I am actually starting to narrow it down. "I think" . I have a BT Elm327 that I use with my phone and car. I have been driving around and looking for a constant that always happens when it is hesitatiing. The constant is, when I am driving around and the Vacuum pressure raises to -2 to -6 it strarts its acting up. I am guessing that I have a vacuum leak somewhere. Also when sitting at idle my Vac pressure is only -9.1 in/hG which stays constant. I believe it is supposed to be higher than that, like -17 to -21

 

mines at the shop right now, i will let ya know what they say.

 

It was an EGR Valve, issue is all resolved. Once I was able to narrow it down to a Vacuum issue and I brought it to my mechanic. He said that EGR valves can show as a massive vacuum leak.. IT has been replaced and is running great now.

 

I just brought my 2nd Monte 2 months ago and the car starts fine, but when you get it up to about 25 miles an hour it feels like someone if pushing back on the throttle and the engine seems to struggle for the car to get up speed.
No power, and no power going up hills.
Also while this is happening it feels like someone is behind the car bumping into the back of the bumper and you feel the car slightly bucking.
I read many blogs and some of the symptoms that some say is a bad Cat.
I also have 2 codes coming up when I hook it up to the ODB2.
P1626 that has something to do with the Fuel and Security passlock II
The other is a U code meaning the class 2 data line has a short, or is open, is a chafe in the wire somewhere in the spaghetti of wires, what a nightmare.
I have read that a bad EGR valve can cause a vacuum leak.
Was yours a bad EGR valve ? or was it the EGR to Intake passage way that get's blocked up with carbon ?
Also the other day I noticed either coolant or oil leaking from the side of the intake next to the power steering pump on a 3.4 L engine.
I already had it set in my mind that no matter where I bought another GM car with a 3.1 L or 3.4 L that unless the last owner had a maintenance diary or wrote down what repairs were done on the car I automatically know I will have to replace the old intake gasket with the new metal gasket so I won't have the dreaded coolant in the oil experience and cost.
At first I was going to pull the upper intake to clean the EGR passageway to hopefully hope that this is the problem with the lost of power issue.
I sure hope I won't have to buy another Cat for my car.
Did you car experience no power getting up to 35 to 40 miles another ? or no power going up hills with the bad EGR ?
The shuttering ? was it more like the car bucking like it wanted to go but felt like a bad misfiring engine ?
I can make the car go faster when this happens but it pings really bad when I do this.
Bad EGR ? Bad Knock sensor ? Bad Cat ?