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Misfires

Crisbol

New Member
Joined
Nov 9, 2019
Messages
5
We have changed everything on this truck that we where told to replace. Compression is good, computer changed 3 times and it burned up a starter. At a loss, keeps coming up misfire p300 or p306 Help! Could it possibly be the wiring harness that's made in Mexico? Anyone have any ideas. It has 115 000 miles.
 
SO please list what you replaced?

Any intake leaks?
What year?
What engine?

What is compression?
 
kinda low compression...

normally you like to see 145 - 160 PSI.

now what all parts did you replace?
 
Intake manifold leaks are fairly common on these engines.

The OEM intake gaskets are not known for their longevity.

Intake manifold leaks can cause misfires.

You did not specifically say the intake manifold gaskets had been replaced.

If they have not, I would suggest replacing with a set of Fel-pro intake gaskets.

Performing a smoke test can help to determine if a leak is present.

What other parts have you already replaced?

It may be important to note cost wise that the coils on these engines seldom fail and almost always not all at once.

Proper troubleshooting can determine if a single coil has failed and only that one defective coil should be replaced.

The OEM coils can easily outlast the entire truck.
 
have changed coils spark plugs and wires crankshaft sensor camshaft sensor. Mass air flow computer relays have not changed intake manifold gasket so we will do that and I will get back to you. Thank you.
 
What about Fuel injector?
What is fuel pressure at idle?

Do the things these websites list:
 
Is it an actual felt misfire (ignoring the SES light)? If not, the misfire detection system may be malfunctioning and the misfire code not real.

When you change or disturb a crank position sensor, you have to do a crank position sensor relearn of the output waveform. Comparing to the stored waveform is the way the PCM knows when there is a misfire. If everything isn't perfect for whatever reason, the PCM reports a misfire (even if it is something else).

PCM can even pick up on the change in waveform from excessive main/rod bearing clearance so it is very sensitive. That was a problem I had with my Blazer when I first bought it (only 65K miles). Other things like belt driven accessories, and flex plate can also cause misfire codes by jacking with the engine balance. This is no secret and is in the factory manuals.

Not sure why everyone always goes for the crank sensor replacement first. It is a really trouble-free simple devices with no electronics in it.
 
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