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  1. #1

    Default Engine runs lean

    Hello:

    The computer on the 86GT says it is running lean. Gives codes 41 and 91. Here's the story so far.

    In March I replaced the injectors with a rebuilt and tested factory set. I replaced the under body fuel lines, both supply and return. I cleaned out the other hard lines on the engine including the fuel rail. I also installed a new fuel filter. Finally I installed a new pressure regulator. It is visibly shorter than the factory unit I removed. I kept it just in case. Both are Ford parts. Actually Bosch but with the Ford part number.

    I replaced all this as the car had over 225,000 miles on it at the time. Car runs better, has more power, and sounds better.

    I even resealed the air box as it was loose and made sure the air tube was tight at the throttle.

    I checked the Thermactor system and it is functioning properly. All the lines and hoses are tight and there are no leaks. That includes the metal tubes that attach to the back of the engine and the line down to the cats.

    I checked fuel pressure and here are the results:

    KOEO. Cycled the key on and off three times. Pressure= 38.5 psi.
    Engine running (ER). Idle for about two minutes. Pressure= 33 psi.
    Engine off, key off, then key on. Pressure= 37.5 psi.
    Key off, waited one minute. Checked residual. Pressure= 37 psi.

    No smell of fuel anywhere, especially at the regulator. Pulled the vacuum line and checked. Looks good. No leaks anywhere.

    After a test drive of about five minutes, I ran the KOEO and ER tests again.
    KOEO= Pass code 11.
    ER gave codes 41 and 91, nothing else. Ran the cylinder balance test. Code 90= Pass.

    Unless the new regulator is the problem, I'm stumped. Perhaps a mystery vacuum leak somewhere.
    Any ideas? They are most appreciated.
    Thanks.
    PS- Can't load photos now. Looks like the factory pressure regulator I removed has part number E3EE-9C968-AA. The new one has F4CE-9C968-AA. Motorcraft number CM4764, which is listed as correct for my car.

  2. #2
    FEP Member
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    May 2016
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    Lake City Pa 16423
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    313

    Default

    More info about the engine would help.
    Is it all stock, except for the injectors?
    How old are the O2 sensors? I hope their not original with that mileage, they should have been replaced about four times by now.
    Old MAP sensor, could be a problem.
    What's the throttle sensor voltage reading at idle?
    How are the spark plugs?
    Like I said more details please.

  3. #3

    Default

    Check to see if the air injection system is routing air to the cylinder heads after warm-up. There should be
    no vacuum at the diverter valve when the engine is running in closed-loop. The '86 TAD solenoids have been
    known to leak, which causes upstream air when there should not be. The upstream air causes the O2 sensors
    to report a lean mixture.
    Cheers,
    Jeff Cook

    '85 GT Hatch, 5-speed T-Top, Eibachs, Konis, & ARE 5-Spokes ... '85 GT Vert, CFI/AOD, all factory...
    '79 Fairmont StaWag, 5.0, 62K original miles ... '04 Azure Blue 40th Anny Mach 1, 37K original miles...
    65 coupe, 289 auto, Pony interior ... '67 coupe 6-cyl 4-speed ... '68 Vert, Mexican block 307 4-speed...
    '71 Datsun 510 ... And a 1-of-328 Deep Blue Pearl 2003 Marauder 4.6 DOHC, J-Mod, 4.10s and Lidio tune

  4. #4

    Default

    The O2 sensors have about 40,000 miles on them. Both Ford units and both replaced at the same time. The air injection system is working correctly. Upstream through warm up, then down to the cats.
    I need to recheck the O2 sensors under the car. The shop manual calls for a Break Out Box but I have another book that outlines some electrical tests. Might get to it this weekend. I did manage to find a fuel pressure regulator that is exactly the same as what came off the car. But I'll check the O2 sensors first.

  5. #5

    Default

    And look real carefully at your TAB and TAD solenoids. Reading glasses, magnifying glass, whatever you've got. The plastic bodies can develop cracks and become a vacuum leak, in spite of still working enough to not cause codes related to the Bypass Valve and Diverter Valve functions which they control.

    How about the vacuum harness? Are the colored vinyl lines still supple and leak-free? How about the line to your MAP?

