what makes a 911 surge?

biosurfer1 · 01-25-2005, 07:20 PM

i know from my 914 days that a common cause of surging idle is a vacuum leak, but what else could make a 911 do this? it isnt a very big surge, maybe 2-300 rpms, but i'd still like to get it taken care of.

TIA

ZOA NOM · ZOA NOM's Garage

If it's a motronic, try the CO mixture, idle control valve, or airflow meter. I had all three in various stages of failure. The airflow meter had a worn spot on the resistive material inside that provides the reference signal to the DME. The idle control valve can get gunked inside and the butterfly can stick.

Early_S_Man · 01-25-2005, 10:01 PM

Sure would be a big help, and time saver ... if you included the year of your car and engine [in case of a transplant] in your signature or post!!!

ESP isn't working, and your question is just to vague without more info!

dickster · 01-25-2005, 10:08 PM

i'm guessing its a DME car?

aigel · aigel's Garage

It's a 79 SC

http://forums.pelicanparts.com/porsche-911-technical-forum/203093-fan-belt-question.html#post1725668

The mixture may be too rich, if it surges coming off throttle. You drop it down from about 3000 rpm and it will dip down to 500rpm, like it would like to stall?

In any case, adjust the mixture, check for vacuum leaks and take it from there.

George

Paulporsche · 01-26-2005, 06:23 AM

Second a too rich mixture. Try turning the screw accessible @ the top of the mixture control unit counterclockwise about 1/32 or 1/16 turn. That just might do it. Let us know.

Superman · 01-26-2005, 06:41 AM

That would not be my guess. In my experience, surging almost always is a too-lean mixture. If an engine is way too rich, it will lack power and can also stumble, cough, that sort of thing, particularly coming off the throttle, as George says. But surging....where the car subtly accelerates and decelerates when it should be cruising smoothly.....has been a lean mixture in all of my experience. That's why vacuum leaks cause this. They're creating a lean running condition.

Superman · 01-26-2005, 06:45 AM

But, umm Warren may come back with some more specific things to look at. If a car suddenly starts running lean, it's probably not because the mixture adjusting screw has moved. Probably, a component is failing or something. So, just turning the mixture adjusting screw might be trying to make a right out of two wrongs.

With CIS cars, you can always just do the step-by-step, and that's what I recommend. First make sure your ignition system is working perfectly. Cuz that's the easiest. Then follow the troubleshooting steps in a book about Bosch CIS FI systems. You'll need a fuel pressure tester and a sniffer. That's the REAL way to find and address engine tuning problems.

Paulporsche · 01-26-2005, 07:13 AM

Biosurfer,
As you can see from the replies, it could be a number of (contradictory) things. We need a better description. Are you getting a rhythmic fluctuation in idle speed? Are you getting the drop in speed as you come to a stop after driving, or do the revs drop then swing back up after you have revved the engine, as George mentions? If that is the case then your mix is probably too rich. If your car surges while cruising it could be either too lean (usually) or too rich (sometimes).

Superman,
You are certainly right in suggesting a systematic diagnostic approach.I would never take issue w/ that. As you know, quite often, though, these things can be solved w/ something like the simple screw adjustment.
If that works, great. Very little time lost and it's back to driving. If there is no change, the screw can be put back in a matter of seconds, and it's back to diagnosis.

In this instance, the poster said his revs changed @ idle, not while cruising.

jluetjen · jluetjen's Garage

I second the lean cause for surging. One of the symptoms is that as you close (back off) the throttle, the car surges forward a little like if it is briefly making more HP. What's happening is that as the throttles close, rather then causing a brief rich condition as the air is cut off the engine actually picks up HP as the mixture transitions from lean across the ideal and then settles back to being lean at the new throttle setting.

biosurfer1 · 01-26-2005, 07:25 AM

i'm gunna have to get back to all of you, i think there are too many things that still need to be adjusted after the repair work, i realized yesterday a few things that were not adjusted back to normal once everything was put back together. thanks for the suggestions, i'll let you know how it runs once things are tuned...