Engine - rough idle & low temp

I just replaced the fuel pump in my '92, and it's running again (yeah!) but idles rough.

It was idling rough before the fuel pump died, so I thought that the pump was probably the cause. Not so much.

It has also been running on the cold side - something that it has never done.
For example, this morning the air temp was 30 degrees but even after driving for 25 minutes the engine was only at 135 to 140 degrees.

Is it reasonable to suspect the thermostat for both issues?
I just replaced the thermostat 4 or 5 months ago, but maybe I got a bad unit.

I also replaced my cap, rotor and spark plugs along with the fuel pump (and filter). Have also recently run some injector cleaner through the system (before the fuel pump bought the farm).

Thermostat shouldn't have anything to do with rough idle or running temps that low. Running rich would account for both. Possible O2 sensor, possible MAP sensor - when's the last time you serviced either of those? Does the exhaust smell rich?

lilredwagn said:

Thermostat shouldn't have anything to do with rough idle or running temps that low. Running rich would account for both. Possible O2 sensor, possible MAP sensor - when's the last time you serviced either of those? Does the exhaust smell rich?

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Replaced the O2 sensor in the spring of '05, but I've never done anything with the MAP sensor.
Exhaust doesn't smell rich, but it did billow a bit of smoke when I first started it up after replacing the fuel filter.

My father-in-law has a Snap-On scanner, and there were no codes or anything out of the ordinary as far as readings were concerned

OK, so I'm thinking that my thermostat (even though relatively new) might be stuck in the 'safety open' position. Been driving around this afternoon (55 degree air temperature) and never even get close to normal operating temp.

I'm also going to get a new MAP sensor. It's about 15 years old, so it's a pretty safe bet and not too expensive even if it's not the fix.

UPDATE:
Replaced the thermostat - good thing, because the existing one (only 4 months old) was wide open. That's what I get for buying the 'premium' unit with the 'opens when broken' feature.
Engine is now running steadily at a warm & fuzzy 195 degrees - just in time for fall!

Also replaced the MAP sensor with a Borg-Warner unit.

Running better now, but still has a slight miss at idle.
Haven't replaced the o2 sensor, because it's not that old and there is no check engine light.

Any other possible causes for the miss before I drop some cash on an o2 sensor?

My brother was having problems with an erratic idle on his '98 TJ, and it turned out to be a severely cracked header. I'll have to do a visual inspection of my header tonight...

Find anything out? My cylinder #1 injector is causing a similar miss-at-idle problem, (determined by pulling the connectors one at a time.)

Spidey said:

Any other possible causes for the miss before I drop some cash on an o2 sensor?

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Try cleaning the throttle body and IAC (careful not to hit the TPS) if you haven't yet.

Does it run fine when it's not idling?