Jolt when I let off the gas

[Splinter]

Whenever I let go of the gas completely, the car gets a sudden jolt. No idea what causes it. It's sort of annoying, although it doesnt seem to affect drivability or anything.

Ideas?

[rallysam]

normal

[Legacy777]

could be surge caused by the bypass valve not working properly....

[entirelyturbo]

You haven't played with any factory settings under the hood have you (i.e. TPS sensor, throttle body stopscrews, IAC valve)?

[wiscon_mark]

ummm, turbo aside, its probably normal. You can get STi pitch stop mounts to clear it up probably.

[Legacy777]

ummm, turbo aside, its probably normal. You can get STi pitch stop mounts to clear it up probably.

Mind telling me how an sti pitch mount will help? The sti one is plastic, ours are metal.

The motors do not have much if any longitudinal motion.

[Splinter]

You haven't played with any factory settings under the hood have you (i.e. TPS sensor, throttle body stopscrews, IAC valve)?

Nothing

Im not that adventurous yet

[wiscon_mark]

ummm, turbo aside, its probably normal. You can get STi pitch stop mounts to clear it up probably.

Mind telling me how an sti pitch mount will help? The sti one is plastic, ours are metal.

The motors do not have much if any longitudinal motion.

sorry, I didn't say that out of personal experience...I guess I read the information wrong....

Edit: Just had a thought. If the pitch is more flexible, maybe it doesn't jolt the car as much when you let off the throttle....dunno if that'd make the drivetrain more flexible though...

[entirelyturbo]

Well, the reason I asked was because I got too adventurous a few times with stuff like that. I played with the throttle body stopscrews and this and that, and the same thing was happening to me: jerking between slight and no throttle application.

What happens is, the ECU has a set method of cutting the fuel injectors when you let completely off the gas, and it needs proper readings from the TPS and IAC valve in order to do so. Once the TPS sensor tells the ECU that the throttle plate is shut, the ECU then tells the IAC valve to open so the engine can get air. If these settings are played with, the info that the ECU reads is skewed, and it might cut the injectors too early or too late, which will result in jerkiness.

If you haven't touched anything, you might have an IAC valve that's a little sticky, or you could have a faulty TPS. If you want to experiment, get a throttle body and IAC valve from a junkyard car, and mix and match and see if any of those cure the problem.

[All_talk]

Hey Splinter

I had that same problem with that car, some PO had opened the throttle stop screw. I adjusted the screw back so that it just touched with the plate closed (not sure how to properly adjust it) and that fixed it. Odd its returned, like subyluvr2212 said, maybe some more tweaking is in order.

Gary

[THAWA]

ummm, turbo aside, its probably normal. You can get STi pitch stop mounts to clear it up probably.

Mind telling me how an sti pitch mount will help? The sti one is plastic, ours are metal.

The motors do not have much if any longitudinal motion.

It doesn't matter that ours is metal. The STi mount is stiffer. The mount is only as stiff as its softest piece right? Bushings are what cause the main difference. Then there's the added metal inside the bushings. The plastic is also VERY stiff, and lighter. Our mount is not a solid metal either, it's two C-style circles welded to a hollow rod. That doesn't sound stiff to me. STi doesn't make items that don't improve performance in some way shape or form. I can stick a screwdriver through the holes on the stock one and feel the play in the bushings. Can't do that with the STi one.

I bought the mount mostly for the bling factor. I installed it not expecting much, but it's much stiffer. The engine moves slightly less yes, but the tranny is where it feels better to me. Shifter wobbles less, clutch feels SLIGHTLY better. It also increased NVH a lot. The starter sounds louder, the clicking CV sounds like it's banging, the gearbox whines much louder, even the turbo sounds a little louder. Some people say it helps with the jolting/bucking, but I can't tell the difference.

Don't discount it just because it's not metal.

Pics here: http://www.thawa.net/gallery/STi-mounts

[wiscon_mark]

well, I was doubting myself there...thanks for clarifying THAWA

[entirelyturbo]

I had that same problem with that car, some PO had opened the throttle stop screw. I adjusted the screw back so that it just touched with the plate closed (not sure how to properly adjust it) and that fixed it. Odd its returned, like subyluvr2212 said, maybe some more tweaking is in order.

Vikash and I agree on this: Once the throttle body stopscrew has been tampered with, remove the entire throttle body assembly and place it in the nearest trash receptacle. It's no longer good

Best bet would to just swap a JY throttle body on.

