ScannerDanner, where are you?

[Merkava_4]

This would be a perfect case for ScannerDanner: A 2000 Buick LeSabre with the 3800 Series II V6.

Car runs perfect at all engine speeds except full throttle. At full throttle, the car will buck like a very angry Texas longhorn bronco bull. Let off the throttle, engine returns back to running perfectly normal like nothing happened. A P0102 code was set one day last week, but the next day, the computer clears the code all by itself! Thursday evening I took my neighbor for a ride in it; he's a GM drivability tech at Madera Chevrolet. He took his OTC Genesis Evo scanner along for the ride. All the data PIDs were normal even while the car was bucking like the previously mentioned bronco bull at full throttle. He's at a total loss for the cause as well as I am.

Here's a clue in case we have any drivability techs on BITOG: When the car is at full throttle and is bucking like a bull, it acts THE SAME as it does when it hits the 108 mile per hour computer programmed vehicle speed limit.

 

[Warstud]

Disconnect the MAF Sensor to see if it runs any better.

 

[mmavet]

Check the wiring to the MAF and make sure it is ok.
Clean the sensor with MAF cleaner only if the wiring is ok. Would be a good time for air filter replacement and check for any air leaks.
If it still has no throttle response at high rpm after the first 2, replace the MAF sensor. Rarely.

 

[901Memphis]

Just replace the MAF as PM

 

[hansj3]

Check the knock sensor

 

[Ojustracing]

I've hit the GM speed limiter before. It just shuts off like you turned the key off no bucking.

The PO102 is a Maf code and could well be the issue. But I have seen more of a loss of power than bucking. Bucking to me that spark is going where it shouldn't. But if its lean from the MAF code and has marginal coil/spark plug wires that could really get things going.

I would first clean the MAF sensor then road test. If it still does it, disconnect it. If it runs better replace the MAF. It might need one anyway based on its age.

When the last time the spark plug wire/plugs were changed? Fuel filter too? Many of the ones I had would misfire under light load, torque converter locked going up a hill.

 

[Ethan1]

At full throttle, this car probably goes into open loop and dumps as much fuel as possible. It's clearly starving for fuel, especially since you mention that it acts just like the rev limiter. Cleaning the MAF is never a bad idea, but you should look to the fuel system. I wonder if a fuel pressure gauge is something you can rent at the parts store?

I know that dealerships get a lot of hate here, but they'll give you a correct diagnosis for $100. I hear horror stories all the time about indy mechanics throwing parts at driveability problems.

 

[Merkava_4]

Originally Posted By: Warstud
Disconnect the MAF Sensor to see if it runs any better.


Can the car go to full throttle with the MAF Sensor disconnected?

 

[michaelluscher]

After the MAF keeping fuel away, I'd be checking fuel pressures at WOT under load

 

[Merkava_4]

Originally Posted By: mmavet
Check the wiring to the MAF and make sure it is ok.
Clean the sensor with MAF cleaner only if the wiring is ok. Would be a good time for air filter replacement and check for any air leaks.
If it still has no throttle response at high rpm after the first 2, replace the MAF sensor. Rarely.


The wiring at the MAF sensor is good.
Pin A - Signal Wire is 2.6V
Pin B - Ground Wire is good (checked by providing battery voltage with test light)
Pin C - Supply Wire is 14.5V

I haven't cleaned the MAF sensor, but a visual inspection with a magnifying glass shows the two resistor wires to be spotless. The engine was acting up when I would wiggle the MAF pigtail, so I replaced the MAF pigtail. Now the new pigtail passes the wiggle test, but I still have the full throttle issue. Incidentally, I have good full throttle response, but only in between bucks - if that makes any sense.

 

[Nick1994]

Now I'm no mechanic, but maybe it's possible the cat is plugged up enough that it doesn't have the flow at full throttle, it chokes the engine and makes it buck around.

 

[Merkava_4]

Originally Posted By: Ojustracing
The PO102 is a Maf code and could well be the issue.


But the computer cleared the P0102 code before I had a chance to.

 

[Merkava_4]

Originally Posted By: Ethan1
I wonder if a fuel pressure gauge is something you can rent at the parts store?


I have a fuel pressure gauge, but not the kind you can see while driving.

 

[Merkava_4]

Originally Posted By: Nick1994
Now I'm no mechanic, but maybe it's possible the cat is plugged up enough that it doesn't have the flow at full throttle, it chokes the engine and makes it buck around.


A clogged converter would cause a loss of power at all engine speeds. This car has good power except when the engine cuts out under full throttle.

 

[Nick1994]

Originally Posted By: Merkava_4
Originally Posted By: Nick1994
Now I'm no mechanic, but maybe it's possible the cat is plugged up enough that it doesn't have the flow at full throttle, it chokes the engine and makes it buck around.


A clogged converter would cause a loss of power at all engine speeds. This car has good power except when the engine cuts out under full throttle.
Not always, sometimes a clogged cat will be so bad it'll be glowing red under a car and the car can have plenty of power.

 

[Thax]

I have seen this in a Jeep 4.0L before. It was a very dirty MAP sensor , granted this was a speed density system. I fixed the same issue with a thorough cleaning of the MAP sensor with electronics cleaner. Without seeing the data , I would start at the basics. Change the fuel filter if it is due or it is unknown when it was last replaced. Fuel pressure is adequate? At open loop at full throttle it would seem like a fuel issue not air, unless the air filter is stupid clogged. One last possible cause ... maybe a faulty fuel pressure regulator on the rail , check it with vacuum .

 

[Luisraul924]

I am no zoologist but I'm pretty sure a longhorn, bronco and bull are all three different animals...

 

[Ethan1]

Originally Posted By: Luisraul924
I am no zoologist but I'm pretty sure a longhorn, bronco and bull are all three different animals...


Wuuuuut?

 

[bullwinkle]

Weak fuel pump, partially clogged fuel filter, and/or bad fuel pressure regulator.

 

[HerrStig]

Originally Posted By: Luisraul924
I am no zoologist but I'm pretty sure a longhorn, bronco and bull are all three different animals...
You ARE a waste of bandwidth.