New to the site, ran across it while searching for some troubleshooting tips. My problem is my recent tow pig acquisition is giving me fits. This is a 1997 Chevy C2500 2wd with a 5.7 vortec. Truck had been sitting for a while when I got it. I had to put a fuel pump in it to get it running. Runs fine, EXCEPT at idle.

Throws a P300 random misfire code, and a P304 code for a specific cylinder misfire, but it will rotate cylinders, and different sides of the engine. It had these codes when first fired up, and all efforts so far have made no change. Replaced the dist. cap. rotor button, plugs and wires as normal maint, after it was first fired.

The gas was jellied in tank when I got it, we cleaned it out, and replaced fuel filter when replacing the pump. Replaced spyder injection with an unknown used set, did not help.

Bought another complete intake off a running engine. Swapped EGR, new PVC, coolant sensor, replaced turtle from intake I bought, transferring all sensors as well. Same result. Did not change lower intake as it did not seem to be anything wrong with it.

Carried truck to local "Guru" as I am only out about 40 bucks in parts so far, (intake was $25, and the PVC was new, plugs and such were routing maint). DId not want to throw parts at it. THey swapped the distibutor and the MAF from a truck in the shop, same result, mine ran fine on the other truck.

Here is where the fun part comes in. While troubleshooting further on my own I found the truck PULLS air into the engine via any of the vacuum related items. The oil cap will 'whistle' when loosening it while truck is running, will not die when it is off. I pulled every vacuum related item at different times, with no change.

Unplugging the MAF will not change idle, but it will throw a code for it. Unplugged all the vaccum items and it will still run.

There is a tube from the platic air intake elbow to the passenger valve cover, if you unpluf the tube, and hold your thumb over the tube going into the valve cover, it will pull MAJOR vacuum.

Compression is fine on all cylinders, no water in oil, or vise versa, pressure test of coolant is fine as well.

One last thing, while the engine has a rough idle with the oil filler cap off, it will NOT throw either of the misfire codes. Within 30 seconds of reinstalling it, it will come back.

 

Is there alot of blowby with the oil cap off? You say PVC is it PCV valve you mean? A slight vacuum leak would be the symptom of a rough idle. Is there another tube (unless this is the one you a refering to) from the air intake to the engine or rocker arm cover? With engine running, pull pcv valve, with your thumb on it, can you feel a good vacuum? I suspect vacuum leak in the pcv valve tubing. Replace the vacuum hose to it. The other tube I was asking about is the clean air inlet to the crankcase, it should have no vacuum.

 

No blowby. It is pulling air in, not pushing out. PCV valve is correct. Does not change it at all. And the tube you mention is the correct one. It is pulling through it instead of the throttle body. Only thing I can come up with is a leak into a runner from the intake to the lifter valley area, the compresson check is throwing me though...

 

Still pulling my hair out on this...

 

When did this start, and was there anything done prior to this? I read where you put you thumb over the tube from the air inlet to valve cover you have major vacuum. You should not have vacuum in the engine.

 

On another note, is the spark plugs oil fouling?

 

Truck was not running when I got it. I know the previous owner though, and he was not having any issue when he got it. Just kept it for a back up when he got a new truck. Never drove it, and the fuel pump died. I put a pump in and have had this issue since I got it running. The vacuum issue wht I am trying to figure out. Going to pull the intake and check lower gaskets and pull valve covers as well to see if I can see anything. Trucks run fine above 1200 RPMs...

And plugs are fine, compression is even across the board as well...

 

Update, pulled the intake, and it appears to be a rare original that made it through the TSB, lol...

It was blown between the center cylinders to the lower edge of the intake on both sides. Engine is clean as a whistle inside the lifter valley and the heads. Got it back together and fired it up with no vacuum issue, or misfire. Got to replace the heater hose connection on the intake and button it back up and road test to be certain. Thanks everyone for your help.

 

Not sure what you mean in the comment above. What was blown between the cylinder

 

The factory plastic gasket failed. The gasket broke lengthwise across the ports and the lower portion of it pushed out between the intake and head and fell into the lifter valley...