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minime331

Engine question

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so i was wondering. sometimes if im stopped i will watch the rpm's and there at like 780-800 and then i will see the rpm's go down to like 740-750 and then as soon as that happens the car starts to shake like its out of sync or i can feel the engine shake thus shaking the rest of the car. its not that big of a problem but i was wondering if this is normal and if not what could be done to help this out.

 

minime331

chris

STella- 14' Focus ST Roush CAI, Cobb access port stage1 tune, custom Ptuning 3" exhuast, Levels FMIC, Turbosmart BOV, rally armor mud flaps, more to come.

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Is your A/C on? When the compressor kicks on it'll cause the temporary RPM drop at idle. There isn't as much torque at the lower RPM so that could cause the shake. . .the engine is chugging trying to keep spinning.

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My '04 MCS does what you describe. Seems like it might stall, but doesn't. I have never gotten a good answer, but because a simple blip of the throttle immediately makes the problem go away, I haven't worried about it.

JAB 67

Fairfax, VA

2004 MCS, PW/B

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Is your A/C on? When the compressor kicks on it'll cause the temporary RPM drop at idle. There isn't as much torque at the lower RPM so that could cause the shake. . .the engine is chugging trying to keep spinning.

 

now that you say that with the a/c it does happen when i have it on

STella- 14' Focus ST Roush CAI, Cobb access port stage1 tune, custom Ptuning 3" exhuast, Levels FMIC, Turbosmart BOV, rally armor mud flaps, more to come.

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its kinda funny bc ill be sitting at a light then all of the sudden the car starts to shake not violently but just a little

STella- 14' Focus ST Roush CAI, Cobb access port stage1 tune, custom Ptuning 3" exhuast, Levels FMIC, Turbosmart BOV, rally armor mud flaps, more to come.

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This has happened recently to Austin as well. I am going to check the ignition coil contacts, as I know they are prone to corroding - maybe that is causing the stutter?

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I obviously can't speak for the 1st Gens but both 2nd Gens that I've had do the same thing and the folks that I've talked to about it just refer to it as "idle hunting" or "idle surge". The ECU tuners have been looking at it and they can't figure out a rhyme or reason for it so it is probably as suggested, the A/C compressor kicking in or some high amp draw system like the PS or cooling fan. The big power systems make the alternator jump back on at a lower RPM and it puts a momentary stress on the engine and then the ECU corrects the amount of fuel to the idle circuit and the RPMs come back up to where they should be.

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