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  1. #1
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    Surging engine RPM when under light load

    I've got this in my build thread but decided it's a problem worthy of its own post.

    The 5.9L has surging RPM when under light load (going up an incline or driving on the highway) with a steady throttle, varying between 50-100 RPM.

    It had a similar problem when we first got the ZJ, but only when using the cruise control, and the surge was more pronounced between 200-300 RPM. I quickly diagnosed it to specifically be a bad TPS related to the torque converter unlocking. It came back about 8 months later and I switched it for a MOPAR TPS.

    Now it does it ALL THE TIME (both when using cruise and not), but not as severely as before (like 50-100). I got a code referencing the torque converter unlocking when I plugged in the scan tool, but it hadn't thrown a CEL. I did all new transmission electronics a year ago (including the BW pressure solenoid and the TCC), so I don't have any idea what it could be.

    Scanning the Internet suggested it was probably a bad brake light switch, which when it shorts can cause the converter to lock/unlock. I replaced it with a MOPAR switch, but it made zero difference. Other Internet opinions suggest it could be: either/both O2 sensor(s), crank position sensor, IAC motor, bad PCM, broken clutch plate in the torque converter. I'm loathe to start randomly tossing even more money at expensive sensors, but I'm not sure I have much choice.

    Any thoughts?

  2. #2
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    I solved my own problem. I eventually found a guy on a Dodge site who was having the same problem right after a transmission service. Long story short: he tightened his TV cable by a couple clicks and the problem went away.

    So Monday night after work, I checked the TV cable and thought it was adjusted correctly, but after playing with it for 30 seconds, I realized that there was about 1/8"-1/4" of take-up before serious resistance started, in addition to the outright slack. When I'd adjusted the cable before, I figured that the light resistance still "counted" for the actual valve, so excluded it as the slack during adjustment. It turns out I was wrong. So I also tightened the cable by a couple clicks to get rid of the light tension, and did one more click for good measure, and took it for a spin: problem gone.

    But I also checked the fluid and it was REALLY dirty, so I didn't do a long test drive. Last night I dropped the pan and did a transmission service. The fluid was dark, but the particles in the pan actually didn't look bad. There as a coating on the magnet, and on the drain-plug magnet (super happy I installed a drain plug last year since it made the service nice a clean!), but no major particles or large chunks in the pan. I also noticed that either myself or the transmission place I went to right when we got the Jeep hadn't pulled the crappy cork gasket that seals the filter to the valve body, so it's possible (probable?) that the filter didn't have a good seal.

    Either way, I got the filter changed. Also note: Autozone is carrying a Raybestos filter kit for the 46RE that is the good plastic/metal filter, AND comes with the metal/rubber reusable gasket! It's listed as Duralast 997607, but it's not a Duralast part. Although I had the good gasket already, I don't know how many times it'd been on/off, so I sprung for the pricier kit at $40.

    New fluid just shy of 7 quarts of fresh ATF +4 and went for a longer test drive. Transmission shifted great, held all gears as expected, cruise control worked, no surging RPM, and the torque-converter lockup unlocked when tapping lightly on the brake at speed.

  3. #3
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    Thanks- I'm going to try that. Mine's done the unlock only a couple times, just when the throttle is at almost zero on the freeway. Thought I had adjusted mine too...
    My tranmission took a full 8 quarts, but I let it drain with the pan off overnight - it will keep dripping for hours. Then had to top it off with the last half of the 8th quart after driving it a bit.

  4. #4
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    Unless the lockup clutch is slipping from inadequate line pressure, the TV cable isn't going to affect it. It doesn't affect when the TCC engages or disengages, only the fluid pressure available to it.
    1998 ZJ 5.9 Limited - Deep Slate
    Mods: Big trans cooler, 231 swap, Indy MA-X heads prepped by IMM, Comp 20-744-9 cam, 1.7 HS roller rockers, 52mm TB, Airgap manifold, DT headers and full 3" exhaust, SCT tune homebrewed by me, Martin Saine valve body, B&M tranny pan, magic suspension made from unicorn tears, power steering cooler, lots of lighting mods

  5. #5
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    I left the pan off for like two weeks when I was waiting on the accumulator spring to arrive, and then to find the time to get back in the garage, and I added the OEM auxiliary cooler. It took 7-3/4 quarts to fill that time.

    Quote Originally Posted by comptiger5000 View Post
    Unless the lockup clutch is slipping from inadequate line pressure . . . .
    I think this was probably the issue. It wasn't jumping by 300 RPM like it does when the TCC dis/engages. It was only surging by 50 or so RPM, but it was enough that I would feel it, see it on the tach, and knew something wasn't right. (As of last week, I completely believed it was still related to the TCC or a broken clutch.) This would also explain the dirtier fluid.

  6. #6
    RallyJeep GO
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    Yeah, if your TV cable was as loose as you describe, it was probably slipping a bit. But for anyone who's cable isn't way off in the loose direction, that won't be the fix.
    1998 ZJ 5.9 Limited - Deep Slate
    Mods: Big trans cooler, 231 swap, Indy MA-X heads prepped by IMM, Comp 20-744-9 cam, 1.7 HS roller rockers, 52mm TB, Airgap manifold, DT headers and full 3" exhaust, SCT tune homebrewed by me, Martin Saine valve body, B&M tranny pan, magic suspension made from unicorn tears, power steering cooler, lots of lighting mods

  7. #7
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    Quote Originally Posted by comptiger5000 View Post
    Yeah, if your TV cable was as loose as you describe, it was probably slipping a bit. But for anyone who's cable isn't way off in the loose direction, that won't be the fix.
    So what could be the other fixes? I'm just assuming my cable isn't loose, partly because I've checked it before, and it's never the first thing you try. Mine only does it with very light loads - just north of coasting, and only a couple times. It will stay locked in overdrive up a hill all day without a hiccup. My first thought was TPS, or governor solonoid.
    Last edited by carlmon; 07-02-2015 at 08:45 AM.

  8. #8
    RallyJeep GO
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    Hmm... IIRC, when I was running stock TC lockup programming, under super-light throttle (barely touching the pedal), it would usually be un-locked just like when off-throttle. It might be normal, but otherwise, TPS is the most likely cause (TC lockup points are determined by trans output shaft RPM and TPS voltage).
    1998 ZJ 5.9 Limited - Deep Slate
    Mods: Big trans cooler, 231 swap, Indy MA-X heads prepped by IMM, Comp 20-744-9 cam, 1.7 HS roller rockers, 52mm TB, Airgap manifold, DT headers and full 3" exhaust, SCT tune homebrewed by me, Martin Saine valve body, B&M tranny pan, magic suspension made from unicorn tears, power steering cooler, lots of lighting mods

  9. #9
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    Did the OP ever figure out he had a bad TPS??

  10. #10
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    Quote Originally Posted by StPaul59 View Post
    Did the OP ever figure out he had a bad TPS??
    HA HA HA HA HA.

    Went through two TPSs, including a MOPAR. No impact on the surging. (See the notation in my original post.)

    I'm thinking I had something let loose in the torque converter, which eventually got stuck on something else in it. Either way, the problem more-or-less went away on its own. Although I haven't checked for stored codes lately, so who knows. It just hasn't bugged me when I've been driving it lately.

    It also could be that I used a non-MOPAR TCC solenoid.

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