P0300 - MULTIPLE CYLINDER MISFIRE

I just took my car out of winter storage. She sat there for 5 months.

When I left the car the engine was running just fine and dandy.

Fast Forward to today and soon after starting her up I have a P0300 code. I drove her home at crawling speeds (I live rather close to the place where I store it). Initially the engine was a little shaky but after a min or so it smoothed out.

While in storage I had stuffed rags in my exhausts and intake just to make sure no rodents get in there and cause havoc...

I cannot exactly pinpoint where the issue is, and I will look into it further tomorrow.

I don't think the flow from the exhaust is quite as high as I would expect it to be.. Would I be able to drop the exhaust after the flex or so and see how she runs?

What are the things that I should be looking for?
Backpressure (how can I measure that ?? Is there a port? Do you remove an O2 sensor and plug in a presure meter? )
spark plugs / ignition coils
Engine Compression

 

Check to make sure no rodents got under the hood and chewed on any wires...and I mean ANY wires, not just coil packs.

You can also try clearing the code and trying again.

Another thing to try, and I'm not sure how it worked, but my friend took his Firebird out of storage and it wouldn't run right, so he left the charger on the car with the keys turned to the ON position over night, and the next day it started right up and ran great. Don't know how it worked, but he's a licensed mechanic so he must know what he's doing.

 

Check under the hood for chewed wires.

I had that issue many times before as my Mustang spends more time in a storage unit than on the road. Rodents used to get under the hood and chew on wiring.

Mothballs under the hood do help...if you can live with the smell.

But yank the plastic cover and start looking for chewed wires

 

Since you indicated that you stuffed rags into intake and exhaust, make sure nothing left in those areas. Anything blocking the exhaust would sound like misfiring.

 

I looked around right after seeing the problem and didn't find any left over rugs or anything like that.

I am wondering if bad fuel can cause this issue..It has stabilizer in it but still.. I will look at the service manual tonight but in case you know, how easy is it to siphon?

 

How long was the gas in the tank?

I've gone 9-12 months with gas sitting with no issue.

Entirely possible though and in fact the first place I'd start. You def want to get some fresh gas in there ASAP

 

The gas was in the tank only for 5 months. As long as I stored it..

 

Full tank? Try and get some fresh gas ot mix in there and go from there. It is possible that 5 months is long enough for the gas to degrade slightly

 

Is it possible that I have a clogged catalytic converter? Should I look into measuring the back pressure of the engine??

 

I wanted to give you a quick update in case anyone else encounters this problem.

After some debugging I found out that I have a bad ignition coil on cylinder 4. The misfires sounded rather rhythmic when the engine was running so I was suspecting a single cylinder misfire.

I had removed the air duct and the MAF sensor in order to gain access to the coils. I installed the new coil and started the engine. It appeared to idle perfectly. I buttoned the whole car up and put everything back together. I started the car and let it idle while I was cleaning up.

It idled for a couple of minute and developed a misfire again ! At this point I was perplexed as to what was going on. A short slow drive around the block (with misfires) and as I was just getting ready to shut the car off but the misfires disappeared. I believe I *MIGHT* have some sort of intermittent issue but I am not certain.

I drove it to and from work today without a single hick-up or any issue of any sort.

My explanation for this is:
I never disconnected the battery through this whole process. My assumption is that when the engine was operating in open loop feedback system it was running without making use of the variety of sensors available in the vehicle. As soon as the engine warmed up it switched into closed loop and the previous stored engine configuration was causing a misfire. The short drive allowed the ECU to "relearn" the engine and cause it to operate as intended.

Either the above explanation makes sense or I am full of crap and I have some sort of intermittent issue that will bite me in the butt sooner or later.

TLL Bad coil on a single cylinder and disconnect your battery while working on the engine.