Jerky throttle

entirelyturbo · Post by **entirelyturbo** » Tue May 25, 2004 3:46 pm

I've had this problem ever since I've had the car. If you're in a low gear (not really a problem above 3rd) and the clutch is engaged, and you let off the throttle completely, no matter how carefully or slowly you do it, the car jerks a bit when you do it. Same for when you get back on the throttle.

I normally like to use engine braking, but I just put the clutch in sometimes because I don't like this jerky feeling. It also can be just a tad dangerous in more spirited driving, sometimes when I'm hot into a turn and get on the gas at the apex, the car jerks and it gets a tad upset. Not smoky drifts or anything, just gets unsettled a bit.

Thing is, I have no idea what it could be. I was thinking TPS for a slight possibility, but I have no codes whatsoever, and I have not touched that sensor ever.

Any other suggestions?

Brat4by4 · Post by **Brat4by4** » Tue May 25, 2004 4:07 pm

more than likely it is the motor and tranny mounts allowing lots of movement. we are talking about your legacy, correct?

THAWA · Post by **THAWA** » Tue May 25, 2004 4:51 pm

I always thought this was a semi normal operation in all cars. I've never driven one that didnt do this to some extent.

vrg3 · Post by **vrg3** » Tue May 25, 2004 5:35 pm

Yeah, almost all cars do it. It's partly due to the EFI system being imperfect. It's just a weird situation when your throttle is going between fully closed and not fully closed, especially when the ECU is in charge of idle speed control and dashpot functionality.

You feel it a lot less in automatics because of the torque converter; I've taught several people to drive stick and had them remark "my car doesn't do that."

I do remember reading a review in Top Gear (I think) of some Peugeot (I think) and one of the things they said was that it had the best fuel injection setup they'd ever experienced. It never hesitated on weird throttle transitions, and never seemed like it didn't know exactly what to do.

Post by **Legacy777** » Tue May 25, 2004 5:45 pm

Yeah I'd definitely agree it's semi normal.

Other thing is the gearing. The lower the gearing ie 4.111 vs the 3.90, it'll probably be more noticable.

the wagon I picked up does the jerky thing. The shifter linkage & clutch are worn to poop....so finessing it is a little difficult to do on a regular basis.

evolutionmovement · Post by **evolutionmovement** » Tue May 25, 2004 6:12 pm

I've learned to drive around it. Strangely, replacing my temp sensor the other day seemed to reduce the let off jerking.

As I understand it, its very difficult to program FI for good acceleration enrichment as it requires thinking ahead. Electronic throttles will probably be able to fix this, but with maybe a lit of lag between gas input and delivery. Its one way well-tuned carbs still hold an advantage over FI.

Steve

entirelyturbo · Post by **entirelyturbo** » Tue May 25, 2004 6:52 pm

If I jumped off the throttle all of a sudden or just jabbed it quickly while in gear, I could understand the jerkiness. But I come off or get on VERY gradually; to try to put it in some sort of perspective, if I were to go all the way to the floor at the rate at which I apply throttle, it would take me a couple MINUTES to get to WOT. I try to apply throttle that gradually and still, the moment the car knows I'm giving it throttle, it jerks forward. I haven't driven any other MT car that jerks this bad...

Also Brat, every mount in my drivetrain is new, OEM liquid-filled motor mounts about a year and a half ago, a WRX tranny mount about 6 months ago.

scottzg · Post by **scottzg** » Tue May 25, 2004 7:07 pm

I always figured it was an awd thing. Every legacy i've driven has had this more pronounced than most cars, my folks camry (92, 5mt 4cyl) hardly does it at all. I figured it was the heavy drivetrain, the cars are otherwise very similar. It wouldn't surprise me if Toyota FI was better designed than subaru's though. Toyota is good about that kind of thing.

I never run into the jerky thing, so when other people comment about it driving my car, it confused me. I slip the clutch when i engine brake in 1st and 2nd without thinking about it. It works pretty nicely.

