I have a 03 blazer xtreme that i recently replaced the engine with a used motor that has 60k on it because the one i had had bad compression on a cyl. Well ive basically replaced everything on this engine now and im having a P0300 code on highway speeds that flashes and slight miss at idle. Now ive replaced fuel pump, mass air flow, fuel injection spider assembly, all ign parts, ign coil and module, cleaned throttle, new battery, belt, fuel filter, air intake system, the works. The one thing i didnt use OEM parts were the plug wires and intake pressure sensor on the upper plastic intake. Ive checked compression and oil pressure and all are perfect. Now when your driving around town its fine but when you try to hammer the gas and have some fun with it it feels like it hits a wall and has hardly any power. Oh ive also replaced exhaust and cat. Now i took it to a shop locally and he says it only has misfire counts on cyl. 1 and 4.

So i finally hooked up a fuel gauge to it and drove around. At idle cold its at 53 psi, when it warms up it drops to 51 psi. Under moderate acceleration its at about 55 psi, under heavy it gets to about 58 psi. At WOT it maxes out at 60 psi and wont ever get any higher. Now when i took it on the interstate and got it up to 75 and put cruise on the psi was at 56 psi then would slowly drop to 53 psi and thats when the SES light would start flashing, then after a minute or so the psi would raise to about 56 again and would keep doing this the whole time driving the highway. I also noticed that when i take my foot off the gas the psi drops to about 49. I aslo tested when the vehicle is off and it holds pressure fine, but noticed when you turn the key to position 2 and pump primes itself it goes to 55psi and then after about a second or 2 it drops to 50 psi everytime before you try to start the vehicle. Anyones advice is much appreciated.

 

Put the gage on it, turn key on, prime, read gage, don't touch anything, wait ten min's read gage. Post numbers.

 

Im just curious what will that tell you? The regulator and spider injector assembly is new also if thats what your curious about. Just wondering

 

The numbers you referenced in your first post sound like a fuel delivery problem. Try working through 96+ Fuel System Diagnosis and see what you come up with. Let us know.

 

Read post #85 as it may apply to your problem

https://blazerforum.com/forum/2nd-ge...e-35827/page9/

 

Well i dont have the gauge on it anymore, it was done at a local shop. I rode with them and watched the gauge so they could show me what it was doing.

 

All of your engine running fuel pressure tests are worthless. GM does not publish any engine running fuel pressure specs for your engine. ALL fuel pressure and leakdown tests are done with the engine off.

I agree with spittybays, it does sounds like a fuel delivery issue. Key ON, engine OFF, fuel pump running, pressure must be 60psi to 66psi and must remain above 55psi for at least 10 minutes after the pump shuts off. Post the results using this method. Some of the auto parts stores rent fuel pressure testers. If you're planning on working on this vehicle yourself, spend the $40 and buy one, (you'll use it more than you think you will ).

 

Ok before we disconnected it i know with key on engine off it only primed to 55 psi then after bout 2 or 3 secs it dropped to 50 psi but did not moniter for 10 more mins.

 

There's the problem. Fuel pressure is too low.

To pinpoint the problem you'll need to do a couple more tests.
Check fuel pump maximum output pressure at the fuel filter. You'll need to modify the tester so fuel pressure/flow ends at the tester. Pressure must be 73psi to 108psi and must remain above 55psi for at least 10 minutes after the pump shuts off. If it fails any part of this test, check the electrical connections at the fuel pump module. Key ON, the grey wire must show battery voltage for 2 seconds then drop to zero. Both of the black wires must show less than 5 ohms resistance to ground. If the electrical checks out ok, the pump is faulty.

EDIT: There are countless threads on the forum warning against using aftermarket fuel pumps. Use AC Delco or Delphi only.