    And does the exhaust smell lean? Or does it actually smell rich?
    Last edited by grabbergreen84; 08-07-2016 at 02:48 AM.
    '88 Mustang GT convertible, T5, 3.08:1 gears. 5.0 Explobra Jet: A9L Mass Air conversion, Fenderwell Mac cold air intake, 70mm MAF meter = 4.6 T-Bird/Cougar housing + '95 Mustang F2VF-12B579-A1A sensor, aftermarket 70mm throttle body and spacer, Explorer intakes, GT40P heads with Alex's Parts springs and drilled for thermactor, Crane F3ZE-6529-AB 1.7 "Cobra" roller rockers, Ford Racing P50 headers, Mac H-pipe, Magnaflow catback, Walbro 190 LPH fuel pump, UPR firewall adjuster and quadrant with Ford OEM cable, 3G conversion ('95 Mustang V6), Taurus fan, rolled on Rustoleum gloss white paint...
    Past Four Eyes: Red well optioned '82 GT 5.0, Black T-top '81 Capri Black Magic 3.3L 4 speed, Black T-top '84 Capri RS 5.0 5 speed.Over 200,000 miles driven in Four Eyes, and over 350,000 in Fox Body cars.

  6. #6

    Default

    I plan on installing a factory fuel pressure regulator tomorrow, just like the one that came off the car rather than keep the new one, despite it being the "correct" Ford part. After that, I'll check fuel pressure. If things are still not solved, I'll update. Looks like all vacuum hoses are good but I'll give them the once over tomorrow.

  7. #7

    Default

    Found the problem. Seems the fog lights quit in the Spring. I looked over the entire system and the only thing I could come up with is the fusible link. But as the EVTM is wrong, it was a challenge to track down. But this site is invaluable. Someone else already did that and the thread is from 2007 if I remember right. Not good at posting links. Seems the fusible link for the fogs also is connected to the O2 circuit. I checked where the thread said to look- near the brake booster. Sure enough there are two links there and one is shot. Problem found. Now I need to repair the link and see if this clears the problem. And fixes the fog lights.

  8. #8

    Default

    Ah, maybe your O2s weren't heating up?
    '88 Mustang GT convertible, T5, 3.08:1 gears. 5.0 Explobra Jet: A9L Mass Air conversion, Fenderwell Mac cold air intake, 70mm MAF meter = 4.6 T-Bird/Cougar housing + '95 Mustang F2VF-12B579-A1A sensor, aftermarket 70mm throttle body and spacer, Explorer intakes, GT40P heads with Alex's Parts springs and drilled for thermactor, Crane F3ZE-6529-AB 1.7 "Cobra" roller rockers, Ford Racing P50 headers, Mac H-pipe, Magnaflow catback, Walbro 190 LPH fuel pump, UPR firewall adjuster and quadrant with Ford OEM cable, 3G conversion ('95 Mustang V6), Taurus fan, rolled on Rustoleum gloss white paint...
    Past Four Eyes: Red well optioned '82 GT 5.0, Black T-top '81 Capri Black Magic 3.3L 4 speed, Black T-top '84 Capri RS 5.0 5 speed.Over 200,000 miles driven in Four Eyes, and over 350,000 in Fox Body cars.

  9. #9

    Default

    Seems that way. Codes 41 and 91 refer to lean conditions that come from too low a voltage signal, according to my books at least. If no voltage, then very low signal, and codes are sent. Hope to find a factory 20 gauge fusible link soon to see if this cures the problem.

  10. #10

    Default

    I know this thread is old but since I have the car finally back together after nine months or so, all seems well until the KOER test and bingo- codes 41 and 91.
    I pulled the hoses off the diverter valve and moved the throttle after warm up. It seems to switch air correctly but the engine runs hot under hood. I suppose I may not have a Thermactor problem. The check valve for the upstream air pipe had vacuum on it while the engine ran. I put my finger over it and felt vacuum. Should this be the case? Actually they both had vacuum. If the diverter valve is bad, that would explain the extra air going to the headers. I also found a very small hole in one of the upstream cats and can hear it, too. Maybe this is contributing? I will check the air diverter and air bypass valves tomorrow to make sure they are functioning right. They might be the originals. Next step is to test the O2 sensors.

