1999 GMC Jimmy hesitation issue

I used the search feature and got a few hits with the same exact issue but I feel I must start my own thread for advice before I just throw parts at my truck. I have a 1999 Jimmy and I have done extensive maintenance to it. I bought it with about 170,000KM on it. It was well card for by the original owner that drove it up to 165,000KM. The next owner had it for 3 years and only drove it for roughly 5000KM. I believe all the second owner did was put gas in it. Anyhow, I have touched nearly every system in this truck, changed all the fluids and tune up parts. You name it, I have touched it even if it was just for inspection. But I digress.....

After I start up the truck and go, I get a strong hesitation. Does not matter if the truck is warm or cold as well, the hesitation is always there. After a few minutes it will be gone and the truck is as strong as an ox. I am getting expected gas mileage as well, I'd dare guess around 20mpg combined. I even just changed the intake manifold gaskets as they were leaking. Any ideas?

This just has me perplexed as the engine runs beautifully and starts within 2 seconds of turning the key even on the coldest morning. I should also add it is not throwing any codes at all as well. I have driven on the highway for many hours with no issues. Same with towing my 2000lb camper, never an issue. This only happens for the first minute or so after starting. If I warn the truck up for 2-3 minutes it does not occur.

Check fuel pressure: ignition in RUN position, engine off, fuel pump running, must be 60psi to 66psi. Pressure must remain above 55psi for 3 to 5 minutes after the fuel pump shuts off. Post your results.

OK, I have access to a pressure tester and I will check it tomorrow and post my results. I have to say I am also thinking fuel pressure, the only thing that keeps me guessing otherwise is that I seem to have plenty of volume as I can hold 75mph on the highway for hours on end with no issue. I am positive there are no internal/external leaks and the fuel filter was changed by me less than a few months ago. Thanks for the advice and I will be certain to post back.

UPDATE:

OK, brought my truck to my buddies shop. We put a Tech 2 scanner on it and a fuel pressure guage directly on the rail.

Fuel pressure jumps straight to 65PSI when started and drops to 60PSI at idle. The second you hit the gas the pressure builds back up to 65. Took the truck for a ride and the pressure never dropped a single PSI, even when it was hesitating. It would even get as high as 65PSI under WOT. During the entire process it remains above 60PSI. Does not look like a fuel issue.

However, after checking the Tech 2 which we had taking a movie while driving, we find that the EGR valve had opened up during one frame when it started stubling whereas the PCM was not asking it to open. There were a few other frames of erratic EGR behavior as well.

Now my catalytic converter DOES have some issues. The media inside it is loose and it rattles around. My thoughts on the diagnosis as of right now is a partiall clogged catalytic converter. All other parameters on the TECH 2 seemed within spec during the testing.

Before I get a new cat I am going to do a back pressure test, but is there anything else anyone can think to look at while I am here? I want this problem solved before camping season starts since I tow a 2000+ lb trailer and my wife and 2 kids as well.

Thanks in advance!

The fuel pressure and leakdown specs are for KOEO - key on, engine off. Not while running.

If you have some rattling in the cat I would not mess around, I would get it replaced. I just went through this with a car. The cat would rattle until warm, then it would be gone. Performance didn't seem to suffer. A couple of months later it plugged up, engine would barely hit 3k in neutral. A bunch of catalyst came flying out of the exhaust and it ran fine again. A week ago I got a call saying the car died. I went to look at it and sure enough cat was blocked again, but car wouldn't run this time. I tried to pull the O2, but it was rusted in place. So I pulled out the drill and made a couple of 1/2 inch holes in the pipe. It ran, but was missing. Limped it to my place, pulled the cat and replaced it. Started it up, bad miss. Threw it on the scanner, miss-fire on 3,4 and 5. Pulled plug 4, the ceramic was broken. Pulled plug 5, the ground was touching the electrode. Pulled plug 3, same as plug 5. Replaced all plugs, runs great again. Point is, the clogged cat caused this, and the only symptom was a slight rattle at start-up.

The fuel pressure and leakdown specs are for KOEO - key on, engine off. Not while running.

I should have clarified, I did test it in the manner you are talking about. I also did a test that requires putting a temporary shut off in the return line that was suggested to me via PM. All my testing on the fuel system checked out fine. Key on I got around 65PSI and key off it never dropped under 60PSI for over 10 minutes. I repeated this test 3 different times during all my testing of today.

As for the catalytic converter, I do have one on order (only $400 too @*#*&()_)!#&$@^&). I really hate how expensive parts are in Canada. Anyhow, I will report back after I install the cat if it fixed it or not. Either way, I do understand it needs to be changed.

OK, got a chance to work on this issue today. Truck is fixed. Mass Airflow sensor was a mess. 15 minutes of patience, some q-tips and some rubbing alcohol has fixed my hesitation issue and my truck has much more power off the line as well.