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What Causes Engine to Surge or pulse very Slightly?

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Old 08-03-2005, 02:12 AM   #1
Robb W.
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Default 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
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Old 08-03-2005, 03:43 AM   #2
Steve Weiner-Rennsport Systems
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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.
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Old 08-03-2005, 10:28 AM   #3
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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.....

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Old 08-03-2005, 11:00 AM   #4
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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,

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Old 08-03-2005, 11:30 AM   #5
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John Walker on the Pelican board gives the same advice as Steve for 3.0 cars, to disconnect the o2 sensor, but I think John suggests CO of 3.0, which is a little richer.
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Old 08-03-2005, 11:17 PM   #6
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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-)

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Old 08-04-2005, 11:49 PM   #7
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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
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Old 08-05-2005, 03:14 PM   #8
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you pedel linkage, that was one of my problems, also my cat started colapsing. check O2 sensor also
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Old 08-05-2005, 04:09 PM   #9
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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.
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Old 08-11-2005, 06:12 PM   #10
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Originally Posted by Robb W.
Thanks again. I'll let everyone know what I find out.

Robb Williamson
Centennial, Colorado

Hmmm, well...what was it?
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Old 08-15-2005, 06:45 AM   #11
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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.

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Old 08-17-2005, 10:35 AM   #12
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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.
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Old 08-17-2005, 01:41 PM   #13
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If you are still running points, check for excessive play in the Dist. bearings. That was my problem.
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Old 09-27-2005, 03:21 AM   #14
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Did you get it fixed Rob?
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