NEED HELP Engine bogs under throttle

[LT1transAM]

03 Trailblazer LS

Engine idles fine and drives alright under light throttle. As soon as I give it a little more, it bogs down and is very sluggish. The sound of the car gets really deep while this is going on. No codes at the moment and all coils and plugs are good. Just tested fuel pressure and it's good. Transmission fluid good as is the oil.

There was a code for the catalytic converter but it hasn't come up since I cleared the codes a little while ago. I just feel like that's the issue. Does this sound like a blocked up converter?

Thanks in advance for the help.

 

[AWD V8]

Typically a blocked up convertor will idle fine and drive not too bad with an easy throttle at low speed, say under 30 MPH, but if you jump on the throttle it will initially take off from a standstill not too bad, but the engine RPM will seem to hit a governor at about 2500-3000 RPM. It may misfire at that point.

You may or may not get a code for the convertor, maybe one for low flow at the MAF.

The sound at the tailpipe will be weazy, like you have a potato stuffed up your tailpipe.

If you unscrew the O2 sensor a head of the convertor and tie it up out of the way, you can drive it to see if it has more power. That little opening in the exhaust will provide a very definite increase in power if it is the convertor clogged. It will be noisy when it runs, and you may get a code for that sensor not reading.

Do you have a scanner that reads live data?

What was the fuel pressure and did you test it under load, as in driving?

 

[LT1transAM]

Did not test under load. It read 50psi key on, engine off. When I pulled the vacumm line from regulator, the pressure did not drop like the manual said it should.

 

[LT1transAM]

No scanner.

 

[Tiggerr]

Sounds just like a clogged converter would act...it'll run like an old VW Bug running on 2 cylinders...sound like it too..
As said above pull upstream O2 see if any difference. Or get backpressure test...drive it a bit after dark... try to run it hard for a bit...check it and see if it's glowing cherry red too..sure sign its toast...
I set the carpet in an old cutlass of mine on fire that way yrs ago lol

 

[AWD V8]

No scanner.

My concern is that the fuel pump may be week. It should be higher, my opinion is 55-60 psi. I don't car what the factory specs are. The manual says to just turn the key on and read the pressure. I know from experience that if the pump is weak that it may ONLY put out 50 psi tops. So what happens when it's running? I know from experience that at 48 psi the injectors will not spray

I had a GM that was supposed to be tested with just the key on, and 55 psi. The problem was that the pump only put out 55 psi. When I drove it of course the pressure dropped because testing pressure with the engine off is not real world. Under load the pressure dropped to 48 psi and the truck fell on its face. I dead headed the pump and still only got 55 psi. These pumps dead headed should put out 70-90 psi.

If you can test the pressure dead headed, that is, disconnect the line at the fuel filter and put the gauge on the line to the back and run the pump.

Or, if you had a scanner that reads live data you can do a test to determine if the engine is getting enough fuel or not, without even getting your hands dirty.

If you have an Android phone you can buy the Torque app for 5 bucks, and an OBD2 adapter for about 10 bucks and you are ready to diagnose.

 

[Mooseman]

A clogged cat will give low vacuum readings at higher RPM.

But the definite answer would be an exhaust backpressure test.

And in this day and age, you need either a scanner or, as indicated above, an OBD bluetooth interface with Torque, which will give you much more info for cheap.

 

[LT1transAM]

I just purchased an exhaust pressure tester. I'm about to order a Bluetooth interface and use torque on my phone.

 

[LT1transAM]

What am I looking for while hooked up to Bluetooth and on the torque app?

 

[LT1transAM]

Ok. I did the exhaust pressure test and it looks fine.

 

[Mooseman]

So what's your definition of "fine"? How did you do the test?

 

[AWD V8]

If you suspect the convertor is clogged you can simply unscrew the O2 sensor before the cat, tie it up out of the way and drive it. It will be loud, but opening the exhaust up before the cat will improve exhaust flow enough that if you have an exhaust restriction the power will now be back. If the power doesn't improve then you do not have a clogged cat.

 

[LT1transAM]

Ok. I'm going to say the cat is fine.

 

[LT1transAM]

Idle was way below 0. 2500rpm was 1 1/4.

 

[Mooseman]

OK, so I'd be checking fuel pressure at higher RPM. I have had instances where it was fine at idle and low speed but would fall flat on it face if you gave it the beans at all. What about the fuel filter? Fuel pressure regulator is also a possibility.

 

[LT1transAM]

Fuel filter just changed. I will get a pressure gauge and try to check it while driving.

Thanks for your help so far.

