5.7 Hemi Misfire Woes

[kapitanius]

Hey everyone,

My hemi 5.7 throws a P0300 code at RPMs above 3K. It sounds and runs fine, no issues idling nor is there any lack of power. There are no other associated fault codes, just the P0300.

I have already replaced the camshaft position sensor.


Thoughts..?

 

[mizewilliamc]

P0300 is the multiple cylinder misfire code correct?


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[mizewilliamc]

There’s so many things that come into play with Hemi misfires, I had one today in the dealership that was missing because the rings fell out of one of the pistons, if you have specific cylinders in question you can try swapping one of the coils to another cylinder and the spark plugs to a different one as well, if the misfire moves to that cylinder you’ll know if either the coil or the plugs are bad. Spark plug gap specs on Hemis are .043 I believe so you can make sure they have the right gap, you can also move an injector around to another cylinder to see if maybe it’s a fuel issue. Hope this helps.


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[kapitanius]

Thanks for the help guys. Unfortunately it is a P0300 - multiple misfire code, and no other (e.g cylinder number). I think Ill pop the hood open tomorrow and check for vacuum leaks.

 

[Fast69Mopar]

Thanks for the help guys. Unfortunately it is a P0300 - multiple misfire code, and no other (e.g cylinder number). I think Ill pop the hood open tomorrow and check for vacuum leaks.

How many miles are on this 5.7?

Have you ever replaced the spark plugs?

Have there been any major mechanical repairs since you became the owner?

Like the earlier poster said, I would get the spark plugs out and record their condition and plug gap.

With a P0300 Multiple Cylinder Misfire DTC and no other single cylinder misfire DTC's set, the best way to determine if you actually have a misfire is with a good scan tool. With a scan tool we could look at the misfire monitor and see which cylinders were actually misfiring and record the RPM at which the misfire occurs.

Some other things that you can do to determine if you have good cylinder integrity is to perform a compression test and a cylinder leakdown test.

A compression test can only tell us so much. A cylinder leakdown test can tell us if we have a mechanical problem with the cylinder itself i.e. piston/rings/cylinder wall or if we have leakage at the intake or exhaust valves on a particular cylinder.

You can rent a leakdown tester from your local auto parts store. You will need an air supply like from an air compressor or an air tank to perform a leakdown test.

 

[Jason G Slabaugh]

Let's start with basics first. Make sure you have good spark and fuel pressure/volume first

 

[kapitanius]

Hey guys, I figured it out and thought id post an update for all to see: The issue was the programming!

It was tuned with a Superchip 87 gas saving program, which was causing the misfire codes. I set it back to the original and its good to go!

I assume the 'gas saving 87' program that I was using caused fuel scheduling issues, or lack of expected fuel & power return for fuel used, etc etc (I have no idea if the engine software works this way, but i'm basing this on my experience with aircraft engines).

As I was getting a single 'misfire on all cylinders' code and nothing else, to me this makes sense but if i'm wrong please correct me!

I don't think keeping the gas saving 87 program would do much harm, as I rarely ever get in the 3K RPM range, but I think Ill stick with stock tuning for a while!

 

[MikeT]

Excellent, nice easy fix, thanks for posting the results. Leaves a starting point for someone else.

Mike