Whatdoyacall that thing which older engines do when you take foot off gas?

[keeweechris]

This is a random question about something I've noticed in a lot of older engines... might take a bit to explain though...

You drive a new car (manual), and everything feels tight. You accelerate, then take you foot off the gas, and the car gently stops accelerating and deaccelerates.

You jump in your old pile-o-shite and try the same hardcore move, and you get a lurch as the engine goes from driving the car, to the car driving the engine. It starts deaccelerating, so you give it some gas. It lurches again, as the engine starts pushing the car again.

I've noticed this with all 10 year old cars I've owned (Ford Laser, Mitsi GSR, and my current WRX). It must have a name or something. And more importantly it must be caused by something.

Is it simply caused by the play in the drivetrain? (Gearbox, diff gears). Funny thing is, I did an engine oil change the other day, and for the first few weeks the problem almost went away! Now its back, but seems new oil has something to do with it.

Anyone else know what I'm talking about? Hard to google this one, as I don't really know the right words to describe it.

Chris.

[Guest boostcut]

fu ked engine mounts lol?

[Guest boostin]

Old cars shut off all injectors at once when you lift off the gas. Newer cars drop injectors one by one to soften the driveline jerk/snatch/whatever other sexual term you can come up with for it.

Plus older cars with have worn bushings etc and that won't help.

[Guest boostcut]

oh and new cars usually have a diaphragm that doesnt let the throttle plate slam shut, it slowly closes to stop the jerkyness from weak asian knees

[ReubenH]

My GTB does the whole jerking from accel to decel thing too, sometimes. But that was after i put the STi ecu in, i think the V3 STi ecu's like doing that sorta thing... It gives the feeling the car only likes to be on or off, and doesn't care how it switches between the 2 :

[keeweechris]

Yeah, thats the one. On or off. Not great for passenger comfort. Can counter-act it by clutching everytime I change speed, but thats a full lower body workout.

I hear all those new-car tricks (sequential inj shutoff, etc) but this is an aged engine thing I think.

[Guest]

I'v noticed it as well, every Friday afternoon traffic from Manukau to Torbay nth shore, a whole 2hours of it! lol

It is on or off, there's no smooth accel or decel in 1st gear in an RA with lower springs, the suspension does'nt absorb any of it......i feel it in the neck once i get home, haha.

[ReubenH]

If i'm trying to be smooth, i simply be smooth on the gas pedal. It's quite a mission in rush hour, while trying not to kill your right leg to death... Still, it doesn't bother me... much.

[log1call]

Misadjusted TPIs cause those symptoms, or TPIs that are out of adjustment because of wear.

I'm not saying it's the only thing, but it can cause it... Simple to check too.

[keeweechris]

TPI's? What tisit?

[Guest]

 log1call said:

Misadjusted TPIs cause those symptoms, or TPIs that are out of adjustment because of wear.

I'm not saying it's the only thing, but it can cause it... Simple to check too.

how?

[Guest]

My GTB does that. I figure it is me not driving smoothly enough + all that drivetrain rumbling around having little bits of slack in it.

I can generally avoid it with careful throttle control.

[keeweechris]

 log1call said:

Misadjusted TPIs cause those symptoms, or TPIs that are out of adjustment because of wear.

I'm not saying it's the only thing, but it can cause it... Simple to check too.

Hey Log1call, what's a TPI?

[funkytown]

 keeweechris']

[quote name='log1call said:

Misadjusted TPIs cause those symptoms, or TPIs that are out of adjustment because of wear.

I'm not saying it's the only thing, but it can cause it... Simple to check too.

/quote]

Hey Log1call, what's a TPI?

TPIs or TPS is the throttle position (Input) sensor.. sits on the manifold on the side of the throttle body. An out of range or badly adjusted one makes things real jerky and can even cause surging (like mine currently is) if the voltage is flucuating

[Guest keltik]

Mine does it, ten times worse with the air con on. I put it down to my auto ECU and shot transmission mounts.

[Guest boostin]

They all do it, so that makes it a feature, not a fault.

[log1call]

Keeweechris, the TPI is, as has been said, the throttle position indictor and is on the side of the throttle body. They have a variable resistor which indicates to the ecu how much throttle you have on and if that is playing up it can cause faulty mixtures, particuly as you are accelerating because they use the speed of throttle movement to calculate out flat spots in the mixture that could be caused by the lag between air going past the air flow meter and it's getting into the engine. That's one part of the system.

There is another part though, and it's this part that generaly causes the hesitation/surge effect as you ease the throttle off idle. There is a set of contacts in the TPi that close as the throttle goes right back to idle. That signal goes to the ecu and tells it to set the ignition timing to a fixed setting. They do that to prevent the O2 sensors feedback causing surging which would then cause advance of the ignition which would cause reving which would cause more fuel which would cause more advance till the revs got too high then the whole process of feedback of the mixture and advance would reverse and cause a slowing down to almost die-out. So... they fix the timing! If the contacts in the TPI are not adjusted correctly they can cause either no advance till you open the throttle quite a bit, at which point the ecu suddenly decides to advance the hell out of it to catch up, or, the contacts are open at idle and you have the surge described.

Two simple processes, but a shit load of explaining. Hope it makes sense. Ask questions if it doesn't!

[Guest keltik]

What i find interesting/annoying/silly is how my car will drive fine with no Map sensor, TPS, Idle controller and intake air temp connected. Yet if one of those sensors is misbehaving but still connected its a dog.

Same goes for a mates old non turbo legacy. Was a right bitch and would cut out randomly when accelerating due to a nakkered AFM. Unplugged it and the car was perfectly drivable for a couple weeks till he could afford a new one.

Another thing which will cause this nasty jerkiness getting onto the throttle i think is badly adjusted timing on a older carbed car

[log1call]

Yes about the old carbied models.

If the Afm is faulty they use the revs and TPI to calculate mixture.

If the TPI is faulty they use revs and AFM to calculate mixture.

I thought they would start, falter and then die with neither attached. Live and learn.

[keeweechris]

 log1call said:

Keeweechris, the TPI is, as has been said, the throttle position indictor and is on the side of the throttle body. They have a variable resistor which indicates to the ecu how much throttle you have on and if that is playing up it can cause faulty mixtures, particuly as you are accelerating because they use the speed of throttle movement to calculate out flat spots in the mixture that could be caused by the lag between air going past the air flow meter and it's getting into the engine. That's one part of the system.

There is another part though, and it's this part that generaly causes the hesitation/surge effect as you ease the throttle off idle. There is a set of contacts in the TPi that close as the throttle goes right back to idle. That signal goes to the ecu and tells it to set the ignition timing to a fixed setting. They do that to prevent the O2 sensors feedback causing surging which would then cause advance of the ignition which would cause reving which would cause more fuel which would cause more advance till the revs got too high then the whole process of feedback of the mixture and advance would reverse and cause a slowing down to almost die-out. So... they fix the timing! If the contacts in the TPI are not adjusted correctly they can cause either no advance till you open the throttle quite a bit, at which point the ecu suddenly decides to advance the hell out of it to catch up, or, the contacts are open at idle and you have the surge described.

Two simple processes, but a **** load of explaining. Hope it makes sense. Ask questions if it doesn't!

Thanks heaps man! So to save pissing around, I'd be doing the car a favour to replace its 200k TPI sensor. Do you happen to know how much this fancy var resister costs?

[Guest boostin]

Get it tested before you replace it.