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Juan Crespo
Juan Crespo, Tech Trainer
Category: Car
Satisfied Customers: 1526
Experience:  A.S.E. Master Technician, Advanced Level, Emissions - Asian, Domestic, & European
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2009 f 250 5.4 wont idle. keep throttle above 1200 rpms runs

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2009 f 250 5.4 wont idle. keep throttle above 1200 rpms runs fine as soon as you take foot off peddle spudders and stalls only code is p0446 evap circuit. comes as soon as you clear it. other than that nothing. tried disconnecting mass air but no change in order to start need to go to wot. I'm stumped.

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That indicates a leak in the vacuum system.

Check the PCV and look over the vacuum lines real well. One spot that tends to leak a lot is the line that goes under the intake from the left side of the engine. It has a funny elbow and it tends to leak at that elbow. The plastic will break under there and cause a lot of the problems you are having. It is super hard to get large hands in there to check it. I just pull it out and look it over, then put it back on. It is a fight to do it, but it does cuase a lot of the vacuum leaks on these engines.


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Customer: replied 8 months ago.

ok I wasn't thinking vacume cause I usualy get a high idle issue, I have a scan tool I can read live data is there something in the fuel trims I can look for to better diagnose? or anything else?

Customer: replied 7 months ago.

hi you still there?

Customer: replied 7 months ago.


It sounds like purge valve is stuck open allowing to much fuel vapor into engine. Completely disconnect purge solenoid lines and plug them off, if it runs better then replace the purge valve. This would also cause the code you are getting. Let me know how it goes or if you have more questions, thanks.

Customer: replied 7 months ago.
disconnected purgr valve, pluged lines, no change hooked vacume guage directly into intake plenum and my vacume guage at idle is dancing around like nobodys business lol. I nevewr seen a vacume guage moving so much. that's supposed to be steady wtf? when I give it gas the vacume seems to sedle down and engine sounds better, but let off gas and vacume guage starts doing the waltz all over again. checking the cylinders through the scanner and only one cylinder is not misfiring ( I just changed the plugs also ) the misfires seem to slow as I give it gas. ( also changed oil with new filter wen I did plugs) any thoughts?

With vacuum gauges jumping around that would sound PCV related or intake gasket related. Block off PCV to see if it steadys out if so replace iwth one from dealership. If not you can spray water around intake gaskets to see if it steadys out if so look for leak in that area. Let me know how it goes or if you have more questions, thanks.

Customer: replied 7 months ago.
I tried sparaying with brake cleaner around intake gasket while looking at fuel trims in scanner but there was no change. ill try blocking off pcv. I'm starting to think the cam phasers are on the fritz. cause of the dancing vacume readings.

That is possible but usually sets a separate code.

Customer: replied 7 months ago.
ok no change with pcv. however again the vacume guage is realy dancing perfectly equally up and down. I can clearly here with the intake snorkel off what sounds like an intake valve leaking either cracked or unseated, its like you here it. also the customer complained that just before he pulled over and cut off ignition the engine backfired so bad he described it as a gun shot. I have several good scan tools I can run test just not to sure were to go.

Hi there. Different expert here.

That sure sounds like a mechanical issue. I'd recommend a static (and possibly even a running) compression test to pinpoint which cylinder/valve is acting up.

Please let me know what the compression test(s) reveal.

Best Regards.

Customer: replied 7 months ago.
cylinder 1 has no compression. driver side bank is all fine. passenger side head all are about 20 psi lower than driver side and again cyl 1 has no compression. it sounds like the air is clearly coming up through the intake I stuck a very good hd camera with reverse camera angel and it seems as if as thought the intake valve seat on cyl 1 is gone. all others seem fine. your thoughts? cure?

I was afraid of that... The head would have to come off and be repaired or replaced accordingly; timing components would need to be inspected as possibly being the cause of this issue.

Use your camera to inspect #1 cylinder wall. If scratched, it might be cheaper to get a used engine than to repair that one.

Please let me know what you find.

Best Regards.

Hi again. Haven't heard from you so I thought I check; everything OK? Need anything?

Juan Crespo, Tech Trainer
Category: Car
Satisfied Customers: 1526
Experience: A.S.E. Master Technician, Advanced Level, Emissions - Asian, Domestic, & European
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