2002 GMC Envoy XL No Power. Restricted exhaust?

[George B]

2002 GMC Envoy, 4X4, 4.2L I6 262,000 Miles
No DTCs and none pending
Solid 58 PSI fuel pressure while running. Stays there when exhibiting the problem
New set of Delco Iridium plugs
clean fuel and air filter
Oil has 1,000 miles on it.

The wife said her Envoy didn't seem to have the power it used to. Since I am driving it while my truck is in the body shop I noticed it is worse than just less power than it used to have. This poor thing won't rev above 4,000 rpm! It also has a pretty regular stumble off idle but not all the time. I threw a set of plugs at it since they had of 80,000 on them, checked the air filter, checked for a clean TB and checked the fuel pressure.

I am thinking a plugged cat or plugged muffler but ran out of time to separate the exhaust behind the cat to test the theory. It is currently our only vehicle for the week so I may have to wait until the weekend and just drive it nicely until then.

So without any codes what is the chance its the cat and not the muffler? Wouldn't a plugged cat throw a code? Unfortunately I don't' have a very good scanner at the moment as I have yet to order a Tech2. I would like to diagnose this without if possible.

 

[George B]

Got up early and pulled the up stream O2 sensor. Seems to have picked up some power from opening that port. I am leaning towards a plugged cat at this point.

One thing I read was that a stuck thermostat can contribute to a cat failure. This truck had a failed thermostat that didn’t really show up until last fall when she said her heater wasn’t very warm. Not sure how long it was like that but may have been all summer. Never threw a code for it since it was close to temp but not quite getting there.

 

[OR VietVet]

Old plugs/wires/coils, misfires and unburned fuel, thermostat, O2 sensors and just age can contribute to clogged cats. If you do the cats, do the sensors at the same time. What brand of thermo did you install? Check with a temp gun and report back with inlet and out temps on the cats when completely warmed up. Get a back pressure reading at a shop you trust. What is the vacuum reading when you accel the engine and experience the loss of power? Does it drop when you accel and then climb back toward normal when you maintain speed. Back pressure can damage engine internals.

 

[George B]

Old plugs/wires/coils, misfires and unburned fuel, thermostat, O2 sensors and just age can contribute to clogged cats. If you do the cats, do the sensors at the same time. What brand of thermo did you install? Check with a temp gun and report back with inlet and out temps on the cats when completely warmed up. Get a back pressure reading at a shop you trust. What is the vacuum reading when you accel the engine and experience the loss of power? Does it drop when you accel and then climb back toward normal when you maintain speed. Back pressure can damage engine internals.

I installed a Stant thermostat back in November. The seal ring around the valve plate had partially popped off and wouldn't allow it to close all the way. I bet she drove it all summer like that. I am going to read the vacuum this evening and I can get a temp reading as well. Wish I had a back pressure tester... Maybe I can borrow one form my friends at the parts store.

I have a new cat on the way anyhow and will continue to try to confirm before I go ahead and break all the exhaust manifold studs off LOL.

 

[OR VietVet]

Don't forget the O2 sensors.

https://www.rockauto.com/en/moreinfo.php?pk=373054&cc=1401275&jsn=2211

https://www.rockauto.com/en/moreinfo.php?pk=437886&cc=1401275&jsn=2212

 

[George B]

Swapped the cat over to a walker unit tonight. Wasn’t too terrible but I cheated and slipped it out without taking the cross member out. Just jacked the trans up and let the mount flop.

It did have quite a bit of restriction. We used the leaf blower to compare it to the new on e and it was apparent it had an issue.

With that done almost all the power is restored and it runs out on the highway like it used to. One remaining issue is some hesitation off idle when taking off from a stop sign. I cleaned the throttle body up real well on the bench and reinstalled it. After 30 min with the battery unplugged she is better but still not quite there.

Against my better judgement I reused both O2 sensors but am considering replacing sensor 1 in case it is lazy but I am not very optimistic it is the issue. Sensor 2 only reports the cat function so I am not worried about that one.

 

[George B]

A follow up. Running much better now. I did a throttle body cleaning and left the battery disconnected for an hour. Now that it has relearned it runs great.