Envoy surging/hesitating at high speeds

PHEOBE456

MEMBER
2003 GMC ENVOY
135,000 MILES

We bought this vehicle used last month and since then it has spent more time at the mechanics and dealership than it has on the road.
It drives well, until it hits 80-100km/hr then it feels like it is surging and doesn't seem to have the power it should. We tried to tow our camper with it but it was a struggle to get it to highway speeds. It felt like it wanted to change gears.
No check engine light or codes present. Not sure about fuel mileage as we haven't had it very long.

It has 216KM on it and the following has been done so far:
Changed spark plugs (AC Delco)
Cleaned Throttle body
New air filter
Changed Transmission fluid
Replaced Shift Solenoids
Diagnostic done, no codes
Back pressure test done for Cat.
Upgraded Tranny to Engine software

One thing we do know is that it needs a new manifold, due to ticking and a visible crack.

Took it to the dealership a few times, no help. Also took it to a local Transmission shop for diagnostic, no codes and was told it had "mushy" shifting

We really have no clue what to do now or where to turn. Any ideas would be greatly appreciated!

7 Replies

JACOBANDNICKOLAS

MECHANIC
109,885 POSTS

Which manifold is cracked?

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Monday, July 15th, 2013 AT 8:22 PM

PHEOBE456

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4 POSTS

The exhaust manifold. We haven't replaced it yet as we have been trying to figure out the other problem. Not sure if we should even keep the vehicle. So frustrated!

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Monday, July 15th, 2013 AT 8:25 PM

JACOBANDNICKOLAS

MECHANIC
109,885 POSTS

Okay, I was thinking intake manifold. The exhaust could cause an issue if it is leaking bad enough. The O2 sensor wouldn't know what to do. Have you checked the fuel pump pressure to make sure it is within the manufacturer's specs?

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Monday, July 15th, 2013 AT 8:29 PM

PHEOBE456

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4 POSTS

Haven't checked the fuel pump pressure. Does that have to be done at the dealership? Would the O2 sensor or the fuel pump throw a code? If its the fuel pump would it only happen at higher speeds or when trying to get up and go?
Sorry for all the questions! I really appreciate the help.

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Tuesday, July 16th, 2013 AT 5:50 AM

JACOBANDNICKOLAS

MECHANIC
109,885 POSTS

I'm glad to help. As far as throwing codes, the O2 sensor should if it is working outside its parameters. However, the fuel pump will not.

As far as checking fuel pump pressure, it is easy. You will need a gauge to check it with, but most parts stores will lend you one. Refer to our home page under the DIY section. There you will find both directions and a video to help you check the pressure.

As far as the O2 sensor, you need to have a real time scanner that will show you what parameters the sensor is working within as the engine runs. That one is a little harder to do.