FIXED! 1998 chevy k1500 5.7 vortec misfire

Can someone please help with what I should check next, I have a 1998 Chevy K1500 5.7 Vortec with a misfire, P0300 code for 3 months. runs a little rough, while driving under normal conditions it sputters, hesitates, and also will backfire, under heavy acceleration no noticeable skip, no sputters, and does not hesitate or backfire. Before I started doing some testing I changed some parts. Here is what has been replaced to try and stop this misfire: spark plugs, spark plug wires twice, distributor cap twice, rotor, ignition coil, EGR, CTS on intake manifold, coolant temp gauge sender, fuel filter, fuel pump, MAF sensor, oil change. Here are the tests I've done so far fuel pressure test, fuel pressure 160psi while running, then shut off for 20 min and fuel pressure was at 155 PSI. Dry Compression test on all 8 cylinders between 150-160 psi. A week ago bought one of those obd2 code readers for the cell phone and here is the info from when the code appeared, that is P0300and P0420, Calculated engine load 13.7, Engine coolant temp 188.6, short term fuel trim B1 +7.03%, Short term fuel trim B2 + 2.34%, long term fuel trim B1 +16.41%, long term fuel trim B2 + 6.25%, intake manifold absolute pressure 46 kpa, engine rpm1566, vehicle speed 47.2 mph, MAF rate 26.04 g/s, throttle position 9.8%. So yesterday disconnected negative battery cable to shut engine light off, Drove it today CEL came back on and hooked up obd2 reader again here is what it says this time , P0300 with no P0420 code, engine load 15.7%, coolant temp 188.6 degrees, short term fuel trim B1 +3.13%, short term fuel trim B2 +3.9%, long term fuel trim B1 -5.47%, long term fuel trim B2 -10.16%, intake manifold absolute pressure 52 kpa, engine rpm 1517.5 rpm, vehicle speed 42.85 mph, MAF rate 29.73g/s, throttle position 11.8 %. If someone could please HELP it would be greatly appreciated, this truck is driving me NUTS. I don't know what else to check maybe the spider injection, I'm all out of ideas. Thanks for taking the time to read this mess.

Dude, 150 psi of fuel pressure is extremely high, perhaps even impossible. I would suspect your pressure gauge. Dead head pressure may reach 100 psi. These vortec engines are well known for injector/poppet valve problems, so you may want to consider the upgraded injector assembly that eliminates the poppet valves. This assembly is readily available and actually cheaper than the stock setup.

Keep throwing parts at it. You're bound to fix it sooner or later.

Hey thanks for that post Hash Cash about the fuel pressure because I screwed up with those numbers, so my wife made me recheck the fuel pressure. Ok the first time I checked it a few weeks ago I was running 60 psi and after 20 min sit would drop to 55 psi, which to me is pretty good. But yesterday when I re did the fuel pressure check, I found something interesting.
First check- cold engine 55 PSI running - 20 min sit dropped to 41psi
second check-warm engine 55 Psi running - 20 min sit 52 PSI
third check- normal operating temp 55 psi running - 20 min sit 55psi, didn't drop
4th check- cold engine- 58psi running - 20 min sit 48psi
5th check warm engine - 55 psi running - 20 min sit 42 psi
6th check warm engine 56 psi running - 20 min sit 40psi
7th check normal operating temp 56 psi running - 20 min sit 41psi

Does this sound like a stuck open or closed poppet or the fuel pressure regulator?? Thanks again

If I cant figure it out, that's the plan

Also at idle its rough but breaking up pretty bad between 1500- 1600 rpm a lot of popping coming out of exhaust

one more thing all the spark plugs are 3 months old and all 8 spark plugs have a brownish blackish tarry streak down the sides of the porcelain below the electrode and its just a streak maybe 1/8 to 1/4 wide that travels to the bottom of the porcelain and the rest of the porcelain is clean.

