Misfire, codes PO300 and PO303

Tiny
CHUCKMALLORY
  • MEMBER
  • 1999 LINCOLN NAVIGATOR
  • 5.4L
  • V8
  • 2WD
  • AUTOMATIC
  • 2,300,000 MILES
Trouble codes PO300 and PO303. What would cause this? Can I get Mr. Ken (Admin)?
Monday, November 5th, 2018 AT 6:42 PM

3 Replies

Tiny
STEVE W.
  • MECHANIC
  • 13,465 POSTS
The P0303 is a specific cylinder misfire (Cylinder 3) while P0300 is a random misfire code. A common cause for the random misfire on a 5.4 is the intake manifold gaskets. They will get hardened and then they do not seal fully. The other common issue is the plastic manifold cracking on the underside. Both of these can cause a vacuum leak which causes the random misfire due to it not being air that was measured through the mass air flow meter. Usually in the case of the gaskets the engine will idle poorly while it is cold and get better as the engine warms up and the parts expand slightly and seal better. The cracked intake however usually just causes the random miss due to the lean fuel mix.
Depending on which issue you have it could also cause the P0303 code if the gasket at that cylinder is leaking more than the rest.
Easy way to test for this issue is to get a can of starting fluid. Then with the engine cold, start it and immediately spay the fluid at the area where the upper and lower intakes meet, as well as up under the intake. Any change in how the engine runs or if it suddenly revs up will point at what and where the problem is.
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Monday, November 5th, 2018 AT 9:01 PM
Tiny
CHUCKMALLORY
  • MEMBER
  • 9 POSTS
Hi Steve thank you for your input. I just got done installing a new heater core and backing the coolant system. Water got in side the coils well and fried number three coil all fixed. The coil cover had come loose that is how the water got in. Thank you for your time.
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Tuesday, November 6th, 2018 AT 1:35 AM
Tiny
STEVE W.
  • MECHANIC
  • 13,465 POSTS
Water intrusion will cause a myriad of problems. Good to hear you found it. Thank you for using 2CarPros.
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Wednesday, November 7th, 2018 AT 10:25 PM

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