renix running issue

I've been having an issue with my 1990 limited for the past 2 months now after driving it for over a year mostly problem free. When I first start it, it runs terribly. It almost feels as though a cylinder isn't firing.
However, once I have been at operating temp for 5 to 10 mins of driving the problem completely goes away. I tested the Intake manifold temp sensor as well as the coolant temp sensor and they're both working the way they are supposed to. Plug and wires are new as well.
I am at a loss at this point, I can think of reasons why it would run badly, but not many reasons why the problem would go away after the jeep is warm. Is there a sensor that is used during closed loop that drops out in open loop? Any help would be greatly appreciated. Thanks

check ur PM's

O2 sensor has an internal heater. Either it is not getting the 12-14 volts for the internal heater,or the internal heater is bad, is my best guess. Once the engine gets good and hot the O2 sensor stays hot enough to reach proper operating temperature.
Easy to test this theory with a volt meter. Analog meter works best for testing the O2 sensor output. Yours should be running from 1-4 volts at idle, and about 2.45 volts at 2000 rpm. One wire, large gauge, usually orange is 12-14 volts for the heater. Wire 2 is ground, wire three is O2 sensor signal.

I guess a leaking injector, stuck injector, my be un-sticking when hot, or if it does only with a hot engine start up, I would guess a heat soak problem, vapor lock on the injector?

The key is the fact that it goes it aways. Try disconnecting the O2 sensor (engine off) and see if runs fine right at start up. IF it does, the O2 sensor is your problem, or the wiring to it.

got a new o2 sensor, still has the miss when it first starts but now it goes away quicker. I'm thinking something might be up with fuel pressure.

You could have an intake manifold leak that closes up when the metal expands. Check the tightness of the mounting hardware.

Sounds like the O2 sensor helped?

Might still have a leaky injector that bleeds off the fuel rail pressure after shut down on one cylinder, then takes a 5 to 15 seconds for the engine to get over the excess fuel and odd data it produces.

Yeah I was thinking along the lines of an injector issue. Luckily I have tons of friends with mustangs. Two of them with modified 5.0 mustangs. They happened to have stock injectors laying around so I'm getting a set of those for $40 tomorrow and putting them on. :greensmok

Which coolant temp sensor did you test?