Jerky deceleration

[R. Aster]

Hi guys, a little expertise if you don't mind. On my almost 100% stock 89 3.2 Targa (41k miles), I get these symptoms:

Very jerky motion (like a horizontal yo-yo) sometimes on decel (say, in 2nd) with foot off throttle from 4k down to 1500
Seems a little worse when air and engine are cool / cold
Worse when the AC is on
Some popping
A little jerky at low RPM constant speed, but drivable
Very occasionally hunts at idle from normal 900 up to 2000
Accelerates very well and runs well at cruising speed

More data:

If I pop off the oil cap to force it a little leaner, it seems to help somewhat.
It does do it from complete cold start, so does that mean it can't be the O2 sensor on an 89?
This car was just chipped (SW standard), but it did it somewhat before. Perhaps a little worse now.
Feels like something in the drive train has got some slack, contributing to the yo-yo. What would that be, guys?
Maintenance has been kind of desultory on this car, which I would like to blaim on the PO, umm, except I am the OO... Have not had the 30k maint done yet, so IIRC rotor, cap, plugs, wires, are original. Oil and air filts have been changed regularly.

Based on reading here, I'd guess it's running a little rich and the drive train slack is emphasizing the problem, so,

- Does that sound right?
- What would need R&R in the drive train?
- I read my Bentley's and comments here, but still not clear if there is any mixture adjustment that the DME won't just override
- I should check the head temp sensor, right?

I don't mind a shop trip but have not been very delighted with the dealer or the couple local shops I tried. If someone's got a good shop in MD near DC, I'm all ears.

Thanks.

RA

[dickster]

sounds fuel related to me - leaky injectors?

[R. Aster]

Originally posted by dickster
sounds fuel related to me - leaky injectors?

Hi Rich,

How would I diagnose this, or is this a shop kind of thing?

Thanks,
RA

[Paulporsche]

Sounds like your diagnosis of too rich is right.

Unfortunately I don't know much about your fuel injection. Do you have an O2 sensor? These can go bad on the SCs , causing a too rich condition.

[john walker's workshop]

the clutch disc's rubber or spring hub may be an issue.

[R. Aster]

Originally posted by john walker's workshop
the clutch disc's rubber or spring hub may be an issue.

Yup, that's just what it feels like, something winding up and releasing. Clutch has quite a bit of chatter last 10k or so also. But I assume since a clutch is, what, an $1100 job, I should just drive until failure -- not worth it to pull the engine and look at this on the chance it might be failing, right, since that would be $600 or so right there? (JWW, I am aware that lots of things can cause / cure chatter from your other excellent posts.)

I guess I'll work on the engine side of this issue this weekend -- see if I can find a fault with the usual mixture suspects (leaks, O2 and CHT sensors, AFM).

Paul, thanks for the diagnosis validation. You'll be receiving my copay in the mail

RA

[dickster]

what mileage have they done?

i recently found out injectors should be replaced on 150k km.

popping
jerky at low/constant rpm
hunting

thats what made me think fuel related.

[DinFtown]

I know I'm running a completely different set up.... 2.7CIS, but I had the same problem right at 2K rpm on decel I would get a real bad jerking. I had a crack in a rubber boot that was sucking air in after the TB. It was making the car run way lean on decel. It would cause light backfire, idle hunt, and the jerking. I know I running CIS, a completely different system, but just check to make sure you have no air leaks. The CIS is very picky because it is unable to monitor the burned/unberned fuel so it is unable to make corrections. Any air leak, even little can cause a poor running setup. Your O2 sensor will monitor your fuel to air ratio by the unburned/burned oxygen in your exhaust and computer should correct the fuel output for the air leak, but it will not go to far off it's fuel map in relation to your TPsensor or else with a air leak you car would never idle down. Just use a can of starting fluid at idle, and spray around you intake manifold if you find a area that causes the idle to pick up, then you are sucking air in the intake in that area. That is a cheap, and easy way to check. A shop can do a smoke test for you, that makes it easy to see right where the leak is. A few gaskets later and your golden.
Good luck

[R. Aster]

Din,
Yah know, occurs to me after reading your note that perhaps I have an air leak as you guess and that the O2 / DME sense the lean state on decel and try to get it richer and an oscillation sets in. Certainly seems the engine is changing states rapidly causing the jerky effect. I'll try the starter fluid check and also do a visual.

Thanks. Also, I see you up the road from me. Found any good shops near us?

[john walker's workshop]

try the starter fluid if you want a nice flash of flame in your face.

[R. Aster]

Originally posted by john walker's workshop
try the starter fluid if you want a nice flash of flame in your face.

Well, I'd prefer not to lose my eyebrows, beard, and perhaps nose hair... Is there a safer way?

