throttle body injection

tom36 · February 07, 2006, 03:47:55 PM

I have been helping a friend wire his '38 Chevy with a 350 TBI motor. Today was the moment of truth, and all lights, accessories, etc. worked fine. However, when the moment of truth arrived (will it start?), the engine turned over but no spray from the injectors. I checked and rechecked all the connections and sensors and all appear to be OK. Finally I put some gas in the throttle body and hit the key to see if it would fire-- it did and the injectors sprayed and it ran fine. Shut it off, and tried to start it, and the same scenario, no fuel spray from the injectors. Prime it and it starts and runs. A call to Painless ( it's their TBI wiring setup) and was told it's probably dirty injectors from the engine sitting (approx. 18 Months). Well, I suppose, but if that was the case why does it inject and run smooth once it's running? The car has a new Tanks tank and in tank pump for a TBI engine. Tomorrow I thought I would check the fuel pressure and if worse comes to worse, I have acess to a running 350 TBI in a pickup and will swap the TBI's and see what happens.

Meanwhile anyone have any suggestions? things to check? Thanks, Tom..

1FATGMC · February 07, 2006, 06:37:47 PM

I don't know what it is, but my guess is that the computer is seeing something after it fires that lets it fire the injectors and when it is cranking something is in a mode (sensor?) that tells the computer to not fire the injectors.

I just can't think of what, buy maybe someone can. Let us know what it was.

c ya, Sum

DrJ · February 07, 2006, 07:17:49 PM

Is there an oil pressure sensor that may be keeping it from squirting gas until it has pressure?
If so should this have a start up over ride?

Just talkng out of my cachucha, since I only know enough about TBI and TPI to stay away from them...

tom36 · February 07, 2006, 07:33:26 PM

Quote from: "DrJ"
QuoteIs there an oil pressure sensor that may be keeping it from squirting gas until it has pressure?

Well, that was one of my thoughts, but I couldn't find anthing in the install manual referencing it. I still think that is a good possibility.

If so should this have a start up over ride?

QuoteJust talkng out of my cachucha, since I only know enough about TBI and TPI to stay away from them...

You and me both! I kept telling him he should have gone with a carb and it would be running now!

Jbird · February 07, 2006, 08:58:45 PM

It could be the oil pressure switch wired incorrectly. Also, make surethe hot wire to the injectors is hot when you're cranking it over. I had that problem with my truck, it only took me two hours to figure it out.
Jbird

enjenjo · February 07, 2006, 09:21:09 PM

When the key is turned on, the fuel pump should run for 5 seconds. This pressurizes the system. Then when you crank it it has enough pressure to inject fuel for the startup. There should be a computer connection to an oil pressure switch, that will shut off the pump when the engine stalls. It actually does this through the computer, by shutting of the fuel pump relay ground.

So if it runs after you prime it, it sounds like you are not getting the 5 second run with the key on. If you are, then check the injector. I blieve only one is used for startup, bot are used for running. If one injector is bad, and it's the startup injector, after it's running, the other injector will supply enough fuel to keep it running.

One other thing, there is a coolant temperature sensor hooked to the computer too. If that is defective, it may not supply enough fuel for a cold start, but usually will hot start pretty good.

If you don't have the correct oil pressure switch, or coolant temperature sensor, it may cause this problem too. The one for his gauges won't work, so he needs two oil pressure switches, and two temp sensors.

tom36 · February 07, 2006, 09:34:06 PM

Quote from: "enjenjo"When the key is turned on, the fuel pump should run for 5 seconds. This pressurizes the system.

The fuel pump runs when the key is turned on, but I never heard it shut off--just stayed running-- it did have pressure at the TBI though, I checked. I thought the pump should shut off myself, but this electronic stuff is a first for me. I'll have to do some more reading

If you don't have the correct oil pressure switch, or coolant temperature sensor, it may cause this problem too. The one for his gauges won't work, so he needs two oil pressure switches, and two temp sensors.

The senors are the ones GM had on the engine and the Painless connectors snapped right on as original.

Thanks all for the tips- I'll keep you updated as to what we find. Tom..

enjenjo · February 07, 2006, 09:47:23 PM

One other thing, I have wired one in the past, where when you are cranking the engine, it shuts off power to the computer. Make sure the computer is powered when cranking. The ignition is a separate circuit from the computer, and may get power to fire, even if the computer is not supplying fuel injector pulses

tom36 · February 08, 2006, 11:56:53 AM

Quote from: "enjenjo"One other thing, I have wired one in the past, where when you are cranking the engine, it shuts off power to the computer. Make sure the computer is powered when cranking. The ignition is a separate circuit from the computer, and may get power to fire, even if the computer is not supplying fuel injector pulses

That's exactly what was happening! The computer was hot in the "run" position but not in the Start position. Fixed that and, "BRRRMMM!!!
Thanks Enjenjo

enjenjo · February 08, 2006, 12:34:04 PM

See, I did learn from my mistakes

tomslik · February 08, 2006, 12:39:58 PM

Quote from: "enjenjo"See, I did learn from my mistakes

been there, done that, but you type faster...

enjenjo · February 08, 2006, 12:51:34 PM

Quote from: "tomslik"
Quote from: "enjenjo"See, I did learn from my mistakes

been there, done that, but you type faster...

I've made a couple mistakes since then too.

58 Yeoman · February 08, 2006, 02:26:34 PM

Quote from: "enjenjo"See, I did learn from my mistakes

So did we.