Random Misfire On 5 & 8, Running Rough

[96Burbo]

About a year ago, I cracked the heads on my 96 K1500 5.7L Suburban due to a faulty thermostat. Took it to Performance shop for a rebuild and figured what the hell, while I'm at it, I'll get it stroked to a 383, a bigger cam, Wait4metune, new CAI, you know the first step of upgrades. Truck ran great for about a year, then started running really rough, felt like I was only running on 7 cylinders. I changed plugs, wires, cap, rotor, distributor. Did a pressure check, checked injectors, fuel lines, you name it. Everything was checked twice by two different shops, one a GM dealer. Both shops said the only thing they though it could be was a problem with the cam. Oh, and it wasnt throwing misfire codes.

 

So I got a local mechanic to drop engine, then I took the block back to the Performance Shop who in turn said there was nothing wrong with it, cam looked fine. Well, since they had her opened up, i added .2 H series main bearings, got a roller cam, and they re did the valves and everything. Picked up the engine, had the local mech who dropped it reinstall it, and it's been running bad from the beginning. This time throwing the misfire codes and still running rough. Since the engine was re-installed,

 

Here's what I've checked/changed:

 

New Distributor

New AC Delco cap, wires, rotor, plugs, and coil

 

Compression Test:

1. 155

2. 155

3. 160

4. 160

5. 175

6. 150

7. 160

8. 160

 

* Fuel Pressure - 65 PSI

* Fuel Pressure Regulator - OK

* PCV Valve - OK

* Wires Connected correctly

* No Vacuum leaks found

* Put Seafoam in tank

 

Replaced Fuel Filter, put in new injectors...MPFI upgrade

 

 

After all that, It still runs like ish, still blinkin ses light and running like its on 6/7 cylinders. Any ideas?

[Sly]

About a year ago, I cracked the heads on my 96 K1500 5.7L Suburban due to a faulty thermostat. Took it to Performance shop for a rebuild and figured what the hell, while I'm at it, I'll get it stroked to a 383, a bigger cam, Wait4metune, new CAI, you know the first step of upgrades. Truck ran great for about a year, then started running really rough, felt like I was only running on 7 cylinders. I changed plugs, wires, cap, rotor, distributor. Did a pressure check, checked injectors, fuel lines, you name it. Everything was checked twice by two different shops, one a GM dealer. Both shops said the only thing they though it could be was a problem with the cam. Oh, and it wasnt throwing misfire codes.

 

So I got a local mechanic to drop engine, then I took the block back to the Performance Shop who in turn said there was nothing wrong with it, cam looked fine. Well, since they had her opened up, i added .2 H series main bearings, got a roller cam, and they re did the valves and everything. Picked up the engine, had the local mech who dropped it reinstall it, and it's been running bad from the beginning. This time throwing the misfire codes and still running rough. Since the engine was re-installed,

 

Here's what I've checked/changed:

 

New Distributor

New AC Delco cap, wires, rotor, plugs, and coil

 

Compression Test:

1. 155

2. 155

3. 160

4. 160

5. 175

6. 150

7. 160

8. 160

 

* Fuel Pressure - 65 PSI

* Fuel Pressure Regulator - OK

* PCV Valve - OK

* Wires Connected correctly

* No Vacuum leaks found

* Put Seafoam in tank

 

Replaced Fuel Filter, put in new injectors...MPFI upgrade

 

 

After all that, It still runs like ish, still blinkin ses light and running like its on 6/7 cylinders. Any ideas?

 

Have you checked the cats? Are the misfires random, on one bank of cylinders.....?

[96Burbo]

Haven't checked the cats, misfire is on 5 & 8 which are separate sides

[98 Z71 SLT]

1-8-4-3-6-5-7-2

 

is your firing order.

 

1-8-4-3

6-5-7-2

 

You stated cylinders 5 and 8 are misfiring.

 

5 and 8 are "mating" cylinders.

 

My guess is something ignition-wise is causing this.

 

Make double-d**n sure you have a GM distributor cap. I've seen alot of misfire situations caused by aftermarket distributor caps.

 

EDIT-just noticed you had a Delco dist. cap. Go back and get another one...