  11. #11

    Default

    Well the Thermactor system checks out fine. Ditto for the check valves. Next step is to check the O2 sensors. They have maybe 40,000 miles on them, have to look that one up. The shop manual lists some checks for the HEGOs with codes 41 and 91 but most require a Break out Box- something I don't have. Yet. Any suggestions on checking the O2s? Also checked the PCV valve. Its good and seated just right. No other vacuum leaks. Thanks again.

  12. #12

  13. #13

    Default

    Be aware, just like the tool that wide band air:fuel ratio sensors and their gauges represent, the oxygen sensor is simply sensing how much oxygen is contained in the exhaust. The causes for excess oxygen in the exhaust can be more than simply that the engine is running lean. Maybe it actually is running lean, all over the place, but combustion misfires (ignition itself, or rich or lean misfires) will all also send an abundance of oxygen out the exhaust, which the tool interprets as, and sets a (basically brain dead) code of "system lean"...

    Here are some tests for checking single wire and heated oxygen sensors:



    Last edited by Walking-Tall; 01-22-2018 at 01:17 AM.
    Mike
    1986 Mustang convertible 3.8L ---> BUILD THREAD
    1983 Mercury Cougar 3.8L LS
    1986 Ford Thunderbird 5.0L ELAN
    1966 Ford Fairlane sedan 200-6
    1966 Ford Fairlane GT (390ci 4spd)
    1981 Mercury Marquis Brougham
    1980 Capri RS Turbo
    1971 Mustang Fastback
    1974 Pontiac Luxury LeMans

  14. #14

    Default

    Thanks for the nice info. I did manage to try the two bottom tests listed on the picture of page 248 above. I got nothing for the heater element test. For the Key Power test, I got maybe 5.9 volts. I recall a problem with the fog lights that is in the thread above from 8-11-2016. I found the factory fusible link and wired it in and the fogs worked. But the repair was difficult as I had to splice a wire with little wire sticking out of what looks like a splice. Check out the photos below. Is the thing with the bare wire exposed just a splice? One side has a blue wire, the other is gray with yellow stripe. Doesn't look like a fusible link. If it is, and since the wire is exposed again, I'll have to repair it but maybe not with factory stuff. Hard to find. Splice or no? Thanks.Name:  HEGO wire 2.jpg
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    This is all back by the brake booster at the firewall. Thanks again. And if anyone has one... PM!

  15. #15

    Default

    Thanks for the nice info. I did manage to try the two bottom tests listed on the picture of page 248 above. I got nothing for the heater element test. For the Key Power test, I got maybe 5.9 volts. I recall a problem with the fog lights that is in the thread above from 8-11-2016. I found the factory fusible link and wired it in and the fogs worked. But the repair was difficult as I had to splice a wire with little wire sticking out of what looks like a splice. Check out the photos below. Is the thing with the bare wire exposed just a splice? One side has a blue wire, the other is gray with yellow stripe. Doesn't look like a fusible link. If it is, and since the wire is exposed again, I'll have to repair it but maybe not with factory stuff. Hard to find. Splice or no? Thanks.Name:  HEGO wire 2.jpg
Views: 176
Size:  99.6 KBName:  HEGO wire 3.jpg
Views: 174
Size:  95.5 KBName:  HEGO wire 1.jpg
Views: 173
Size:  100.3 KB
    This is all back by the brake booster at the firewall. Thanks again. And if anyone has one... PM!

  16. #16

    Default

    UPDATE: I checked the wiring diagrams and the item is indeed a splice. Hopefully this takes care of the codes 41 and 91.

  17. #17

    Default

    ANOTHER UPDATE: I repaired the damaged previous repair and the fog lights work fine so that circuit likely is repaired. However, when I retrieved the KOEO codes I now have a code 31. Again. Seems that comes up often on my car. I've already went through maybe three of the EGR valve position sensors in perhaps ten years. When the code 41 and 91 problem started and was 'fixed' previously, the same thing happened. Code 31 and a new sensor which cured the problem. I'm more than curious. If my lean codes are indeed gone, what might the connection be to the EGR sensor circuit? Voltage spike or amp surge? I may put on a quality parts store sensor to see what happens before I put on the Motorcraft one. No need to waste money. Ideas are most welcome. Thanks all.

  18. #18

    Default

    And yet one more time. I applied vacuum to the EGR and made the car stall so the EGR should be fine. I jumpered the EGR sensor connector from VREF to EVP signal like the shop manual says to try to get a code 35 and that worked. Manual says to replace the EGR sensor and then try again. That's why I want to use a more affordable part at this time.

