1999 Vortec 135k mi hesitates at hwy speeds...

[DeadheadNM]

Anyone deal with a similar issue? The vortec 350 in my 99 Suburban hesitates/stutters briefly when under hard acceleration at highway speeds and when accelerating uphill but otherwise runs great. The motor was professionally tuned about 15k miles ago. Fuel filter recently changed. No check engine light.

Thanks, Pete

[B. W.]

I've had that issue several times on different rigs. It's usually the intake manifold gaskets failing. The engine will also be going through a little coolant. If it's setting a 420 code & no misfire codes it can be the pressure regulator (it's inside the manifold on the side of the spider)

[sweetk30]

years ago i did a tune up and used a non-factory spark plug brand . tryed to save a few bucks and pinch my pennys .

well my 96 350 vortec had a skip at highway speed and some load .

well for kicks i swapped in the old spark plugs orignal acdelco and it went away . so i got a new set of acdelco and installed them and no problems .

my figuring is there designed so tight these days with all the sensors and feedback that it didnt like the other brand spark plugs .

i have also fixed a few for people with the same non factory plugs and skips . put in factory brand plugs and problem gone .

this might be your problem or not . just a idea .

[LBJ]

Believe it or not, your gas cap may need replacing. Had the same symptoms before. Cap wasn't venting properly. Worth a look.

[95 S_Trucker]

Originally Posted by LBJ

Believe it or not, your gas cap may need replacing. Had the same symptoms before. Cap wasn't venting properly. Worth a look.

On a pre emissions carbed motor that may be true,

But not on a fuel injected truck that has fuel vapor management

Originally Posted by B. W.

I've had that issue several times on different rigs. It's usually the intake manifold gaskets failing. The engine will also be going through a little coolant. If it's setting a 420 code & no misfire codes it can be the pressure regulator (it's inside the manifold on the side of the spider)

If a code did set with a leaking fuel pressure regulator, it would be a misfire code, or a P0172(Rich B1) or a P0175(Rich B2).

A P0420 is a catalyst efficiency code(for bank 1, P0430 is bank 2)



OP: Check all parts of the ignition system. Plugs, wires, cap, and rotor. Its amazing what a little corrosion on the terminals in the cap can do. Like stated earlier, these trucks are picky about plugs. Aftermarket caps and rotors do not last like the originals did.

The poppet fuel injectors can stick too.

[Tom]

Fuel pressure.

[DeadheadNM]

Thanks for the tips guys. I'll run through these items next week when I'm off and will report back. I've used only AC Delco parts on the motor fwiw. Pete

[DeadheadNM]

Found two spark plugs that were really oil fouled. Maybe these were misfiring. I'll give it a run on the highway tomorrow now that fresh plugs are installed. I suspect the valve stem seals are bad...

[DeadheadNM]

Ran fine at highway speed with new plugs. It's headed to a local garage for new seals. Thanks for all the input, I appreciate the help. Pete