Sure hope there's still people watching this forum as it's way down on traffic.

I am trying to diagnose a no fire on cylinder #4. It has compression, spark (new plug & wires), but no fuel, so it is not firing at all. I have done a continuity test on all the injectors and they are good. OHM test is also good. So I don't know if the injector is getting voltage or not.

Several months ago the injectors were sent out for test and cleaning. I am only able to test through the harness connector as I don't want to remove the intake plenum. The manual says the injectors fire by a signal from the ECU which provides "battery voltage". I am assuming that means 12 volts.

I want to momentarily apply voltage to each injector to see if I can hear the click, but I don't want to over voltage. Can anyone confirm that 12v from battery will be safe to fire the injectors?

Also would it be possible to connect my noid light to each pin (+ grnd) of the harness connector and turn the engine over to confirm power?

Thanks

OK sorry, but I was able to get them to fire with a 9 volt battery.

But I still need to see if I can confirm the 12v supply voltage from the ECU by testing at the connector harness. I have a noid light but also a 12 volt LED that I may be able to back probe into the connector. I would prefer to connect to the disconnected harness and just turn the engine over.

Anyone think that will work?

LOL, I guess I can't wait for a reply. I have worked it out.

While idling was able to back probe the harness connector then using alligator leads connect my home made back probe pins to the noid light. Back probed both sides of the harness and got pulses.

So now I know the computer is working and there is a connection to the injector because of the 9V battery hookup that gave me the clicks. Still no fuel on #4.

Can only be a blocked injector preventing fuel from squirting.

No breaks on this job, will have to pull the intake plenum and get #4 injector checked again.

Maybe this thread will help someone else.

As you found a 9V battery is a preferred method to open an injector.
The injectors get 12V from the main relay and then are grounded by the ECU to fire them.
The injectors have a last chance filter in them, this can be checked once removed from the fuel rail. The filter is a tiny basket slid in place from the top.

You can swap the injector positions around to see if the dead cylinder follows, damn lot of work on these 6 cylinders.

You can also backflush the injectors, rig up something to spray cleaner into the outlet and open the injector with the 9V battery. Let the cleaner blast through.

Thanks for follow-up.

Been a frustrating year on this truck since I dropped a valve into the cylinder during timing belt service with valve seal change. Replacement water pump shaft seal failed and then new head gasket failed with coolant into cylinder #4. Eventually used "Steel Seal" which has fixed the coolant breach but now this no fuel from injector #4 seems more than a coincidence.

I wasn't sure if injectors fired with grounding or applied 12v, not much in the FSM. With ignition on I get about 9 volts at the harness. Having to remove plenum for injectors is a pain, but compared with my two previous tear downs the past few months not that bad. It's as if the truck has a death wish.

I hope I find something definitive when I pull the injectors. Hate testing and finding nothing.

9V in the harness seems low. The engines utilizing Low Z injectors have a resistor mounted to the firewall to drop the injector voltage but yours should be battery voltage. I would check your harness and battery cables to see if you have voltage drops somewhere, I have found resistance in the engine ground cable down by the AC compressor that made all sorts of issues. I doubt this is your current problem but you should be seeing close to full battery voltage there.

I hope you did not use a Felpro head gasket, I have had a 100% failure rate with them. For these engines I use the later MLS gaskets with good luck.

Yes, Felpro :>( I hope this breach is the last, no way I'm going to tear this down again.

I usually keep my grounds in good shape. The AC ground cable has been done twice since June 2016 due to two WP replacements, but I'll further check it.

I wondered about the 9V but since the other injectors are firing properly and all harness pins gave 9V I didn't think much about it since I was just on ignition, not engine running. I'll retest and see if I can get 12V.

Good info, thanks

OK, I cleaned the ground cable connections, even the one that bolts to the frame about midway to the battery.

Retested and got 9.5 volts from + pins to injector ground at harness. Then I grounded direct to the engine and got 11.5 volts. Looks like I may need to clean the computer ground.

Check the voltage from engine block to battery negative again, but also from Bat + into the fuses. The stacked terminals at the battery can hide problems.
Check bat+ to where you got the 9V at the injectors, if you are loosing power somewhere you want to check Pos to Pos and try to find all the ¼ V drops. May years ago I was working on a customers problem child Volvo, it had little hidden issues that could not be found till you looked for a drop along each wire or across each bolted terminal. I have found voltage drops in crimped connectors, not the push together part but between the wire and the crimped on terminal. These can be cleaned with an acid or using CLR cleaner.

OK, so cleaning the computer ground got me to 10.5, another volt. I also cleaned the negative battery terminal.

Yes the ground strap to the AC bracket/frame/body has crimped connectors for the bolt connection and I can see where that could have corrosion inside the crimp. I cleaned as well as I could with carb cleaner and wire brush as well as sandpaper for the bolt connection.

I'll check your other suggestions. Even though with a measured volt drop the engine runs fine except for the #4, which today I double confirmed is the dead cylinder. Would I get a full 12v when the engine is running? Don't know how to test as the ground is switching too fast for DVM. So would muriatic acid be OK to clean those crimped connections?

This truck has been in NC for the past 20 years. No real salt/water corrosion caused issues. Whole body has no rust.