98 Chev Vortec 5.7 Misfire

I have a 98 K1500 Vortec 5.7L with 244,000 miles and a misfire issue. Idles great but when demanding power it starts misfiring. The issue came on suddenly after it had sat for a few weeks unused. Replaced distributer, cap, rotor, wires, plugs and it acts exactly the same (good wires and Delco plugs). Fuel pressure ignition on 60, idling 55, under power 55-60+, minor bleed down 3psi after 1 hour. Leaving it overnight for a long time bleed down check tonight.
At the point now where I need assistance, any suggestions for the next check??

Iv'e had the same issue with my 88 k2500 when i let it sit for weeks at a time.

I think the issue I had, was that when it sits like that, the injectors get clogged up or some other similar effect? I dump a bottle of octane booster/injector cleaner in my gas tank, and it clears it up after a few miles.

Hope this helps?

I have a 98 K1500 Vortec 5.7L with 244,000 miles and a misfire issue. Idles great but when demanding power it starts misfiring. The issue came on suddenly after it had sat for a few weeks unused. Replaced distributer, cap, rotor, wires, plugs and it acts exactly the same (good wires and Delco plugs). Fuel pressure ignition on 60, idling 55, under power 55-60+, minor bleed down 3psi after 1 hour. Leaving it overnight for a long time bleed down check tonight.
At the point now where I need assistance, any suggestions for the next check??

Your fuel pressure is low 5-10 lbs.
Was camshaft retard setting correct and checked on a engine capable scanner?
Is fuel system running lean?
What codes are you getting?
Is miss ramdom or a certain cylinder?
Delco dist cap?

Had my truck hooked up to a solus about 12 months ago with a random cylindar misfire code, turned out to be bad dist cap. At that time the camshaft retard was good, fuel system was good, never checked fuel pressure at that time.
I know I have a fuel evap code but I have a buddy scanning for new codes today. The check engine light is not flashing, like it does with a bad misire.
Not a Delco dist cap but no change in performance after changing dist.
I will scan again and hope for new codes.
fuel pressure low, would it be the fuel pump even though it held pressure and didnt bleed down? I replaced the fuel pressure regulator 12 months ago.

Had my truck hooked up to a solus about 12 months ago with a random cylindar misfire code, turned out to be bad dist cap. At that time the camshaft retard was good, fuel system was good, never checked fuel pressure at that time.
I know I have a fuel evap code but I have a buddy scanning for new codes today. The check engine light is not flashing, like it does with a bad misire.
Not a Delco dist cap but no change in performance after changing dist.
I will scan again and hope for new codes.
fuel pressure low, would it be the fuel pump even though it held pressure and didnt bleed down? I replaced the fuel pressure regulator 12 months ago.

Low fuel pressure CAN GIVE MISS FIRE.
Non OEM dist cap can give misfire.
Other things are as.
Low compression .
Vacumn leaks.
When ever dist is replaced the camshaft retard need ckecked/reset.
Bad or cracked plug or bad plug wire or weak coil.
Maf sensor.
Distb rotor.
Bad gas
Non oem type air filter /systems.
bad o2 sensor.

Let us know how the code check and setting cmshaft retard goes.

with this issue post the DTC engine failure codes found. this can narrow down whats really going on.


what was the ignition wire resistance ? new wires does not mean there good wires.

Ok I found a couple issues. First was a bad plug wire. Second I have a bad O2 sensor down stream. replaced the wire and it definetely runs better but when it warms up it starts missing again. I will replace the O2 sensor tonight and see if that fixes it entirely.
The only codes read were misfire, fuel evap (known), and heater on O2 sensor.

Ok I found a couple issues. First was a bad plug wire. Second I have a bad O2 sensor down stream. replaced the wire and it definetely runs better but when it warms up it starts missing again. I will replace the O2 sensor tonight and see if that fixes it entirely.
The only codes read were misfire, fuel evap (known), and heater on O2 sensor.

Is it getting good hot blue spark to all plugs when hot?
What was wrong with the bad wire?

What was code no?
Evap system could aso cause missfire.
What is problem with it?

As in post 5 your fuel pressure is low?
Low pressure can cause miss and aso have you checked other tings from post 5???????? .

Ok I found a couple issues. First was a bad plug wire. Second I have a bad O2 sensor down stream. replaced the wire and it definetely runs better but when it warms up it starts missing again. I will replace the O2 sensor tonight and see if that fixes it entirely.
The only codes read were misfire, fuel evap (known), and heater on O2 sensor.

I agree with mt 2500 that posting the complete DTC failure codes will aid the forum members to best help you !

Ok I finally have everything sorted out. It turns out I had 3 bad O2 sensors which seems strange to me. The 2 downstream sensors tripped codes, one bad heater, the other low voltage. When it was still running bad and no more codes I had a buddy come over with his solus and take a look. Bank 2 was running rich and the B2S1 was not reacting very well so I replaced that one and new plugs again and now it is running strong as ever. I do still have an intermittent evap code but I don't know the number and I dont know why.

Ok I finally have everything sorted out. It turns out I had 3 bad O2 sensors which seems strange to me. The 2 downstream sensors tripped codes, one bad heater, the other low voltage. When it was still running bad and no more codes I had a buddy come over with his solus and take a look. Bank 2 was running rich and the B2S1 was not reacting very well so I replaced that one and new plugs again and now it is running strong as ever. I do still have an intermittent evap code but I don't know the number and I dont know why.

Thanks for posting back how it went.

If you want help evpa problem with post back code no.