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fleetsidebob 04-18-2013 11:46 AM

TBI fuel delivery problem when starting
I am resto-moding a 69 C10 using a GM 350 TBI engine/700R4 setup. I wired the truck with Painless harnesses including the TBI harness. My problem is starting. It turns over fine, and if I prime the throttle bodies, it starts and runs with the injectors supplying a good stream of fuel. However, if I don't prime them, it won't start and no fuel comes out of the injectors during the start process.

I'm stumped. Anyone have any ideas?

Sleeper68 04-18-2013 11:58 AM

It sounds like a fuel pump problem. What type of fuel pump are you running?

richard stewart 3rd 04-18-2013 12:17 PM

I agree with Sleeper, you need a pump that has at least 15# of pressure.


fleetsidebob 04-18-2013 12:20 PM

It's a new Airtex E8094. Specs show it pumps 50 GPS @ 17psi. For a TBI, I would think that is plenty.

Of course, it works fine when the engine is running. It just won't pump, or the injectors won't request fuel while starting.

Sleeper68 04-18-2013 01:02 PM

Well, given the pump operates properly when the engine is running, I think there may be an electrical problem; mainly, when the stater is turning the engine over. You may not have enough amps from the battery and the starter is pulling power from the pump. If that's not it, you may not have a full +12 volts to the before ignition.

Does the pump prime the lines when you turn the key to the on position before you try to turn over the engine?

fleetsidebob 04-18-2013 01:17 PM

When I turn the key to the run position, the fuel pump runs for a couple of seconds and shuts off. I was assuming it stopped running because it was pressured up.

How can I test to see if there is actually pressure at the injectors during start up? I hate to disconnect fuel lines and have gas shooting all over the place.

The pump itself is not very accessable, but I'm wondering if I could use a clamp meter around the power wire to the fuel pump and see if we have current while the starter is turning. Would that make sense?

richard stewart 3rd 04-18-2013 02:06 PM

When you put this together did you run a return line from the throttle body back to the tank?
You can attach a meter to the hot going to the pump at the relay to check for power while trying to start it.


fleetsidebob 04-18-2013 02:59 PM

yes, I did plumb in a return line. I'll check for power. I'm thinking that the pump might be pumping but the injectors are not getting the command to inject while starting. I'll see if I can check for power to the injectors while starting too.

64nailhead 04-18-2013 03:51 PM

I assume you're running the stock ECM. Sounds like the pump is not running during cranking. A quick way to check it is jump power to the pump prior to cranking (for a second or two), and then begin cranking. If it starts, then that is the case.

I have to admit, that I don't know what controls the pump during cranking, but I'm sure I can find out. It must be either the ECM or oil pressure sensor. At least I think.

Someone else on this forum must know. I'm sure someone will chime in.

Good Luck - Jim

LATECH 04-18-2013 04:22 PM

2 things: as suggested check for fuel pump running when cranking
Also check for injector pulse while cranking

my guess is no injector pulse while cranking
I had one several years back that the module was bad and wouldnt fire the injectors but would fire the coil.
One other time It was a bad pickup coil...but did the same thing.
I would try a module first if there is no injector pulse.

fleetsidebob 04-18-2013 04:27 PM

Yes I am running a stock ECM although not the one that came from the donor car. That vehicle had the theft deterent system. I replaced the ECM with one from a late 80's truck that didn't have the feature. It coincided with the ECM that Painless said I needed.

The funny thing is that I'm pretty sure the pump is working. I just think the injectors aren't getting a signal to pass the fuel on to the TB. How do I check and/or correct that?


fleetsidebob 04-18-2013 04:53 PM

Actually, I have a spare ECM so I might try that. In fact, I might first try switching the PROM from the spare to the one in it now to see if that is the problem.
When you suggest check for injector pulse, are you saying check for injector fuel flow to the TB, or are you speaking of an electrical pulse? If so, how would I check that?

PS, I like your 64 Tempest. That was my first car.

LATECH 04-18-2013 04:55 PM

Check injector pulse with a noid light. Dont use a test light as the bulb could draw to much current and burn the injector driver out in the ECM.
You can also use a logic probe (red/green LED test light type deal). They are safe as they draw virtually no current.

ssmonty 04-18-2013 05:41 PM

I'm pretty sure that the computer(ECM) gets a signal from the distributor's reluctor while cranking to tell the computer to pulse the injectors. Its not a large (less than +5V) pulse that comes from the reluctor. I believe it goes to the ignition module, is amplified, then goes to the ECM, but I'm not certain the module is involved.
Which ever route it takes, a signal gets to the computer from the distributor, sort of like a tach, and the computer pulses the injectors.
Look closely at the reluctor's magnets for cracks, wires/terminals for corrosion, wobbly distributor shaft, and corroded ignition module terminals. A little rust can reduce the signal level enough that its not efective until the running voltage(+14V) increases the amplification just enough to trigger the module or ECM.
There are many different distributors on computer controlled ingnitions. Are you certain the one you have is correct for the harness or ECM?
Hope it helps more than it confuses.

LATECH 04-18-2013 05:54 PM

Yes...the module handles the signal from the pickup coil, and when cranking ...the ecm sends a 5 volt signal to the module to "bypass" the computer and fire the ignition on its base timing, leaving the signal to the ECM to fire the injectors. In bypass mode I have seen that circuit fail to run the injectors, but once the engine starts and the key is in run , the bypass voltage goes away and the module is now in the run mode and runs from the reluctor signal passed through the module then to the ECM and then back to the module.I Know ...confusing.
Anway...injector pulse is checked at the injector connectors. There is a positive and a ground which is the on/off side of the injector circuit. THAT signal is provided by the ECM which gets it from the distributor Module.
Check for injector pulse by using a small bulb like a 194 . Use a buld and socket with 2 leads, shove the 2 leads into the connector for each injector and crank the engine. If they flash on and off as the engine turns
over then the pulse is fine.If not...that will be the trouble

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