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Old 11-11-2019, 12:01 PM   #1
RRRM8E
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slight bucking at 1500 - 2000 rpm

I'm getting a slight bucking at low rpms, steady or slight throttle. Noticed it today on 3rd gear in 25 mph zone. reminds me of carbureted motor loading up at low revs. Seems to pull OK otherwise. I changed DISA and various rubber boots a few years ago, and cleaned the ICV. In hot weather , I occasionally get the dreaded idle drop and recovery, so I have a new ICV to install sometime soon. No codes. Thoughts?
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Old 11-11-2019, 12:16 PM   #2
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I would check for vacuum leaks first, if none found then I would change the vanos seals and update the ecu
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Old 11-11-2019, 01:02 PM   #3
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I'm not sure there's a problem, 3rd gear at 25 mph sounds like you're lugging the engine.
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Old 11-11-2019, 03:17 PM   #4
RRRM8E
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I'm not sure there's a problem, 3rd gear at 25 mph sounds like you're lugging the engine.
I would have thought 1800 in third was acceptable. Fourth or fifth, yes.
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Old 11-11-2019, 03:19 PM   #5
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Any service on the coils? I had a reproducible buck around 1700 in a higher gear with no codes that completely cleared up after a coil replacement (#3 specifically).
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Old 11-11-2019, 03:39 PM   #6
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I would have thought 1800 in third was acceptable. Fourth or fifth, yes.
Re-reading your original post I see you are talking about light throttle, not hard acceleration, so maybe not really lugging. Though I try not to get on throttle under 2k rpm if I can avoid it.

The only other bucking I've experienced is lash from a worn drivetrain.
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Old 11-11-2019, 08:19 PM   #7
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Any service on the coils? I had a reproducible buck around 1700 in a higher gear with no codes that completely cleared up after a coil replacement (#3 specifically).
How did you home in on #3?
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Old 11-11-2019, 09:13 PM   #8
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I use 2000 RPM as my shift point and 25mph seems slow for third gear. I would check the lower boot and make sure that it's seated correctly. If you don't have a CEL, odds are a little more throttle will smoothen it out.
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Old 11-11-2019, 10:30 PM   #9
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How did you home in on #3?
Because any time I drove over 80 indicated, or did an especially hard pull, the car would drop into limp mode and throw P1347. I swapped the coil with #5 and the code followed the cylinder. Replaced coil, no more issues---including the buck around 1700 rpm.

I would do a smoke test first just to make sure the engine is leak-free, though.
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Old 11-12-2019, 09:42 AM   #10
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Because any time I drove over 80 indicated, or did an especially hard pull, the car would drop into limp mode and throw P1347. I swapped the coil with #5 and the code followed the cylinder. Replaced coil, no more issues---including the buck around 1700 rpm.

I would do a smoke test first just to make sure the engine is leak-free, though.
OK, originally you said no codes. I think they are original - will check records. Thanks
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Old 11-12-2019, 09:49 AM   #11
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i use 2000 rpm as my shift point and 25mph seems slow for third gear. I would check the lower boot and make sure that it's seated correctly. If you don't have a cel, odds are a little more throttle will smoothen it out.
cel?
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Old 11-12-2019, 10:10 AM   #12
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cel?
Check engine light
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Old 11-12-2019, 10:42 AM   #13
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OK, originally you said no codes. I think they are original - will check records. Thanks
I didn't get a code when the engine bucked at 1700 (I think remember seeing the SES light flicker for a split second on occasion), only when I drove over 80 indicated and the engine went into limp mode. If I hadn't had that last issue, I would never have suspected the coil...
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Old 11-12-2019, 10:48 AM   #14
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I didn't get a code when the engine bucked at 1700 (I think remember seeing the SES light flicker for a split second on occasion), only when I drove over 80 indicated and the engine went into limp mode. If I hadn't had that last issue, I would never have suspected the coil...
Sounds like OP should go get a speeding ticket and see if he gets a misfire code hahah.
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Old 11-12-2019, 11:03 AM   #15
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When's the last time the fuel filter/regulator was serviced?
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Old 11-12-2019, 12:47 PM   #16
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When's the last time the fuel filter/regulator was serviced?
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Filter and pump are fresh. Car seems fine otherwise. Pulls well. Only problem has been the idle dropping to near stall then recovering from time to time. That's more warm weather issue with AC on. Otherwise, idle steady at 600. Starts right up, highway cruise is smooth.

New ICV on hand. Plugs were done a few years ago. What are the DISA valve symptoms? Higher RPM as I recall.

I guess no CEL is a good sign.
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Old 11-12-2019, 02:15 PM   #17
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Filter and pump are fresh. Car seems fine otherwise. Pulls well. Only problem has been the idle dropping to near stall then recovering from time to time. That's more warm weather issue with AC on. Otherwise, idle steady at 600. Starts right up, highway cruise is smooth.

New ICV on hand. Plugs were done a few years ago. What are the DISA valve symptoms? Higher RPM as I recall.

I guess no CEL is a good sign.
OK. Check air intake system and the ccv/hoses for a major leak. Usually with disa issues, lean codes, rattle noise, rough running, lacking power, etc. You might clean out your idle valve to see if that improves things.
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