P0300 and fuel pressure
 

I've got a friend with a 1999 Tahoe with a 5.7 equipped with the stock CSFI system. About two weeks ago, the vehicle began to misfire and illuminated the CEL. It is throwing a p0300 Random Misfire code. We hooked it up to the computer with my scan tool and saw a lot of misfires. The misfire count seems to be increasing faster on the rear cylinders (5,7,6,8) and is not isolated to one bank. All cylinders were showing some misfires.

The idle is very erratic and the truck has a definite hesitation while driving. The exhaust smells very strongly of unburned fuel and in fact appears to spray fuel out of the tailpipe when accelerating after being stopped.

We went through the entire ignition system to see if that was the problem. Coil seems to be fine, as do all the plugs and wires. Wires, distributor cap, and plugs have recently been replaced (Approximately 1k miles ago). With the ignition seeming to be ok, we went ahead and moved on to fuel delivery.

We hooked a fuel pressure tester to the fuel line and immediately thought we had found the problem. Upon the key being turned on and engine still being off, the fuel pump will pressurize to 55-60 psi, but that pressure bleeds off completely within 15 seconds. When running, fuel pressure seems steady around 55 psi. I think that's within specs (53-60 psi right?), but everything I read states that the fuel pressure should hold constant after the key is turned off.

With this new data, we replaced the fuel pressure regulator. Unfortunately, this has not changed anything. Fuel pressure still bleeds off immediately after the key is turned off and the engine is still running rough. Worried that one or several of the injectors might be stuck, I pulled all of them out of their ports. Not a single one leaks a drop of fuel when the key is turned on.

At this point, I'm stumped. Everything I read points towards this being a fuel delivery problem, more specifically, the pressure regulator. Is the CSFI system shot? Is my friend at a point where an MPFI conversion is justified?

Thank you for your time.

-Brett

You more than likely have bad fuel. Add a can of seafoam or injector cleaner to the gas, then go from there.

If it's not the injectors leaking, then gas is feeding back into the gas tank. Since you should be able to rule out it being the pressure regulator, it must be the check valve in the fuel pump isn't holding. But, since you're keeping good constant pressure while the engine is running, I don't see that as being the cause of the random misfire. Your problem could be with restricted or clogged injectors. Been there, had that, and injector cleaner additive didn't help for squat. Cleaner in the tank certainly won't help if you have a totally clogged injector, since it can't run through the injector. When you have a clogged injector, or more, your system doesn't know which one is causing the problem. Then the O2 sensors pick up a lean condition and tells the computer to richen up the air/fuel mix. Now the good injectors are spraying more than they should, and you have a confused mess, and a reason for the random misfire code. Best bet would be to remove the injectors and clean them by hand. Go on youtube, there are videos on how to clean the injectors while you activate them to really get them flowing well again.

Thanks for the advice guys. I'm going to go ahead and clean the injectors out manually. After I reinstall the CSFI unit, I'll run a can of cleaner through to clear out anything I missed. Hopefully we'll see some results!

I'm probably going to take care of everything Thursday evening. I'll let you know how it goes.

Did that work ??
 

I have a 04 Chevy Tahoe z71 and that same code is giving me nightmares. Did cleaning the fuel injectors fix the problem ?

No, I ended up trying to clean it and it didn't fix the issue. Be extremely careful if you end up trying to clean them. It's incredibly difficult to get the poppets out . Ended up that my friend had screwed up the distributor cap when she changed it. I should have figured as much. It's always the last thing that was worked on.

Have you checked the fuel filter? I have a 2003 suburban that had crank and no start. I replaced the fuel pump but then she died. I then replaced the battery but she was losing power. Finding a circuit gremlin in the cluster I replaced that. Then she gave me the po300 code. I had already done all the sparks and plugs and replaced both knocks. I replaced the fuel filter cause that's all that was left and now she runs like a beast.

Question regarding codepo300 post
 

[QUOTE=mm66;241736]No, I ended up trying to clean it and it didn't fix the issue. Be extremely careful if you end up trying to clean them. It's incredibly difficult to get the poppets out . Ended up that my friend had screwed up the distributor cap when she changed it. I should have figured as much. It's always the last thing that was worked on.


So just checking are you saying that the screwed up distributor cap was causing the po300?

Yes, the new distributor cap had significant carbon tracks from being installed incorrectly. Removed it and replaced it with a new unit and all was well with the car and the world.

I can't believe that this thread actually got another reply 7 years later 🤣. That Tahoe is still on the road in case anyone is interested. I don't see it very often as its owner is now my ex, but last time I talked to her it had about 350k miles.

Sweetlilnitemare, that was one of my first thoughts but it was quickly put to rest by attaching a fuel pressure gauge to the injector rail. Drove it around the block with the gauge under the windshield wiper and the laptop logging misfires. Pressure stayed strong but misfires happened seemingly at random. It was 100% the distributor cap being damaged from incorrect installation. Thanks for your thoughts though!