The Hurky Jerky's

[ivangene]

F!!!!
I have been chasing this for a while and read a lot of posts..Cory for one (tippy) and know there is an answer but son of a B!!! I am not liking this -

idle's a little rough, not bad
Power to boot...she goes like a raped ape
but 2500 rpm slightly on gas cruise is horrid!

so far, new plugs, cap, rotor, wires, valve adjust, CO multiple times on a machine (after each change) compression and leak down looked fair to good, injectors all fire but have not check spray pattern, new chip, MAF (remove AFM) another chip...back to stock AFM and stock chip, cat delete, new muffler, check and adjust idle stabilizer, IDK probably a few other things....checked micro switch, used break out box on DME checked all inputs.....

I WILL GET THIS!!

so this is more of a venting thread...

Think I will change the injectors next!!....maybe a vacuum leak though I have looked! - fuel filter is relatively new, fuel pump and DME relay are new have not checked fuel pressure...

anyhow its a warm day so the car is clean....done for now - maybe

[Steve@Rennsport]

Hi Ed,

I'd love to help you, however I would need a lot more detailed information about how the car runs at light throttle and cruise than simply "horrid",...

Barring that, I'd have the injectors professionally tested and cleaned, look for any small vacuum leaks as that's common on these cars with some miles on them, and clean the grounds on the left side intake manifold.

[ivangene]

Thanks Steve...

Horrid:
seems to come on and off throttle abruptly causing a jerking motion rather subtle sometimes and rather violent others. Seems worse on a slight down hill where I am just on the gas enough so it is not using compression to slow down the car...rolling.

coming off throttle I get back fires on a semi regular basis

maybe "missing" at cruise its tough for me to know since I dont have any other 911 driving experience to compare to

[Josh D]

Sounds like timing too advanced or a vacuum leak.

[john walker's workshop]

broken rubber clutch hub? unplug O2 sensor and see if it's better. any dead or near dead holes at idle? a lower intake gasket being sucked inward is common, killing a cyl at idle.

[fitchn]

Subscribed, my parents 78 SC also has (what sounds like) this exact problem. So far no luck diagnosing after new injectors, distributor rebuild, plugs/wires etc.

[ben parrish]

Lean condition at idle. Does it lurch when idling along in first? Most likely a vacuum leak somewhere. Use carb cleaner and spray it at the base of each intake runner while at idle...it may not change the idle is the cylinder is "dead" but you should see the cleaner disappear and smell it in the exhaust. If no leak is found at the runners, start spraying it at each vacuum line....you'll
Soon either find a leak or move on to the next set of options.

[Tippy]

No way Ed?!?! You're a victim too?

I am at the point of pulling all of the Motronics off mine and going MegaSquirt.

When mine starts surging, I see the AFR's go very lean and then back to somewhat normal over and over again - maybe what your's is doing too.

PM me, I'd love to talk about this with you. "ischmitz" and "scarcellar" have been knowledgeable and patient with me and my questions.

[Arne2]

Originally Posted by ivangene

Horrid:
seems to come on and off throttle abruptly causing a jerking motion rather subtle sometimes and rather violent others. Seems worse on a slight down hill where I am just on the gas enough so it is not using compression to slow down the car...rolling.

coming off throttle I get back fires on a semi regular basis

maybe "missing" at cruise its tough for me to know since I dont have any other 911 driving experience to compare to

Horrid seems a little strong for what I'm noticing on mine. Definitely jerky feeling at low RPM, throttle barely cracked and (as Ed mentioned) not quite under compression. I have no backfires - none at all. Drove it to the coast and back today. (Beautiful weather, great drive.) Not a miss or backfire one.

To me it feels like the a over-sensitive fuel cutoff (like many Motronic systems do under compression) causing the drivetrain to rock badly. I'm wondering if I need new engine mounts.

In any case, subbed.

Originally Posted by Tippy

When mine starts surging, I see the AFR's go very lean and then back to somewhat normal over and over again - maybe what your's is doing too.

That's also a thought. I had a similar issue with a prior Motronic-equipped non-Porsche. Our old 16v Jetta (bought new in '91, gave to son for graduation 19 years later, he still has it) had an episode where if it got started bucking, it was like it had positive feedback and got worse and worse. Turned out it had a leak in the intake boot after the AFM. The engine would rock a bit under torque which would open the leak, the engine would go way lean and lose torque, the leak would close and the engine regain power which would open the leak to allow the engine to go lean and...

I don't know how possible something like this might be, give that (unlike the Jetta) the intake is all mounted to the engine on the Porsches. (Or is it? I need to look into that as well.)

[Tippy]

Originally Posted by Arne2

Horrid seems a little strong for what I'm noticing. Definitely jerky feeling at low RPM, throttle barely cracked and (as Ed mentioned) not quite under compression.

Since day 1 I've owned my car, it has had a small surge/oscillation of RPM at light/part throttle. PO experienced the same thing the entire time he had owned the car too.

All of a sudden, it got really bad to the point it was jerking, then started cutting out, then got so bad it started backfiring.

