P0301 code during warmup

[bigperm2]

As the title states, I have been having a P0301 DTC (with P0316 sometimes) come up on my car, so misfire on cylinder #1. This is on a bone stock 5.0 GT performance pack, manual transmission car with 130k miles. The problem started all of the sudden (parked the car running normal the night before. The next morning rough idle and MIL/CEL)
This is what I have done.

Replaced all plugs. No change, P0301 DTC.
Swapped coils with other cyl's. No change, P0301 DTC
Checked and Swapped injectors with other cyl's. No change, P0301 DTC.
Ran compression check on cylinder #1, 180 psi cranking compression.
New tank of gas with added injector cleaner, seems to have improved, but still will flash DTC P0301. Only difference is I don't always have to clear the code. It will flash while it stumbles, but MIL turns off right away. If I check for the code, it is in there.

I am at a loss. After warmup it runs great. I would guess 1 mpg improvement due to the new plugs. No funny noises, and the plugs read consistent among cylinders. Has anyone had this happen? Any guesses as to what to check next?

Potential issues I can think of are:
Intake manifold gasket leak or cracked intake???
faulty wire or burned out ECU component???

Thank you for any help before I have to send it to the dealer.
-Dan

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[SensesFail]

Have you checked if the all the runners are opening and closing properly in the intake manifold?

 

[mustangdriver]

I like your idea of a vacuum leak. Try brake clean with the red tube stuck in the
nozzle. Spray in short bursts all around the intake. You will hear rpm change if
you have a leak.

 

[Buldawg76]

If only happens after sitting for extended periods of time like overnight or during working hours I would suspect possible coolant leak in cyl 1 creating misfire until coolant is burned out of cylinder. Is coolant reservoir level low or dropped since issue started, may need to have cylinders scoped after overnight period to see if coolant is present in cylinder. Could also check exhaust for white sweetish smelling smoke/steam at first startup.

BD

 

[bigperm2]

Have you checked if the all the runners are opening and closing properly in the intake manifold?

This, makes sense. I had first written it off since most that report this also get the general miss code. Also because the IMRC should be on the entire bank. But perhaps the the IMRC shaft broke between cylinder 1 and 2. Or, it just is just a fluke that only cylinder 1 is the only one recording.

I like your idea of a vacuum leak. Try brake clean with the red tube stuck in the
nozzle. Spray in short bursts all around the intake. You will hear rpm change if
you have a leak.

Thanks for this, I will test this also. Either one will require intake manifold removal to repair.

If only happens after sitting for extended periods of time like overnight or during working hours I would suspect possible coolant leak in cyl 1 creating misfire until coolant is burned out of cylinder. Is coolant reservoir level low or dropped since issue started, may need to have cylinders scoped after overnight period to see if coolant is present in cylinder. Could also check exhaust for white sweetish smelling smoke/steam at first startup.

I have a scope from work and will check this out also. I have not noticed any smoke at startup. Also, the miss lasts full warmup and so I am hoping it is not this!

If I was gambling man, I would bet its the IMRC. Thanks everyone, I will post updates when I can verify the problem!
-Dan

 

[bigperm2]

Update! Pulled the intake of and imrc is intact, no flaps broken, great shape, move like they are supposed to. Scoped the cylinder, looks good, and the same as a “good” cylinder. Spark plugs all still look brand new. I am at a loss??
I have read a few posts about cracked intake manifolds, but no conclusion as to where they crack (unless accompanied by broken imrc). Does anyone know where these intakes crack?? I can’t find a crack but there are a few seams that could leak.
Any advise is always appreciated!

 

[NGOT8R]

What about the catalytic converters being worn out? 130K miles is a lot of miles on the factory cats.

 

[Call_Me_Bruce]

I’d check the wiring. Make sure you have good power and grounds for the coil and injector for #1. You have compression, problem remained with known good coil pack and injector. Make sure the pins and connectors are free of corrosion and are making good contact.

 

[bigperm2]

Update, still at a loss.

Bruce, I think your suggestion was a good one and I checked what I could.

I have narrowed it down to an injector issue. with the coil pulled and a dummy spark plug shows good spark.

Plugging in a dummy injector to connector on running car does not inject fuel. (have a spare injector I ran fuel with pressure. No flow or clicking sound on #1. Did have flow and clicking sound on #2)

tested voltage, gets 14 volts on positive side on #1 and #2.
Using a new pigtail and with pigtail wires plugged directly into the injector connector. Voltage across connector reads the same on #1 and #2 both under load and unplugged. I understand that injectors are PWM and voltage readings are not real. But I would expect a low/no voltage condition for the injector not firing or at least a different reading from #1 to #2

At this point I don't see a connection problem, but I am at a complete loss. Probably heading to the dealer when I can.

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