"Surging" at Constant Speed

[porsche0nut]

This is something that I have mildly noticed ever since getting the car, and am curious to hear your responses on.

While driving at a constant speed, I can feel what I think is best described as consistent "surging" every few seconds. Almost as if some invisible hand is giving me a slight push. The rpm needle stays put and does not hunt at all. It's more noticeable in the lower gears, but even along the highway in 5th I can feel it slightly.

Some things that ran through my mind (and then were dismissed):

- Bad fuel.... but after many many tanks of fuel I have sensed no difference

- Clutch.... could it be that there is a worn spot where the clutch is slipping? Counter argument: if so, wouldn't I notice it more than every few seconds as the clutch is spinning at engine speed?

- Brakes.... perhaps they are rubbing and the surge is a sudden lack of friction? Then again, I would think the wheels are turning too quickly for it to be every few seconds, and would hear a noise associated with brakes rubbing (my brakes also aren't wearing quickly or getting overly hot etc etc).

- Inherent characteristic of the engine?... thinking here is that the uneven length headers and the exhaust design with the crossover pipe (giving one bank a much longer path than the other)... perhaps back pressures in the exhaust system vary and the result is this small surge of power every few seconds?

I also just had my car in for a tech inspection before a track day and nothing came up as out of place. Slight front wheel bearing adjustment was the only thing needed. I have new tires, all balanced.

I want to reiterate that it's not a large surge, and when accelerating I don't notice at all. I notice it most whenever I'm driving a consistent speed for a prolonged period of time, especially through a town where I'm cruising in 2nd or 3rd.

Interested in hearing your thoughts/experiences!

[JohnJL]

I believe surging can be caused by both lean conditions such as a vacuum leak, or by too advanced ignition. Both can have a 'cycling' effect as the rpm moves out of the effected range, then appears again as the rpms come back down.

[david.avrahami@]

cruise control?

[JAR0023]

One possibility is a lazy oxygen sensor. As they age the rate at which they cycle slows down. Easy enough to check. Disconnect the sensor at the harness. Left side of engine bay. Go for a drive and see if you still feel the surge. -J

[rsscotty]

The oxygen sensors were supposed to be replaced every 30k by Porsche service specs. Not many shops do this.

[porsche0nut]

Originally Posted by JohnJL

I believe surging can be caused by both lean conditions such as a vacuum leak, or by too advanced ignition. Both can have a 'cycling' effect as the rpm moves out of the effected range, then appears again as the rpms come back down.

Hm, I'll check for vacuum leaks. I would be surprised if the ignition was advanced too far as I have the stock chip still. Rpm's don't seem to change (according to the rev counter) but I guess if they only changed slightly it wouldn't be noticeable on the gauges.

Originally Posted by david.avrahami@

cruise control?

My cruise control doesn't work that I know of, unless you are suggesting it's having some affect in the background due to not working properly?

Originally Posted by JAR0023

One possibility is a lazy oxygen sensor. As they age the rate at which they cycle slows down. Easy enough to check. Disconnect the sensor at the harness. Left side of engine bay. Go for a drive and see if you still feel the surge. -J

Thanks, I'll try disconnecting it.

Originally Posted by rsscotty

The oxygen sensors were supposed to be replaced every 30k by Porsche service specs. Not many shops do this.

I changed my oxygen sensor last summer (say, 10,000km ago) when I removed the cat and put a bypass pipe on, but I guess it could be faulty! Good news is that with the new pipe a new O2 sensor will be easy to install - unlike the old one which was seized to the cat.

[Bob Kontak]

Lorenfb beat me to death once when I mentioned that surging may be related to a lean condition.

Resounding feedback was it is a rich running condition.

I don't know what can cause that in a 3.2.

However, I had about a year or more of pulling my hair out with my SC. Surging when stopped in hot weather. Little putt putt backfires out of my exhaust. Ended up being an intake runner air leak at the head.

[porsche0nut]

Maybe I should just order my wong chip and see if that helps - loren would love that solution!

I've often thought my car ran rich, just because of the smell of the exhaust and the backfires (M&K muffler/cat bypass). I don't mind the little putt putt backfires, but the surging I would love to fix! Perhaps the wong chip would tune my mapping properly for the setup I have?

[JAR0023]

Agree on checking for vacuum leaks. On a 3.2 pay attention to the gaskets and spacers between the intake and heads. Wouldn't hurt to check and retorque them. I found a couple loose. I also found a blown out gasket on #3 which is under the aux fan and the hardest one to check tightness on.

