Every so often my 2003, 3.7 Jeep Liberty will fail to start. The engine cranks, and just keeps on cranking but wont start. If I try again right away, the problem repeats but if I wait about 20 to 30 seconds it will start and there will be a heavy gas/exhaust smell.

For some background, I have done alot of work on this jeep. The issue seems to have originated during a problem with my cam and crank sensors. I have replaced both of those as well as all the EVAP lines in the engine compartment including the purge vent solenoid. Right now I am leaning towards a vacuum leak somewhere in the fuel line, or maybe some spark plugs. Any suggestions or ideas anyone has would be appreciated.

Bumping to see if anyone has any ideas.

I'm definitely not a mechanic, but I'd recommend going for the cheap options first. Change your spark plugs and run some sea foam through the system and see if it gets better. It'll cost $30 and it probably needs to be done anyway. I think the spark plug recommendations on the KJ is every 30,000 miles. Also, check your air filter and make sure you don't have a colony of mice living in there.

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"BERT" 2003 3.7L Sport Gasser with the 5-speed stick
Frankenlifted (plus 1 extra rear iso. and a 3 nut clevis), Al's gen 4.5 UCA's, Full Mopar Skids, Kurt Hitch, Soft 8's, BFG AT's 245 75 16, Rock Lizard Front Bumper, fence top-rail crossbars (unbent!), some stuff inside...