Post by **Legacy777** » Tue May 25, 2004 9:12 pm

michael, wonder if your tps is not reacting as quickly as it should....

entirelyturbo · Post by **entirelyturbo** » Wed May 26, 2004 12:42 am

I'd kill for a Select Monitor right now

Can I determine that by testing the electronics? vrg3 explained to me the basics of a multimeter, so I do have the ability to test it and make the right decision based on the data I collect...

vrg3 · Post by **vrg3** » Wed May 26, 2004 1:44 am

All the Select Monitor will tell you about the TPS is the signal voltage. You can measure the same thing directly by measuring the voltage between ground and the TPS signal wire (either at pin B58.2 of the ECU, or at the black wire of the TPS).

Or bum a laptop off of someone and build the scantool.

mhrallyteam · Post by **mhrallyteam** » Fri May 28, 2004 3:28 pm

My 92 awd leg does the same, so i investigated. Replaced all tranny mount, no fix. So the most possible thing would be the idle solenoid. It seems to be the cause of the jerking, it would come on and off because of a glitch in the efi.

If that doesn't fix it it's gonna be the rear diff mount. Has anyone tried that

entirelyturbo · Post by **entirelyturbo** » Sat May 29, 2004 12:40 am

I don't think I'm describing the problem accurately. It's not a driveline jerk, my entire driveline feels great. The engine just doesn't know how to respond to gradual throttle application. It automatically reacts as if I just slammed the gas instead of nudging the gas. It is definitely a problem with the throttle somehow or another...

After the Dragon run, I might do some more investigating.

evolutionmovement · Post by **evolutionmovement** » Sat May 29, 2004 7:05 am

Mine's a little eager, too, but you should be able to get it smooth. Is there a kink or something in the throttle cable? Maybe some lube'll fix it.

Steve

1990 Legacy L Owner · Tue May 08, 2007 10:12 am

Has anyone found a real solution to this?

My car does the same thing when I let off the gas. You can feel it way more in lower gears, but it does it in all.

I also had this problem when I had my 63 bug, I ended up puting a tranny strap on it and it stopped happening. I'm thinking that same strap I bought at a VW parts store would work just as well if it's actually the problem. I thought I read that somebody else had done the same thing on this forum, but I couldn't find it.

BB · Post by BB » Tue May 08, 2007 3:05 pm

Idle control is an automated system coupled to an engine, which is connected in turn to engine/trans/diff mounts which act essentially as a 'spring/damper' system. Unfortunately IMHO, Subaru didn't foresee the two systems not getting along - which they don’t. 1st gen legacies are some of the worst 'bucking' cars I've ever driven.

The bucking coming off/on throttle is normal, I believe there is nothing wrong with your sensors or your ECU. To make this go away, you need to change the parameters of the IAC (which you cant), change the mass of the car (which you cant really), or change the stiffness or damping characteristics of the mounts, which will help, but may not totally fix your problem. All my mounts are shot, and I plan to put in some rubber engine mounts instead of the fluid villed viscous type, a filled in trans mount and new rear diff mounts when the new motor goes in.

Basically, slip the clutch, or just don’t drive at low RPM and let that boxer howl.

BB

If you want to know more go to:

http://en.wikipedia.org/wiki/Vibration

and scroll down to “Forced vibration with damping”. That’s what I’ve got in my head when my car bucks.

IronMonkeyL255 · Post by **IronMonkeyL255** » Tue May 08, 2007 4:20 pm

I was in a Saturn a few days ago that bucked SO freaking hard. When I fully let out the clutch in first, however I tried it, the bastard would buck like no tomorrow. I had to let off of the gas and wait for it to calm down in gear before going.

I swear I don't suck at driving a stick. I may not be the best, but I don't suck.

AWD_addict · Post by **AWD_addict** » Wed May 09, 2007 1:04 am

evolutionmovement wrote:Strangely, replacing my temp sensor the other day seemed to reduce the let off jerking.

I've noticed an increase in the bucking/jerking since my car has been having cold starting problems. I think it's the coolant temp sensor, which would have something to do with the fueling I guess.
Since replacing your sensor helped the jerking, the relation makes more sense.

evolutionmovement · Post by **evolutionmovement** » Wed May 09, 2007 1:58 am

BB - I never slipped the clutch - I just let it move off on torque at around idle speed and rolled on the gas with no issues. I don't have a pitch stopper attached either. I also regularly let the car drive along at idle speed or below (by lightly applying the brake) in various gears in traffic and I never looked like a learner driver. Maybe it's just the transmission and motor mounts.

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Jerky throttle

Jerky throttle