1999 5.7 vortec misfire

operator556 · May 27, 2015

New plugs, wires, cap and rotor within the last 1,000 miles. Started misfiring after an hour drive of running great. Only misfires between 2-3k rpm. From a dead stop it will run and accelerate normally in 1st then after it shifts it will misfire. It needs an idle air control valve I know that but would that cause a misfire? If I put the gas to the floor it'll run and pull like it normally would. The codes I'm pulling are P0300 and P0303 which is random misfires and a misfire on cylinder 3. This has me stumped!

sputtle · May 28, 2015

Classic fuel delivery problem. First thing to check is proper fuel pressure. You can rent these at O'Reilly's or stores of the like. Pressure should be around 60 PSI, 55 minimum.

If pressure is low:

Fuel pump will likely need to be changed, along with the fuel filter. I'm pretty confident this is where your problem is because this happens at higher RPM when fuel demand goes up.

If fuel pressure is good (60 PSI or better):

Injectors and poppet nozzles are likely the cause. Also the fuel pressure regulator could cause this. Injectors are usually recommended to be replaced, however I haven't had to do this yet and they are a bit spendy. I had a similar problem on my GMC Jimmy. If you're on a budget, I recommend this. Buy a new fuel pressure regulator and a set of gaskets for removing the throttle body and intake cover, and also one for the spider injector assembly. Then take it all apart. I was able to find lots of YouTube videos to help me out. Once you've gotten the spider injectors visible, have a buddy help you to verify they're all working. Just pull the ignition fuse and crank the motor while the other guy watches the nozzles spray. If you find one or some are weak, or altogether they aren't consistent with each other, then you can clean them. I used a can of starter fluid and tapped the nozzle on something sturdy. I did this until I was satisfied they were all clean and spraying consistently with each other. This completely fixed the problem and has lasted over 20k miles and counting. You could try a brush to help cleaning, but it wasn't necessary for me. This is also where you'll find the fuel pressure regulator, so if that is the problem, you'll have to do all of this anyway.

operator556 · May 28, 2015

I suspect it's the injectors because they've never been changed and at 166k that's a longer life than usual. But what has me scratching my head it that it runs damn near perfect at WOT above 3k. I'm debating just swapping the whole spider unit this weekend. Is cleaning them really worth it?

sputtle · May 28, 2015

If they have never been replaced, then buying new is the safest route. When I worked on mine, everything was in pretty good shape. The fuel tubes weren't brittle and the gasket for the injectors was good, so mine had been changed at some point. If you take it out and everything is dirty, brittle, brown fuel lines, then your best bet is to put in a new unit. It's also a judgement call. I have more time to work on things than I have money to throw at vehicles, so I don't mind taking the chance and seeing what happens. If you just want it done with, then I'd recommend buying new injectors.

operator556 · June 1, 2015

Ok so the fuel pressure is good and I changed the fuel filter. But on my test drive the ses light started flashing yet the truck was still running good. Is it only because there's raw fuel being dumped into the cat?

How can I be sure it's the injectors? I don't want to spend $280 on something that may or may not be the cure

sputtle · June 4, 2015

The only way to know for sure is to take it all apart and verify it visually. I've heard that a trained ear on a stethoscope can tell without disassembling anything, but mine is not trained and I'm not sure how to do that. Getting to the part and seeing it malfunction is the best way I know of.

1997SierraSLT · June 4, 2015

I finally gave up after owning this truck for 16 years and with 167,000 miles on odometor trying to hunt down this issue. I've replaced fuel pump (Delphi), replaced fuel filter every 30,000 miles, replaced CSFI system with upgraded Delphi MFI system, and also replaced numerous sensors. No luck. That Godforsaken check engine light starts blinking and eventually stays lit every time I romp on the throttle going down the freeway (highway speed) and hold it there until I reach about 85-90 MPH. Each time I check the the OBD-II code it shows P0300 (random misfire). However, from a standing start all the way up to about 50 MPH, it runs like a bat out of hell with no check engine light. It only comes on at around 3rd gear shift when the vehicle starts to pull a load.

Since the day I've owned it, I've referred to it as my rolling POS and just chalk it up to good ol' GM engineering at it's finest.

operator556 · June 4, 2015

Well I've been running a shit ton of injector cleaner through it and it's gotten much better. It's now only some of the time that it misses instead of all the time. It's got to be the injectors. I'll rip them out this weekend and I had a '97 Tahoe that did exactly that I did everything to it and the problem never went away until it finally broke a valve spring and grenaded the motor.

ELF 16 · July 4, 2015

My truck is a 1997 4x4 extended cab.165,000 miles.only misfires from 1500rpm to 2000rpm I changed the whole spider assy. Tune up.cap rotor wires plugs fuel & air filters. Vacuum sensor. I have a knock sensor comming tomorrow.i think I'm just gonna buy every sensor in the pos. New cats.I did see on here it could be a camshaft gear or crankshft sensor. Is there any ****ing thing else it could be ???...I dont get the check engine light anymore. But still misses just a little.It does have a ton more power since spider change. FML...THANKS For any help.