I have an 05 gmc jimmy 5 speed manual, it starting hesitating on acceleration up hills and now it bogs down on any significant throttle imput, if i feather it super light i can get it to climb but it mostly sound like its underwater with any throttle, wont go above 2k rpm, i have replaced plugs, wires, distriutor/rotor, fuel filter, air filter, throttle position sensor, brand new exhaust from the manifold back, have done a fuel pressure test and it is acting normal. Cannot for the life of me figure out what is, my dad was a mechanic in the Canadian miltary for 20 years and a gear head long before that and it has him stumped aswell any help would be greatly appreciated

 

1. Do the fuel pressure leak down test.
2. Do you have a scanner? Any codes? What are your fuel trims
3. Pull the pre cat O2 sensors and see if it runs better

George

 

Originally, when I plugged in the scanner and I had an p0300 engine misfire code and a fuel trim rich engine code. I think it was P0172 but didnt write it down or save it anywhere,
Both of those led me to replace the distributor cap, rotor, plugs and wires. Thought process being the misfires are causing the engine to run rich, i have not I've gotten a chance to run or drive the vehicle long enough since clearing codes for engine light to come back/ fixed the misfire issue and as of right now there are no codes showing up on the scanner

Pre cat O2 sensors plugged in and unplugged no difference

Fuel pressure 55 psi ish consistent through idle and throttl imput takeLeak down test without starting engine start 54 psi
1min 54
2min 54
3min 54
4min 54
5min 54ish
6min 53
10min 52
15min 52ish
17min 51

I can provide videos aswell if it helps diagnose

 

I’m saying to physically remove the pre-cat O2 sensors, not unplugged the wiring harness

George

 

when mine did that, it was a bad catalytic converter. came with a light rattling sound.

 

after plugging the o2 sensors back in i pulled the drivers side but wasnt able to pull the passenger, and decide to try it anyway, turns out it was a bad o2 sensor on the drivers side. When i pulled the sensor it reved all the way up to red line like it was brand new with some backfire, when i put the sensor back in the issue of the rpm loss and bogging down came back. I have a sensor coming tomorrow and hopefully it will solve the issues if not atleast i know the location of the problem

 

i was leaning in that direction but the problem is i had a new exhaust sysytem from the manifold back installed for a saftey inspection less then 6 months ago, im jot saying mechanics are perfect but i dont want to believe they installed a faulty cat with out covering my bases, and i didnt have the rattling sound in the exhaust, i made sure to give the cat a decent shake to make sure

 

This experiment was to see of you have a backpressure problem. usually from a plugged cat. if uplugging the electrical connector but leaving the O2 sensor in did not improve the situation then its probably not the O2 sensor. You need to move to the next step which is a back pressure test.

George

 

Okay just to clarify and dumb it down for my sake, if the sensor has been pulled but is still plugged in and its running better it indicates a possible back pressre problem?

 

Let's make sure that we are on the same page with our terms.

1. Unplugging the electrical connector on the O2 sensor but leaving the O2 sensor installed physically is one thing. If the truck runs better doing that then there may be a closed loop fuel control issue. If that's true does the truck run OK with a cold start in the morning and then run crappy as soon as it transitions to closed loop?
2. Physically unscrewing the O2 sensor is a cheap and dirty way to see if you have excessive back pressure. The electrical connector is not connected to the O2 sensor for this test.

If the truck runs better doing #1 and then doing #2 does not improve things, then you likely do not have a back pressure problem. In that case it can be a few different things and not necessarily the sensor itself. We would need to graph the sensor output in live data and if its faulty, check the power, ground, signal wire and bias voltage from the engine computer to the pre-cat sensors.

George