Might be worth checking fuel pressure too. If you are having intermittant low fuel pressure, that will effect the spray pattern.
CK5
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fuel injection rejection
- Thread starter wheels87k5
- Start date
Here's why the light trick won't always work when testing fuel injectors. The injectors are pulsed, which to them is turning on and off. To a slow responding item like a light bulb, this is like a DC voltage. At idle your pulse width is low, so the duty cycle is maybe 10%. To the bulb this looks like 10% of the supply voltage, which is 1.3 Volts or so, which might not produce any visible light.
So at low rpm / low load (i.e. idle), you could measure from the injector low pin (ECM side) to ground. Instead of 10% of supply voltage, it would be 90%. So it seems like you could see the bulb "dim" slightly when the injector starts pulsing. Unfortunately, the supply voltage is also changing when you start the engine.
Plus, if you're going to hang something from the ECM injector drivers, you should make sure it is a small bulb. You don't want to burn that out or prevent the injectors from opening properly.
An oscilloscope is the best option. If you don't have one of those, try an analog meter. The thing is, if you unplug the injector to do the test you change the conditions. You lose your "pull-up" to B+ as well as the circuit load (an injector is Ohms and a multimeter is Millions of Ohms). You should see a low voltage which increases with rpm (voltage is proportional to duty cycle). Also, blipping the throttle should make the voltage spike up a little due to acceleration enrichment.
So at low rpm / low load (i.e. idle), you could measure from the injector low pin (ECM side) to ground. Instead of 10% of supply voltage, it would be 90%. So it seems like you could see the bulb "dim" slightly when the injector starts pulsing. Unfortunately, the supply voltage is also changing when you start the engine.
Plus, if you're going to hang something from the ECM injector drivers, you should make sure it is a small bulb. You don't want to burn that out or prevent the injectors from opening properly.
An oscilloscope is the best option. If you don't have one of those, try an analog meter. The thing is, if you unplug the injector to do the test you change the conditions. You lose your "pull-up" to B+ as well as the circuit load (an injector is Ohms and a multimeter is Millions of Ohms). You should see a low voltage which increases with rpm (voltage is proportional to duty cycle). Also, blipping the throttle should make the voltage spike up a little due to acceleration enrichment.
I've tested via the bulb, it works. Even a dim light is something. The voltage is always there when the key is on, but the ground isn't. It lights up, not dims as the injector pulses. Pulsewidth is milliseconds, right? That's controlled via the injector driver on the ECM, how is it going to ground anything other than the 14V no matter where you measure? (or whatever supply voltage is) The length of time to ground is what the ECM modifies, not voltage.
Since the poster mentions the problem does NOT any longer follow one injector connector, it's not likely an injector wire issue IMO. ECM injector driver (of one driver for both injectors, like TPI batch fire) injectors, or intermittent bad ignition as Thunder said.
Fuel pressure is good to check, but I wouldn't hesitate to get my injectors serviced or replace them either, especially with age. Yeah, you could get an o-scope to test further, but the cost for one time diagnosis isn't worth it. Nor is cleaning IMO.
Since the poster mentions the problem does NOT any longer follow one injector connector, it's not likely an injector wire issue IMO. ECM injector driver (of one driver for both injectors, like TPI batch fire) injectors, or intermittent bad ignition as Thunder said.
Fuel pressure is good to check, but I wouldn't hesitate to get my injectors serviced or replace them either, especially with age. Yeah, you could get an o-scope to test further, but the cost for one time diagnosis isn't worth it. Nor is cleaning IMO.
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Thanks for the replies and help.....The fuel injectors and fuel pump and fuel filter are brand new....my short term memory is kinda blah so bear with me....when I first got the blazer, it didn't have any injector problems...just transmission....I replaced the transmission and decided to put in new plugs, wires, cap and rotor because of unknown milage....the problem developed after that...I thought maybe I knocked something loose or accidently broke something but couldn't find anything odd....So I checked all connections and for vacuum leaks...found nothing...I took the strobe light and checked for a consistent cone spray and when you give it half throttle, you can see the cone spray in one injector, then maybe the other, but not both at the same time...it starts good and idles good with a very light surge...I did adjust the timing a tad but didn't think that would cause what it's doing now...I put the timing back as it was and still same problem...I will keep plugging away until I find the culprit...when I do, I'm going to have some drinks and enjoy my 4x4...when I do find the problem I'll let you know what it was...keep the replies coming. Thanks Wheels 
