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I installed a Kenne Bell supercharger last year, and have been chasing a couple very irritating issues ever since. One of those issues was an intermittent erratic idle. Sometimes the idle was perfect, and sometimes it would "hunt" for a while when I pushed in the clutch. It would go down to as low as 200 rpm and as high as 1400, and eventually settle back to about 830 rpm. Many times when it went that low it would stall.
Then when I took the car on the highway, I noticed some light surging. Furthermore, when slowing down, if I went to neutral, the car would inevitably stall. Because of a TSB on the stalling after a highway drive, I replaced the fuel pump with no changes. I also chased it by replacing the Throttle Body, hunting down vacuum leaks, and in general tearing my hair out. There is another issue and I never knew whether it was related or not (cold run problem), so in chasing these problems I also replaced the injectors, the fuel filter, the plugs, and the MAF sensor. I also looked at the EGR and other things that might cause the problems I was experiencing.
Recently, while revisiting the issues with Kenne Bell's tuner, Ken Christley, I let him know that I had an aftermarket bumper and the intake filter was right smack behind a big opening in the bumper. I asked if it might be causing a problem, and he responded with an overwhelming "yes".
So tonight I picked up a sheet of aluminum and jacked up the front end. After a couple hours of working the sheets into a two piece air dam, covering both the front of the filter, and also doing a cover for the bottom of the filter, I took the car out for a test drive.
The weather is particularly windy right now, but it was dry and cool. I figured if turbulence was the issue, the wind would make it even worse, so my fix was going to be tested under extreme conditions.
I took the car out toward the highway. I paid close attention to the idle, and never saw any issues.. Then I got out on the highway and opened it up.. I set the cruise to 80 in 4th at first, and there was no surging. The MAF was reading very little fluctuation, where before it was all over the place. The AF gauge was reading the typical fluctuations but nothing as extreme as before. I cruised for a few miles, and then at 80 mph pushed in the clutch.. it went to 800 rpm and stayed solid, something it NEVER did before.. Then I went to 5th gear and set the cruise at 70, very low RPM and small fluctuations in the road (and gusting winds) will add a big load to the engine, which I figure was a good test. Not only ran great, but I reset the fuel mileage gauge and was getting over 25 mpg at 70mph in gusting winds.. Very nice.
I drove for another 15 minutes heading back home, and the idle was solid, the surging in cruise control was gone, and in general the car drove a lot more consistently. Before this fix, after a time the driveabilty would change.. I figure it was because after a time the PCM was so confused at the inaccurate readings from the MAF sensor that it was doing all sorts of oddball things to correct for it, and that affected the overall driveabilty after a time. Now it ran the same after driving aggressively for over 30 minutes.
So now I am just down to the cold run issue, and can troubleshoot it independently again. What a weight off my shoulders. One more problem to fix and the car will be perfect, and it will be time to start upping the boost and tweaking the tune.. haha.
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I have had some similar quirks. My CAI filter is in the normal location, but I have this giant *** vented cervini hood. I will try closing it up.
For you - now that a lot of your problem is fixed, try this for cold starts. Reset you fuel trims and let the car re-learn its fuel trims. They're surely confused from the turbulence that's now gone. To do this, disconnect your batter for a while, apply the brakes to drain any juice left in the car, reconnect the battery, crank and let idle for 10 minutes.
Then see how it works. 'Cept you'll have to wait for the engine to cold cold to test the cold start. :-)
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