Misfires, hesitation and lack of power when hot

Here is one of the nicest-running van we ever had with a 3.1L V6 TBI Chevrolet engine that always ran too good to be true.

Few days ago it developed a misfire condition that is very pronounced when the engine is hot/at operating temperature - under load it hesitates like it's running in 4 cylinders out of 6. When cold it starts right away and runs good & smooth.

Now the van can't go anywhere - it misfires so severely that the engine stalls and dies at idle.

What we tried so far, to no effect, was:
- changing the spark plugs & wires
- replace the distributor cap & rotor
- replaced the EGR valve
- replaced the IAC valve
- replaced the MAP sensor

Any ideas what we should be looking at next?

We had a lot of 3.1L Chevy TBI engines and none ran so smooth like this one and, all of a sudden, none ran that bad in such a short time...

You may be running lean.

Check your fuel pressure . Change your fuel filter if pressure is down.
CHeck your fuel pressure regulator if your engine has one. CHeck it for leaking gas= bad diapraghm(3800 series have these).

Our 3800 van was having problems some years ago, and the fuel pressure as
around 20lbs at the rail...changed pump ASAP

Have you checked coil output? Coil external or internal to the cap? If external check spark right out of the coil...otherwise at the ends of each ignition wire...

If spark is ok, see if spraying a little carb cleaner into the throttle body helps(if it does, then fuel pressure or injectors or even the ECM could be the problem)....

If still ok, check for vac leaks....check all vac hoses for cracks....

Check vac line to f/p regulator for fuel....

If still a problem, then we could have a mechanical problem...you will have to do a compression test....

Fuel pressure with the TBI is much lower than with the 3.8 injectors...around 12 psi ...
And just to be sure, Meridivs.......as you have discussed that before...you are not running on propane...are you ?

This one was running only on regular gas. The engine worked smoothly like a charm until 2 weeks ago when it started to act-up.

Checked all vac hoses, they appear in good order, replaced MAP, IAC, EGR and the whole ignition system (plugs, wires, cap & rotor) just to rule them out.

The only thing that improved something was changing the fuel filter and filling the new one with injector/valve cleaner before installing. Car ran great for two hours after that then it started to misfire again.

I'm thinking about intake gasket (checked it with a propane source and we can't notice any change in rpms).

A mechanic suggested that two valves might be acting up (losing compression in the cylinders) but I doubt that this can happen out of the blue... Any idea on what might be causing these misfires?

The only thing that improved something was changing the fuel filter and filling the new one with injector/valve cleaner before installing.

Makes me wonder if a fuel injector is going bad.....

...The only thing that improved something was changing the fuel filter and filling the new one with injector/valve cleaner before installing. Car ran great for two hours after that then it started to misfire again....

I'm tempted to think the new filter ran easy first then once soaked became harder again to pass fuel through.....500hpgtx suggested to verify fuel pressure...12 psi for the TBI. ...I'd verify. Fuel pump might be tired....or at least verify new filter..and the old one too...for easy flow through...

I had a similar problem with my 3.8 years ago...and the problem was the MAF sensor gone wild and not measuring air flow correctly, delivering lean gas mixture. The TBI 3.1 does not have MAF but it has a MAP sensor ...manifold pressure...and a false data might procure same consequence.
Will check for ways to determine MAP condition...

MAP sensor was already replaced just to rule it out of the equation.

The only thing still standing right now is the fuel system/injectors or some mechanical issue (valves not closing).

I would give the fuel pump/pressure a test and take it from there...

Could lack of fuel cause misfires in the cylinders/exhaust? Interesting enough all spark plugs look good and clean even if the engine misfires badly, even under load.