[Splinter]

Damnit

What's that gonna cost me?

[All_talk]

Vikash and I agree on this: Once the throttle body stopscrew has been tampered with, remove the entire throttle body assembly and place it in the nearest trash receptacle. It's no longer good

There is NO way I believe that's true, the TB is a simple mechanical device, no magic involved. Are you saying that if I turn the screw 1/2 turn in, then 1/2 turn out so that its back in the precise location it was originally it will never be right again?

All we lack here is the proper procedure to set it... or a close inspection of an unaltered TB (OR a few) to determine the proper air gap. Does the FSM outline this setting?

Gary

[vrg3]

Wait, that's not what I said -- it's what Subaru says!

Subaru does assert that it's not possible for you to recalibrate a throttle body that's been tampered with.

There isn't any "magic" to it, but apparently Subaru uses some very precise measuring and adjusting instruments when calibrating the throttle at the factory.

It's not clear that you could just compare to a factory throttle body to figure it out... but I guess there's no way to really know.

Both my SSs' throttle bodies have been tampered with. I think I have them more or less where they need to be, but I do experience some random driveability problems sometimes. It's not implausible that they could be due to something weird going on with injector shutoff and/or IAC behavior.

[Splinter]

Damnit

What's that gonna cost me?

^

[All_talk]

Wait, that's not what I said -- it's what Subaru says!

Subaru does assert that it's not possible for you to recalibrate a throttle body that's been tampered with.

There isn't any "magic" to it, but apparently Subaru uses some very precise measuring and adjusting instruments when calibrating the throttle at the factory.

It's not clear that you could just compare to a factory throttle body to figure it out... but I guess there's no way to really know.

Interesting, I wonder if it’s calibrated to a certain flow rate? I looked through a PDF version of the ’92 FSM and found no info on setting the TB stop screw or IAC valve, it does appear that Subaru considers these “no monkey” items. I’d really like to eyeball an unmolested TB so I could at least approximate the setting.

The FSM does indicate that the IAC provides “a dashpot function during the time the throttle valve is quickly closed”… this is most like the function behind the jolt.

Gary

[Legacy777]

.....
Don't discount it just because it's not metal.

Pics here: http://www.thawa.net/gallery/STi-mounts

Do you have any other sti mounts besides the pitch mount? With my car and its setup, I just can't see it adding that much. I suppose when I finally get the car running, I could give it a shot and see if it makes any difference.

edit: just looked at the pics. Your mount looks different then mine.

[wiscon_mark]

STi motor and tranny mounts. The tranny is supposed to make the biggest difference. It makes shifter feel better, and stops the tranny from buckling on takeoff, elminating some of that "lag" when you don't gas the crap out it....

[THAWA]

.....
Don't discount it just because it's not metal.

Pics here: http://www.thawa.net/gallery/STi-mounts

Do you have any other sti mounts besides the pitch mount? With my car and its setup, I just can't see it adding that much. I suppose when I finally get the car running, I could give it a shot and see if it makes any difference.

edit: just looked at the pics. Your mount looks different then mine.

Just the pitch stop at the moment. The tranny mount is an 02 WRX mount with about 30k miles, the engine mounts are 90 LS mounts with about 171k miles.

There are two different STi Mounts. There is the ST4100055140, and there is the ST410404S000. I assume you have the first one. That's the older one, they stopped using that somewhere in the GDB era of STi's. The new one has been used since GDB-D I believe. Maybe older. As far as the difference between them, hell if I know. I can't say for sure if one is stiffer than the other, but I'm fairly confident both are an upgrade in stiffness to stock stuff.

[Splinter]

Guys?

Cost?

[entirelyturbo]

Of?

[skid542]

If you're looking for a throttle body I have an extra that has a TPS on it but I can't verify if the TPS works. I haven't touched any of the adjustment screws or anything, just swapped it out with my old one when the new engine idled high and threw TPS, IAC codes but I don't know if those codes were real since I got thrown a TPS code today.

[Splinter]

If you're looking for a throttle body I have an extra that has a TPS on it but I can't verify if the TPS works. I haven't touched any of the adjustment screws or anything, just swapped it out with my old one when the new engine idled high and threw TPS, IAC codes but I don't know if those codes were real since I got thrown a TPS code today.

How much?