98 Ford Ranger 4.0 litre. Intermitent problem where it is very rough running at start- usually after truck sits for several days without starting it. It clears up after warming up about 30 seconds to 1 minute. Also when this happens and I got going sometimes it will cut out under acceleration. I have replaced plugs/wires and did lower intake gasket but no help. It is throwing no codes. Could it be fuel regulator?

 

The fuel could be leaking back to the tank. To check, turn the key to ON for 4-5 seconds, then OFF multiple times before trying to start the truck. That should allow the fuel pump to run and refill the fuel rail. If it now starts without missing, you know what to do next time you let it sit.
tom

 

You may have more than one problem.

After one of those sitting spells, do a KOEO & KOER fuel pressure check, before starting the engine & post the readings.
If you don't have a fuel pressure test gauge, most autoparts stores have a kit in their "Loan-A-Tool" program, for a refunable deposit.

Watch the fuel pressure reading after engine shut down, it should hold pressure for several minutes.
The pressure should be 64 +/-8 psi, the KOER figure may be a little lower than the KOEO figure.

If the fuel rail pressure is low, maybe suspect a clogged fuel filter, weak pump, or faulty pressure regulator, or pinched or crimped line.

The fuel pressure regulator is integral with the fuel pump & is located in the fuel tank.
Sliding the bed back to access the pump is said to be easier than dropping the tank.

If the fuel pressure checks ok, a couple of other things come to mind on the engine cutting out, maybe suspect a acting out fuel pump power relay, so try swapping it for a like one not needed to run the engine, like the A/C power relay.

Maybe a bad electrical connection to the fuel pump.

On the rough idle, if the fuel pressure checks out ok, maybe suspect an acting out/sticking IAC.

More thoughts for pondering, let us know what you find.

 

Get a used oil analysis (UOA) done. It will tell you things, like whether there is any coolant mixing in with the oil (possible cracked head or failed headgasket).

Blackstone Laboratories - Gas Engines

 

Thanks guys. I'll try the fuel pressure check first then go from there.