94 K2500 350 starts cold but won't keep running

[jbarker]

OK, just put rebuilt engine in. K2500 4WD TBI auto OBD-1. Timing is solid at 0 TDC with magic wire disconnected. When the truck is stone cold it starts right up and runs good. 40-60 seconds later, idle rpm drops, starts running rough and then quits. Maybe get it to run a few revs floored but that's it. When it runs this way, exhaust plume is grey (it's clear when running smooth).
No codes after a no-start (other than 12 which is normal). ECM codes seem to be working because it threw a code 42 when I was timing it with the magic wire disconnected. I did get a code 15 "coolant temp sensor indicates temp <-27F for 30 sec after engine running for 30 sec" just after it quit the last time. I can assure you it's not that cold in NH yet! Disconnected the coolant sensor, still no start.
Both injectors are working - they pulse against closed butterflys when trying to start. Entire fuel system from tank thru fuel regulator is brand new. EGR is new. Have spark. Gas in tank is fresh. Dist cap, rotor, wires, plugs all new. Battery fully charged & helped with 275 amp charger/starter. OK, so the moon is not full. Should I replace the coolant temp sensor? Help!
Thanks,
JB

[jbarker]

Just a little more info on this post.
There are two coolant temp sensors on this engine.
One is threaded into the head midway back on drivers side. Single wire. This one is brand new because I stepped on it & broke it during install.
The other is threaded into the intake manifold next to the thermostat gooseneck. This is a two-wire device and has not been replaced.
I'm told that one of these run the electric fan, which does not exist on this truck. Assume the other runs the temp gage.
JB

[1500cheyenne96]

Could try pulling a plug before you start it and hook it to its wire and let it run untill it quits to see if it is a spark or fuel issue cause it kinda sounds like a faulty ICM or pick up coil

[Kennyray]

the dual wire sender is for the computor, single wire for the gauge

[crabtruck]

The ECT sensor will give that 15 code for lo temp or high resistance.
Also known as no connection. Test it and connections before replace.

ECT sensor uses Yellow to detect temp and the Black is an ECM ground.
Located at thermo housing.
The ECT sender uses a Dk Grn and grounds via block.

Disconnect and Ohm the sensor.
If sensor is problem it will be over 52k ohms.
At 32deg F (freezing) it should be around 9k ohms.
At 60deg F it should be around 4,400 ohms.
At 100deg it should be around 1,500 ohms
At 185deg it shpuld be around 300 ohms.

An Aux Cooling fan ECT switch would be midway on the right/pass side of block.
Uses a single Lt Grn wire that runs fwd to join ECT sensor harness.

Can never have enough ECT's.

Kenny you're going to have to make more of an effort to be more long-winded
er, I mean full of it, er, umm, informative - ahh, that's the one, than me.

[Kennyray]

I'm the type that mothers warned their daughters about, ya know the quite ones.

[crabtruck]

Yeah I do, and mother's didn't say long-WINDED

[Kennyray]

I got the double whammy, I'm also a preachers kid

[jbarker]

Two wire sensor at the thermostat housing was open (resistance = infinity). New sensor ohmed out around where crab had it - makes me feel better.
Now what makes me feel worse - truck now WILL NOT START AT ALL.
No codes. Bounces a little occasionally so I know it's got spark; also triggers my timing light OK. Has gas. Tried disconnecting the sensor and reconnecting the old (open) one to see if that made a difference - nah.
I'm going to go back to the basics - fuel, hard spark, etc. - once it stops raining here.
Thanks,
JB

[jbarker]

So it stopped raining finally. Per my previous post I'm going to check for fuel, hard spark, etc. Pulled two plugs - a little carbon on the gap electrodes due to the previous run-rich issues but not a big deal. Sandblasted them clean. Then noticed that the wire end was not shiny - due to sitting around in my parts bucket in the barn for six months. Not rusty mind you - just not shiny metal, so I blasted them also. Repeated for balance of plugs. Did not regap - had done that at install. Charged battery (which I had done all previous start attempts).

Hit the ignition - instant blammo, running smooth, hitting on all 8, no codes, clear exhaust. I did nothing else beyond the above. This thing would not start for love or money after trying for an hour the day before and a couple times before that (it's not the rain in other words).

Final chapter - still occasionally throws that 15 code that crabtruck described - not sure why as the sensor is new. Still, I ran it for like an hour today and drove it around and it's purring, and temp gauge is working.

Many thanks to all you Chevy forum guys and gals who helped me out on this.
JB