Bad news: Gas tank’s got to go

Dear Mike: The fuel gauge on my 2000 GMC Savanna works fine when the tank is full, but once it drops below three-quarters or so, the needle starts bouncing all over the place. Sometimes it will register full when it has only a quarter tank of gas.

The gauge never seems to drop below a quarter of a tank even when the vehicle runs out of gas — which was not funny to my wife when she made a trip to town and ran out of gas. When I came to her rescue, the gauge was still showing a quarter tank!

Can you please help? I’m sure the sender must be in the gas tank. Does the tank have to be removed?

A: Bad news: The tank has to be removed from the vehicle. Worse news: The sender is part of the fuel pump assembly, which for your car costs $562.

However, you can buy a grease pencil to write the mileage of the last fill-up on the windshield for only 25 cents.

Dear Mike: I have a question about my 1999 Suburban. Many times when I accelerate from a complete stop, I hear a clunk noise from the rear. Two mechanics have checked this out. The dealer serviced the rear end and checked the U-joints. Another mechanic checked the rear wheel bearings by jacking the truck up and shaking the rear wheels. Still nothing indicates where the noise is coming from.

What is causing the clunk? — F.E, via Internet

A: This is one of the five most common questions I get, so I answer it every few years for the benefit of those who do not read my column every month.

Your clunk is probably from the splines on the transmission output shaft inside the tailcone. When you stop, they bind up on the mating splines on the driveshaft’s front yoke. Compliance in the springs changes the depth of the engagement of the splines. When you reverse the direction of the torque on the driveshaft, the binding releases all at once, causing the clunk — which always seems to come from the differential.

The cure is to pack the splines with a special grease that won’t dissolve in the transmission lubricant. This involves removing the front universal joint to get the front yoke loose. It’s an annoying job, too annoying to do every couple of months. And that is what’s necessary because this fix usually lasts for only a few thousand miles.

You might also try replacing the front transmission yoke in the hope that the new part’s clearances are tighter. Or just live with it.

Dear Mike: I was driving along the interstate last week when my car started to slow down all by itself. When I finally got to an exit and pulled over into a gas station, the brakes were glowing a dull orange color and smoke was pouring out of the wheel well. The mechanic there replaced the master cylinder while I waited, but before I got home a few miles later, the same thing happened, only not as bad — no orange glow but plenty of heat and enough smoke that I didn’t want to park the car in the garage.

When I called the gas station back, the owner told me the mechanic wouldn’t be in until after dinner, and said that I must be holding my left foot on the brake pedal as I drive. My car has a manual transmission, so I doubt that I’m doing that. — M.T, Carlsbad, Calif.

A: Your brakes are dragging. Your letter didn’t specify whether one or both brakes were involved, and I can only assume that it’s the front brakes and not the rear ones — but it really doesn’t matter now. If it’s both front brakes, a number of things may be causing the pads to drag: a poorly adjusted master cylinder pushrod, a sticky brake pedal pivot point, a bad proportioning valve, a bad check valve, a pinched brake pipe or several other things.

If only one wheel is involved, it might be a sticky caliper piston, sticky sliding pins or ways, a pinched brake line or a half-dozen other things. In this case a bad master cylinder is not likely. Sounds like the mechanic just had a master cylinder he needed to sell that night.

I’d try to get some money back. And you probably shouldn’t let this mechanic work on your brakes again. If the brakes became so hot that they glowed, the pads should have been replaced. The disc or discs may be warped, and if that’s the case they are certainly more likely to crack.

A competent, honest mechanic will be able to advise you if the disc or discs need to be replaced — after he determines the source of the problem. All said and done, my guess is you may well need to replace the discs and calipers, considering their bake session, regardless of the cause of the problem.