Nearly impossible to start; idles rough; revs fine?

Took forever to crank over today. Seemed like no spark, or only a couple cylinders firing. Eventually it started but idled roughly and a bit low on RPM. Throttle responds normally and revs just fine. I only moved it around in a parking lot and didn't attempt driving it down the road.

No CEL, so where to begin?

 

Coolant temp sensor, maybe

 

So if I can get it started, run until hot and see how it is?

Or, once I learn where that is I suppose I could put an ohm meter on it?

Wouldn't a bad temp sensor default to making it do the opposite? (Start fine cold but not ever tell the ECU engine hot?)

 

If the ECT is reading too hot when cold its not going to richen the fuel mixture like normal. Best way to see is if you hook a scan tool up and look at your coolant temp.
The gauge on the dash is a seperate sensor so dont rely on that.

 

I don't have a scan tool. But if it's mostly likely to be a bad sensor and nothing ignition or fuel system related, I might as well just buy the sensor and get back on the road.

 

I'm also wondering, if due to the ECU being lied to by a bad sensor, why my air/fuel ratio is bad from the start (and at idle), but just off idle and at higher RPMs the ECU making bad decisions suddenly does not? See "Throttle responds normally and revs just fine" from my original post.

It's got an IAC valve, right? I realize symptoms could be different from what I'm experiencing. High idle and such that aren't affecting me. If the rain lets up today I may try and run the engine until it warms up and/or drive it around to better assess all behavior. Again, there's no CEL.

 

How many miles on the spark plugs?

 

No clue, why? Do bad plugs only make it run bad at idle or start?

 

I’d start a plugs then look for vacuum leaks

 

This morning it immediately started up and ran fine, if not perfectly. The cold idle speed was low, around 500 but everything else worked as expected. After the engine was hot everything was still great and the idle, even better. Drove it around town, nailed the throttle, everything OK. Restarting was not a problem; always had plenty of power, no misfires.

I'm ready to declare the IAC is at least dirty, if not its plunger actually having an intermittent problem engaging on some mornings.

I'm out of town this week and will tear it down when I get back. My understanding is that there's about a gallon of antifreeze to purge, the throttle body to remove and so on.