closer to reality than you realize. True story: Every day I descend the same mountain road on my way home from work. When the car was new I would have to downshift to 4th to keep avoid the need to use the brakes. It would maintain about 70 mph in 4th. Gradually, as my snow screen clogged, the engine braking improved until I could maintain the 65 mph speed limit while coasting in 5th gear! I never noticed this change creeping up on me, but yesterday I certain noticed it! I kept checking to make sure my car was in gear because it continued to speed up beyond it's "usual" speed. Then it finally dawned on my why I was going 85 in 5th gear instead of 65! Duhhh!

-mickey