Looking for diagnostic help

[angminas]

2002 Buick LeSabre, 120k miles. Seems in decent condition. Have had it less than two months, so I don't know it well.

When you start it, the motor hesitates several times and then runs more normally. If you approach a hill, partway up, when you'd think it would downshift, it bucks. Release accelerator for a second and reapply and it's better. Sometimes you have to do it more than once for a long hill. Power and fuel economy are less than I'd expect.

Fuel filter was very dirty, so I had it changed. No impact.

Then I had the codes pulled. No codes. No CEL.

Changed the air filter this morning. Seemed moderately dirty, but the gasket was split in a few places, and there was a wasp nest in there, about the size of two golf balls. After the new filter was in, the car was much smoother and more powerful. No hesitating or bucking.

That only lasted for 10-15 miles, and then it went back to the old business. It's slowly getting worse over the past few weeks.

Ideas?

 

[John Connor]

Don't know much about vehicles, but I gotta wonder if it's the fuel pump or injectors?

 

[DaTT]

Don't know much about vehicles, but I gotta wonder if it's the fuel pump or injectors?

Does sound like a failing fuel pump. When there is demand for fuel, it just doesn't have the output the engine wants. That's where I'd start.

 

[boomerang]

I think checking fuel pressure would be a good step to take but I doubt a problem that rears its head under load would also be present at startup. But a cold start with a fuel pump that has been sitting idle could be a circumstance where it takes a while to build pressure. Based on the age of the vehicle it could very well be time for a fuel pump. That's something I wouldn't replace without doing the proper diagnostic work. There is a fuel pressure regulator that if faulty could give one the notion that the fuel pump is the culprit. Also, dropping the gas tank can be a chore especially on an old car if rust is an issue.

It could be the mass air flow sensor. If it's out of whack, it won't set codes it will just be giving out the wrong information. The computer supplying fuel based on incorrect air flow information can result in the problems you're having.

A throttle position sensor could also be the culprit. If the readings are skewed, the computer doesn't know any different and no codes will be set.

Lastly, the knock sensor. If it is supplying bad information the timing will be retarded and their could be a loss of power. But your problem is both at startup and under load so I would rule that out.

I would replace the Oxygen Sensor based solely on the age of the vehicle, the mileage and unknown history. They get lazy after a while and give out bad information. As long as they are once again, operating within a range that makes the computer happy, no codes will be set. A bad or lazy sensor will not be in play at startup, only when the motor has warmed a bit. This, I would replace as a normal maintenance item on a car that I didn't know the history factoring in the age and miles.

 

[hanoverphist]

o2 sensors maybe, maf/map sensors couldd be. could be a clogged cat, that doesnt always pop a code. sometimes not until it is beyond gone. how much exhaust pressure do you feel at the tail pipe? can you feel it from 10-12" away from the pipe end?

 

[JCH13]

I agree with the fuel delivery diagnoses. If the filter was really dirty then the pump, regulator, and/or injectors might be real sad too. I'd start with the pump.

IMO it would be worth replacing plugs and wires (if appropriate) if you don't know when they were last done.

 

[olds]

I'd recheck fuel filter to see if it's picking up more debris to rule out garbage in the tank and then (since the filters indicated zero maintenance) I'd check and probably replace the plugs and wires.

 

[angminas]

Thanks, all. I will see what I can do and probably check back here later.

 

[mindless1]

Being New To You, you need to go over it with a fine tooth comb. You can check fuel pressure on the fuel rail valve. You can do a cylinder compression test when you take out the spark plugs, before putting new ones in. Probably good to replace the wires while you're at it.

Look around the engine for minor vac leaks. Those 3800 engines are quite robust but subject to intake manifold leaks too. You might at least grab a torque wrench (as the plastic manifold is fragile) and torque down the intake manifold to spec.

Clean the MAF sensor, then disconnect battery for ~10 min. then drive it for a good 20 min. to relearn parameters.

If it had been sitting before you bought it, as used cars for sale often do, the fuel might've built up some varnish. Put a container of Techron fuel system cleaner in 1/2 tank of gas.

See if the cold idle hesitation coincides with the compressor cycling on and off. Presumably it has an Idle Air Control valve, if so then clean that out.