99 C1500 Suburban P0300

[needlenose]

Nope wires are correct. Some interesting developments:

Pulled plug 5; it's black as sin. Almost a light fuzz. But there is a tiny clean spot in the center of the electrode. Like a spark has been popping right there. Pulled cyl 1 plug(replaced at the same time) and it just looks like a used plug. Slight discoloration, nothing remarkable.

Then, as I sat with the laptop connected and the key on adding inj p/w monitors, I heard a weird random clicking under the hood. I crawled around looking for it and it's coming from under the plenum. I checked the pressure and it's falling fast. Like 15lbs in about 5 min. I think the injector is firing by itself and dumping fuel. A couple of times it also makes a long wailing stuttering squeak(for lack of a better description). I bore-scoped cyl 5 and the piston is glossy black. Cyl 1 is flat black. I don't have a high quality scope, but I think I can see little sparkles on the piston. Maybe fuel running down the piston?

I guess I will have to remove the top of the engine again, pull the injector, prime the system, and see if it opens by itself when the key is on. Interesting that it doesn't do it when the key is off.

Sheesh......

 

[Schurkey]

How does #7 look?

 

[needlenose]

Completely clean.Shiny. Smelled faintly of fuel. Plug tests fine off the vehicle. Inline tester flashes regularly when it runs.

Part of my fuel leakdown was due to a low battery. My needles started flapping and the cluster finally went dark. The battery wouldn't hold a charge at all, even with a charger on it. So I went back to Autozone and warrantied that as well.

Now there's no clicking and no leakdown with the key on/engine off.

Also, I checked cyl 5 plug again after running it with the new battery, it looks normal. No black fuzz.

 

[needlenose]

Bought a noid light and the ECU is pulsing both injectors. Bought another Delco MPFI and pulled the old one today.

I learned you can pulse the injector on an MPFI unit by applying 5v to the injector pins. I tested it today on my old unit. I found a Multec document online that appears to support this. Apparently it pulses the inj very quickly with up to 60v after the pintle is lifted with 5v to hold it open and cut down on power consumption and heat. Or something like this.

Unfortunately, cyl 5 & 7 are clicking fine on the old unit. So I may have purchased an MPFI I didn't need. One detail is that the cyl 5 injector looks to have some black debris around the nozzle. I guess just because the inj is clicking doesn't mean the injector is nozzle is clear. Hoping that is the case....

 

[needlenose]

The source of all my anguish....

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[needlenose]

Just to circle back; the reman head is on and the truck runs perfect.

Moral: always do a compression check to diagnose a miss. Even if the fuel pressure guage says low; it could be lying to you. Mine was.

Thanks for all the help!