454 tbi questions

[ryanroo]

Hello,

i am new here, but noticed that there is alot of informed people on here with experience in the TBI world. i am a compotent fab guy and do pretty good with mechanicals as well, but i cant figure out what the heck is going on with my 454 tbi. the engine is an 86. the tbi stuff is a bit of a hodge podge. TB is from a 94 and the computer and harness are from a 91. i reworked the harness to get rid of all the stuff i didn't need. after a few other gremlins were chased off i am left with one headache.

the truck will almost never start without ether. it will not idle. it runs great above idle and will smoothly come up to ~5k rpm if the throttle is opened gradually. if you crack the throttle quickly it stutters and pops at about 3k rpm. i have had it up to temp and it runs fine then as well.

i have a heated O2S in the header collector, timing was set at 4* btdc and the knock sensor is hooked up. has a new CTS and i have checked the IAC motor, 46 ohms across a&b and c&d. TPS was in range.

if anyone has iny ideas i would be greatful for the help

thanks
Ryan

[EagleMark]

The few big block TBI I have played with do not have a knock sensor?

Also the 94 injectors are bigger than the earlier ones...

Big Block IAC wiring is also different than small block, two wires are reversed...

Have you checked for error codes?
Have you checked fuel pressure?
Some of your symptoms also point to a vacuum leak... HTH

[ryanroo]

Originally Posted by EagleMark

The few big block TBI I have played with do not have a knock sensor?

Also the 94 injectors are bigger than the earlier ones...

Big Block IAC wiring is also different than small block, two wires are reversed...

Have you checked for error codes?
Have you checked fuel pressure?
Some of your symptoms also point to a vacuum leak... HTH

my ecm has knock sensor compatibility. the harness and computer were both for a big block. i can only assume that the IAC wiring is correct. it matches all of the diagrams i have looked at. i used a 92 GM truck manual when i reworked the harness.

i need to wire in a aldl before i can hook a scanner up to it, and i havent checked for codes yet either. that is my plan for today.

no FP check in the truck. TB came from a functioning truck. so i assume the regulator is good. the pump is new and is the ford truck fuel pump i forget the model. it is supposed to be good for ~80psi i believe.

i have all of the vacuum ports on the throttle body hooked to something or plugged. i also used new gaskets under the adapter and the TB. i do know it runs much worse with an intentional vacuum leak.
Ryan

[Ethan B]

Codes are easy. Is your check engine (SES) light on when running?

[ryanroo]

yesterday i hooked up the aldl and a ses light. no codes only a repeated 12. once it is warm it runs smoothly but warmed up it still wont idle, and when you open the throttle quickly it hits 2700-3k romand then instead of popping out the exuast like it does when its cold, it just falls off and would die if i didn't let of the throttle. you can see the injectors spray less fuel abruptly when it hits that magic rpm. it will still run fine up to ~5k rpm if the throttle opening is slower and steady.
Ryan

[Ethan B]

Did you read AND follow Bill's instructions in the "Stickies" FAQ?

Setting the IAC (Idle Air Control) is important, among other things.

[ryanroo]

i just read them. i can see where the IAC could need set since it wont idle, but will that have an effect at higher RPM with a sharp opening of the throttle?

i will try to set the IAC tomorrow and see where that gets me
Ryan

[Bill usn-1]

verify fuel pressure on inlet and return.

The ecm can not monitor the fuel system and the sensors are not popping a code so you need to look at things the ecm does not control.

vacuum leaks, fuel pressure., bad injector/s.......

And is your timing right?

[ryanroo]

fuel pressure is 32.5 lbs.
timing is correct.
there are no vacuum leaks that are obvious. but i have the correct air cleaner base no so i will do a better test.
i am starting to to thing the injectors may be a little bad, but i cant tell you what exactly is the correct way the spray should look. they dont drip, but the pattern looks wet to me

[EagleMark]

32.5 fuel pressure is a problem!!! Should be close to 13 PSI

Disconnect your return line from the TBI and run it into a bucket and check again. A blockage or restriction on the return will cause to high a pressure. If that is not it you have a problem in the regulator located in the TBI unit.