Truck keeps cutting off.

Post by **ZombieCleaner** » Mon Dec 31, 2007 6:16 pm

This has been going on and off (every other month or so) for the past year.

I drive a 2001 GMC Sierra, in fairly good condition.

What happens is, it will start to "jump", meaning the RPMs will flucuate, causing it to lurch foward and back (while driving) and eventually cut off.

This happens when I apply the breaks too hard (not slamming them), or just randomly.

It's happened multiple times while driving, and let me tell you, it's a bitch to pull over when the power steering is cut off and the breaks lock up (power breaks, power steering). The air or heat will stay on, as will the lights (they dim a bit) and the radio will continue to play.

We just replaced the fuel filter, battery, and took it to the dealership to get some kinda part replaced ($200+ for some shit that didn't even fix it).
I use BP gas (supposedly the cleanest you can use) and maintain the truck very well.

So what the hell is wrong with it?

EDIT: I've noticed that when this happens, the gas gauge will go UP, damn near a quarter tank, and then return to normal after a while.

Post by **wester** » Mon Dec 31, 2007 6:35 pm

Will it start back up after this happens, or do you have to try it a few times?

Post by **ZombieCleaner** » Mon Dec 31, 2007 6:36 pm

The last time, it started right back up.
But there have been other times where I've had to try up to 4 times.

Post by **SamuraiBobX26** » Mon Dec 31, 2007 7:12 pm

When you turn the key do you here the fuel pump engage? It sounds like a humming noise and last only a few seconds. If you don't hear it after it happens again you might want t take a look at it to see if you are getting a good connection between the wiring and the pump. Other than that, I have no other advice, plus there is a good chance that I am wrong.

Post by **ZombieCleaner** » Mon Dec 31, 2007 7:41 pm

Haven't really heard anything of that nature.
Wasn't really listening though lol.
I was trying to get off the road before I got hit.

Post by **SamuraiBobX26** » Mon Dec 31, 2007 7:57 pm

I can understand that. I had a car that had a similar problems and it was something wrong with the fuel pump. But it was an intermident problem. Hopefully you will get it figured out before something bad happens.

Post by **ZombieCleaner** » Mon Dec 31, 2007 8:00 pm

Well that's just it...we took it to the dealership and they thought they knew what the problem was, so they "fixed it", and yet it still persists.
Guess it's just a sign to get rid of the sum'bitch.

Post by **crypto** » Mon Dec 31, 2007 8:51 pm

I'm voting fuel pump too. Maybe there's a pinched wiring bundle back there that's shorting the fuel tank sending unit and the fuel pump at the same time.
That would account for the sudden 'surge' of fuel you see, and the stalling.

I would get under the back end of the truck and start looking for problems, pinches, and chafed wire coming out of the gas tank. Follow it all the way up to the front of the truck, and inspect every inch. Maybe something is getting squeezed or rubbed when the suspension flexes or the bed moves on its rubber bushings.

If you still cant find anything, you may be in need of a new pump and/or sending unit (many fuel pumps are in the gas tanks now, but I dont know if GMC does that or not.

Oh, and one more thing.

B R A K E S .

Post by **wester** » Mon Dec 31, 2007 11:02 pm

I was thinking fuel pump as well. Chevy/GMC trucks have had some notoriously loud pumps, you should be able to hear it from the cab.

Post by **eugene** » Mon Dec 31, 2007 11:05 pm

dealerships suck at diagnosing problems, if they plug it in the computer and its giving the code the books says then they replace it.
could be as others said, fuel pump, a pressure gauge is cheap to buy and screws onto the fuel rail easily to check it.

Post by **crypto** » Mon Dec 31, 2007 11:30 pm

The problem here is that it's an intermittent thing, the problem only manifests itself under braking. I'm thinking its an electrical problem of some sort.

Post by **Ellie With An Axe** » Mon Dec 31, 2007 11:39 pm

Oxygen sensor?

Fuel pump?

Is your computer going kaput?

What I know about car computers isn't much since my car is 40 years old. Is there something that could cause a short when you apply the brakes (and thus the stop lights, which sends a zap to the _____, which shorts the system (which causes the needle to jump) and kills the fuel pump.)

Last idea, check the alternator.

ETA: Some suggestions here also... a couple of people mention the crank positioning sensor.

Post by **Apollo-11** » Tue Jan 01, 2008 8:23 am

Intermittent problems are a b*tch to find and diagnose.

I have had problems like this in the past and they are usually electrical. The car is one big computer, with sensors all over the place. If a wire is chafed, or you have an intermittent bad contact somewhere, the computer will lose contact with that sensor. Usually the car will keep running, unless it's something critical like the throttle position sensor. If it's critical, the car will die immediately. You need more data in order to find the problem.

You might get a code scanner and leave it plugged in to the dash socket all the time. Then when the car dies, look at the code. At least that's a start.

