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Discussion Starter #1
This one ought to be fun. It's the last of my "fix 'er up" quests since I got it. There has been this whomping sound from the engine, accompanied by bucking. It happens at any speed in any gear, but only if the RPMs are between 1300-1700. I first suspected something to do with the clutch assembly, but if it's in neutral and I rev it up, as it falls down, it makes the "whomping" noise as well once it hits that mid-1000's spot. As I said, it happens in neutral or in any gear, so I'm suspecting engine issues. Also, the lower the gear, the harder the bucking is. 1st gear feels like a rodeo and 5th gear is almost unnoticeable. Ideas?
 

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Do you still have the factory catalytic converter installed? It could be clogged, although I think that would impact the car across the entire RPM range, though.

If you know you are getting proper fuel and spark, might be the AFM contacts are not getting a good connection at a certain part of the sensor sweep, or possibly the throttle position sensor contacts have some carbon on them.
 

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I'm not clear yet - is the problem that the engine accelerates and decelerates quickly, making the car "buck"?

If so, I'd be looking at the fuel system. A stray thought - is the accelerator linkage free of obstruction and binding? No cruise control cable hangups? (Think simple first...)
 

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Discussion Starter #5
No obstruction or weird issues like that. The problem isn't when accelerating and decelerating quickly, it's when you drop through 1300-1700 RPMs at any speed. It's only that range.
 

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First guess is a dead cylinder. Since it isn't constant, go over all your ignition components, including plugs and timing. I'd also check the cam timing.
 

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Dead cylinder would show up in a few ways, missing and ragged idle. Compression test would answer that question in a matter of minutes. Without video mostly wild guesses.
 

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Discussion Starter #9
I attempted to get a video but the sound is very low pitched and the tach doesn't give the bucking feeling much justice (as in it doesn't move too much) so it would sound like normal to you guys. It starts and idles perfect, in fact it idles so smooth that you would think it's brand new. It's just when it's decelerating between that RPM range when it acts up. I also forgot to mention that it is most prominent when the car is hot. After looking at a few similar posts I'm thinking maybe it's the TPS, since when I give it throttle in that SAME 1300-1700 range it accelerates totally fine? The TPS says "JECS" on the back, so I'm wondering if this is an OEM part and it's getting to the end of its useful life.
 

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Do a leak down on all 4 cylinders to rule out any major issues.
TPS would usually create an unstable and High idle because it affect the Timing in Fuel injected cars.
If it’s on deceleration perhaps O2 sensor?
 

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Discussion Starter #12
Just changed the O2 sensor a few months ago with a high-quality unit. Just pulled it out of storage again today so I will be taking the TPS apart tonight to see if there's any corrosion.
 

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Discussion Starter #13
TPS looked a bit oxidized. Even after cleaning it there is no change. Even unplugging it as a whole doesn't change the idle either.
 

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I'll throw this out there. Is the throttle plate or linkage loose at all. "Theory" being with the vacuum on deceleration the pull against the throttle plate might be strong enough to hold it tight until vacuum reduces enough at mid 1000s to let the plate flop a bit causing an air flow distrubance. More noticeable in first gear maybe as the engine is being driven by the rear wheels at a faster revs/sec than in fifth where the blip in air flow might laze through the whomp as the air flow required is less because of lower revs/sec.
 

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Discussion Starter #15
I like that theory.... Unfortunately I ran out of "fun" time this weekend so I'll have to wait for a bit until I can work on it agian. That'll be the next thing I try though. Thanks!
 

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Discussion Starter #17
Ok so I spent today taking apart and working on the throttle body and it was a bit mucked up from what I'm assuming was blowby from the PCV, but nothing even near the amount that would cause me to think that the throttle is getting sticky. Nevertheless, I cleaned everything out. The throttle plate is nice and strong so I don't see any issues there. Next, I checked the coolant head temp sensor and this is where it gets interesting.... I took out the sensor fully expecting coolant to come out right behind it but nothing came out at all. Is this normal for the sensor? Unless I'm misunderstanding its purpose, I thought it would be fully submerged in coolant to... you know... measure its temperature. Also, the sensor connector (white piece with terminals) and the very tip of the sensor that goes into the head appear to be free from the body of the sensor (where the threads are) and you can freely spin them around. Is something amiss here??
 

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Discussion Starter #18
Bump: Changed the CHTS anyway but still get the whomping and bogging every once in awhile. And I emphasize the issue only presents itself once HOT. Any more ideas?
 

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Discussion Starter #20
BUMP: Well bois, I think I have a solution! I gave in and took it down to a shop and after both the mechanic and I poking around in the engine bay, he disconnected the vacuum line to this part (below) and to my surprise, the whomping was almost completely eliminated! Like it was gone! Now the question is, for my good friends here at ZCar, what is this part called and what does it do exactly? We had a general understanding that it probably lets air back into the throttle body but more explanation would be nice. Thanks!
106756
 
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