Fixed the Hesitiation

[c928jon]

I thought i'd just do a quick update on the problems I was having with my 99C2. I searched the board for suggestions when trying to diagnose the problem and posted, but didn't manage to get much light thrown on the subject, so i'm trying to write this including all the key words I searched for :)

The car would pull like a train sometimes, then feel flat at others, often with a hesitation at about 3500-4000 rpm.

It never threw an CEL or did it store any fault codes, I guess it wasn't quite bad enough to go out of limits.

I started with the simple things such as air leaks, and soon found that the oil filler tube had split, from the posts and bulletins on caps I guessed this wouldn't be helping matters. Changing the tube was quite simple, once you suss shifting the alternator.

With this fixed there was an improvement but still the hesitation, whilst changing the tube I'd noticed Oil in the intake, coming from a crankcase ventilation tube. so I guessed that the external oil separator had failed, again thanks to this board and many bellows references :)

Changing the separator is a bit more involved and I think any long serving Porsche mechanics must have arms like orangutans. I'll happily write up some DIY's if anyone is interested?

When I eventually got the separator out, the rubber bellows section had failed, another air leak fixed :)

Then came the test drive with two fairly serious leaks fixed and i'm expecting all to be well...so I warm the car up and away we go...and it hesitates just as bedly as before :censored:

I've obviously read about MAF sensors failing and now I start thinking Oil mist in the intake may have contaminated the sensor. Out it comes and lashings of electrical contatct cleaner later it's refitted.

It was like having a new car, it's amazing how you will drive around a problem if it's getting worse gradually. :)

In hindsight my hesitation was the change from open to closed loop,the air leaks and contaminated MAF giving me a lean mixture on WOT, ok on partial throttle but no good when having fun and fine when cruising on the motorway.

The main reason for my post is that I feel that the operating parameters may be different in the UK. From the posts I've read CEL's would have been lighting up like a christmas tree, but not a glimmer from mine. If i'd had a CEL, with lean condition I would almost certainly have changed the MAF, and never found the air leaks. I would have been happy with the result and considered all fixed!

If that had been the case I'd never have found all the extra power and almost certainly been changing another MAF in the future.

Both sources of my air leaks are easy to spot, if you know where to look, It may be worth checking these if your changing a MAF.

I've had some fun with the sunroof this weekend and may have found a quick fix for a rattle, I'll put than in another post though, this ones getting a bit long!

[ianwallwork]

Interesting!

EXACTLY the same problems I had with my last P-Car (a '99 C4). Split oil filler pipe and failing bellows (3 times on mine)! Ran like a dog at part throttle until the air leaks were sorted. Then ticked over fine but still no good at full throttle. Guess what? Yup, MAF!! As you said - like driving a new car once this is replaced.

The issues with deteriorating MAF sensors are genuinely dangerous IMHO. At part throttle, the MAF measures the amount of air entering the engine and the DME calculates the fuel required to deliver the correct mixture. This is then 'checked' by the lambda sensors and corrected in 2 ways - a 'long term fuel trim' that tends not to vary much and compensates for manufacturing tolerances in the MAF and a 'short term fuel trim' that instantaneously corrects the mixture and goes up and down like crazy!! This correction ensures the part throttle mix is OK even if the MAF is off calibration by a small amount.

Once above part throttle, however, there is no mixture correction by the lambda sensors and the MAF governs air/fuel ratio. If it is out of calibration, a lean mix will result. Performance suffers and more importantly LEAN MIXTURES BURN VERY HOT! And this at full throttle when the engine is being worked its hardest. This fault can cause local or general overheating and I wonder if it is in-part responsible for some of the engine failures we read about? My engine had always run a little too warm on the gauge and this stopped immediately on replacement of the MAF.

Anyone else experienced similar?

Ian W

[c928jon]

As you say Ian, interesting, did you get a CEL?

As for it being dangerous, i'm quite mechanically sensitive and "felt" a lean condition so drove on part throttle most of the time with occasional bursts to help diagnose. If you weren't of a similar mindset and opted for the thrash it and see if it clears approach, you could be looking at a rod through the block!

My personal experience was that every now and then I'd forget it was doing it and go to overtake somewhere that only cars like this can, and then feel like i'd stopped half way past...

I think my underwear suffered some serious overheating at times like that!