rhythmic surging sound/sensation

[kuehjo]

My 1981 4.5 928 is back together, and I'M back from deployment, so I'm finally getting to drive her a little bit. It is a JOY at 60+ MPH

But I have some ongoing issues - the most troubling of which is a sort of rhythmic "surging" sensation, especially at low speed. The cycle does NOT seem to change with engine speed, though it is MUCH less noticeable at speeds over about 40 mph. The cycle is about every second or so (maybe slightly faster than that, actually) and its probably as little as 100 rpms in difference - kind of a rrrRRRrrrRRRrrrRRR sort of sensation - and if you're in drive, and sitting at a stop light (foot on the brake), the car sort of "pushes" a little.

If you've ever flown in a twin engine airplane - its like that harmonic you hear when the two engines arent quite in sync...

For what it's worth - it may be idling a bit fast as well, and there's sort of a "bogging" sensation when I first step on the accelerator. I suspect I do NOT have the fuel injection dialed in just right. It DID pass California smog, though, so I can't be THAT far off - can I?

Oh yeah - and I just picked up a Boxster with what might be a bad IMS bearing - I am a GLUTTON for punishment!! (Its cool - the "entry fee" was low enough - I won't get hurt!)

Thanks for any thoughts/opinions!!

[kuehjo]

Shoot - was hoping SOMEONE might have a thought....

Well I found this on-line - not specific to Porsche, but it certainly gives me some direction
And if I were to GUESS - then I would guess that the cycling i feel/hear is likely the ECM searching for the correct fuel management

But lots to troubleshoot...

http://www.2carpros.com/articles/engine-surges

[MainePorsche]

Curious, last time fuel filter changed ? Fuel tank/strainer status evaluated ?

[kuehjo]

You know - its funny you ask that - i was trying to remember if I did the fuel filter when I did everything else - I THINK I did - but right after I got the car all buttoned up I got deployed, and that was 10 months ago...

Cheap fix though - and will probably be one of the FIRST things i do.

[MainePorsche]

Originally Posted by kuehjo

You know - its funny you ask that - i was trying to remember if I did the fuel filter when I did everything else - I THINK I did - but right after I got the car all buttoned up I got deployed, and that was 10 months ago...

Cheap fix though - and will probably be one of the FIRST things i do.

Before I change out the external filter, I would want to see if the fuel tank lining is soiled/varnished. If it is you will shoot your external filter early.
Go in rear hatch, open, and look down the sender hole. If you think clean, then go to filter. If you think soiled, treat it first to kick out the junk to the existing external filter, THEN change it out. If it is soiled you can change out the strainer too when you're doing the job.
When tank half full add 1 bottle Techron and 6 oz. MMO (Marvel Mystery Oil). The solvents in both will naturally clean, and the MMO will help lube some of the workings in the fuel pump. Put some miles on the car. Do it 5-6 times ! Then change out both pieces.

I had a similar surging pattern from 600-800, and would not want to start briskly. Went through the electrics and my Dx was fuel starvation at start up. Now starts that way it should - briskly. Idles like a burring cat at 600.
Cheers
Craig

[Fixer]

Could possibly be the TB failing.

[Maleficio]

Title sounds sexy.

[curtisr]

Originally Posted by Maleficio

Title sounds sexy.

At first, I wasn't sure the original poster was in the right forum! Err, sorry, mom...

Second thing I thought of was the fuel filter.

[robucop]

Originally Posted by kuehjo

But I have some ongoing issues - the most troubling of which is a sort of rhythmic "surging" sensation, especially at low speed. The cycle does NOT seem to change with engine speed, though it is MUCH less noticeable at speeds over about 40 mph. The cycle is about every second or so (maybe slightly faster than that, actually) and its probably as little as 100 rpms in difference - kind of a rrrRRRrrrRRRrrrRRR sort of sensation - and if you're in drive, and sitting at a stop light (foot on the brake), the car sort of "pushes" a little.

Sorry, i'm not trying to hijack this post, i'm just curious.

That sounds like the same issue i have with my "79 auto i'm pretty sure it's mechanical and originating from the rear of the car. I was thinking maybe from the diff or CV(s) going bad.? I only hear it when the car is in motion (below 100 kph or 60 mph). Sound familiar so far?

If i ever get around to diagnose it i'll let you know.

[kuehjo]

No worries - I don't feel hijacked at all

Thanks for the thoughts gentlemen - and MainePorsche I will try that first. That's a simple enough thing to do, and if I HAVEN'T replaced the fuel filter, I certainly need to.

I will get back to you all with an update when I have a better answer

[Fixer]

When a motor with chain driven over head cams has stretched chains which need replacing, one can experience surging. Couldn't a 928 TB do the same..

[kuehjo]

I don't know - but I just did heads, timing belt, and the Audi tensioner, so I don't think that's it...

[Fixer]

Good luck figuring it out and keep us posted.

[kuehjo]

Will do - unloading tools from the moving POD now, so hopefully in the next week or two I will be able to get on this and get a solid answer.

Thanks for all the thoughts gentlemen!

[Fixer]

Gosh it may be your auto trans, check the fluid level?

Thank you for your service to the Country also.

[kuehjo]

excellent thought - and I have NOT looked very carefully at that yet. Its on my to-do list - but its quick and easy - I will bump it up in priority and let you know...

And it has been my pleasure!

[Fixer]

Good Luck with it, i didn't realize you owned an Auto until the other day.

I do have a factory manual, i will look into your symptoms when i get a chance.

[kuehjo]

I had a thought this morning, driving to work...

The 928 only has 73K miles on it
How much would you bet that the transmission fluid (and filter probably) have NEVER been changed, and I'm driving on 31 year old fluid?

That will definitely go on the to-do list...

[dcrasta]

OK before you over think this.

Have you replaced ALL of the vacuum lines and connections?

A slight leak can make this happen.

Also, there are an Oring on the intake and several gaskets that can lead to vacuum leaks.

http://www.porsche.com/all/media/pdf/originalparts/en/E_928_86_KATALOG.pdf shows the parts Im talking about on illustration # 107-05

Before you start taking things apart however, make a 'test rig' and test for vacuum leaks.

You can put a mightyVac or vacuum gauge on some of the vacuum lines and confirm you're pulling 18-21 hg of vacuum.

You can rig up a plug that plugs into the Throttlebody (some plumbers PCV pipe and some imagination) to pressurize things to make sure yo don not have leaks.

All in all surging is usually vacuum leaks or fueling issues.

Transmission service should be on your list, however the amount of fluid is more important than age. Old fluid (unless the trans has been abused or overheated because of low fluid or other problems) is not the problem, low fluid is.

[kuehjo]

Great advice - and I think as you do... A vaccuum leak seems MUCH more likely. I have replaced many of the degradable pieces, but not all (some of them were literally disintegrating in my hands) including all of the high pressure fuel stuff.

It just struck me this morning that this car is 31 years old, and I have NO knowledge of prior service history. It only has 73K miles, so I would not be entirely surprised if the transmission fluid was the original fluid. Although I think that is less likely to be the source of the problem, it HAS to be one of those things that is way over-due. I just meant that it needed to be on my to-do list, regardless. And I don't know how well tranny fluid holds up over time - but i can't imagine that anyone would RECOMMEND running it on that longer than you have to if its that old.