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Abrupt power cut on/off when feathering throttle
I've searched the forum pretty thoroughly and have only found one thread that describes the same problem I'm having... unfortunately, it's a dead end.
In the last 6 months or so, my 8 has developed an issue where it feels like the fuel cutoff is rather abrupt when feathering the throttle. It's as if you had a dimmer switch where the last 10% didn't exist and it jumped from 10% to 0%. I can reproduce this at will about 75% of the time and the cutoff is often abrupt enough that I can feel and hear shock in the drivetrain. It also does the inverse when applying the throttle, but it doesn't happen nearly as often. The only thing I can tell you with some certainty is that the issue doesn't seem to present itself above 3200 rpm and the gear I'm in doesn't matter. There are no CEL's, I cleaned the MAF, reset the ECU, run on quality premium fuel, and take immaculate care of the car. Plugs, coils, and wires have about 12000 miles on them and all the solenoids were replaced pretty recently. Anyone have any thoughts as to what might cause this or tips on how to troubleshoot? |
Have you tried removing the air box and accordion, and having someone slowly play with the throttle while you watch the throttle plate in the throttle body? You can see if it is acting strange.
If it is, something is probably sticking. If it isn't, then the idle air bypass may be blocked, leading to a sharper-than-expected airflow drop when the plate closes completely. |
Originally Posted by RIWWP
(Post 4626207)
Have you tried removing the air box and accordion, and having someone slowly play with the throttle while you watch the throttle plate in the throttle body? You can see if it is acting strange.
If it is, something is probably sticking. If it isn't, then the idle air bypass may be blocked, leading to a sharper-than-expected airflow drop when the plate closes completely. How easy is it to check the idle air bypass? I'll see if I can dig up the service manual. |
From what I'm reading, the 8 doesn't have an independent IAC -- the TB performs those functions.
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Yeah, you are right. Visually test the throttle body then :)
Technically, this is approximately the same issue with a stall at idle, since it would be a too-significant restriction in the airflow. The workshop list of possible causes for that problem is significant. So if the throttle body operates normally, start working through this list. Some of these wouldn't apply, like fuel delivery ones, since there isn't any fuel delivery anyway. No battery power supply to PCM or poor GND Vacuum leakage Air leakage from intake-air system parts Air suction at intake-air system (between MAF sensor and intake ports) Air cleaner restriction Improper operation of drive-by-wire control system Throttle body malfunction (stuck closed) Poor fuel pressure Inadequate fuel pressure Pressure regulator malfunction (integrated in fuel pump unit) Fuel pump body mechanical malfunction Fuel pump resister malfunction (open) Fuel leakage from fuel injector Fuel injector (FP1) (RP1) clogging Jet air mixing system malfunction (restriction or leakage at air passage) Open or short circuit in fuel pump body and related wiring harness Restriction in exhaust system Purge solenoid valve malfunction (stuck open) Improper air/fuel ratio control (abnormal signal from ECT sensor to PCM) MAF sensor improper installation No signal from eccentric shaft position sensor due to sensor, related wire or wrong installation Electrical connector disconnected Low engine compression Engine internal malfunction Abnormal engine oil condition (viscosity, deterioration) Low oil pressure Excessive fuel pressure Air mixed in oil line Metering oil pump malfunction Leakage or clogging in oil pipe Leakage or clogging in oil nozzle A/C system operation is improper Engine overheating Abnormal engine oil condition (viscosity, amount, deterioration) Spark leakage from high-tension leads Ignition coil malfunction Immobilizer system and/or circuit malfunction Immobilizer system operates properly (Key is not registered.) |
I snagged the service manual last night and did a little troubleshooting.
Couldn't find any vacuum leaks. Air filter is in good shape. MAF has been cleaned and ECU reset a couple times in the last few months. The car is an 08 with 72000 miles on the original engine and has been very well cared for. Cranking times are a wee bit slower than they used to be, but not enough for me to think it's a compression issue. I premix religiously and the car idles beautifully. Plus, I've never had any CEL's on the car. If it isn't a TB or TB control issue, my gut tells me it could be the fuel pump, a faulty MAF, or maybe an issue with the CAT. Granted, it could be anything on that list... fun... |
Oh, one more thing that MIGHT help with the diagnosis -- the car backfires once in a great while.
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No, that doesn't help :)
All rotaries inherently have all the attributes needed for backfiring. If you have a cat installed, it shouldn't, but still might even if everything is working correctly. Without a cat, most rotaries will backfire all the time. |
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