Loosing speed/power

[Unbreakable]

2005 Envoy with 4.2 and 138,000 miles. The issue is it's loosing power and/or speed. When I am driving on the interstate, I always have the cruise control on but it starts to slow down or loose power, it down shifts to compensate for the loss of speed, then hangs around at high rpm for a while, then up shifts, and finally the torque converter locks up. All this happens in 30-45 seconds and happens on hills or flat roads.

I've tried driving without the cruise and nothing changes. I had to replace the water pump 2 weeks ago and while I was there I replaced the temperature sensor and thermostat. I replaced the fan clutch last summer, and while I was swapping the water pump, I took off the throttle body and cleaned it.

After I replaced the water pump, it seemed to straighten up, but it has started acting up again today. So, do I look toward the catalytic converter or throttle body sensor? While I was swapping out the water pump, I did disconnect the negative cable on the battery for about 2 hours. I've ran Sea Foam and then Techron through the fuel system as well as replacing the air filter.

 

[Mooseman]

Have you done any other maintenance like the plugs, fuel filter, tranny fluid, etc,?

To rule out the cat, you'd have to do a back pressure test.

 

[Chickenhawk]

Why did you replace the thermostat? Was it running cool? (Below the center line of the coolant temp gauge?) If so, and if it has been running below the center tick line of the gauge for a while, your cat is becoming plugged.

Replace the cat and all will be good. Cats run in a VERY narrow range of mixtures, and the slightest deviation in the long term will clog it quite effectively. (Or running with a misfire and a flashing check engine light - even for a few minutes - will also cook it badly.)

The thing as, as this platform ages, more and more people will be reporting a loss of power. It starts at highway speeds and manifests itself by more throttle needed to climb hills and downshifts at highway speeds for no reason.

Once you get the old cat off, shoot a flash photo of the inside and post it for all to see.

It is almost important to point out that a clogged cat will NOT usually throw a check engine light. Remember that check engine lights are NOT there to identify mechanical problems; they deal with emissions. Your cat could be almost completely clogged but unless it stops cleaning the little bit of exhaust that makes it out the back, it will not throw a light.

Let us know how this works.

 

[Enlightened1z]

2005 Envoy with 4.2 and 138,000 miles. The issue is it's loosing power and/or speed. When I am driving on the interstate, I always have the cruise control on but it starts to slow down or loose power, it down shifts to compensate for the loss of speed, then hangs around at high rpm for a while, then up shifts, and finally the torque converter locks up. All this happens in 30-45 seconds and happens on hills or flat roads.

I've tried driving without the cruise and nothing changes. I had to replace the water pump 2 weeks ago and while I was there I replaced the temperature sensor and thermostat. I replaced the fan clutch last summer, and while I was swapping the water pump, I took off the throttle body and cleaned it.

After I replaced the water pump, it seemed to straighten up, but it has started acting up again today. So, do I look toward the catalytic converter or throttle body sensor? While I was swapping out the water pump, I did disconnect the negative cable on the battery for about 2 hours. I've ran Sea Foam and then Techron through the fuel system as well as replacing the air filter.

Having some similar issues on my 02 Envoy and 02 TB. What did you find out with the Cat?

 

[Unbreakable]

The problem seems to have corrected itself, although I plan to replace the cat before winter sets in.

 

[Sparky]

FWIW When my 2002 did that it ended up being the upstream O2 sensor failing.

 

[Unbreakable]

I replaced the thermostat and the temperature sensor as a measure of caution since I already had the front of the engine apart. Funny thing is in the last week my temperature started fluctuating from normal down to 150 and back, so yesterday I replaced the thermostat again, this time with genuine AC Delco (the last one was an O'Reilly brand) and it still fluctuates. Going to replace the temp sensor next I guess since I got it at O'Reilly too.

I'm going to also pull the front o2 sensor and see how she drives, it's gutless but gets 18-19 mpg, just not as peppy as my wife's explorer that has the same size engine.