For another data point for these cars; since I installed the Innovate Motorsports AFR wideband, I've been watching AFR's very intently. One thing I noticed on mine is when you start the car cold and it is running in the 12's range for AFR's, it will briefly go up to 18.XX:1 AFR's and stumble and cough and then go back to say 12.XX:1 AFR's and run fine.

If it is an intake leak from your experience, it should never fluctuate back to a normal AFR IMO - it should always stay lean......at least in my cars since it does not have an oxygen sensor for feedback.

Something goes wonky in these cars.....and I've yet to figure it out........

GL Ed, I feel fer ya

[ivangene]

all good reading....all good!

Cory, I remember when mine did it...I got on the throttle on an onramp and heard a strange noise..a noise I have never heard before, and it sputtered a couple times...since then it has done this....

I changed out the AFM and it was exactly the same

intake leak is high suspect

John's comments have me wondering though...can you elaborate on the clutch and the lower intake gasket

since mine was all of the sudden and made a noise, the clutch thought seems plausible...but how can a clutch effect running - give the the dumbed down version

[ivangene]

for your viewing pleasure.....

my 86 next to a friends new GTS Saturday....
the 993 is another friend, he also brought his 996 Turbo - sucks to be him huh?

[Tippy]

When mine stutters and hesitates, it is running very lean at that point. Goose the throttle, AFR's come down to 12-14 and all is good.

[ivangene]

BTW new motor and trans mounts (club sports)

and the O2 sensor plugged or un-plugged = same smell (an old saying for "same thing")

[Targalid]

Ed,
In the old days this jerking was caused by a carburetor adjusted too lean. Simply richening the mixture screw would make it stop. For this reason, I suspect an air leak giving you a lean mixture with your fuel injection. Possibly, as suggested above, it is intermittent.
Al

[Ruflife]

Originally Posted by ivangene

F!!!!
I have been chasing this for a while and read a lot of posts..Cory for one (tippy) and know there is an answer but son of a B!!! I am not liking this -

idle's a little rough, not bad
Power to boot...she goes like a raped ape
but 2500 rpm slightly on gas cruise is horrid!

so far, new plugs, cap, rotor, wires, valve adjust, CO multiple times on a machine (after each change) compression and leak down looked fair to good, injectors all fire but have not check spray pattern, new chip, MAF (remove AFM) another chip...back to stock AFM and stock chip, cat delete, new muffler, check and adjust idle stabilizer, IDK probably a few other things....checked micro switch, used break out box on DME checked all inputs.....

I WILL GET THIS!!

so this is more of a venting thread...

Think I will change the injectors next!!....maybe a vacuum leak though I have looked! - fuel filter is relatively new, fuel pump and DME relay are new have not checked fuel pressure...

anyhow its a warm day so the car is clean....done for now - maybe

Have the same issue. Pulled fuel injectors and had witchhunter clean them, all ignition parts new, DME relay .. blah,blah,blah, etc. Checked for vacuum leaks - none with carb spray as anticipated but the only 3 things remain. I'm going to replace all the intake gaskets and the plastic risers, ( check ), replace fuel pump, ( check ), then I'll give up and take it to my independant and leave him with my problem. I've got maybe $1200 into this problem but the parts were 25 years old so probably needed to be changed anyway. I have a feeling checking for vacuum leaks at idle or at light throttle is hard to do with the car sitting still and that a vacuum leak at part throttle does exist at moderate/part throttle at the base of the intake runners. All I can say is that you're not alone. Keep searching and checking in between each fix.

[Rick K]

Similar issues in the 84. Ended up being a faulty ICV - tried cleaning, then bought new - which helped a bit. But ultimate solution was replacing the AFM with a rebuilt unit (the wiper track on the AFM was worn).

Good luck!

[ivangene]

Rick, I replaced the AFM with a MAF unit - there was no change. back to stock now, has anyone changed the idle possition switch? Also there is a detailed section in the Bently on setting these things up with a "gap" to the micro switch - the process is a little confusing (like the first clutch adjustment was) but I wounder if there is something to that series of adjustments too...linkage and stuff....

if nothing else its frustrating as all get up - having fun, wish you were here..... LOL

[Tippy]

I really think there is an inherent bug in the Motronics that produces this phenomena. I've replaced EVERY part in my Motronics except the cylinder head temp sensor to no avail. It specs out everytime to the manual so I didn't replace it.

[Tippy]

Originally Posted by ivangene

Rick, I replaced the AFM with a MAF unit - there was no change. back to stock now, has anyone changed the idle possition switch? Also there is a detailed section in the Bently on setting these things up with a "gap" to the micro switch - the process is a little confusing (like the first clutch adjustment was) but I wounder if there is something to that series of adjustments too...linkage and stuff....

if nothing else its frustrating as all get up - having fun, wish you were here..... LOL

Ed, I simply left the screws slightly loose and then pushed the microswitch up until I heard the click and then tightened them down.

You have to have 3 hands to do the manual's method.....