1998 B1500 (5.2l) stumbles periodically at part throttle. Feels kinda like it starts losing fuel pressure, but letting off the gas and repressuring the gas pedal typically resolves the issue. Happens a few times a day.

Thus far I have done a tune up, sans coil, and checked the fuel pressure during driving.

I am leaning toward a vacuum leak, or ecm issue, but am curious what you people think. Thanks for any reply.

 

Did your tune up include new spark plug wires?

You could be running lean. This can be caused by coolant temp sensor out of whack, 02 sensor out of whack(use NTK or Denso, not bosch), or inadequate fuel pressure

 

I had stumbling and stalling at idle before my ECM went out. Keep an eye on rokhard's thread posted this week for more info.

 

The stumbling is worsening this week (after replacing TPS). Typically it begins after periods of unuse and the engine is already warm. Also, at startup, it has been finicky. Repressing the gas pedal helps periodically.

 

Sounds like the "classical" CAT is clogged issue.

Go under the van and use a screw driver handle to whack the CAT. If it meows (haha) really, rattles, then the CAT needs replacing. A clogged CAT will prevent the exhaust pressure from properly being released out to the muffler and the rear pipe.

 

Last item to add in reference to the issue. It seems to has a synchronous stumble. If I hit the gas pedal, approximately 1/2 a second later it begins to lean out. This can be reproduced several times when rehitting the gas pedal. After about 8-10 times the engine seems to get the hint to work properly.

 

Test the map sensor. It is located on the front of the throttle body/air horn.

 

Replaced MAP sensor, no change. Replaced temp sender, no change (although the existing one was "flimsy", and had to remove the alternator to access :/ )

Ordered new O2 sensor BEFORE changing out CAT.

 

If the van has more than 110k - 120k miles on it (and still has the factory cat) you can safely assume that your catalytic converter is now clogged.

You can check for back pressure due to a clogged cat very simply. Check for pressure before the cat by drilling a small hole or remove the front 02 sensor. Check for pressure with a $10 vacuum/pressure gauge. Any reading over a few psi indicates a backpressure problem due to clogged catalytic converter and/or clogged muffler.

Or just remove the front 02 sensor and drive it around the block. The PCM will go into limp mode without that sensor installed so it won't run great but if the stumble is gone, you have found the problem.