I have some symptoms and not sure if it is the plenum gasket or some other problem.

When accelerating, after the truck shifts, it will go to whatever rpm and it feels good, then after a few seconds, it drops about 200-300 rpm and then it has no power. I can move the gas pedal and it does not seem to do much, barely even changes the sound of motor. Truck has a hard time just pulling its own weight up the hills. It is almost like the truck is detuning itself or something.

What is weird is it always does it the same every time, almost like there is a timer and a switch, like clock work. I can tow my utv, but I have to keep power in a sweet spot. Truck seems really under powered, if I am in overdrive, it won't accelerate unless it is flat, and it does so very very slowly.

The impression I get from reading about the cat and plenum gasket is it is just under powered all the time. Mine seems like the ecu is cutting power after it shifts.

Truck is stock except a flow master muffler and 33s. 70k miles. 1999 sport 5.9

 

Don't tow in overdrive. Good way to smoke the trans.

Has your plenum been fixed yet? May want to check that out at least.

How long between first starting, and power falling off? Does it do it in every gear? Or just O/D?

 

I'll check plenum today. Does it in every gear, and it does it the same way in every gear. It really feels like the ecu is pulling timing or something. It is that precise every time. I can make it happen. I don't tow in od, truck will barely pull itself in od unless on flat.

I have read about a "death tune" or something. Is that possible?

 

Sounds to me like you are feeling the torque converter doing its job. It is locking up in D and OD to save fuel and that is normal. However I can still accelerate even in lock up. So x2 on the plenum issue.

 

Could be the converter. I never felt one do that before, it feels like it shifts twice for every gear, it is that apparent. I sure hope it is the plenum, not being able to pull small hills with a light load is not acceptable......

 

I also think it's the plenum and clogged cat. unscrew the precat o2 sensor and take it for a ride. It will be loud, but it is a good way to check the cat.

 

Ok, well....FML.....I got the plenum kit in the mail. Took it all apart and low and behold...the guy before me already did it!!! I looked in the intake when I took the carb off and it didn't look bad, in fact I thought to myself." That looks like an aluminum plate in there ", but I was that far so I took it off anyways. So at least I know my problem isn't the plate. I did not notice a a difference when I unplugged the 02 sensor either.

When I put it all back together though, there are two hose ports on the drivers side of the intake. One goes to the brake booster but I don't know where the other one goes, and I don't have any hoses to put on it. I don't remember if there was something there or not, but that could have been my problem cuz it is a pretty big port. It is pretty loud leak too, I would think I would have noticed it before. Anyone know where this port goes? I have looked at some pics and some only had one port there.

 

I'm not sure what year the PCV went over to the driver side but the brake booster and PCV line are the only large hoses on the driver side that I know of....

 

My PVC goes into the air cleaner. It is a 99

Maybe this truck is ok and I just want more power, it is my first truck. But towing my UTV with trailer, can't be more than 1700 lbs. and it is a dog. Slows Down really bad on the slightest hill in 3rd. Any length to that hill and I have to go to second.

 

Ah. Can you post a pic? Use photobucket or the like to link the img source...