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Dakota runs rough - sometimes

My truck occasionally will not accelerate when I press the gas. In fact, it “gags” and actually slows down or coughs. Most noticeable in 1st or 2nd gear (manual trans, but it will happen at any speed. It never stalls, although it will idle rough sometimes. I can cure it by flooring the gas pedal. I replaced plugs and wires and throttle position sensor, but it hasn’t helped. Someone has suggested something loose in catalytic converter blocking exhaust. Any thoughts?

Last ‘tune up’?

How old is the fuel filter and air filter?

You may have a dirty throttle body or a faulty IAC valve.

I took it in for a tune up last May because of this problem. Still acting up. New plugs, wires, air filter - but they couldn’t get it to repeat the problem. Mechanic said the fuel pressure was ok, and they checked for vacuum leaks, but I can’t say that they replaced the fuel filter. What is an IAC valve? Do you discount the catalytic converter blockage as a possibility, as that would be an expensive experiment.

What year is the Dakota? Is it carburated? Throttle body injected? Multiport injected? All those things make a difference. The best test for a blocked cat. is to drill a hole in the exhaust just prior to the and look for a pressure of 3???PSI or higher. You might get with a shop and just ask them if they can perform that test. I think it is 3 PSI, maybe it’s 2, either way you’ll need a gage that can read that low PSI, when you’re done just put a sheet metal screw in the hole to seal it up. Usually with a blocked cat, you cannot “cure it” by flooring the go pedal.

It is a 1999 with a 3.9L SMPI V6 ohv 12 valve engine. I think it is multiport injected, as there is a rigid tube that goes to each cylinder at the spark plug.

Good idea on the catalytic converter. I question whether that can be the problem, but measuring a spike in pressure would confirm.

I would think a problem in the fuel system would be more likely. Any other suggestions?

Was a new fuel filter in there too? Sometimes people will use ok fuel pressure as a reason to say the fuel filter is fine. But it is also the case that fuel pressure testing is often not done under load - do you know how the fp was tested? Or whether you got a nee fuel filter or not?