Ok, for starters, very frustrated with this truck. And sorry in advance if this is an old topic. I searched the forum but didn't find an answer to my problem. Here is the history: truck has about 160k on it. It ran fine until i took it on a 1100 mile round trip.

On the way back it started running really rough, was afraid it would stall at every stop so i just kept her moving as best as I could at 55 and drifted up to red lights so I wouldn't have to stop. When I got it tested finally it was throwing the multiple misfire code. I began by looking here (months ago) and elsewhere trying to fix the problem.

I changed the plugs, wires, cap, rotor to no avail. I bought a fuel pressure tester, was below 55 and would suddenly drop below 40 and stall. Figuring it was a bad pump, I replaced that. Pressure back up to 60, but still drops to 0 when stepping on accelerator. Ok, thot it might be pres. regulator or bad spider so I went ahead and replaced that with the upgraded one. It will idle now, roughly, but fuel pressure is still dropping to zero when i hit the gas and it will stall if i keep stepping on the gas. It's acting like it has a massive vacuum leak but since I just changed the spider, I didn't see anything.

I haven't checked the codes again, but I don't have a tester and the truck won't make it to Advance Auto to check it. After about $500 and shipping I'm out of ideas.

 

These things are real fussy when it comes to tune up and fuel delivery parts. If they aren't AC Delco or Delphi, chances are pretty good that you'll have trouble. The fact that you've replaced the fuel pump, and have done the injector upgrade, means we need to start at ground zero with diagnosis because there's a problem with fuel delivery, and you've already replaced most of the system.

P0300, (random multiple cylinder misfire) is almost always caused by a fault in secondary ignition: spark plugs, plug wires, distributor cap, rotor, and ignition coil. Low fuel pressure, low volume, or excessive leakdown can also cause it to set along with a worn distributor gear and or bushings, and incorrect camshaft retard, to name a few. Your fuel pressure drop to zero on acell is a problem that needs to be addressed first.

When you initially turn the ignition to the RUN position, (do not crank the starter) the fuel pump should activate for ~2 seconds and shut off. Fuel pressure must be checked during the 2 second time frame, while the pump is activated, and the engine is off. Pressure must be 60psi to 66psi, and it must remain above 55psi for at least 10 minutes after the pump shuts off. Post your readings with the pump activated, and again 10 minutes after the pump shuts off. The results will tell what we test next.

 

It stays at or above 60 PSI.

 

Just to verify until the captain gets back, you're saying that fuel pressure stays above 60 PSI ten minutes after the pump shuts off?

 

Yeah, it holds. Except for the tubing, everything is new in the fuel system. It damned well BETTER hold! (btw, I forgot to say I changed the fuel filter). It just drops to zero when I hit the accelerator. If I let off the accelerator, it jumps back up. Something is telling the pump to shut off, assuming the computer.

 

Sounds like like your checking the fuel pressure with it running? Should be checking it with key on, no run. Key on pump shuts off after 2 seconds

 

Are you stabbing the pedal to the floor?

 

Nah, that's with the engine NOT running. It's a brand new pump, new spider, no leaks. So why shouldn't it hold pressure? Engine just runs like crap, holds idle (badly) and stalls if I hit the gas, even just a little. The fuel pres. gauge shows it dropping every time i hit the gas so I'm wondering if something is telling the pump to shut off. I'll probably end up taking it to the shop, but I thought I'd come here and give it one last shot to see if you guys could help. Thanks in advance!

 

Ok, just to be sure, I ran the 10 min. test again. pump running, over 60 psi, then dropped to 58 and held for ten minutes.

 

I dont see how touching the gas with the engine not running would drop it to 0. the pumps already off