Originally Posted by matt167
The '95 Chevy 1500 I just bought ( 305 V8 5spd 4x4 ) takes a little longer turning over to start than I think it should, but once running it runs great... I'v heard Chevy TBI, is a 'dumb' system, not having many sensors and relying on a lot of fixed values.. but what could cause the hard start? I'm used to Carburators ontop of Chevy's.. haven't messed with many TBI engines really, and never on the electrical end
Not really that dumb, it computes fuel delivery using computational methods instead of direct measurement such as used by Mass Air Flow (MAF) sensing systems common to early TPI.
It carries a map in a PROM memory consisting of many cell numbers that correspond to a fuel injector signal and a spark timing. When in closed loop operation it uses an oxygen sensor to sniff the exhaust and uses the mixture data to trim the fuel schedule from the cell. When it encounters the same differential repeatedly it reprograms the cell, but the degree to which it can is limited and always defaults in direction that creates minimum emissions.
Basically the TBI is whats called a Manifold Absolute Pressure system or MAP. It calculates the air mass flowing thru the engine rather than measuring it as in MAF systems. To do this it's measuring engine operating temperature, RPMs, transmission gear selection, sometimes torque on the trans output shaft, manifold vacuum, and throttle position. From this input data it computes a cell identification number and instructs the computer to go there from the fuel delivery and ignition timing data.
MAP systems are sometimes called Speed Density systems as well. Actually the MAP and MAF sensing systems of measurement and calculation have sloshed back and forth on TBI and TPI type systems over the years as the manufacturers have tried to find the least cost solution sufficiently effective to meet government emissions and fuel economy standards.
Actually the MAP/TBI computer and sensors can be used to drive the GM TPI port injection system with surprisingly little modification. The big issue with MAP systems regardless of whether they are driving a TBI or TPI system is the sensitivity of the sensors to reading small changes and the programming that is looking for specific cell numbers based on computation of known relationships of RPM, manifold vacuum, and throttle blade position as the prime measured variables. When changes are made mostly to the cam timing or rocker ratios, this affects the relationship of manifold vacuum to throttle position and to RPM. This causes the computed cell to be in error against what would be the ideal fuel delivery and ignition timing selection for the engine. The solution here is to reprogram the chip to reflect the changed relationships of vacuum, RPMs and throttle position. To some extent here MAF offers more flexibility since it reads mass airflow as the resistance in temperature changes of a heated wire which varies directly to the amount and temperature of the air flowing past the wire. These systems, however, require an air temperature compensation unit be added since inlet air temp will have as much an effect on the wire's resistance as the amount of air flowing past it.
Holley makes a TBI system for race cars that is simpler than OEMs having sometimes only a throttle position sensor. A little upgraded system employs manifold vacuum sensing and RPMs for more discrete fuel delivery and they add on an O2 sensing option if passing emissions becomes an issue for the user.
All in all these things are pretty good, but the world is going to direct port injection. These offer better emissions, improved fuel economy, and a lot more power than can be had with any sort of carb or injection system where fuel is introduced before the intake valve as the space occupied by fuel within the intake tract is now available for just air and the injected fuel within the cylinder actually adds to the compression ratio. A double sided win for power output.
There are many reasons why your TBI may be hard to start, after-all it ain't no spring chicken at this point. So anything from a worn out engine with low compression, to failures in the pump electrical circuit, dirty fuel filters, to gunked up injectors, to needing an ignition tuneup or a new timing set can be at fault.