98 K1500 5.7L Cylinder Misfires

[Beaner81]

I have a 98 K1500 with 240000 miles on the 5.7L Vortec. I start getting misfires very shortly after startup, especially when towing large loads. Idles rough and spits black smoke while misfiring. I had my buddy hook up his solice and cylendar 2, 4, 6, and 8 are all missing. Both bank 2 O2 sensors are reading bad and I have O2 heater error. Sensor 1 is reading around 200 and sensor 2 is reading 700 +/- 20. Less than a year ago I replaced the cap, rotor, wires, and plugs so I'm thinking its not the cap. My thoughts are possible fuel pressure regulator leak? All vaccum lines look good. Does anyone have any suggestions before I dive into this one. It seems unlikely that both O2 sensors on that bank go bad at once while all cylendars on that side are misfiring. Again to me this looks like a fuel delivery issue.

[MT-2500]

Originally Posted by Beaner81

I have a 98 K1500 with 240000 miles on the 5.7L Vortec. I start getting misfires very shortly after startup, especially when towing large loads. Idles rough and spits black smoke while misfiring. I had my buddy hook up his solice and cylendar 2, 4, 6, and 8 are all missing. Both bank 2 O2 sensors are reading bad and I have O2 heater error. Sensor 1 is reading around 200 and sensor 2 is reading 700 +/- 20. Less than a year ago I replaced the cap, rotor, wires, and plugs so I'm thinking its not the cap. My thoughts are possible fuel pressure regulator leak? All vaccum lines look good. Does anyone have any suggestions before I dive into this one. It seems unlikely that both O2 sensors on that bank go bad at once while all cylendars on that side are misfiring. Again to me this looks like a fuel delivery issue.

What codes does it have?
How many misfires on each cylinder?
What is camshaft retard reading?
What are fuel trim readings?
Sounds like time to run a complete fuel pressure test.
The only true test on a fuel pump is to check the direct fuel pressure from fuel pump.
A quick test is to block off the return line and see if fuel pressure comes up to 75-85 lbs.
But do not run the pump at full pressure very long.
Also when testing fuel pump and pressure you need to tape a gauge to outside windshield or outside mirror and drive it on the road for 20 -30 minutes
until the pump gets has run a while to check for a pump fading out after hot.

Hard to start cold or hot and fuel pressure testing guide lines.
Check cold start fuel pressure.
Check engine running fuel pressure.
Check engine running on the road fuel pressure.
Then shut it off and watch for fast leak down.
Pinch off at rubber part or block off return line and check full pressure.
If pump has full pressure with return line blocked and low pressure without it blocked most usually the fuel pressure regulator is leaking or not holding pressure.
Later V6 and V8 gm engines should have 60-66 fuel pressure.
Cold start should be 64-65 lbs of pressure.
If you do not have full fuel pressure on a cold start the injectors will not squirt fuel.
Do not leave home without it.
Engine running should be 60-66 lbs pressure depending on engine load.
Full pressure with return pinched off should be 75-85 or more.
And if it has a fast leak down after shut off you have a leak in system.
Post back fuel pressure readings.

[Beaner81]

The codes were, fuel evap (which ive had for about a year), O2 heater, and random misfire. The cylendars misfired thousands of times each, we cleared the codes and watched 2, 4, 6, 8 misfire 50+ times in the first 30 seconds at idle. The engine does start great all the time.
I had a mechanic friend check out the camshaft retart reading and the fuel trim readings and he didn's see anything out of the ordinary. He wants me to replace the O2 sensors and then he wanted to take off the upper plentum and pressurize the fuel system. I will get some pressure readings within the next week and post back but in the mean time do you think i need to get O2 sensors or are they reading bad due to the possible fuel issue? The fuel pump was replaced 2 years ago.

[MT-2500]

Originally Posted by Beaner81

The codes were, fuel evap (which ive had for about a year), O2 heater, and random misfire. The cylendars misfired thousands of times each, we cleared the codes and watched 2, 4, 6, 8 misfire 50+ times in the first 30 seconds at idle. The engine does start great all the time.
I had a mechanic friend check out the camshaft retart reading and the fuel trim readings and he didn's see anything out of the ordinary. He wants me to replace the O2 sensors and then he wanted to take off the upper plentum and pressurize the fuel system. I will get some pressure readings within the next week and post back but in the mean time do you think i need to get O2 sensors or are they reading bad due to the possible fuel issue? The fuel pump was replaced 2 years ago.

Airtex pump?

Confirm fuel pressure test before pulling intake.

[MT-2500]

Engine miss and fuel pressure can throw off 02 sensor readings.

[tblake]

Did all the missfires start at the same time? Or have you noticed misses progressively getting worse over time? Mine had misses on two cylinders which turned out to be the darn CPFI injectors. I fixed the whole can of worms with the MPFI upgrade which included a new fuel pressure regulator. However it may be more money that you may want to spend on your truck with 240k.

[Beaner81]

Yes Airtex pump.
Misfires started all at the same time about 2 weeks ago, its hard to say if it is getting worse because it just plain runs bad. I don't drive the truck much but it does better without pulling any load. Once I pull a trailer it really acts up. I heard about the injector update kit and the series of issues with the poppet style injectors but then I saw it was 4 cylendars at once misfiring and I figured I would dig in deeper before pumping cash blindly into the engine. Being one bank of the engine could it be a bad distributer cap after less than 1 year?

[jdmccright]

I would check items that affect the whole bank, namely the O2 sensors being the most obvious culprit. Switch them left to right (start with upper and then do lowers, not both sets at once) and see if the misfires change over to the left bank. If no change, then disconnect the y-pipe before the cats...if the right-side cat is plugged the back pressure will affect the whole bank.

I'd find it hard to believe that all 4 right bank poppets started sticking all at once.

[Beaner81]

Ok I got her fixed. First I replaced the fuel pressure regulator and noticed some washing under the intake. It was better but still idled rough and slightly misfired while driving. I wasn't towing anything so who knows how much better it really was. I replaced both O2 sensors on bank 2 and not everything is back to running great.
Thanks for the advice!