@Bob one vacuum leak that could lead to rich vs lean is the Y shaped line that runs from back of throttle body to the fuel pressure regulator and damper. Active vacuum pulls the fuel pressure down. A leak here would cause pressure to rise and give more fuel (i.e. rich).

I think the Motronic cars are pretty amazing in their ability to run pretty damn well with any number of minor maladjustments, vacuum leaks etc. When I bought my car the PPI checked out and all was good and I thought it ran well. It did just not 100%. Then I started noticing all the little things. I got tired of chasing this or that small thing and ultimately I went through everything from the intake gaskets back to the air filter and clean up all the vacuum leaks, sensors etc. Couple hundred bucks and a couple of weekends and all was right with my world. -J

[Bob Kontak]

Originally Posted by porsche0nut

I don't mind the little putt putt backfires, but the surging I would love to fix!

That is good info. Putt putt backfires are really worth beating on the basics to find. Buy some starting fluid or carb cleaner and spray away on the leak spots. Do a Tony if you want to do it right (air pressure/air leak test).

One leak in the wrong place and it can make a world of difference.


Thanks JAR0023 for the info on the fuel pressure.

[john walker's workshop]

like previously mentioned, unplug the O2 sensor and test drive it.

[Swiftmotoring]

If the unplugged 02 sensor gets rid of the surging is it OK to run the car without an 02 sensor? My SC is having the same problem...

[porsche0nut]

Thanks for all of the replies!

I did try unplugging the O2 sensor, and it didn't change the "surging" feel. The O2 sensor was replaced last summer so unless it was faulty I would be surprised if it had failed.

Now that winter is upon us, I will follow JAR0023's and maybe drop the engine and do a good "once over" on gaskets and torquing bolts/studs and looking for cracked hoses that may be the culprit of leaks.

BOB:

What am I looking for with the spray test? The engine sucking the mist in through vacuum lines? Will I notice a surge in rpm?

[jlex]

Yes. The ingestion of carb cleaner or other volatile spray will cause the RPM's to increase.
I had a bit of surging that I'd experience in third gear between 2000 and 3000 rpm's. On a certain stretch of road, I could always count on it happening. Went on for a number of years ( I bought my 2005 3.2 in 2001 ). Did the usual things of changing filter, cleaning injectors, changing O2 sensor, idle control valve, etc. & didn't see any real improvement.
Bought a Steve Wong chip & installed it this Spring. Car was much better behaved around town at lower speeds & the surge was eliminated. Don't know what it may have done specifically (maybe changed timing) so I can't analyze how it got rid of it, can only report the results. The surging thing wasn't all that bothersome; it was the jerking at slow speeds & me having to constantly use the clutch to assure a smoother ride around town that was greatly improved. Just my experience.

[theiceman]

Jonathon i suggest before you drop your engine you get a hold of an LM1 or equivalent and do a base CO check , once the engine is out you obviously cant do that and will always wonder. Even if it is fine it will be good to check. you dont have a cat so it should be easy.

also does that car have the single wire CHT or the dual wire, . if it is the single i would bypass it with a paperclip while warmed up and go for a drive. If that ensor is beginning to take a crap it will richen your mixture as it thinks the engine is colder than it actually is. Rich mixtures can cause hunting and surging.

i also have a timing light if you want to borrow it to check your advance. I dont know if the Carrera uses centrifigal weights to control advance but if they do then its worth checking in the off season. I am sure someone here will verify this for you.


Good luck.

[Joe Bob]

02 or vacuum leak...oh and Bob, Loren can go screw himself.....

[theiceman]

i just noticed the back fire .. through the intake or the exhaust ? i am thinking you are going to say exhuast

[Roger 911]

I was having a similar issue with my '84 3.2. I wound up adjusting the idle and the mixture. This solved the problem for me. In my case, someone had previously monkeyed with the mixture, since the little inspection cover was already removed from the adjustment screw location. You can adjust the mixture by hooking up a voltmeter to the O2 sensor output. With the engine warmed up, you should be running between 0.1 and 0.9 volts. Do some searches and you can find a lot of info on this.

Roger

[jlex]

I'm sure Roger is right-on with his advice: idle & mixture adjustment if everything else (vacuum leaks/O2) checks out. By switching chips I probably just masked the problem. Go with his suggested adjustments.

[AVIMAX]

I had the same surging with my 86 3.2. I checked everything, changed many parts but couldn't solve the problem. Then one day my alternator died. After repairing the alternator, the surging was gone. I think the wrong voltage to the DME may possibly cause this surging. I haven't tested this theory, just going on my experience. It may be a long shot, but check that your alternator is putting out the right voltage. If it is the original alternator as mine was, there is a good chance the brushes in the voltage regulator are nearing the end of their life.