Toyota Repair: 3VZE Starting Malady, engine temp, loop mode

Question
Hiya, Ted,
I have a problem with my 1990 SR5 4x4, 3VZE-powered truck and have checked each and everything imaginable (all of which are within spec) and I have narrowed the problem, or what I think is a problem of my hard starting, to the "STA" circuit in the ECU. I am no novice when it comes to EFI systems but this one has me stumped so I am soliciting your help for a possible fix. Basically, when the key is turned from "start" to "run" the engine will stumble then recover. When it is cold outside and the engine is naturally harder to start the engine will quit a coupla times before it finally holds. Once warm it fires immediately but the little "hiccup" is still present. The cold start injector is functioning properly and the ignition is up to par. ALL of the obvious has been checked so I know that it is narrowed down to the following. Toyota (and many others) use a specific parameter within the ECU that is dedicated to priming, cold start, & after start enrichment. It is usually triggered by 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 VAFM) along with a few more calcs and the engine begins it''s after start warm-up in open-loop mode until the engine temp is to the point where the 02 sensor takes over and maintains the 14.7 stoic in closed-loop mode. This is a brief summation but it is basically how all EFI''s work as I am sure you will concur.

Aside from the current "malady" this truck runs real good and no trouble codes ever show up.

What my engine is doing is basically either very quickly shutting the injectors off when the transition from sync to a-sync is taking place when the key is released or too much fuel is being introduced as if they were all open for a split second and flooding. BTW these are all new injectors that have been in service for 10K miles so far. How I narrowed it down to the STA circuit is I clipped the wire going into the STA pinout thus disabling the prime/after start "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 falter 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. I have even swapped out ECU''s with the same result

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


Answer
This engine uses a cold start injector for starting, the injectors are still fired in pairs on cold start with the cold start injector providing additional fuel, maybe that's where the problem is, the cold start injector receives the start signal from the compter to turn it on so you need to look there, the cold start injector is located on the intake chamber on the right side.