  19. #19

    Default

    Or, before buying anything, test that EVP sensor's variable resistance. Remove (or use your vacuum source while leaving it installed), connect a multi-meter to measure ohms of resistance as below, moving it's stem to see if it's b*tched or not...

    Mike
    1986 Mustang convertible 3.8L ---> BUILD THREAD
    1983 Mercury Cougar 3.8L LS
    1986 Ford Thunderbird 5.0L ELAN
    1966 Ford Fairlane sedan 200-6
    1966 Ford Fairlane GT (390ci 4spd)
    1981 Mercury Marquis Brougham
    1980 Capri RS Turbo
    1971 Mustang Fastback
    1974 Pontiac Luxury LeMans

  20. #20

    Default

    I managed the resistance test as above and found the ohm way too high. Sensor is bad. So I somehow found a very affordable (pun intended) EGR sensor and after arrival, tested it and bingo. I'll put it on when the weather improves around here and go for codes again. Plus the bad sensor was not a Ford part so maybe that was the problem.

  21. #21

    Default

    Woops. Had to edit the post. Trying again.
    Last edited by Magnetic82; 02-04-2018 at 01:30 PM. Reason: spelling

  22. #22

    Default

    New EGR valve position sensor installed and tested. KOEO test code is 11 but the KOER test still gives codes 41 and 91. I noticed the upper radiator hose doesn't get hot until maybe ten minutes after turning the car on so if the thermostat is opening late, that might explain the codes. The engine coolant temp sensor is in the heater tube but the engine temp gauge sender is in the intake and the temp gauge on the dash shows in the upper range until the thermostat opens, then drops to the lower side of the gauge. It sometimes moves during warm up. If it takes that long for the thermo to open, that might contribute to the lean codes. A new stat is on its way so I'll see if that helps.

  23. #23

    Default

    I would like to mention a small detail that gets overlooked sometimes for troubleshooting these things: most especially for KOER, the engine should already be good and warmed up. Because... who knows what mode of mayhem is all going on prior to being fully warmed up to operating temperature, at least thermostat temperature... and FWIW, when EEC-IV in general senses that the engine's too hot, it interpreting it as being too lean, it'll add fuel (enrich) in effort to cool it, not take fuel away (lean)...
    Mike
    1986 Mustang convertible 3.8L ---> BUILD THREAD
    1983 Mercury Cougar 3.8L LS
    1986 Ford Thunderbird 5.0L ELAN
    1966 Ford Fairlane sedan 200-6
    1966 Ford Fairlane GT (390ci 4spd)
    1981 Mercury Marquis Brougham
    1980 Capri RS Turbo
    1971 Mustang Fastback
    1974 Pontiac Luxury LeMans

  24. #24

    Default

    It wouldn't report hot as a lean code though unless it had a signal from the HEGO right?

  25. #25

    Default

    Quote Originally Posted by emerygt350 View Post
    It wouldn't report hot as a lean code though unless it had a signal from the HEGO right?
    ECT has influence on EEC's dictated running air:fuel ratio too... and who's on first? The leg bone's connected to the hip bone, and the wheels on the bus go 'round and 'round. It would report lean as a lean code if signaled so from an oxygen sensor, yes, but all of these variables are so very interrelated, and the point is that none of these sensors or the computer itself can be trusted without testing or verifying they are working correctly ... not simply take whatever they tell us or the computer or be set in stone as to what to replace next so that the obviously blocked toilet returns to proper smooth but brisk powerful flushing, lol... any/all of it is only a general direction to begin looking when there's a problem... not if there's a problem with EFI, but when... always only just a matter of time...
    Last edited by Walking-Tall; 02-07-2018 at 08:48 PM.
    Mike
    1986 Mustang convertible 3.8L ---> BUILD THREAD
    1983 Mercury Cougar 3.8L LS
    1986 Ford Thunderbird 5.0L ELAN
    1966 Ford Fairlane sedan 200-6
    1966 Ford Fairlane GT (390ci 4spd)
    1981 Mercury Marquis Brougham
    1980 Capri RS Turbo
    1971 Mustang Fastback
    1974 Pontiac Luxury LeMans

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