I am testing my fuel pump and the pressure in the vehicle. At Key on, engine off, it goes to 61 psi which is correct. No leak down problem afterwards. But, at idle, the pressure is a steady 52 psi. That sounds kinda low. I am getting a P0300 code (random misfire) even though the engine has no noticeable issues. If the pressure is too low, it may cause the injectors not to spray every so often causing this P0300.

If I can some how figure out where to pinch the return fuel line a little bit, it will raise the pressure and I can see on my phone app if the misfires go away. But the lines are ALL steel or that hard plastic. You can't pinch those lines. There are no rubber, flexible lines in the whole car!

How do you pinch the return fuel line?

 

Simple answer: You don't.

You have another problem, as possibly the fuel pressure regulator itself.

 

Yes, there is a problem. But nobody knows what it is. How can you possibly cross out the fuel pump as not being weak? It needs to be *tested* to be crossed off the list. And pinching a fuel line (if there was a rubber one) to test the pump is WAY easier than removing the intake manifold to get to the fuel pressure regulator. So thats why I'm trying to do it first.

 

If you're trying to destroy the fuel pressure regulator, that's about the easiest way Restricting the return line will expose the diaphragm in the regulator to more than ~75psi, and it will rupture. Don't try it. This will help you diagnose the fuel delivery system: https://blazerforum.com/forum/2nd-ge...eakdown-88305/ Post all 4 readings and we can analyze them for you.

 

Okay, here are the results:

62.0 psi - Pressure at fuel rail service port, engine off. PASS.

55.5 psi - Leak down value after 10 minutes at service port. PASS.

88.0 psi - Pressure at fuel filter. PASS.

77.0 psi - Leak down after 10 minutes at fuel filter. PASS.

All these are good values.

I also took some fuel pressure readings with the car idling:

53 psi - Car just turned on, idling, engine cold.

52 psi - About 10 minutes later when operating temp reached, idling.

50 psi - After about 30 minutes of idling. Remained this way and never went any lower.

Here's where things get interesting. I was able to come up with a method to vary the return fuel pressure with an inline ball valve. Near the fuel filter the return line has a junction. I undid that junction and put a ball valve in. So when the car was idling for 30 minutes with 50 psi fuel pressure, I slowly closed the ball valve watching the pressure gauge at the service port. It did in fact start to rise as I closed the ball valve. I put it at 62 psi. I looked at my scanner while it was at 62 psi instead of 50 psi to see if there was a change in the random misfires. There wasn't. The misfires did NOT go away so that told me that operating at 50 psi should be fine. Then, I got further clarification that 50 psi is okay because the fuel trims went way into the negative when I turned the pressure up to 62 psi. They got better (more towards nominal) when I lowered the pressure back to 50 psi.

This is EXACTLY why I wanted to test this out instead of going through the massive headache of replacing the fuel pump!

So the question remains, what is causing the misfire? Now that this fuel pressure post is finished, I guess I will have to make a new one about the misfire. : /

I love my ZR2 Blazer, but man, sometimes taking a sledge hammer to it sure seems like a lot fun.

 

There are a TON of things that can cause a misfire, not just the fuel delivery system. Is the check engine light on? What DTC's are in memory? Under what conditions does it misfire?


These engines are very fussy when it comes to ignition parts. Aftermarket stuff doesn't play nice with them at all. If there are any installed, save everyone a lot of time and frustration and replace them. It might just fix the problem Here's a list of the "basics" that are necessary before diagnosing a misfire:


No DTC's in memory, or pending
No check engine light
AC Delco 41-993 plugs with a gap of .060"
AC Delco distributor cap & rotor (dab of silicone dielectric on each terminal, inside and out)
Good quality, name brand plug wires, (not a store brand)
Camshaft retard adjusted to zero degrees (+or- 2 degrees)
Current crankshaft position sensor relearn data in the PCM (requires relearn)
Good distributor bushings and drive gear
No vacuum leaks
good, fresh fuel


Let us know when you have the basics taken care of.