Post by **Gargoyle** » Tue Jan 01, 2008 1:20 pm

This may sound stupid, But check your gas cap.
When they are not venting properly they can cause problems.

When you open if you get a gasp of air its probably fine , If there is any kind of pressure built up its no good.

Post by **eugene** » Tue Jan 01, 2008 4:43 pm

crypto wrote:The problem here is that it's an intermittent thing, the problem only manifests itself under braking. I'm thinking its an electrical problem of some sort.

How about a vacuum hose? small leak that doesn't cause problems until the extra load from the power brake booster. My old s10 kept stumbling and throwing the map sensor code even after I replaced the sensor, turned out to be a tiny hole in the hose going to the sensor.

Post by **eugene** » Tue Jan 01, 2008 4:46 pm

see if you can find a real garage with a scanner. Right after I put plug wires on my wife's car it would start to stall. I couldn't find anything wrong so my mechanic cousin plugged hig snap on scanner in and we went for a drive. It kept reading a misfire on one cylinder so we swapped the plug and it was still misfiring. Turned out to be a tiny bad spot in the plug wire whenever it would flex to the right spot.

Post by **BAD BOY** » Tue Jan 01, 2008 10:03 pm

I have a similar GM product and had the same problem you mention. For myself it ended up being the battery cable. I'd check both for bad connections. Unlike older cars the alternator alone can not keep your truck running without the battery hooked up. Start the truck and giggle the cables, if it shuts off then you've found your problem.

Good luck!

Post by **eugene** » Tue Jan 01, 2008 10:07 pm

Ohh, I forgot about the battery cable, and 2000-2001 were the problem years. The bolt was made too long and it bottoms out in the lug on the battery and cracks it. The battery leaks fluid and you get a bad connection.

Post by **Steev** » Wed Jan 02, 2008 2:37 am

Crank position sensor.. 4.0L Jeeps act the same way when the CPS is on its way out.

Post by **Valarius** » Wed Jan 02, 2008 3:40 am

Ohh, I forgot about the battery cable, and 2000-2001 were the problem years. The bolt was made too long and it bottoms out in the lug on the battery and cracks it. The battery leaks fluid and you get a bad connection.

Huh. A 20 dollar pair of bolt cutters may permanently fix that problem.

Post by **AwPhuch** » Wed Jan 02, 2008 1:52 pm

SamuraiBobX26 wrote:When you turn the key do you here the fuel pump engage? It sounds like a humming noise and last only a few seconds. If you don't hear it after it happens again you might want t take a look at it to see if you are getting a good connection between the wiring and the pump. Other than that, I have no other advice, plus there is a good chance that I am wrong.

+1 on checking the fuel pump

I had the fuel pump go out on my Dodge Dakota 318V8...it did the EXACT same thing...however I did a few things to remedy the problem

A. Get a can af Acetone from Walmart, put 1/4 cup per 10 gallons in your tank for up to 3-4 tanks to remove ALL water from the tank
B. After the 1st or 2nd tank re-change your fuel filter (it emulsifies all the poo in your tank and it goes RIGHT into the fuel filter
C. If it continues to have problems...then your fuel pump IS going bad and needs to be replaced..period...too bad so sad..they wont last forever

I guarantee you have water in your tank..sitting still the pickup for the fuel is above the water that is heavier than gas and sits in small puddles on the bottom of the tank...when you stop hard it slooshes around to one big fat puddle that gets sucked into the lines and settle in the fuel filter

Post by **MI-1Honkey** » Wed Jan 02, 2008 5:31 pm

I hate to be the lone voice of opposition here, but im thinking its an electrical problem. Im leaning towards electrical because of the guage fluctuation. Maybe a corroded terminal on top of the tank, or in the pump wiring.

Mechanical things(like fuel pumps) usually don't die then come back to life.

I'm not a Chevy guy, but i would check for:

Correct voltage at pump, while manually moving the wiring harness.

Check the ground for the pump, and the sensor while moving the harness.

if these check out, id would change the fuel pump relay.

if this showed up on the scanner, the kids at the dealership would've fixed it.

Post by **ZombieCleaner** » Wed Jan 02, 2008 6:31 pm

I dont exactly know what showed up on the scanner at the dealership (they said they saw where it took multiple times to crank) and they replaced something with the fuel pump, but that didn't do shit.

Its below freezing here with the wind blowing in strong gusts, so I'll have to wait till the morning to try some of this stuff out.

Hell we even got a little snow earlier today

Post by **eugene** » Wed Jan 02, 2008 9:39 pm

Valarius wrote:
Ohh, I forgot about the battery cable, and 2000-2001 were the problem years. The bolt was made too long and it bottoms out in the lug on the battery and cracks it. The battery leaks fluid and you get a bad connection.

Huh. A 20 dollar pair of bolt cutters may permanently fix that problem.

basically, you either file the end off the battery bolt or replace it with the right length then buy a new battery. Most people are unaware of the bolt problem so they say crappy oem battery and replace the battery but the problem comes back.