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TBI - How many pulses per second?

I am working on a 1991 Chevrolet S10 2.8L with throttle body fuel injection.
I have used a 194 light bulb to detect PCM pulses to the injectors. But as I recall, they might have been too far apart.
How many pulses per second should I be getting when I am starting the engine? The engine will not start. Also, the injectors will not spray but drip, once or twice per second.
I am wondering this, because looking at videos of TBI operation, it looks like the spray cone is uninterrupted, which should mean that pulses should be fairly frequent. I believe I was getting one or two pulses per second.

The frequency of the fuel injectors will be one half of the engine RPM’s while starting.

So if the engine cranks over at 700 RPM’s, the injector pulse signal will be at 350 Hz

Or, 350 times/second.


Can’t speak to that specific design, but injector firing is often tied to the crank position. Fired either every tdc, or every other tdc. If the crank speed is 300 rpm, then in the 150 to 300 per minute range; i.e I’d be expecting 3-6 injector firings per second.

Ok, so the worst case scenario, 300 rpm, divided by 2, divided by 60, is 2.5 pulses per second. Hmm, that is pretty slow, and is similar to what I am getting. Then why do the spray cones appear continous/uninterrupted?

That would depend on how long (in time) the injector pulse is. The injectors could fire infrequently, but for a long enough time that one pulse almost abuts the next, so it would appear to be a continuous injection stream.

So what is your fuel pressure during cranking?

I have yet to put the fuel pressure tester on it.

Yes, the pulses can be long. But my bulb only showed very short flashes. As well as looking on the web for more information, it seems the pulse width is in the few millisecond range, which is very short.

That would be my next step.

Can you hear the pump run for about 5 seconds if you turn the key to run position?

Yes, I can hear the pump run when I turn the key on, and when I am done cranking. Whether it is putting out enough pressure is to be determined.

How old is the fuel filter? Replacing it might worthwhile. And I believe a special adapter is needed to connect a pressure gauge to the fuel line on that throttle body.

I have replaced the fuel filter, which turned out to be quite a job. I think I will test my fuel pressure between the pump and the filter, as it is much easier to work with and I won’t have to fight the steel fuel line. I wouldn’t think there would be more than 1 psi pressure drop through the new filter.

That should be 350 times per minute, not second.

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While the fuel spray from a GM TBI injector appears to be continuous it does pulse but I recall it being much slower than 350 times per second. Maybe a rate of 300 times per minute.

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The pulse repetition rate is not as important as the pulse width. The GM TBI system is Pulse Width Modulated (PWM) signal to the injector. The pulse width will be much wider at cranking and lower frequency or PRR.

The fact the injector dribbles instead of sprays says one of the following is messed up- low fuel pressure to injector (some implementations had a second, sintered filter at the TB you need to check for obstructions), the injector is failed/plugged and/or the injector signal is incorrect/failed (the ECM grounds the injector to fire it. It needs good +12 during cranking which IIRC is a separate power lead IGN2? and the MOSFET in the ECM must provide good ground for injector)

The lamp is not the best method for testing electrical conditions but if it is at least illuminated, I’d start looking at fuel feed.

Here’s another quick test just as a sanity check- does it sputter and try to start or momentarily run if you spray starting fluid into the TB? Go easy on that stuff, just enough to test and leave air cleaner off during cranking…

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Seems we’re getting too complicated too soon. Always start with the basics. I would be checking fuel pressure before I checked injector pulse. I will bet the short piece of hose between the fuel pump and pipe has ruptured. So common is this problem, that fuel pressure is the first thing I check after determining that no fuel is the reason it’s not starting. 9-13 PSI is specs.

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Lighter fluid works very well for that and is not nearly as explosive as ether based starting fluid. I used to give lawnmower carburetors a squirt of lighter fluid and cranked it to prove to the customer that his year old gasoline had gone bad.