DME Actual Values

[tholyoak]

Have you checked your fuel pressure?

Todd

[jcorallo]

Also note that since this is a 1997 RoW model, there is no secondary air injection. It also very rarely throws a code since its pre-OBD2 compliant. It certainly does not throw a code as its running now.

well that certainly makes things a bit more complicated! can you describe the throttle hesitation bit as best you can? does it happen from a stop? from any constant speed to acceleration? on decel? this is a manual trans, correct?

hi insite.

Symptoms are as follows and totally repeatable:

From cold, either when left overnight, or on carpark at my workplace: start engine let it idle and settle by itself. Whilst in neutral, very slowly increase revs towards 1400rpm. As soon as revs touch around 1400rpm engine totally cuts like I turned the ignition off and revs drop to around 600rpm for a brief moment, then revs regain back to around 1300rpm/1400rpm - all this happens in less than one second. Now trying this again for a second time, since engine has been running for about 30seconds at this point, each time engine is pushed slowly to 1400rpm, as soon as it hits around 1400rpm, the revs drop to 1300rpm. Now holding it at this RPM the car shakes like its mis-firing and the engine jumps about a bit and sounds like its struggling.

When fully warmed up - the only symptoms are a feeling of surging at lower speeds and part throttle. Say when going along in 4th or even 5th gear at 30mph, you can feel the car hesitating. Its very uncomfortable and annoying. It feels like a misfire.

The only thing I know about that RPM's, is that the VarioCams switch around then. So I think the VarioCam switching is possibly adding to the affect and making it more noticeable perhaps? Just thinking out loud here...

I've not checked the fuel pressure yet - once I've ruled out any leaks I'll look into this.

Cheers,

Jules

[jcorallo]

Yes gearbox is 5sp manual.

oh - and another thing: when coming off decel back onto the throttle, its not always smooth - sometime giving a little jolt.

Jules

Edited March 16, 2007 by jcorallo

[insite]

jules -

i have experienced similar symptoms twice. once, it was bad fuel purchased at a gas station in the middle of nowhere. the high octane they were selling was very old and probably had some water in it. the other time i had similar symptoms, it was the oil separator. the bellows that connects the oil separator to the crankcase was shredded.

when it was cold outside, any attempt to give my car gas would result in the RPMs dropping dramatically, like it was going to stall. once it was warm, it seemed okay, but still hesitant at times. try inspecting your oil separator, especially if it's the original since it's ten years old.

[jcorallo]

Yes the AOS was the first thing I looked at as I did a lot of research before posting.

Since I could not get close to the AOS, I pushed my digital camera up towards it and took a load of pics to view at leisure on my 21inch screen.

The bellows really looked OK, but granted I can only see half of them....

AOS1

AOS2

AOS3

There is a very light misting of oil below the AOS bellows, but none on the bellows themselves - so I had discounted this as being the problem? I would assume that if the bellows were leaking, there would be oil actually ON the bellows?

I could just bite the bullet and change the whole AOS system and pipes - but thats not cheap considering it doesnt look to be the problem?

Jules

[insite]

There is a very light misting of oil below the AOS bellows, but none on the bellows themselves - so I had discounted this as being the problem? I would assume that if the bellows were leaking, there would be oil actually ON the bellows?

I could just bite the bullet and change the whole AOS system and pipes - but thats not cheap considering it doesnt look to be the problem?

Jules -

there's a diaphragm inside the oil separator, too, so just because the bellows is ok does NOT mean the oil separator is ok. i'd go ahead and replace it. also, while you're at it, i thought of another thing you can just replace (cheap): oil filler hose. if this thing cracks, the motor sucks in air and runs funny.

[jcorallo]

OK guys the problem is now solved!

Yesterday I decided to strip off all the intake piping and throttle body and check for leaks.

I found two significant problems and one minor problem:

1) One of the large hose clamps that secures the rubber sleeve to the intake distributor seemed to come undone a bit too easily. I never did check these for tightness... The suspect one was on the left side of the engine serving cyls 3,4 and 5.

2) The idle air control valve, although looked very clean and dry and not gummed with oil, was "crunchy" when I pushed the swing door back with a small screwdriver. There was a layer of carbon causing it to bind slightly. Once cleaned off, the door moved very freely.

3) There was a slight "plug" of thick oil in the signal line to the Fuel Pressure Regulator which I blew out. I also cleaned the throttle body out completely and removed a very thin layer of oil from the inside.

I also hoovered out the air filter sponge and cleaned and de-greased all parts before reassembly.

Once back together, I reset the DME and the difference was superb.

Rock solid idle, no cut out at 1400rpm and part throttle is as smooth as anything. No more surging or hesitation. Transition from idle to any rpm and back is now as you would expect - flawless.

Regarding point 1) above. When I'd put the intake back together and started the engine up, viewing the running engine from the drivers seat (I had both access panels removed) I noticed how much the plastic intake manifolds (intake distributors) moved relative to each other at different levels of throttle due to the vacuum inside. I actually measured it! At idle, the two intake manifolds are a good 2mm closer together. Open the throttle and let air in quickly, and they "relax" outwards. You can also see the flat top section (that says Porsche on) collapses in a couple of millimeters at idle, and springs out when you open the throttle.

Given that these manifolds move about so much, you can see a loose clamp would cause havoc - item 11 in the pic below (the one that was loose on mine was the 2nd one in, going left to right)...

Cheers,

Jules

[Richard Hamilton]

Well done Jules! Perseverence paid off in the end. Now you can get back to enjoying the car!

[jcorallo]

insite, tholyoak and Richard Hamilton - thanks very much for your help and suggestions - really much appreciated! :thumbup: :beer:

I can actually start enjoying the car now - its been like this since I bought it at New Year....!

But I've been steadily working through the list of things to do on it - previous owner loved to clean and polish the car, and it has full service history, but thats about it - I dont think he knew it wasnt running right...

There are a couple more things to work on yet - but I'll start a new thread for those....!

Jules

[insite]

glad you got it squared away. best kind of fix: FREE! enjoy.

[mee]

generic OBD2 scanner give the following Long Term Fuel Trims (LTFT):

Bank 1 LTFT = -2.5%

Bank 2 LTFT = -4.8%

A couple of stupid questions if I may ;)

* What should the LTFTs be and how much bank-to-bank difference is typical/allowed? I think somewhere it said 0.96 +/- 0.03.

* What are the units of the durametric readings? Is 0.96 the same as -4%?