What Causes Engine to Surge or pulse very Slightly?
#1
Track Day
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Join Date: May 2004
Location: Centennial, Colorado
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What Causes Engine to Surge or pulse very Slightly?
The car at idle is very regular and steady. No ups and downs.
But when I am driving steady at 3000 rpm plus or minus, I can detect a faint surge or pulsing of extra oomph or gas. This is on very smooth roads so it's not that.
It is very faint but, I'd rather it not be there.
We did replace the oxygen sensor just in case it was getting lazy.
I also do detect the briefest bit of hesitation under start up of harder acceleration.
It's a CIS 3.0 litre SC upped to 3.2 litre. The engine was just rebuilt 4k and 4 months ago.
Thanks for any thoughts on what might be going on.
I should just get PMO carbs and then I would be in the clear right?
Robb Williamson
Centennial, Colorado
But when I am driving steady at 3000 rpm plus or minus, I can detect a faint surge or pulsing of extra oomph or gas. This is on very smooth roads so it's not that.
It is very faint but, I'd rather it not be there.
We did replace the oxygen sensor just in case it was getting lazy.
I also do detect the briefest bit of hesitation under start up of harder acceleration.
It's a CIS 3.0 litre SC upped to 3.2 litre. The engine was just rebuilt 4k and 4 months ago.
Thanks for any thoughts on what might be going on.
I should just get PMO carbs and then I would be in the clear right?
Robb Williamson
Centennial, Colorado
#2
RL Technical Advisor
Hi Robb:
Those kinds of hesitations are caused by an incorrect fuel mixture.
SC's equipped with lambda controllers (oxysensors) run notoriously lean at part throttle and I'd guess that given your displacement increase, thats exacerbated the inherent problem.
In my experience, these cars run much better in open-loop (oxysensor disconnected) and the CO set manually at the adjusting screw to 2.0-2.5%.
Without question, a properly setup and jetted set of PMO's will make that SC REALLY go, but you may have troubles passing emissions testing (if that applies). We've had good luck with things like that, but your fuel mileage will suffer a bit.
Those kinds of hesitations are caused by an incorrect fuel mixture.
SC's equipped with lambda controllers (oxysensors) run notoriously lean at part throttle and I'd guess that given your displacement increase, thats exacerbated the inherent problem.
In my experience, these cars run much better in open-loop (oxysensor disconnected) and the CO set manually at the adjusting screw to 2.0-2.5%.
Without question, a properly setup and jetted set of PMO's will make that SC REALLY go, but you may have troubles passing emissions testing (if that applies). We've had good luck with things like that, but your fuel mileage will suffer a bit.
#3
Track Day
Join Date: Jul 2005
Location: Nantes, France
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the C.O. rate in france is about 1-1.5%.... but there are no catalyse !!
look also the spark plug, beam ignition.
for check beam ignition, run the motor in the dark and look for sparks near it....
may be stiff injector too.....
A+
look also the spark plug, beam ignition.
for check beam ignition, run the motor in the dark and look for sparks near it....
may be stiff injector too.....
A+
#4
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Join Date: Aug 2004
Location: Indianapolis, Indiana
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Robb,
After throwing parts at mine it ended up being the fuel accumulator. Fuel Filter some help, Fuel Frequency Valve more help, and then the Fuel Accumulator finished the job. My 911sc screams! There is a combo on e-bay now, but, I'd buy new. Start with the Fuel Accumulator if you want to save cash.
Good Luck,
Ron
After throwing parts at mine it ended up being the fuel accumulator. Fuel Filter some help, Fuel Frequency Valve more help, and then the Fuel Accumulator finished the job. My 911sc screams! There is a combo on e-bay now, but, I'd buy new. Start with the Fuel Accumulator if you want to save cash.
Good Luck,
Ron
#6
Instructor
Howdy Robb,
The board's gonna think I'm nuts but this falls into the 'it really happened to me!' category. My '86 3.2 Carrera began to exhibit a throttle pulsation at speeds above, oh, 20mph or so. At higher speeds it became worse and was actually accompanied by a noticeable pulsation of the throttle pedal.
End result was that the left inner CV boot had split and after sufficient rotational speed was reached it created enough centrifugal force to fling a portion of the CV boot up against the throttle rod that runs just above the CV joint. One pulse for each impact of the 'flap' that was sticking out.
Weird but true. A goog hint was that the pulsation increased with the rotational speed of the axle.
Neat huh? Probably not your issue but I just had to share this... 8-)
Barry
The board's gonna think I'm nuts but this falls into the 'it really happened to me!' category. My '86 3.2 Carrera began to exhibit a throttle pulsation at speeds above, oh, 20mph or so. At higher speeds it became worse and was actually accompanied by a noticeable pulsation of the throttle pedal.
End result was that the left inner CV boot had split and after sufficient rotational speed was reached it created enough centrifugal force to fling a portion of the CV boot up against the throttle rod that runs just above the CV joint. One pulse for each impact of the 'flap' that was sticking out.
Weird but true. A goog hint was that the pulsation increased with the rotational speed of the axle.
Neat huh? Probably not your issue but I just had to share this... 8-)
Barry
#7
Track Day
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You Guys Rock!
Guys,
I got sucked into a firedrill at the office, but I wanted to get back as soon as I could and thank you guys for the GREAT feedback.
I will talk to the mechanics at the shop and get it dialed in next week.
I am trying to get a dyno chart here in Denver for the upgraded motor.
I couldn't be happier except for the Very slight pulse.
I can't even imagine adding carbs or a bigger engine because this engine in the SC with a short gearbox and other extras is just plain LOTS of fun.
Thanks again. I'll let everyone know what I find out.
Robb Williamson
Centennial, Colorado
I got sucked into a firedrill at the office, but I wanted to get back as soon as I could and thank you guys for the GREAT feedback.
I will talk to the mechanics at the shop and get it dialed in next week.
I am trying to get a dyno chart here in Denver for the upgraded motor.
I couldn't be happier except for the Very slight pulse.
I can't even imagine adding carbs or a bigger engine because this engine in the SC with a short gearbox and other extras is just plain LOTS of fun.
Thanks again. I'll let everyone know what I find out.
Robb Williamson
Centennial, Colorado
Trending Topics
#9
As mentioned, the frequent cause of surging is the fuel mixture setting.
Another very common source of surging, which is inherent, is the close-loop Lambda
system as the O2 sensor corrects for AFR (air fuel ratio) values. A value of 50 to
100 RPMs variation (surging) in idle is common.
Another very common source of surging, which is inherent, is the close-loop Lambda
system as the O2 sensor corrects for AFR (air fuel ratio) values. A value of 50 to
100 RPMs variation (surging) in idle is common.
#11
Instructor
Robb,
I've had this very similar problem, sounds the same!
I never did get it fixed, even after changing all the recommended things.
Search for my old message "Strange engine problem when driving" if you want to see the advice given to me...
Steve Weiner from Rennsport Systems was quite helpful here.
Cheers,
Justin
I've had this very similar problem, sounds the same!
I never did get it fixed, even after changing all the recommended things.
Search for my old message "Strange engine problem when driving" if you want to see the advice given to me...
Steve Weiner from Rennsport Systems was quite helpful here.
Cheers,
Justin
#12
I recall a John Walker post on the Pelican board that dealt with this subject. The mixture adjustment screw that we all know and love is for the most part an idle mixture adjustment. The WUR has a factory adjustment screw that adjusts the springs inside for throttle up pressures. I think John felt there was a strong possiblity that DIY'ers could really muck up the pressures playing with it but he felt it was the needed adjustment for a clients car that surged.