Okay now you have a tool tool read the fault codes you can get down to tracking down the fault before throwing more parts at it.
With fuel trims what is of interest in the total fuel trim for each bank so taking the first results bank 1 seems to have a total fuel trim of around +23% and B2 around +8%.
+23% is way above what is an acceptable fuel trim and indicates the computer is adding a huge ammount of fuel to counter a lean mixture.
It is a chicken and egg situation misfires make the exhaust oxygen sensor read lean but lean air fuel ratio causes misfires, however as you have eliminated most of the other causes of misfires it looks like lean air fuel ratio is the cause of your problem.

However before jumping to the most obvious conclusion it is Vortect injection poppet problem you I would look at output voltages from the 4 exhaust gad oxygen sensors. Get the engine fully warm at idle speed look at the voltage output from each upstream sensor B1S1 and B2S1. The voltage from both upstream sensors should constabtly jump up and down between roughly 0.1 volts and 0.8 volts, if your app graphs the readings this is the best way to check oxygen sensor activity.
If when the engine is warm the voltage from any of the oxygens sensors reads a constant 0.5 volts you have problem with the sensor or it's heater circuit. Try holding the rpm around 2,500 to 3,000 and look for any life signs from the sensors.

If the output from either B1S1 or B2S2 sensor is fixed low(below 0.5v ) try fluttering the throttle (just very very quick very small dabs on the throttle) this should drive the voltage towwards 0.9v indicating richer fueling.

You could try also disconnecting the Mass Air Flow sensor (MAF) to see if there is any improvement but I don't think this the problem
Also be aware the OEM airfilter and the condition of air tube is critical and can really upset the fueling on a MAF engne.

Check for vacuum leaks this will drive your fuel trims to go positive.


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Hey Andy.MacFadyen thanks for the post, my obd2 reader just gives me the codes and the readings from when the CEL came on, I cant do live readings. You know, now you mention the MAF sensor that was the very first code the computer threw 3 months ago, then it went from that to a p0303 code, then A while later it went to the dreaded p0300 code,I don't remember the exact code from the beginning but I think it was MAF low voltage circuit , so that's when I changed the MAF. I tried unplugging the MAF last night and ran it at idle to see if the rough idle went away but it didn't. any other ideas? THANKS

RolexService thanks for the reply, I'm going to do that , I'm having my wife picking me up a cigar today so I can blow the smoke through the brake booster hose since I aint got one of those smoke machines. I have visually and listened for vacuum leaks but didn't see or here anything. Besides a smoke test are there any other checks I can do to find vacuum leaks? THANKS again.

I think you might have exhaust valves sticking in the guides. The way that would usually present itselfe is on hard accell the light would start flashing then by the time I would get back to the shop the light would be out. Strange because the light in my opinion should have stayed on after a misfire event.

It was kind of a common thing back then.

Sterline80 wrote: Keep throwing parts at it. You're bound to fix it sooner or later.

Come on man...
You've never had to ask anyone for help before?

Keep it constructive or keep it to yourself.

If you download the Torque Pro app for $5 you will be able to read data and graph. I keep the OBD2 connector on me all the time in case something comes up!

I'm suspecting a vacuum leak here since it appears to be impacting one bank. Fuel pressure wouldn't cause a one bank issue.

Set up the app to graph b1s1 voltage and stft B1 and stft B2. With a propane tank unlit, open the valve and "spray" the gas in suspecting areas like the intake manifold and hoses and when you find the spot the stft will instantly dive. The O2 will also instantly go rich and increase voltage due to them detecting the added fuel. The stft will be responding to that be decreasing the out of fuel it is being given.

You can also do Paul's favorite water trick around the intake when the engine is cold. I would focus on that bank that had the +25 trim though. That can cause misfires.

You can check to make sure the O2 sensors are repsonding by pulling a vacuum hose and watching to see if the voltage dives - low is lean. You can also introduce the propane to a hose or intake and see if they rise with the extra fuel. A full throttle run should show rich as well.

Keep at it!

