Rough start and idle jumping issue. - 8th Generation Honda Civic Forum

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Old 12-17-2018, 03:56 AM   #1 (permalink)
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azbritton just put it into 1st gear on 8thcivic
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Rough start and idle jumping issue.

I own a 2008 civic Si.
Fully built head also port and polished.
Up until recently everything with my built head was working fine and then I had a Brand new RBC intake manifold fully port and polished to match a ported ZDX turbo throttle body with a k tuned spacer. Also should mention I'm running ID 1,000 injectors.

Once the new intake manifold and throttle body were put on I have had a world of problems that no mechanic or tuner can seem to figure out. First off in the summer I run E85 so once the weather starts getting cold the car becomes hard to start so I switch back to 93. Now even back on 93 the car is still hard to start. Secondly when the car is sitting in neutral it just sounds like it's running rough. And 3rd but the biggest of the problems is when I am rolling in neutral the car jumps idle from 1000rpm to 2000rpm-3000rpm until my speedometer reads 10mph and then it settles at 1500 which is still too high until I come to a complete stop and then the car drops to 900rpm. Also when I turn the heater off it doesnt idle jump when rolling in neutral.

So in plain:
Hard to start on 93
Sounds like it's running rough
Idle jumping when rolling in neutral

My mechanic said they have checked for:
-vacuum leaks (None)
-reset idle (didnt work)
-tried another zdx throttle body (same issue)
-put stock throttle body back on (same issue)

My tuner tried everything he could to work the issue out on the dyno and the only thing he could think to do was to tell my CPU to stop looking for idle on the throttle body and read it off the gas pedal. It worked for a week or so and now it's back to idle jumping.

The throttle plate is not opening and closing as its suppose to. I did not have this issue before the new intake manifold and zdx turbo throttle body.

Does anyone have any idea what this issue could be? Maybe a voltage issue? 2 bad throttle bodies? A leak my mechanic didnt find?

Any help would be much much appreciated and thank you in advance!

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Old 12-18-2018, 04:51 PM   #2 (permalink)
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jrickerm just put it into 1st gear on 8thcivic
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Interesting. I'm having a new stalling issue myself as of last night, and have had some idle surging recently. I was leaning towards new plugs and O2 sensor for me (I'm SC'd and it beats the crap out of those items) as a start, but you've given me an idea about looking around for vacuum leaks (always a treat to find them). I've had my upgraded TB for years and never had any issues like yours. Good luck!

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Old 12-19-2018, 01:53 PM   #3 (permalink)
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MicoliFA5 just put it into 1st gear on 8thcivic
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I had a similar issue with my 2008 Civic Si after messing up with the throttle body. Revs jumped on neutral and hanged between gears. It turned out to be the TPS (throttle position sensor) the issue was solved resetting the TPS with a capable OBD2 scanner. Hope you find a solution.
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