Welp im back again to ask for opinions because im at a loss as to why i cant get this engine running right. It is a 1991 351w that was put in a 1990 i6 truck, computer and engine harness has been swapped. The issue now is that the idle is loping like the motor has a big cam in it, the engine will only rev to about 3500 rpm, and when the motor is revved the idle air control will open up, stay open, and not let the motor come down from the rev. After i let off the throttle if i wait a second and move the throttle slightly it will come down from the rev and continue its lopey idle. I have replaced the throttle body, the iac valve, and the tps. The only things i can come up with is the eec is not functioning correctly, or the harness in between the eec and engine harness from the straight six truck is pinned differently. The tps voltage is in spec and the iac is recieving about 4-5 volts at "idle" and like 6.4 volts when it is holding the idle high. Thanks.

 

Do you think that it could be the throttle position sensor?

 

The wiring for the dash cluster tach is different for a 6
and a 8 cylinder engine. For a 8 cyl Pin 7 of connector C208 B
on the back side of the dash cluster needs to be grounded.

It's a Black/Yellow wire. You need to verify your new harness
grounds that wire. The wire goes to connector C100, thru the
firewall and then connector C176 which is on the LH plastic
fender apron. Then it's spliced in to some wiring and turns
into a Black wire and goes to G801.

G801 is a ground point at the LH inner fender behind head lamp.

If the wire is not grounded take a piece of wire at the dash
cluster connector and ground the Black/Yellow wire somewhere
inside the cab.

 

No the tps voltage is in spec and ive tried 3 of those so far. And okay ill keep that in mind if i have to change harnesses. Nobody else has any ideas?