1991 4.3l swapped to 5.7l, some engine hesitation questions.

[Foxbody and a Z71]

Hi all,

I just bought a Z71 1991 step side regular cab 5 speed.

I have yet to experience this, but the previous owner mentioned all details about this vehicle and I have one thing I'm not sure of that is a small gremlin the vehicle has.

It has a GM reman. 350 with 60k on it, and has the proper TBI and all that I am aware of.
However, when the truck is cold, the motor doesn't get up and go too quick. It's not a huge issue, it feels like timing has been pulled too much, but when the vehicle warms up it is fine. I have no other ideas how to describe it.

It has new distributor, plugs, wires, timing is proper. I'm not sure what to look into next.

Thanks all.

 

[98_k1500]

Coolant temp sensor would be a prime suspect.

 

[Foxbody and a Z71]

Coolant temp sensor would be a prime suspect.

not that I know this, how does the temperature of coolant (from ECM perspective) alter fuel, spark or air going into combustion? Or is it just an early attempt at telling the motor it can only go so hard depending on its warmth?

 

[98_k1500]

Coolant temp plays a HUGE role in fuel delivery. It "chokes" the engine when cold, and leans out as it warms up. It can read something like -40° to 280°. If you unplug the CTS, it thinks its -40° and will spray so much fuel it'll flood to the point it pops out the intake and catches the TB on fire. The easiest way to check one of these is to plug to a snap on "brick" or other OBD1 scanner, and look at the coolant temp reading the computer is seeing, and make sure it makes sense. That bein said, a CTS is about $10, most folks just change them. Check the MAP sensor hose for leaks too, that can also cause funky issues.

 

[Foxbody and a Z71]

I just brought the truck back home last night, and after driving it for awhile it seems that I am aiming more towards constant rich settings. The truck does not really seem to get up and go after it warms up as PO mentioned.
At idle it smells very very rich, and it seems the idle stays constant when cold and until it is warm, and even when warm it seems to idle around 950-1k rpms (no RPM gauge).
Just to have more options, I will be replacing the coolant temp sensor either sunday, or relatively soon; however, are there any other suggestions of what may be wrong?