99 LeSabre loses power when slowly going uphill, please help!

Dear all,
My 99 Buick LeSabre Ltd runs well and has been well taken care of.
When it slowly goes uphill, it loses power and sounds like it backfires. I have to either press hard on gas to give it more power or downgear. A good local mechanic took a test drive and says it is either the transmission slips or the catalytic converter goes bad. He also says that the engine is still very good. He suggests me to keep driving until the problem gets worse enough to show off and worth to fix. The guy from a muffler shop took a test drive and does not think that the catalytic converter is bad.

Please help!

Do a fuel pressure test with a gauge hooked up to where you can see it (tape to winshield wiper or something to that effect)and drive up a hill watching the fuel pressure.

Which engine and mileage on vehicle. Chgeck for any codes present.

Thank you very much your help. I will have fuel pressure checked asap.
The engine is 3.8 Series II, with 95K miles. It does not burn oil and runs very good on gas.
It does not show any engine error code.

More than likely, you have a secondary ignition problem i.e. plugs, wires or coils. When was the last time you had the plugs & wires changed? BTW, you may want to remove your personal email address from this public forum to prevent unwanted spam. If you need help doing so, let us know.

Poor deliverability of a motor can give symptoms of a bad transmission.

1) Have a back pressure test done your catalytic converters asap, a plugged cat could cause heat damage to your engine.
2) It's cheaper to replace your fuel filter, befor the fuel pressure test.

Dear all,
Sorry for unable to get back here for a while! I would like to tell the story, hopefully, it will help someone.
After I posted the question here, the problem has got worse. It happened more often, even on flat roads. Then, I noticed when the car needed power to keep up speed, if I pressed the gas pedal and it was below 2000 RPM, then it happened most of the time. What I did was to press it hard and got above 2000 RPM, then the car got enough power to run.
I had the same mechanic (also a friend) to check the fuel pressure. He refused to do it as he was certain it would not be problem with the fuel but with the transmission.
Then, I had the car checked at a muffler shop. I was told the the catalytic converter was okay.
A few days ago, the car jerked when I stopped at red lights. Today, I replaced spark plugs and wires. Checking the old spark plugs, I noticed one of them was burned at the ceramic part, near the top. I showed it to the mechanic, and he said what happened was the firing did not go through the trunk of the spark plug, but jumped over from the outside.
Results: The car has no more problem! Yeah! It was just a bad spark plug!
HotZ28, hifi!
I appreciate very much all your time and helps. You guys rock! Automotiveforums is wonderful!

I just want to make a small remark here.....

This is very common on all of the GM 3800 Engines... These all use a Waste Spark System... If one of the Plugs is bad or pissed, or you have a bad wire this will affect the operation of the opposing cylinder due to the resistance in the wire...

2 plugs are fired at once, one on the compression stroke( takes more power to fire the plug ), and the other on the exhaust stroke( takes less power to fire the plug )... Imagine if there is a problem in the way the system operates... You will end up with a Miss under load, and the other cylinder won't be firing as good as it should, so the car will become weak...

a lot of people assume its a trans issues... If you have a 3800 and the plugs and wires have never been replaced and are worn you want to take care of this first to rule them out... All 3800's will have this issue somewhere along the line...

Also just as fair warning, Do Not Use BOSCH PLATINUM plugs in a GM waste spark system...

Thank you for a good lesson.
I agree with the BOSCH PLATINUM plugs warning. I had Bosch Platinum plugs available at home, but my mechanic just shook his head and advised me not to use them for GM engine.

BTW, "If you have a 3800 and the plugs and wires have never been replaced..." <= My car had run 96K miles on original factory wire set; and this was the first time I replaced them.

I just want to make a small remark here.....

This is very common on all of the GM 3800 Engines... These all use a Waste Spark System... If one of the Plugs is bad or pissed, or you have a bad wire this will affect the operation of the opposing cylinder due to the resistance in the wire...

2 plugs are fired at once, one on the compression stroke( takes more power to fire the plug ), and the other on the exhaust stroke( takes less power to fire the plug )... Imagine if there is a problem in the way the system operates... You will end up with a Miss under load, and the other cylinder won't be firing as good as it should, so the car will become weak...

a lot of people assume its a trans issues... If you have a 3800 and the plugs and wires have never been replaced and are worn you want to take care of this first to rule them out... All 3800's will have this issue somewhere along the line...

Also just as fair warning, Do Not Use BOSCH PLATINUM plugs in a GM waste spark system...
how can you tell if its the trany?? i was reading these posts... (new as of today) and i think my trany is slippen... about to buy a trany, but concerned it might not be that now.... so how can a person tell....
thx

how can you tell if its the trany?? i was reading these posts... (new as of today) and i think my trany is slippen... about to buy a trany, but concerned it might not be that now.... so how can a person tell....
thx

I thought my problem a year or so ago was the transmission, but it turned out to be bad wires. How can a person tell? Well not being a mechanic, I am sure there are ways, however if your plugs and wires are original or old, it is far cheaper (and easier) to FIRST replace them, than it is to buy and replace a tranny.