69 1/2 Six PAck Bee

I have since my original post checked each and everything immaginable and I have narrowed the problem, or what I think is a problem of my hard starting, to the "STA" circuit in the ECU. Toyota (and many others) use a specific parameter within the ECU that is dedicated to priming, cold start, & afterstart enrichment. It is usually triggered my the "start" command as is the case with my 1990. WHen you turn the key to the "start" position a +12V signal is sent to the STA pinout and this sets the ECU into the prime mode. It fires all 6 injectors in a synchronous mode (all at once) until the engines fires and when the key is returned to the run (IGN) position the injectors return to their asynchronous injection order. The ECU has calculated the engine temp (THW signal from the coolant sensor) and the ambient air temp (via the THA signal from the sensor in the AFVM) along with a few more calcs and the engine begins it's afterstart warmup in open-loop mode until the engine temp is to the point where the 02 sensor takes over and maintains the 14.2 stoich in closed-loop mode. This is a brief summation but it is basically how all EFI's work.

What my engine is doing is basically either very quickly shutting the injectors of when the transition from sync to a-sync when the key is released or too much fuel is being introduced as if they were all open for a split second and flooding. How I narrowed it down to the STA circuit is I clipped the wire going into the STA pinout thus disabling the prime/afterstart "circuit" from the ECU. I could move the key from run to start a zillion times and the engine would keep running and not even once would it replicate the engine dying that I have. I put +12V right to the STA pinout and it would almost kill the idling engine. You could tell the change in the way it was running by having all the injectors firing at once as the engine acted like it was "loading up". Remove the wire and it would recover. I left the wire off and even though it was harder to start it would never faulter like it does. I did not get, however, a code 43 which is the "no STA" signal but I only started it a few times with the wire clipped so possibly it did not have enough times to register.

Now I am wondering what the root cause of the problem might be.

Do any of you "experts" think of what it might be?

Thanx,
Bud Weaver

zachb55

i had reported in your original thread similar problems to you. I since found that my TPS was crap (even had a code 41 which the light didnt come on for and had to register in the last month or so...) so I went and picked up a new one, threw it on and with a little pop and a sputter the ECU reset and i had a whole new truck. the starts are quick and i have 4 gears again!!! no more sputtering or anything, but my OD still takes forever to kick in so gotta dig into the tranny on that one. good luck figuring out you problem though...