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TBI engine swap, won't start


Fred Guam

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I’m looking for some help with an engine swap project. I’m transplanting a 1994 5.7/350 from a Suburban into my 1990 Toy rock crawler. The install has been a bit of a challenge but the engine and drive train are in the truck. I’m happy to share the mechanical learning points on this swap with anyone interested.

I have a reasonable amount of mechanical experience but this is my first bout with a fuel injection swap.

The current problem I have is that the engine won’t start. I’ll describe what we have done so far and would appreciate any guidance or hints that may help me get it running.

Facts:

. 5.7/350 TBI engine from a 94 Suburban

. Factory wiring harness in excellent shape

. ECM from same vehicle.

. Engine has spark and will run for a few seconds if primed with gas through throttle body

. Injectors will “pre-spray” for a few seconds but there is no injector spray (pulse) when cranking.

. I have 12v at the injectors.

. Fuel pump is wired to 12v ignition on.

. I have not hooked the Chevy fuel pump relay into the ECM circuit as the in tank Toy pump is wired to 12v ignition on.

. Fuel pressure is 11 psi.

. I have traced every engine circuit from the connections to the ECM and all the wiring is sound.

. I have checked the ignition module and it tested ok.

. I have wired the Toy signals such as 12v constant, 12v ignition and 12v ground into the wiring harness as instructed in the conversion manual. I’m pretty confident the basic circuits are wired correctly

. The battery ground goes to the engine, the engine is grounded to the frame, I ran a separate ground from the ECM “box’ to the engine.

. I have an after market VSS to convert from Toy to GM specs wired in correctly. I’ve double-checked with manufacture to ensure wiring is correct.

. I have not hooked up the knock sensor or the VSC (vehicle spark command) as I plan on not running them.

. Oil pressure cranking is +25PSI

Bits and Pieces:

I have full wiring diagrams from Chevy’s tech computer I got from our local dealer and from Mitchell’s electronic diagnoses software so I can trace and discuss the circuits in detail if need be.

The problem appears to be that we are not getting the ECM to provide the ground pulse to the injectors.

Questions:

Does the fuel pump relay need to be wired into the ECM to make the ECM “think” it has fuel and send a signal to the injectors? Or does the ECM just trigger the relay to turn the fuel pump on?

Do you need the VCS to make the engine run? Researching the test procedures for the knock sensor and VSC has lead me to believe they are not required to run the engine.

Following the GM wiring diagram for the oil pressure, fuel relay circuit it appears that line 120 / gray pin number B12 is the fuel pump signal. Does this circuit into the ECM need to be 12v with the ignition on?

I live on Guam and I don’t have access to a bench ECM tester. I can get my hands on an ECM from a 305 TBI ECM if I need to “try” another ECM. How do I identify if the ECM from the 94 5.7 is compatible with the 5.0 TBI ECM?

The vehicle is a pretty extreme rock crawler, full tube bed, tube doors, coil over suspension, roll cage, 38’s ect., and barley street legal, I’m not striving for a daily driver, street or show vehicle just a cleanly wired, reliable, solid swap. I’m not opposed to tricking the ECM if needed as I don’t need any of the ECM driven accessories.

Any and all assistance will be greatly appreciated.

Fred from Guam

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Most GM TBI fuel pumps run about 35psi, and the injectors run off a signal from the ECM, through the pickup coil in the distributor. I know a friend of mine has an 86 Monte Carlo that wouldn't fire, originally the fuel pump was replaced, then the distributor, then the ECM when all it was was a bad ground signal from the ECM to the TBI injectors.

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