Random misfire on 5 & 8 causing ruff ride
 

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

Other tests

* Fuel Pressure - 65 PSI
* Fuel Pressure Regulator - OK
* PCV Valve - OK
* Wires Connected correctly
* No Vacuum leaks found
* Put Seafoam in tank

Today 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?

RE: Random misfire on 5 & 8 causing ruff ride
 

Just for kicks, tonight I changed the PCV Valve. I also hooked up the stock PCM and ran it for 5 minutes....no change in performance. Lastly, I pulled out the PCV Valve, attached a vacuum gauge to the hose that attaches to the PCV, and it read at a lil over 18 which is right where it should be (17-21 being optimum).

I'm stumped.

RE: Random misfire on 5 & 8 causing ruff ride
 

have you checked out your throttle position sensor? or your pickup inside the distributor.

RE: Random misfire on 5 & 8 causing ruff ride
 

No, Will check the throttle Postition Sensor tomorrow.

What do you mean by pickup inside the distributor?

RE: Random misfire on 5 & 8 causing ruff ride
 

the pick up coil, just didnt know if that was replaced during the process of changing the distributor or not

RE: Random misfire on 5 & 8 causing ruff ride
 

pick up coil? I changed the ignition coil, is that the same thing?

RE: Random misfire on 5 & 8 causing ruff ride
 

no the pickup is inside of the distributor. and the ign coil is either off to the side or on top of the distributor depending on the model

RE: Random misfire on 5 & 8 causing ruff ride
 

I'm gonna take it to the dealer on Saturday. I'll have them check the pickup coil and the throttle position sensor.

Question, wouldn't it throw a code for the throttle position sensor?

RE: Random misfire on 5 & 8 causing ruff ride
 

any news?

RE: Random misfire on 5 & 8 causing ruff ride
 

Unfortunately no. Dealer checked coil and throttle sensor, checked timing, re re-checked wires plugs, but can't find the reason. Tech II is telling them code P0300 and random misfire on 5 & 8.