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Ask Chuck Your Own Question
Chuck, ASE Certified Technician
Category: Ford
Satisfied Customers: 2100
Experience:  33 Years Experience,Ford Senior Master,ASE Master,L1 Advanced Engine Performance
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2000 Lincoln LS: 3.9 V8 engine..intake..leak..O2..rough

Customer Question

2000 Lincoln LS with 3.9 V8 engine, had code P0171 lean bank 1 and P0175 rich bank 2
No intake leak, O2 reading on bank 1 very low, engine run very rough, short term trim
on both side -16
Submitted: 7 years ago.
Category: Ford
Expert:  Chuck replied 7 years ago.
What are both long fuel trims reading? And has this vehicle just had timing chains or tensioners replaced? If not do you have any noise from front of engine?
Customer: replied 7 years ago.
Long term trim is zero, timming chain not replace but a littlte noise in front of engine
Expert:  Chuck replied 7 years ago.
Both long term fuel trims are zero?
Customer: replied 7 years ago.
Both long term trim are zero, Both short term trim from - 15 to -20
Expert:  Chuck replied 7 years ago.

I suspect that you have a broken timing chain tensioner. It is most likely the one on the short chain between the two camshafts but it could be one on the long chain also.

That is what will usually cause you to have one side showing rich and one side showing lean. It will usually show up in the fuel trim readings also,they will start to oppose each other. If you have just cleared codes this may be why they read zero? You can verify this with a manual compression test. Just test one cylinder on each bank if one of the cams has slipped due to a broken tensioner one side will read like 190 to 200 PSI or so and the other side will be like 140 or 150PSI.

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Customer: replied 7 years ago.

I run compression test one cylinder on each side, they both show 170PSI

After go to close loop, short term trim read around 22 on both side, long trim around -1 to 4, oxygen read on bank 1 both sensor almost below 450 mA, but on bank 2 front one 900mA, rear after Cat read 1000mA or over

Expert:  Chuck replied 7 years ago.

Those compression readings should eliminate the timing.


Thats strange that you are getting a rich code. You should be getting 2 leans codes with those short fuel trim readings.

The one reading of over 1000ma is a little high but rear O2 would not set the codes you have.


I would check the MAF and make sure you don't have a bug or something stuck on the filament and maybe clean it with some electric or MAF cleaner.

Customer: replied 7 years ago.
I already check MAF sensor and clean it, spark plug on bank 1 very white and bank 2 a little black ( they change plugs befor bring it here)
Expert:  Chuck replied 7 years ago.

that sounds just like a tensioner problem but they will have a difference in compression from side to side.. Have you driven it to see if codes will reset and see what fuel trims are reading then?

You said you have a little noise in the front of the motor does it sound like it is close to the cams?

Customer: replied 7 years ago.
A litle engine noise in front of engine may be close to chain, will put back together, test drive, take reading then get back with you on monday, Thank alot
Expert:  Chuck replied 7 years ago.

What I have seen mostly on these when it does this is the secondary chain between the intake and exhaust camshafts usually on the driver's side head most of the time. The plastic piece on top of the tensioner for the secondary chain will break off. This lets the exhaust cam slip a little maybe a tooth. This will cause it to set the one rich and one lean code. And usually if you drive it or maybe run the RPMS up to about 1500 to 2K and hold it for about a minute or 2 the fuel trims will start opposing each other.

I have also seen one that the plastic piece on one of the cassettes broke off but it made more noise.

Customer: replied 7 years ago.

Will get back with you on monday, Thanks

Expert:  Chuck replied 7 years ago.

sounds good