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roybelding
roybelding, ASE Certified Technician
Category: Jeep
Satisfied Customers: 5425
Experience:  My knowledge of Jeeps from the early 60's till present. Love to help people with there issues.
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Sorry for the length - want to be thorough!! My '98, 4.0,

Customer Question

Sorry for the length - want to be thorough!!
My '98, 4.0, 208k miles, owned since new, meticulous service history (well, except for my o2 sensors!) is giving me an engine issue for first time in 19 years.
I have a very slight, intermittent misfire and occasionally get code P305 (cylinder #5 misfire).
I have performed the following:
New plugs, wires, cap, rotor
Fuel injectors - bench cleaned (ultrasonically) and flow tested.
Fuel pressure test - I have a slightly fluttering needle at idle (+\-2psi from spec), needle steadies above idle (gauge issue?).
Analyzed obd2 PID data - the only thing of note is lazy upstream O2 sensor - a given since I actually never replaced it. I have a new one, just need to unfreeze old one. Probably not causing the issue here anyway.
Compression test - all cylinders 155 psi +\- 5 psi (#5 is 155 psi)
Leakdown test - all cylinders exhibit 8% or less (#5 has 5% leak down reading)
Voltage check at injector plug - good.
Inspected injector harness for abrasion - good
Flexed harness while monitoring voltage - no intermittent issues found.
Did not find any vacuum leaks yet...
Diving into OBD2 Mode 6 data, system shows test failed for evaporate canister purge solenoid (but no evap codes are being set).
I'm at a loss here....I spoke with my dealer and the lead tech said he still thinks I should pull the head off because he has seen slight corrosion, pitting, of 4.0L valves that pass both compression and leak down tests. ?? I am not buying this right now, given the great readings I am seeing.
Can one get good internal,mechanical readings and still have a valve issue?
Any help would be appreciated!
Thank You,
Anthony
Submitted: 8 months ago.
Category: Jeep
Expert:  roybelding replied 8 months ago.

good afternoon my name is ***** ***** I will assist

based on all the info you provided, I agree with the dealer tech

this motor has a history of exhaust valve failing and the guides seizing and the valve not closing quick enough.

I have done many of these for this exact reason. it makes sense especially with the mileage

Roy

Expert:  roybelding replied 8 months ago.

please dont forget to rate me as it is the only way I get paid for my efforts

Roy

Expert:  roybelding replied 8 months ago.

please dont forget to rate me as it is the only way I get paid for my efforts

Roy

Expert:  roybelding replied 8 months ago.

please dont forget to rate me as it is the only way I get paid for my efforts

Roy