The Iron Duke 2.5L TBI lives ... on 12P!

American Delco GM ECUs and PCMs, ALDL, OBD 1.5.
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The Iron Duke 2.5L TBI lives ... on 12P!

Postby Mark_ZZ3 » Mon Aug 20, 2018 3:50 am

After many hours tweaking, testing, trailing ... it is now a running and driving vehicle.

Some history: It's an 1987 S10 Truck with a 2.5L 5 speed running GM's 1277165 ECU TBI setup. Good for 90-ish HP.

- A turbo from a 2000 era Volvo was added along with the matching intercooler (It fit perfectly in front of the S10 rad).
- The stock Throttle body was swapped for a larger unit from a 5.7L using a home made adapter. The reason for this was add extra fuel under boost as the stock TB had no other injectors available that we could find.
- We are running 12P code. No wiring changes needed whatsoever (well except adding the second injector wire). Started with the V8 setup and went from there. Removed lots of fuel calibrations so as to start from a flat line.
- Fuel pump. This was the key to the whole setup.


The first attempts of running this was with the stock GM code. Tried to add fuel in the high rpm, but this was with a 1 bar MAP. Limited success other than it made some boost, but would not hold it in the revs. We jumped into 12P and added the 2 bar MAP and now could tune into boost ... but were plagued with lean issues. Tried different fuel injectors, lots of settings, but nothing worked well.

Fuel Pump! Here is where the story changed. The stock pump is a TBI setup good for 15 PSI, maybe 18 max. Once we hit boost, we would loose fuel due to the differential pressure between boost and fuel. So if we had 3 PSI boost pushing against a 15 PSI fuel system, then we only have 12 PSI of usable pressure. No end of tuning helped here. We would see upwards of 4 PSI peak boost, but it would not hold.

We swapped out the pump for a high pressure multiport setup ... good for 50 PSI. The engine came to life. Plus I had to remove a ton of fuel in the fuel maps in 12P. After 3 hours of driving around with a wideband gauge and running the 12P code through an emulator (Moates APU1), we got a base config. I spent a few hours smoothing out the the VE table and it runs! We have a 2.5L TBI running 12P code with boost. It runs very well.

So here is why it worked. The Fuel pressure regulator is in the TBI unit. In stock form it uses vacuum to reduce fuel pressure since the diaphram is exposed to engine load/vacuum. In a turbo setup, it works to hold the diaphram closed under boost. Now this isn't linear in the sense that 1 PSI boost increases fuel pressure by 1 PSI. Thus far, it works fine up to 7 PSI boost throughout the RPM range. Wideband holds rich (12.5:1).

All of the testing thus far has been in open loop as the narrowband output from the Innovate Wideband doesn't simulate the stock narrowband well. Once a second O2 is added in, closed loop can come back for some fine tuning while driving.

Another Tip. The '165 computer does not like the TBI injectors. They are low impedance. I had to add a resistor in series to simulate the load of the multiport injectors. I realize this sounds odd, but that's what it took to make it work and keep the '165 from burning out it's injector drivers. It works.

So I am super happy with 12P and the '165. It avoided any aftermarket system. It's quite fun to tell people that it's running a stock ECM when they expect you to list one of the aftermarket systems. I suppose it would have taken less time with the aftermarket systems ... but we would not have learned as much.

Hmm ... maybe a Turbo on my TPI engine is next?


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