Hard start when warm 94 5.7l 3500 5speed just installed motor

[Road Trip]

Picked up snap on Solis pro.

Nice. This is one of those pieces of test equipment will more than pay for itself over time by allowing you to use
data instead of parts to narrow down the possibilities. (!)

It's reading -40 so you were spot on. I'll be removing Teflon from the threads that i put on like a dumb ass (rookie mistake)

You are in a good place, for you get to precisely follow the data instead of chucking parts in the general direction of the
problem. As for the 'rookie mistake', truth be told that's how we all honed our repair chops. And posting the truth
allows others new to the old truck hobby see what does & doesn't work even better than just reading about it in a
dry service manual. (!)

Good stuff. Once you post the final fix here, this thread will have gone from just another starting issue dead ended
to a place where there's a hard start symptom > FSM theory > live data confirms theory > physical fix.

And the more that we get to document the way these machines fail (and more importantly how to troubleshoot/fix
these failures) ...the easier it is for fellow GMT400 owners to keep their prized machines out on the road reliably.

And this is a big reason why I joined this forum -- the talent is deep in here, and the info is densely packed.
Great community!

 

[fancyTBI]

Picked up snap on Solis pro

That is a great scanner. It is what I have with 14.2 software. You have done more for your truck by purchasing a great diagnostic tool than you may realize!

 

[chris032188]

I've seen the hard-starting issue on other vehicles. The symptom of "warm starts succeed only with full throttle" - that suggest to me that the engine is being fed a Too Rich mixture. By opening the throttle all the way, you are introducing enough air to offset the too rich condition.

In my experience - this was caused by the engine computer interpreting coolant temperature as an indication of Cold Start condition, so it's trying to feed the engine with a rich air/fuel mixture.
[edit: Note that the OEM configuration cites two coolant temp sensors, the first is the two-wire sensor that feeds the computer (the ECT sensor) and is located by the thermostat, and the second single-wire sensor that feeds the gage on the dash, located on the drivers side down between plugs 1 and 3]


1) Have you verified the coolant temp sensor (ECT) is connected to the engine computer?

2) Have you tested the coolant temp sensor to confirm that is is providing the proper signal to the computer?
Not sure if the service manual has a test sequence for the coolant temp sensor ???

Just put a new sensor in still reading -40, gonna try redoing the wiring

 

[Road Trip]

Just put a new sensor in still reading -40, gonna try redoing the wiring

FWIW, you made a comment about the wiring over in your other thread,
and I responded with the ECT <> computer wiring diagram you are
troubleshooting. (LINK)

 

[chris032188]

Bad connection at splice that PO put in was the culprit.

 

[registered-bum]

Excellent - glad you got 'er done!