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Random misfire help!!!

D

Dallasneon

Member
This is driving me nuts. Driving home from work the other day I felt some slight hesitation while accelerating, it got progressively worse over the next half mile of road, by the end it would barely idle in traffic. I limped it home and started to diagnose. Car will idle but is very very rough. Finally gave me a P0300 code.

Items I've check/tested/replaced:
New Plugs
New wires
Checked spark from coils (good)
Checked hoses for vacuum leaks (none)
Pulled vacuum hose from FPR didn't see any fuel (good)
Unplugged MAF (No diff)
Carb cleaner sprayed on tops of injectors (o rings good)
Carb cleaner around lower intake/supercharger/TB (no leaks found)
Tested fuel pressure - @idle (rough) the gauge bounce from 40-50psi with a little throttle it stabilizes @ about 52psi at the rail. (ggod)
Boost gauge also bounces and fluctuates at idle, a little throttle stabilizes the boost gauge as well.
I did replace the crankshaft position sensor about a year ago with one from autozone. Ran fine up till now.

I'm thinking about putting a crankshaft and camshaft position sensor in just the hell of it to eliminate those 2 items. Thoughts?

Anything else I should be looking at? Any help is appreciated. Thanks guys.
 


Pulled 104's, replaced with new Autolite 104's. Looked at the original plugs when I pulled them but they all looked pretty much looked the same.
 


My vote is a weak coil pack. This happened to me this week, only the code it finally threw was just for #3 misfire. You stated good spark from coils, but did you check resistance on them? It takes only a couple minutes to swap one coil pack, and if you had just one new one you could swap it around to see which old one is bad. I had a spare engine and borrowed a coil from it then went comparison-shopping for new coils.
 
idrivejunk said:
My vote is a weak coil pack. This happened to me this week, only the code it finally threw was just for #3 misfire. You stated good spark from coils, but did you check resistance on them? It takes only a couple minutes to swap one coil pack, and if you had just one new one you could swap it around to see which old one is bad. I had a spare engine and borrowed a coil from it then went comparison-shopping for new coils.
Click to expand...
Good idea. I checked the spark with an hei spark tester, looked good. I do have a new coil pack maybe I'll swap it into all 3 positions and see what happens. Thanks.
 


Well, something went terrible wrong last night. The car was idling in between swapping coil packs and the engine started rattling and knocking. Sounds like something let go inside. Looks like she's coming apart this morning. Wish me luck!
 
thats sounds bad for sure.

make another vid, but hold the rpms to 2500 rpm for a few seconds, then just drop the throttle letting it go back to idle.
 
Scottydoggs said:
thats sounds bad for sure.

make another vid, but hold the rpms to 2500 rpm for a few seconds, then just drop the throttle letting it go back to idle.
Click to expand...

Too late, supercharger is already removed. HAHA! I gonna get to the bottom of this today. Any thoughts from what you could hear?
 


taking the sc of wont let you see any thing damage wise.

sounds like a rod knock. but you can take off the valve covers looking for a broken rocker or spring. if all the rockers and springs look ok its the bottom end for sure.
 
Found it. Valve spring broke. Do I need to tear this engine completely down?
 

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nope. but you will need a air compressor with a threaded end on the hose to air up the cylinder. then you cant take the broken spring off. take a magnet and look for the valve locks. two 1/2 round metal sleeves if they are not still in the valve shaft.

you'll need a valve spring compressor to compress the spring and put it back on. also take that push rod out and make sure its not bent. roll it on a flat table.

and of course the rocker has to come off to do all this.


nice find too. way better then a rod knock.
 
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