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Sluggish acceleration despite high rpm

57K views 16 replies 11 participants last post by  TomasC69 
#1 ·
First time poster, long time lurker. '05 Envoy XL 4.2 Runs like a top except when accelerating to pass another car or up a grade. Becomes very sluggish despite revving to a high rpm. MPG has dropped 2 mpg. Cleaned throttle body and changed plugs (AC iridium). Tranny is fine. Replaced throttle position sensor. No change. Accelerator Pedal Position sensor? Bear in mind, NO CODES thrown. Thanks!
 
#2 ·
this really sounds like a clogged catalytic convertor.
They work ok at regular speeds... but when you step on it- like going up a hill or trying to pass.. you revv-- but suddenly it bogs dows...

its unable to expell the increase in exhaust gases--- so it chokes up--- and loses power...
 
#5 ·
more info

93k miles. Changed pedal position sensor, and as previously mentioned, the throttle position sensor. Again, no codes light up despite the ongoing problem. At level, constant speed, I can feel a slight surge and decline with rpms. Will check the converter/exhaust angle. Thanks for any input as this has me stumped.
 
#10 ·
Same issue

I just replaced my original plugs with 197k miles on them... way to go GM. Cleaned the throttle body as well as new air filter. Same thing... it's like I'm towing something. I put the throttle down and sometimes bam and most of the time I feel I'm going backwards. So, sounds like a catalytic converter but like the previous poster asked, is there a way for us lay people to be sure it is that?
 
#13 ·
The front oxygen sensor is in the exhaust manifold and is easily seen and accessed on the right side of the engine. Yes it will be loud and yes it will throw a code, but it is only temporary; you only need to pull the sensor out long enough to see if there is a difference in acceleration.

Another way is to simply take it to a good muffler shop and ask for an exhaust backpressure test. They will pull the front O2 sensor (you will need a special slotted socket for this) and replace it with a simple pressure gauge. They will read the pressure at idle and again at 2500 RPM. Pressure should be 0 at idle and no more than 3 or 4 PSI at 2500 RPM.

Here is a tip. If your front O2 sensor has been in for 5 years or more, or has more than 100,000 miles on it, you might as well replace it before it seizes to the exhaust manifold. It should only be replaced by an AC Delco sensor, and if you bring a new one to the muffler shop, they can replace the old one with the new one. You WILL need the special slotted socket to install the new sensor, although you can use an ordinary deep socket to pull off the old one if you cut the wires off.

Hope this helps!
 
#14 ·
This sounds like what my TrailBlazer is going through. Slow acceleration, when I try to accelerate, the engine whines, but it doesn't shift into the next gear until I eeeease into it, and it's taking me a while to reach a top speed. And when I go uphill, I feel the truck struggling.

I have an 04 TrailBlazer, by the way.
 
#16 · (Edited)
Found the vacuum leak! FINALLY!!

I have been chasing the lack of acceleration and sluggish I6 for way too long. The issue is a vacuum leak, that no one has seem to of found. I have seriously replaced everything on my TB '04. Never could find the secret vacuum leak. Well I found it and yall will not believe it, but I think your gonna love me. I tested the entire side of the engine. NO vacuum leaks. I tested the brake booster. it held at 20-21 psi (I think that's right, from memory) Never did I test the brake booster check valve. Why would I? We its a very cheap part, so I changed it. My TB runs like a champ. For over 2 weeks now, I have not had any power issues or stumble. All power and acceleration has come back and she runs like she is new. Even brakes and stops like it was new. The reason for the brake booster check valve causing the vacuum leak is this. It is supposed to be a one way valve. It holds the pressure for your brake booster but the valve can get weak or fail. So it either wont seal or stays sealed or open. I don't know what mine was doing, but it sure was not doing its job. I LOVE my TB again. I really hope that I help someone. I could not keep this to myself. I know how many people are out there that have lost their mind over this loss of acceleration and sluggish, junk. I know that a brake booster check valve sounds crazy, but its @ $11-$20 on amazon. Its insanely easy to swap out. No reason not to give it a shot. I seriously was about to take my TB into the transmission shop because it wasn't shifting right or stopping right. I also already have a new brake booster and master cylinder, ready to go, but this little valve solved SO SO many issues.

Just FYI:
My part number for my 2004 LS trailblazer I6 is: ACDelco 179-1266
 
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