1998 GMC Pickup,PO306 Moved from "Venture forum"

[broughy84]

My 98 5.7 litre p/u had a trouble code of PO306 come up after I scanned it. I no that this means misfire on #6 Cylinder. I tuned up the truck and scanned it again after the dash light came on again. The Same code came up (PO306). I sent the truck in to have the injectors flushed but this didn,t cure the PO306 problem. Through process of elimination I replaced #6 fuel injector and nozzle assemble but the code still comes up. I,m stumped at this point and considering removing of the main intake manifold and search for possible vaccum leaks or possible a bad lifter. Does anyone else on the forum have any further ideas what the problem could be or does this code refer to more than just #6 cylinder misfire?

Laverne Thompson

[amac209]

cylinder might be bad. do a compression test. if it's close to zero you probably have dished valves.

[Laverne Thompson]

I had the truck in for injector flushing and they told me that they did a compreesio test at that time. this was only a week ago.

[J-Ri]

You didn't say it, so I have to ask. You replaced the cap and rotor? Did you check for dist. shaft play?

Check to make sure the plug wire isn't arcing to ground somewhere.

[mphmilkman]

I know you won't like to here this but the dealer has a few test procedures that will accuratly diagnose your problem. There is a box with a harness that connects to your injectors and fires them wile you enguage your fuel pump with the scan tool and watch a fuel pressure guage. It rules out the injectors. There may be a way to use a node light to see if that cillinder is getting power to fire the fuel injector.

[Laverne Thompson]

Thanks for your responses, I will check some of the information that you relalted to me and let you no how it works out.

[skipr]

I had exact problem (except mine was P0305) fuel system was compltly ruled out as being problem. I was told commpression may be low in #5. I did a commpresion and a leak down test. It passed with excellent readings. Took to GM dealer three different times, they never found any thing, A year and a half later I got sick of it and traded in for a 2005 K1500.Oh one thing to metion is that it idled like crap but ran smooth and fine after 1500 rpm's. Every thing was checked or replaced (ign., fuel sys, air leaks, ...etc) By the way it was a low milelge 1998 Z71.

[Laverne Thompson]

Well I changed the fuel pump and it seems a little
better but still get the dreaded PO306. Checked for cross fire and seperated all the wires, still same Problem. Check for spark and there is all kinds. But I would like to ask? After I changed the fuel pump I tested the system and the fuel pressure was Approx. 58 pounds. this is with the key tuned to the on postion and motor not running. Is this the proper way to do this or does the Fuel pump have to be jumped-out. Also I didn,t test to see if the injector had power at the connection yet but will check tommorrow and let you,s know.

[jveik]

58 pounds? lmao my ancient cheyenne has about 7 pounds of fuel pressure in it.

i have really no idea what im talking about in respect to the misfire, but i have heard about carbon deposits in the cylinder causing a number of problems including misfiring (pinging, specifically). is it possible that carbon deposits would be a culprit?

[Laverne Thompson]

How would you tell if carbon deposits are in the cylinder? How would you clean this?

[J-Ri]

Run a can of SeaFoam Deep Creep through the throttle body. The directions are on the can.

[jveik]

so they can cause that type of problem? what i did with my lawn mower (LMAO) was i took the cylinder head off and scraped all the carbon off with a screwdriver. yes i accept the fact that i am a redneck

[Laverne Thompson]

Well I finally found the problem. After doing a vacuum test that was in fact showing 20inchs Steady I decided to take it to the shop. They used a smoke machine to test the engine and found a vacuum leak on the lifter galley side of #6 Cylinder. Replaced the intake gasket a presto problem solved. Thanks to all the responsed to this problem and I guess its time to close this thread.