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Old 11-04-2010, 07:47 AM
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how 'dumb' is Chevy TBI?

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

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Old 11-04-2010, 09:47 AM
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I bought an 88 TBI van years ago....I had to crank it about 30 seconds before it would start.......What was happening, was that there was a broken wire right at the fuel pump relay.......The relay wasn't coming on until the engine built up enough oil pressure. There is also a low oil pressure switch that if oil pressure is low, the pump shuts off......So, in your case, I would be looking at the relay or possibly a broken wire.
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Old 11-04-2010, 10:12 AM
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TBI is ok there is a lot of mod's you can do! I would add that you need to check the ground wires obd1 is hard to trouble shoot! But once its working there fine. If your going to keep it then you need the shop manuals a scanner and this web site to get you up to speed www.fullsizechevy.com

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Old 11-04-2010, 10:13 AM
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The TBI fuel pump circuit uses one of two methods to supply electricity to the pump. There is a fuel pump relay that is controlled by the ECU. The ONLY purpose of this relay is to allow the ECU to provide power to the pump for two seconds to prime the fuel system before the engine starts. Once the engine turns over and there is oil pressure, the oil pressure switch closes, bypasses the fuel pump relay, and provides power to the pump. What is happening is that your fuel pump relay (or the wires to it) is bad. This prevents the pump from coming on until you crank the engine long enough to build oil pressure and close the oil pressure switch, powering the pump.
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Old 11-04-2010, 02:29 PM
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Quote:
Originally Posted by joe_padavano
The TBI fuel pump circuit uses one of two methods to supply electricity to the pump. There is a fuel pump relay that is controlled by the ECU. The ONLY purpose of this relay is to allow the ECU to provide power to the pump for two seconds to prime the fuel system before the engine starts. Once the engine turns over and there is oil pressure, the oil pressure switch closes, bypasses the fuel pump relay, and provides power to the pump. What is happening is that your fuel pump relay (or the wires to it) is bad. This prevents the pump from coming on until you crank the engine long enough to build oil pressure and close the oil pressure switch, powering the pump.
X 2
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Old 11-04-2010, 03:32 PM
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Quote:
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.

Bogie
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Old 11-04-2010, 07:05 PM
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Bogie is right. They were (and are ) a dependable system.Easy to diagnose.
Try something OBD 2 with Control Area Network (CAN) system where everything is tied together. You get B codes C codes P codes U codes, it can be kind of tough sorting through the codes.
88 chevy 1/2 ton efi throttle body, pretty much fix it in my sleep. Those are gravy work...LOL
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Old 11-04-2010, 07:59 PM
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Quote:
Originally Posted by latech
X 2
Make that X3 . I had this happen with a Chevy Cavalier I had . Wow ! That made my brain hurt trying to remember how I found the problem . Oh yeah . Just had to plug in the code key on that one and read the flashes . Autozone didn't have the code on it but the local Chevy dealer was nice enough to interpret the code and sell me the parts to fix it . Apparently Autozone only interprets the codes for the parts that they sell .
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Old 11-04-2010, 08:18 PM
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Quote:
Originally Posted by adantessr
Make that X3 . I had this happen with a Chevy Cavalier I had . Wow ! That made my brain hurt trying to remember how I found the problem . Oh yeah . Just had to plug in the code key on that one and read the flashes . Autozone didn't have the code on it but the local Chevy dealer was nice enough to interpret the code and sell me the parts to fix it . Apparently Autozone only interprets the codes for the parts that they sell .
Yeah, i see it a lot. Dude at auto zone pulls a code and sells the guy a part.
Take an egr valve for instance. I get a lot of people who buy a valve to put on cause autozone read the code. 95% of the time it isnt the valve.
Mc Parts sheesh.
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Old 11-04-2010, 08:31 PM
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Quote:
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
How long are we talking about here? If the delay is relatively short, it could even be a build up of carbon in the intake tract and on the back side of the intake valve acting like a 'sponge' that absorbs the first few cycles of the injector until the combustion chamber "sees" enough fuel.

It wouldn't hurt to give it a basic tune up while you're at it- who knows how old the plugs, cap & rotor, etc. are.

AFA the fuel pump pre-start prime goes, you should be able to hear the pump run for a couple seconds when you turn the key on- but don't crank the engine. If you do not hear anything, suspect that circuit. You can also check for an increase of fuel pressure, if you have a pressure gauge.
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Old 11-04-2010, 08:46 PM
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Quote:
Originally Posted by cobalt327
How long are we talking about here? If the delay is relatively short, it could even be a build up of carbon in the intake tract and on the back side of the intake valve acting like a 'sponge' that absorbs the first few cycles of the injector until the combustion chamber "sees" enough fuel.

It wouldn't hurt to give it a basic tune up while you're at it- who knows how old the plugs, cap & rotor, etc. are.

AFA the fuel pump pre-start prime goes, you should be able to hear the pump run for a couple seconds when you turn the key on- but don't crank the engine. If you do not hear anything, suspect that circuit. You can also check for an increase of fuel pressure, if you have a pressure gauge.
Easiest to hear if you take off the fuel fill cap and put your ear to the fuel fill and have someone turn the key on for you .
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Old 11-05-2010, 10:37 AM
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Quote:
Originally Posted by adantessr
Easiest to hear if you take off the fuel fill cap and put your ear to the fuel fill and have someone turn the key on for you .
While you're smoking!

Bogie
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Old 11-05-2010, 10:50 AM
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Quote:
Originally Posted by oldbogie
While you're smoking!

Bogie
LOL . Believe it or not, several years ago I gave an estimate on repainting the side of a full size Bronco that got burnt up by the guy's kid smokng while dumping gas in from a can . Could have been worse .
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Old 11-05-2010, 11:09 AM
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There may also be a check ball valve in the fuel pump that after awhile will bleed the fuel pressure back to the tank overnight. So somrtimes there isnt enough fuel pressure in the rail for the engine to start immediately. Try turning the key to accessory, listen for the fuel pump to whine/run for like 5 seconds. Then try to turn it over.

Kinda the same thing as pumping a carb before you start it. Common problem on s10's abd blazers of that vintage.
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