Random misfire help!!!

**Dallasneon** · 01-16-2015

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.

**Fivefingerdeathpunch** · 01-16-2015

What plugs are they??

Have you pulled them to see which plugs look the worst?

**Scottydoggs** · 01-16-2015

how many miles? maybe the cat is clogged up. the car feel weak at all?

**Dallasneon** · 01-16-2015

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.

**Dallasneon** · 01-16-2015

Originally Posted by Scottydoggs

how many miles? maybe the cat is clogged up. the car feel weak at all?

SD Headers with Magnaflow cat. Has maybe 40k miles on it. Car feels weak now but it didn't right before it started missing.

**Fivefingerdeathpunch** · 01-16-2015

What gap?

And do you have another 3800 around that you can swap the ICM with to see if that is the problem?

**Dallasneon** · 01-16-2015

Gapped @ 55 I believe. Same as before it started this issue.

No other 3800's around.

**Timing** · 01-16-2015

Magnaflow cat is honestly probly your problem. they dont last that long.

**Dallasneon** · 01-16-2015

Originally Posted by Ginseng

Magnaflow cat is honestly probly your problem. they dont last that long.

Really? Never would have thought that. I'll pull the down pipe and see if that fixes the problem.

**idrivejunk** · 01-16-2015

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.

**Dallasneon** · 01-16-2015

Originally Posted by idrivejunk

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.

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.

**Dallasneon** · 01-17-2015

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!

**Scottydoggs** · 01-17-2015

post a vid of this knocking. a few of us are pretty good with engine sounds.

**Dallasneon** · 01-17-2015

Here is the video. Didn't leave it running very long but once the idle starts to smooth you can hear the rattle.

https://www.youtube.com/watch?v=n7xlu6XN6PY

**Scottydoggs** · 01-17-2015

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.

**Dallasneon** · 01-17-2015

Originally Posted by Scottydoggs

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.

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

**Scottydoggs** · 01-17-2015

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.

**Dallasneon** · 01-17-2015

Found it. Valve spring broke. Do I need to tear this engine completely down?

**Scottydoggs** · 01-17-2015

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.