5.7l vortec random misfire

[jdlover1]

I have a 99 Tahoe with the 5.7l vortec in it. Has a random misfire when cruising down the hwy and when going over overpasses. I have done the usual tune-up: plugs, wires, cap and rotor. Fuel filter has been changed and new fuel pump within the last year. I even changed the fuel spider assembly to the updated version and still does it. The only code that is being set is p0300. I am ready to put a stick of dynamite it this thing.

 

[Rancelot211]

Injector pulse? Fuel pressure? Lol there is a lot to check!

 

[jdlover1]

fuel pressure 58 psi with key on/off. Running was 50psi. No problems starting.

 

[ScreaminDuramax]

Its most likely your intake manifold, its either your gasket or the actual intake manifold. They get brittle and crack. Ive done 2 and for this exact reason. But make sure you smoke test it first so you can determine if its the gasket or the Manifold itself as you wouldn't want to re seal it and it still leak. Good Luck!

 

[jdlover1]

I did spray brake cleaner all around the upper and lower intake to see if any changes would happen and engine rpm never changed

 

[skidoosleds3]

Fuel regulator? I had one that acted funny.

 

[jdlover1]

when I put the new style fuel spider injection system in it came with a new fuel regulator

 

[ScreaminDuramax]

Id still smoke test!

 

[Chevy1925]

Id still smoke test!

x2, those intake mani's are notorious for that crap!

 

[duramaxzak]

X3 in the intake manifold/gaskets!

 

[jdlover1]

just for laughs I checked the compression on it last night. Cylinder 1 and 3 and 4 never showed any compression. I tried three different gauges and all three of them showed nothing. even drove down the road to warm up engine. check number one again and still nothing.

 

[Chevy1925]

just for laughs I checked the compression on it last night. Cylinder 1 and 3 and 4 never showed any compression. I tried three different gauges and all three of them showed nothing. even drove down the road to warm up engine. check number one again and still nothing.

And all you feel is a faint miss fire?!

grab a bottle of water in a squirt bottle or pop a tiny hole in a bottle of water in the cap. get the rig warmed up and spray some water on each exhaust manifold right where the exhaust comes out of the head. if the water doesnt instantly steam off and instead sizzles for a while, those are your dead cyl. if you really have no compression, cyl 1,3,4 should show that

 

[ScreaminDuramax]

Or an infrared thermometer

 

[jdlover1]

And all you feel is a faint miss fire?!

grab a bottle of water in a squirt bottle or pop a tiny hole in a bottle of water in the cap. get the rig warmed up and spray some water on each exhaust manifold right where the exhaust comes out of the head. if the water doesnt instantly steam off and instead sizzles for a while, those are your dead cyl. if you really have no compression, cyl 1,3,4 should show that

Sometimes the whole vehicle is hucking and bucking when it is doing it. And it has a bad vibration that you feel thru the whole vehicle. I will try the water on exhaust trick.

 

[Chevy1925]

Or an infrared thermometer

Yup!

 

[Yellow Jacket]

Did you look at the gear on the distributor? I've seen them worn paper thin on GM vortec's before....

 

[jdlover1]

Yep even pulled distributor and gear is in perfect shape

 

[Huffy67]

If the cylinders really have no compression pull the valve covers and look for broken valve springs. If not, you're gonna have to pull the heads. You will have bent or burnt valves. While the valve covers are off have someone crank it over, make sure all the valves are moving up and down, could have lobes wore off cam.


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[jdlover1]

I finally got around to messing with it again. This time I checked the temps of each cylinder idling and after W.O.T. while idling the temps ranged from 113 degrees to around 375 degrees. Drove it down the road and checked it 213 degrees to 500 degrees.
Number two cylinder is the one that had the lowest temps. Number 7 cylinder was the hottest it maxed out my ir gun. With no.2 being the one with the lowest temp I believe my gauges where right on that one have low compression

 

[Chevy1925]

I finally got around to messing with it again. This time I checked the temps of each cylinder idling and after W.O.T. while idling the temps ranged from 113 degrees to around 375 degrees. Drove it down the road and checked it 213 degrees to 500 degrees.
Number two cylinder is the one that had the lowest temps. Number 7 cylinder was the hottest it maxed out my ir gun. With no.2 being the one with the lowest temp I believe my gauges where right on that one have low compression

i would agree