Although the most used Android OBD2 apps Torque and Torque Pro I would not recommend tham as they are a pita to use, selecting PIDs you actually require is very slow, the frame rate is slow and quailty of the graphing leaves a lot to be desired.

The pro version of OBD Car Doctor is much more useable.

I'll have to check that one out. I actually prefer the graphing in torque over my launch scanpad!

Finally fixed after 5 long months of going Crazy. Here is what has happened since I replied last, The skip was getting worse and worse, about 3 weeks ago my fuel pump started acting up stalling the truck and making weird noises, replaced it along with the fuel pump solenoid, After I did that seems like it ran even worse, then a few days after that my radiator blew a hole on drivers side tank, replaced it. Power Steering pump shit the bed after the radiator, so I replaced it also. Then finally we broke down and bought the new injector assembly for $375 with the gasket set, so I dug into that and when I got the upper intake off, it revealed I had a series fuel leak, the front half was nice and black with lots of carbon deposits but the back half towards the distributor end was clean as a whistle, no carbon build up, so I got excited I thought I had finally found the problem, installed new injector assembly with new gaskets , checked it for fuel leaks them put everything back together, and started it. Well guess what, when I started it, the skip got so much worse, but the exhaust smelled cleaner, so the next day, I took it back apart took the spider injection back out to see if I possibly had kinked a fuel line to one of the injectors, so I reinstalled the spider injection again being very careful routing the injectors so I would definatley know it don't have kinks, then I reconnected the fuel rail again while the upper intake was off to triple check and make sure the injection system didn't have leaks anywhere, after I made sure there was no gas leaks, put everything back together again, and it ran just as horrible skipping, sputtering, puffing but I had noticed the loud backfiring had stopped. So anyway as I was checking sensors and vacuum lines to m,ake sure everything was connected, I noticed I had a lower intake leak, leaking coolant pretty bad, I guess from me jarring around the upper intake. Finally I gave up, It seemed every time I fixed something something else broke. We then decided to take it to a mechanic to get the lower intake fixed for the coolant leak, and was hoping that was causing the misfire, 2 days ago we brought it to the mechanic and he said its probably not the intake leak causing the misfire, he said he would hook it up to the computer after that was replaced, a couple hours later he called and said he thinks he had found the problem, when he took the distributor out to change the intake gasket he said the distributor was worn plum out, and had a bad egr valve hose on the intake, Last night we went and picked it up, keeping my fingers, toes, and eyes crossed, started it up and drove off, it is now running like a CHAMP. Funny thing was from the beginning roughly 5 months ago, I said it was probably the distributor, but I kept thinking how could it be bad, I never messed with it. But anyway during 5 months of aggravation and roughly $2500 in parts and this mechanic labor, My truck is running better then ever, the sad thing is should have just brought to a mechanic and had it diagnosed the right way, instead of me and youtube trying to diagnose it and throwing all kinds of parts at it, that it don't need, we probably spent a $1000 on parts it didn't need, everything else it definatly needed. Thanks to all that replied to this post trying to help me figure this out. FINALLY FIXED!!!!!!

That girl hit a long patch of rough road!

Sounds like you've been busy.
Glad it's back in action. Don't get down on yourself for not nailing it down right away. If it was easy, we wouldn't need forums like this or instructors like Paul.

Thanks for getting back to us with the fix.

Hi, I found out that the distributors on the vortecs wear out and cause these problems, some say only 100,000 miles. Mine is 243,000, but a new one fixed it. The OEM are plastic and the shaft starts to wobble as they wear and age and produce misfires. Got a new one made from aluminum with cap and sensor for $34 on eBay, works great. I first had changed everything from sensors to filters and injectors, but it was the distributor. To set the new one on a vortec you have to use a scan tool, you are really setting the cam sensor since the engine is self timing. Instead I bought an app, Car Gauge Pro and a obd2 blue-tooth adapter I got on eBay (BAFX) and set mine for under $35.