[Paulporsche]

Do a search re propane and vacuum leaks.

[R. Aster]

PP,

Yeah, did that. So the concensus is that propane is safe?

RA

[john walker's workshop]

safe as any flammable gas around a potential leaking plug wire.

[DinFtown]

I have used starting fluid countless times, and never had a problem. Don't go run your car so it is red hot then do it, I mean use some commen sence about it. If you are leaking spark out you wires, well then that is sad, and you might want to fix that. Like I said the only way to do it is with something flamable... du. do not use propane(although propane is a great way to clean all the carbon deposites out of your motor. I use fumigation on my cummings, it's great) it will sink in air, and sit in your engine compartment making for a nice setup for a explosion, the starting fluid is a liquid, and it evaporates(the flamable part) the evaporation goes up into the air out of you engine compartment. Just use starting fliud, you do not need much, it is very noticeable when you spray near a leak. If you are still worried about it, take it to you mechanic and have him perform a smoke test, or if you have a smoke machine just use that(it is alittle more dificult on a 911 motor but it works).
Dave

[Paulporsche]

Sounds like smoke is the ticket.

[Jetscape]

RA

Came across your thread searching the forum with same problem. Where you able to revolve your bucking issue?

My 88 runs great. My only issue is at low rpm 3k and below when decelerating. Car jerks/buck intermittently.

Paul T.

Originally Posted by R. Aster

Hi guys, a little expertise if you don't mind. On my almost 100% stock 89 3.2 Targa (41k miles), I get these symptoms:

Very jerky motion (like a horizontal yo-yo) sometimes on decel (say, in 2nd) with foot off throttle from 4k down to 1500
Seems a little worse when air and engine are cool / cold
Worse when the AC is on
Some popping
A little jerky at low RPM constant speed, but drivable
Very occasionally hunts at idle from normal 900 up to 2000
Accelerates very well and runs well at cruising speed

More data:

If I pop off the oil cap to force it a little leaner, it seems to help somewhat.
It does do it from complete cold start, so does that mean it can't be the O2 sensor on an 89?
This car was just chipped (SW standard), but it did it somewhat before. Perhaps a little worse now.
Feels like something in the drive train has got some slack, contributing to the yo-yo. What would that be, guys?
Maintenance has been kind of desultory on this car, which I would like to blaim on the PO, umm, except I am the OO... Have not had the 30k maint done yet, so IIRC rotor, cap, plugs, wires, are original. Oil and air filts have been changed regularly.

Based on reading here, I'd guess it's running a little rich and the drive train slack is emphasizing the problem, so,

- Does that sound right?
- What would need R&R in the drive train?
- I read my Bentley's and comments here, but still not clear if there is any mixture adjustment that the DME won't just override
- I should check the head temp sensor, right?

I don't mind a shop trip but have not been very delighted with the dealer or the couple local shops I tried. If someone's got a good shop in MD near DC, I'm all ears.

Thanks.

RA

[R. Aster]

Paul,

Well, crap. Saw this when you posted then got distracted and here we are months later. Probably don't make no never mind as (1) my Targa stopped doing this at some point but (2) I do not recall when and (3) I do not know what fixed it. Pretty helpful, eh?

I got a great mechanic shortly after this post and he did lots of catch-up maintenance and the issue was somehow fixed. I do not recall a new O2 sensor or other obvious fixes that would change the mixture. I have not had to do anything to the drive train. I did un-install the Steve Wong chip at some point (I was getting ping, surprisingly) but I do not recall that that was proximate to this issue. The car is now about 15K miles further down the road and runs great. (Targa top is up at Dan Dan Petchel's place in NJ and due back this week after a complete refurb so getting quite excited here.)

So, apologies that I am no help on the issue and that I have been a poor Pelican citizen. Did you get your issue resolved?

Regards,
RA

[ant7]

Had Similar issues with my 3.2, see below link to a thread I started on it a while back.
Hope it helps

https://forums.pelicanparts.com/porsche-911-technical-forum/1124854-3-2-afm-inspection-re-set.html

[jlex]

I have an '88. The '85 I had before that had the bucking issue at deceleration, but it went away after installing a Steve Wong chip.
On the '88, I had no bucking issue to begin with, but decided to install the SW chip for the added performance. That's when I had a bit of bucking.... not severe, but noticeable after not having the problem w/o the chip. Emailed Steve Wong about it and he said maybe I have a vacuum leak. Checked for a leak, but found nothing.... so... I decided to simply reinstall the original chip. Bucking gone. Must have something to do with the timing which I suppose is what the chip modifies. Would rather have a smooth ride vs better performance. Is funny that it was a cure on the '85, but caused a bit of a problem on the '88. Weird. Anyone out there want to buy a barely used standard SW chip?