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Thread: Code 91 lean on LEFT side only - CEL comes on after 10-15min driving, runs fine

  1. #1
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    Code 91 lean on LEFT side only - CEL comes on after 10-15min driving, runs fine

    90SC Auto Bone stock, has thrown CEL all the time I have owned it and yet never failed etest.
    Throwing this out looking for ideas on why ONE side might throw a lean 91 code.
    O2 sensor is new/recent.
    Maybe going to spray injectors, and LEFT side areas carb cleaner and see if it affects idle speed to look for a vacuum leak on the left side.
    Could a Missfiring plug cause this- I woudl think not more likely the opposite but guesssing.

    Looked through / searched for similar posts but did not find anything.

    PR
    2018 F150 Super Crew Lariat for when it REALLY snows
    1990 Titanium Auto 110k miles (Winter Toy)
    1992 RED/Black 5Spd 71k miles (Pristine Toy)
    1971 Mustang Convt 5.0L EFI 5 Spd

  2. #2
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    LEFT side areas carb cleaner and see if it affects idle speed
    Usually not worth the chemicals - the EEC might adjust/smooth the idle faster than you can detect a variation. Smoke test, however, is your friend.

    Might be a bad injector and/or leaking o-ring. How many miles on them? How is the fuel mileage? Does the car have EGR?
    Last edited by KMT; 10-05-2018 at 09:39 PM.

  3. #3
    Join Date
    Jul 2005
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    425
    Good evening

    I had this problem before and for me it was an exhaust leak issue. Jack the front of the car and support with stands. Check for the following areas for suspect leak(s):

    1. Exhaust manifold. Manifold could be warped/blown gasket/no gasket was installed. Look for evidence of black exhaust trail from top and bottom of the cylinder head/ manifold ports.

    2. Exhaust manifold to pipe collar leak. Could be "cocked". Remove both left and right manifold nuts. Check manifold stud threads for damage. Re-torque. Don't forget anti seize.

    3. Exhaust pipe to inlet of the cat. Be careful and inspect with the engine running.

    4. With car running listen for a tapping sound around the exhaust areas that changes with increase rpm. Suspect item # 1.

    5. Evidence of ignition wire(s) burned from exhaust manifold.

  4. #4
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    Thanks for some good ideas

    Yes Smoke test might be very helpful, Thanks for the reminder.

    The exhaust suggestions are good too, as I DO have a sound which is similar to an Exhaust manifold leak, just always figured it was piston slap.
    I have a drive on hoist so I can run it and inspect quite closely.

    Will note back when I find something.

    PR
    2018 F150 Super Crew Lariat for when it REALLY snows
    1990 Titanium Auto 110k miles (Winter Toy)
    1992 RED/Black 5Spd 71k miles (Pristine Toy)
    1971 Mustang Convt 5.0L EFI 5 Spd

  5. #5
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    Rattle noise IS definitely the drivers side catalytic converter

    Ran on hoist this morning and the noise is clearly from the DS catalytic.

    Inspected for evidence of leaks none found, will use USB camera next week.

    (oddity - did a movie and noticed boost gauge not going positive yet drive/power feels normal......)

    PR
    2018 F150 Super Crew Lariat for when it REALLY snows
    1990 Titanium Auto 110k miles (Winter Toy)
    1992 RED/Black 5Spd 71k miles (Pristine Toy)
    1971 Mustang Convt 5.0L EFI 5 Spd

  6. #6
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    Quote Originally Posted by Peterdroy66 View Post
    Ran on hoist this morning and the noise is clearly from the DS catalytic.

    (oddity - did a movie and noticed boost gauge not going positive yet drive/power feels normal......)

    PR
    Use an IR temperature gun to check the temp of both cats. See if one is hotter/colder than the other....might be that noise/rattle indicates a converter that has started coming apart inside - could be blocking flow, but in that case, the boost should climb. Ask me how I know... Not sure a rattling cat would throw that code, tho. Seems like a separate issue.

    The lack of boost on the gauge could indicate a leaking hose between the manifold and the gauge, or the bypass isn't closing. It should be open with the engine off, then pulled closed when running at idle/under vacuum. You should be able to watch the lever/plate move as you manually blip the throttle and vacuum drops. That would bleed air into the intake, but I'd expect it to affect both O2 sensors, if at all, not just one.

    Does your SC have EGR?
    Last edited by KMT; 10-06-2018 at 10:08 PM.