 

[MILL2ETG]

I have a 05 envoy thats doing the exact same thing. Have you made any head way on this issue yet. Im tired of shooting the parts cannon at this thing lol.

 

[mrrsm]

You did not mention doing this for your 5.3L Engine... but the clue about another possible place to look is hidden in the title of your post:

"throttle"

If time permits... you can R&R the Throttle Body and give it thorough cleaning... avoiding the plastic cover-motorized section and tidy up the Intake Venturi areas and Butterfly Blades and if you took the Battery (-) Ground Cable off of the vehicle when you started this service... by the time you get done you can re-attach the cable ... fire that motor up and take it for a short Test Drive after letting the Engine get nicely warmed up.

Other possible issues with bogging have been reported involving the heater/AC turning on in combination due to a problem with the environmental control mechanism which can demonstrate a sudden loss of power if the AC Compressor kicks in and cause about a 200 RPM drop in the power band. The other most reported problem happens when the Alternator is replaced or a Battery change-out is performed to cause similar issues. Sometimes... leaving the Battery unhooked for a 30 Minute Period can be all that is needed after performing service in these areas.

 

[MILL2ETG]

I have a 05 envoy thats doing the exact same thing. Have you made any head way on this issue yet. Im tired of shooting the parts cannon at this thing lol.

 

[MILL2ETG]

Thanks for the helpful input. I just put a new TB and AAP sensor plus changed the VVT solenoid cam saensor crank sensor coolant temp sensor AND MAP sensor plus both O2 sensors. I figured if im doning one sensor might as well do ALL of them. Fuel psi is good at all ranges of rmps. Thanks again

 

[webdawg]

A Crankshaft Angle SEnsor (C.A.S.E.) relearn has to be done when the crank sensor is replaced.

I learned the hardway. You still start getting misfires most likley soon.

I had a less then 1 on back pressure and my stuff was plugged so bad.

Check it out:

http://gmtnation.com/forums/threads/2004-tb-i6-4wd-manifold-red-hot.14795/page-2

 

[gmcman]

Idle was way below 0. 2500rpm was 1 1/4.

What does below 0 mean?

Not sure what the 5.3 back pressure should be, but mine (4.2) was less than 1/2 PSI at 3000, averaging around 1/4 PSI.

May not be the cat, but it's pretty high imo, may not be enough to cause your issues.

 

[MILL2ETG]

I pulled the upstream O2 sensor and drove it. Noisy but the power was there through out the entire pedal 100%. It sonded like i was driving an old John deer. NEXT STEP replace or punch out cat. PROBLEM SOLVED. THANKS TO ALL for the input.

 

[webdawg]

What does below 0 mean?

Not sure what the 5.3 back pressure should be, but mine (4.2) was less than 1/2 PSI at 3000, averaging around 1/4 PSI.

May not be the cat, but it's pretty high imo, may not be enough to cause your issues.

He meant the pressure was below 0.

 

[gmcman]

I pulled the upstream O2 sensor and drove it. Noisy but the power was there through out the entire pedal 100%. It sonded like i was driving an old John deer. NEXT STEP replace or punch out cat. PROBLEM SOLVED. THANKS TO ALL for the input.

Don't rule out the cat yet, it's very likely your culprit, but......check the tailpipe for any hair-like strands protruding from the tailpipe.

The packing in the resonator has been known to clog up the tailpipe. This is the less likely candidate, but check first.

I would also replace both oxygen sensors when you replace the cat...personally I use AC but that's up to you...I like OEM when emissions related items are Involved. If you are due, may be wise to replace spark plugs also......AC 41-103.

 

[gmcman]

He meant the pressure was below 0.

That's what I figured but wanted to be sure.....generally don't see a steady vacuum in the exhaust, maybe a needle fluctuation if a valve is hanging but I thought that was odd with our platform.

 

[LT1transAM]

Just got my Bluetooth scanner. I'll try and see what I can find.

 

[LT1transAM]

Just got a scanner that reads live data.i compared my good running trailblazer to this one. Only really big difference I see is the long term fuel trim. Mine sits at -5.5 at idle. And the other one was at I think 3. When I rev the bad one it goes to -15. I don't remember for sure but the good one was going into the positive.

 

[LT1transAM]

Just tested fuel pressure again. Car off and primed, 52psi. Same when driving. It never moves.

 

[LT1transAM]

It's getting to the point now that if you even give it gas, it just cuts out and doesn't want to move.

 

[LT1transAM]

Unbolted the converter at the manifold and let it hang. Took for a ride and the power was restored.