Greetings JPVortex,
If you give yourself the troubleshooting gift of seeing the live data, then I'll bet that
you will see actionable info leading to a fix on the 1st try. For example, assuming that
your vehicle runs OK when fully warmed up (eliminating lots of troubleshooting variables
elsewhere) ...then an ECT sensor which has lost it's calibrated response to temperature
changes would explain the symptoms you describe. (Especially the no fast idle when cold,
but idle OK at normal temps.)
In the '94 GM Service Manual
** the guidance is that when the engine is cold, a good
ECT sensor will measure within 5° F of actual temps. This is pretty tight, so obviously
a relatively accurate temp sensing must have been considered a key component in the
strategy of meeting cold engine customer driveability goals. (!)
So if you have access to the right scan tool, comparing what the computer thinks is
the ambient temp vs reality would be a quick & easy go/no-go check. (ie: may not
prove that the sensor is completely accurate throughout the range, but will find a
gross failure.)
OK, definitely a solid ongoing maintenance item. And even if this didn't clear up the
cold running issue, it wasn't a parts cannon misfire at all. Why? Because it does help
the troubleshooting process to know both what to consider, and as well as we know
what it's not.
Given this, the rebuilt TBI unit is off the troubleshooting table for now. (ie: the problem
definition is more of a incorrect fueling calculation delivered accurately vs a correct fueling
calculation delivered incorrectly due to a malfunctioning TBI assembly. Does this make sense?)
Alright, at this point I'm going to offer up something that is a bit controversial in this forum.
Ideally, in order to find/fix the issue on the very first try, you would start & finish your
troubleshooting session with a proven-good scan tool & monitoring the live data.
(ie: looking over the computer's shoulder and seeing what it's perception of reality is.)
I have no argument with this. Actually it would be my personal preference as well.
But at the same time I realize that there's no scan tool available (or available anytime soon)
you need the truck to go work, and instead of just throwing parts at it out of desperation,
you'd like to at least follow some sort of troubleshooting approach out of the GM service manual.
If you find yourself in this pickle, then if I was your neighbor we'd scare up an ohmmeter and measure your
ECT sensor resistance (still on the engine) against this Temperature vs Resistance chart:
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Cold unstarted engine:
68° F garage?
3,250 ohms
5° F driveway?
21,450 ohms
Engine fully warmed up:
(
195° F thermostat)
241 ohms
Notice we're
not trying to see only a couple ohm difference between a
stone cold unstarted engine vs a happy engine fully up to temp. (I
hate those 'angels dancing on the head of a pin' type measurements. :0)
The resistance readings are large, clear, and unambiguous. This
is a go/no-go check that can be done anywhere that there's a
reliable multimeter available. (Bonus points for verifying/proving these
readings to yourself when your GMT400 is operating perfectly *before*
needing to troubleshoot a genuine cold-start or cold-running issue.)
One more thing. One of the major strengths/advantages of the TBI
setup is it's relative simplicity. When it's all said and done, the 4
primary inputs that the TBI relies upon to work properly are:
* How hot or cold is the engine?
ECT
* What is the load? (inferred from the manifold 'vacuum')
MAP
* What is the RPM? (Ignition)
* What is the driver demanding?
TPS
Pulling this all together, if the engine runs poorly 100% of the time,
we have to figure out pretty much everything, including the mechanical
health of the engine.
But IF the engine runs well at normal operating temps, but only gets it
wrong when it's cold, then that allows us to remove lots of stuff from
the troubleshooting table. Especially when the TBI unit has been recently rebuilt.
Hopefully you get to use live data to prove/disprove that the ECT sensor is no longer
reporting accurately.
But while waiting for the live data stuff to show up, if you feel like it you could grab
an ohmmeter off the shelf & do the quick temp vs resistance check that GM brewed
up for their techs.
And in the perfect world? You would do BOTH, and show the forum how this
cross-check agreed or disagreed with each other.
Best of luck. And let us know what you discover.
Cheers --
**Highly recommend that you download your own copy of the '94 FSM from here. ('94 Factory Service Manuals)