Lab Scope for Injectors

[anotheridiot]

We rebuilt a GM 3.1 M engine for a 98 Malibu. The car starts fine but then starts knocking (hopefully detonation) when it gets to temperature. The colder it is the smoother it runs. Main code is misfire cylinder 1. Yesterday we were backprobing the injectors with the lab scope. The waveforms look like the book suggest from AES, except there isnt a real pintle closing hump, its more of a steady decline. The upward spike varies in height and it stays together on the way back down instead of showing a gap.

The main question I have is does a wave form necessarily mean that fuel is getting thru the injector? Can the filters be varnished up or pintles sticking at temperature and not allowing fuel to get thru? Does the electrical reading really mean an injector is working? Cylinder one is the main misfire and the plug in that cylinder is a wet carbon where the other plugs are just dried black carbon.

These injectors were out of the car for probably around a year. We have used a pressure canister fuel injection cleaner on them and it hasnt helped, although when it was hooked up the car ran extremely smooth.

[greasybob]

Here are two screen shots of two 5.3 injectors, the one plugged with no pintle movement and the other a good injector with good flow. The plugged injector has no hump and draws less amperage. I would not use this as a conclusive test though and not sure it applies to all injectors, other information such as fuel trims, fuel pressures, ignition testing, should also be taken into account. When does the misfire occur ? under load ? at idle ? If the plug is coming out wet I would look at compression, ignition, and possible vaccum leaks. Too bad a functional flow test is not offered for this vehicle.

[anotheridiot]

Thanks for those pictures. Its just idling. Idling cold has no knock, when it gets to temperature the knock starts. We havent even had it out of the shop yet to put it under load. We double checked compression and are between 170 and 180 on all cylinders. I first thought the lifters werent getting enough oil and not staying pumped up, they are new and are the stock hydraulic rollers. We changed the oil pump and got 15 psi at idle and 20 at 1500 rpms.

We have a 97 lumina that has the common piston slap knock cold that goes away when warm and its a completely different sound. I think the car would run great if we could keep the temperature down around 100. So I am thinking the injector gets hot and the pintle starts to stick.

[ephratah service center]

I unplug the harness and fire the injectors with the injector tester from snap on to check for pressure drop. I compare several injectors to see if there is a difference in pressure drop from one injector to the other.