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Discussion Starter #1
I have a 2008 EX coupe. AFTER the car warms up, I am consistently getting hesitation from around 2500-3250 rpm under partial throttle. If I floor it, the hesitation seems to go away. I cleaned the Mass Airflow Sensor (with the proper cleaner), and that did not fix the problem. I ruled out the accelerator pedal sensor by generating the hesitation with the cruise control accel button. The car accelerates just fine when above this rpm range, so I don't think it's a coil, injector, or fuel pump/filter. I'm guessing it could be the Throttle Position Sensor. I am NOT getting any Check Engine Light. I change the oil and filter regularly, so I'm guessing it's not clogged VTEC. The car has 134,000 miles on it.

Interestingly, the car does NOT hesitate until it is at operating temperature.

Thanks.
 

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We can officially rule out the clogged VTEC. What it does sound like is a spark/ignition problem, possibly.

When's the last time you put in new plugs? If it's been a while (or never), that would be a good place to start.
 

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Discussion Starter #3 (Edited)
New NGK Iridiums about 20,000 miles ago. Today, I noticed that it also does not hesitate if I give it very light throttle. So, it only does it under moderate throttle. Light and heavy throttle are both OK.

I should probably clarify that I'm pretty sure it's not a misfire of one cylinder. The period of hesitation is long enough that it's more like the fuel injectors to all four cylinders shut off for almost an entire second. It's my understanding this could happen if the output voltage of the Throttle Position Sensor drops to zero at one particular position, due to malfunction/wear.

From what I can tell, the sensor is not available as a separate part. Rather, Honda sells the entire throttle body assembly, which is a bit expensive. I might try a junkyard throttle body assembly, but my understanding is that I'll also need something like a Foxwell NT510 two-way scanner to do the throttle reset, so I'll need to save a bit of money before I get that.
 

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I've been noticing this about my car too, it does the exact same thing. I've changed the plugs a couple of weeks ago and done a valve adjustment and the throttle no longer feels sloppy but there is still a delay. I wonder if it's a drive by wire thing, and it would seem that it's happening when the car is potentially in "econo" vtec. I read this article on Temple of Vtec, and it's a bit of a longer read but very informative. This delay sounds like its a feature:

Technical Overview of Honda's new R18 i-VTEC Implementation

It would seem like it's how Honda designed the car to give us the good gas mileage. Can anyone confirm this?
 

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Discussion Starter #5 (Edited)
I'm familiar with the Drive By Wire delay, but this is something else. I've owned the car for eight years, and it never hesitated like this until about a month ago. I've been "enjoying" the Drive By Wire delay all eight years, ha ha ;-)

The article you mention might be useful in explaining my car's problem, though. Since I'm not getting the hesitation under very light throttle (when VTEC DBW pushes the throttle open wide, but with the economy VTEC "low" cam online) and I'm also not getting hesitation when I floor the throttle (again, DBW opening the throttle wide, but with the regular "high" cam online) this could support the theory that the Throttle Position Sensor has a bad spot somewhere in the middle of it's movement range.

I'm still open to other ideas, though, or additional support for my theory, either way.
 

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Discussion Starter #6
Update: I found that if I disconnect both Engine Coolant Temperature sensors, the hesitation is reduced, but not eliminated. Does anyone have any idea why this would be the case? It's a substantial improvement.

I also tried unplugging various other things, one at a time:

-- MAF sensor: no effect on the problem
-- VTEC solenoid: no effect on the problem
-- EGR valve: no effect on the problem
 

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Discussion Starter #7
Update: OK, I finally have a scanner now (Foxwell NT520). I have ruled out the Throttle Position Sensor, as it shows good function on the graph via the scanner. The scanner is showing misfires on all four cylinders under part throttle (only). Long Term Fuel Trim is + 0.9%. What else should I check with the scanner? There is no CEL and no codes.
 

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Discussion Starter #9
Solution: It was the plugs! I did not realize that most Iridium plugs don't last very long. My original Denso plugs from the Honda factory were the "Long Life Iridium" Densos. This past summer, I took all the "new" NGK plugs out to do a compression test, and BAM, it was obvious the Iridium tips were gone. So, I replaced them with my original Densos (which I had stashed away), and the problem was fixed.

It's pretty disappointing that the NGK Iridiums only lasted 22,000 miles before they started misfiring badly. The factory Densos lasted over 108,000 miles (I eventually bought some new Long Life Iridium Densos this summer, even though the factory ones from 2007 were still working). What's crazy is that new Densos are actually cheaper than the NGKs. I definitely won't be buying the NGKs again.

NGK Iridiums with about 30,000 miles on them (misfires started at 22,000 miles and progressively got worse). Only tiny bits of the Iridium tips remain on two of the plugs, and completely gone on the other two:

NGK.jpg



Compare that with the Denso Long Life Iridiums with 108,000 miles on them. Gaps were barely out of spec!:

Denso.jpg
 
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