  7. #7
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    Check the ground wire that bolts to the back of the driver side head next to the fuel pressure regulator, could be loose or cut.
    Tom Downs-member#2752
    MPX'd 90 SC Auto~bought Feb. 2012
    RIP MOM!!! I will always miss and think about you 4Ever! 1940-2018
    RIP Rexy boy!!!! Miss my dog 2006-2018
    http://www.sccoa.com/forums/garage_v...vehicle&id=424

  8. #8
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    Next week

    Thanks for the suggestion won’t get back to this till next week.
    2018 F150 Super Crew Lariat for when it REALLY snows
    1990 Titanium Auto 110k miles (Winter Toy)
    1992 RED/Black 5Spd 71k miles (Pristine Toy)
    1971 Mustang Convt 5.0L EFI 5 Spd

  9. #9
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    broken boost nipple on the plenum right by ACT

    SO I found a broken "old repair" the nipple is gone off the plennum this explains the gauge, AND POSSIBLY the lean condition (but only on own bank????)
    Regardless any neat suggestions on a fix for the nipple broken off in the plenum?
    Thought I had a spare but not, may have to remove ACT to get access to work on it, or plug it and T somewhere else for the gauge
    Suggestions welcome

    Pete
    2018 F150 Super Crew Lariat for when it REALLY snows
    1990 Titanium Auto 110k miles (Winter Toy)
    1992 RED/Black 5Spd 71k miles (Pristine Toy)
    1971 Mustang Convt 5.0L EFI 5 Spd

  10. #10
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    Until you can make repairs, do your best to plug the leak, then clear any stored/continuous codes and see if that fixes the lean condition/code.

    As for the nipple, you might need to remove the plenum to do it right...use an extractor to get what is left of the broken fitting out. Brass as I recall, so it shouldn't put up too much of fight. Just remember the plenum is cast aluminum. Be sure to have fresh gaskets, black RTV, etc. if you're forced to go that far.

  11. #11
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    Removing plenum

    Quote Originally Posted by KMT View Post
    Until you can make repairs, do your best to plug the leak, then clear any stored/continuous codes and see if that fixes the lean condition/code.

    As for the nipple, you might need to remove the plenum to do it right...use an extractor to get what is left of the broken fitting out. Brass as I recall, so it shouldn't put up too much of fight. Just remember the plenum is cast aluminum. Be sure to have fresh gaskets, black RTV, etc. if you're forced to go that far.
    I think I am just going to pull the plenum and do it right.
    Been a while, looking at the minimum stuff I can remove to do this, once in hand a good repair will be easy.

    hoping I can get it off without having to move/remove the lower IC tube.

    Not my daily driver anyways.

    Pete
    2018 F150 Super Crew Lariat for when it REALLY snows
    1990 Titanium Auto 110k miles (Winter Toy)
    1992 RED/Black 5Spd 71k miles (Pristine Toy)
    1971 Mustang Convt 5.0L EFI 5 Spd

  12. #12
    Join Date
    Jul 2005
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    425
    Good evening


    Here is option repair for the rubber nipple and or/plastic boost/vac line damaged/missing without removing the plenum. I am guessing not only is the nipple missing but a section of the hose.


    1. You can go to your local auto discount store and purchase vacuum nipples,caps and vacuum hose. I go to the local salvage yard for 90+ F-150. The truck engine bays have plenty
    straight/angled and cap nipples.

    2. Another alternative is your local Home Depot and or Lowe's and find black lawn water dripping tubing with the approx. diameter as the boost/vac hose. Get 3' just in case you have to deal with
    future HVAC, EGR solenoid and or fuel pressure regulator issues.

    3. Install hose to nipple. Install to the plenum. Cut new hose to length of the broken section. Cut a 3" section of heavy duty heat shrink and slide over the 2 hose section. Heat until the
    connection is tight.

    4. Start car and check for leaks.

  13. #13
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    PM sent, if ever in the area.

  14. #14
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    Thanks - all back together and good

    Pulled the plenum (upper IC tube only with blower top).
    Tapped and installed a brass barbed fitting in the old hole.
    All back together running fine (IC tubes sealed).
    Will see if the CE light still does its lean thing - I don't have ALOT of hope that this will fix it, but it may be SOMEWHAT related, we'll see.

    Peter
    2018 F150 Super Crew Lariat for when it REALLY snows
    1990 Titanium Auto 110k miles (Winter Toy)
    1992 RED/Black 5Spd 71k miles (Pristine Toy)
    1971 Mustang Convt 5.0L EFI 5 Spd

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