'97 Gmc K1500 5.7l P0300

[97GMC85]

I just redone the heads on my '97 GMC 5.7L Vortec motor, i got everything back together and started it up, its running really rough and i plugged the SOLUSpro Snap-On scan tool to it and it told me i had a random misfire, so i looked at the misfire data on the scan tool and cylinders 5,7,8 are misfiring real bad, with #8 being the worse, misfiring over 2k times in 1 drive cycle. I have put new plugs, wires, distributer on it still no improvement. To top it off i checked my coolant after i filled the radiator with roughly 3.5 gallons of coolant then topped off the resovior and it was completely dry, i checked my oil and none of the coolant seemed to go to the oil pan. The truck isn't smoking and there is only small amounts of water dripping from the exhaust and condensation is what i believe that to be, but i can't figure out where the water is going, could it be an intake gasket problem causing the coolant leakage and the very bad misfires on those particular plugs?

[L29er]

Have you pulled those plugs to see what coulour they appear? that should point you in that direction if you think it is burning the coolant. Plus the exhaust will smell like burnt syrup, From my experience anyways. If you can check your fuel pressure to eliminate the fuel side of things that would be beneficial, should be 50-60psi key on engine off. I don`t know how often it happens but a bad cap could be a culprit, but doesn`t explain coolant loss...

[97GMC85]

I havent checked the fuel pressure yet, but i did pull the 3 plugs and they are completely black looks like it was burning oil or some sort of liquid the electrodes on the plugs look sort of wet, but isnt wet to the touch, i will do the fuel pressure test and post the results later. I forgot to put it in the previous post but I also have a new cap and button on the distributor and when i first started my truck it threw 2 codes, one being the p0300 and the other being bank 1 sensor 1 no activity but when i cleared the codes and started the truck again the p0300 code was the only one that came back and i checked the o2 sensor with the scan tool and it seems to be operating correctly.

[97GMC85]

checked the fuel pressure on the truck and its at a steady 55psi. I found the water leak, the waterpump is leaking, but still havent figured out why such a bad misfire yet so if anyone comes up with a conclusion to this problem let me know.

[L29er]

have you ohmed your wires? something could have broken during install, have you double checked the cap and rotor aswell? Being that it is a "random misfire" I`d put suspicion into the cap/rotor/distributor or something in the primary ignition.

[97GMC85]

well guys i pinpointed the problem, i done compression tests on the #8 cylinder, and what do you know, no compression at all, so i adjusted those valves, and it cleared up cylinder 8 completely as well as helped clear 5 and 7. now i just have a slight misfire on cylinder 4 at idle, but clears when driving, so im going to adjust valves on cylinder 4 and clear it the rest of the way. so anyone with the random misfires thats redone your motors, be sure your valves are in check!!! save money and extra work!

[L29er]

Good job!