[flash]

while i don't want to scream that the sky is falling, a blown head gasket can do exactly what you are talking about



under load, the head is forced up with more force than it is at idle and such - it also tends to expand the materials more - if there was a hairline crack, or a tiny gap, then you could experience what you are talking about - i've only had this happen twice - drove me nuts both times - one turned out to be a tiny crack in an intake port that only showed up with a magnaflux, and the other was a tiny gap between the head gasket and the block that would form under load that created a path between two cylinders



that's why i suggested a leakdown test - it would tell you if there was an escape that might not show up with no load - a compression test may show the problem, but it may not - it will at least tell you if you are in the ballpark - on a dead cold motor, with the throttle body open, and all plugs removed, 8 "pulses" should bring you at least 180lbs on any motor that runs decently (new and tight is considerabley more)- it should be more when hot too



it is more likely that it is a control or something like that, but it is an easy test that will tell you a lot about the condition of your motor anyway - saved larry a big headache



the chip is such a long shot, that i would discount it completely - they either work, or they don't - they don't change - it can't lose a step in programming and mapping - if it never worked right, would be one thing, but if it worked, it still works

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[Cloud9...68]

Flash,



You're right about the chip. I went ahead and put the stock chip back in just to rule it out, and it made no difference - still wheezing and stumbling under any kind of load.



As far as your head gasket/cracked head theory, I guess I can't refute it, other than to pray it's something else, because something of that magnitude (the head, anyway) would force me to take the car off the road for months; my other car just died, forcing me to buy a new one, which puts a severe dent in the 968 repair and tinkering budget. I'll go ahead and do a compression test just to see if things look tight. If I get 180+ psi in all cylinders with a very tight distribution, do you think it would be safe to put the head gasket/cracked head <img src="/forum/images/smilies/968/ohmy.gif" class="smilie" alt="" /> theory aside for now, or would you do a leakdown test even if the compression test is perfect?



Back to the O2 sensor for a minute, could a bad one (but not bad enough to show up in the flash codes) be responsible for a stumble as severe as the one I'm experiencing? I realize that with 87K miles, it probably wouldn't hurt to replace it, but I'm becoming very convinced that my car's problem has nothing to do with its "holistic health." By that I mean that I've done a lot of stuff that are good overall things for any car (cleaned injectors, new coil, flushed fuel system, new fuel filter, etc.) to absolutely no avail. My car's problem is something very acute (to continue with the medical analogy) as opposed to something chronic. I need (hopefully minor) surgery, not herbal treatments or massage, and I'm afraid a new O2 sensor, as overall beneficial as it might be, won't attack my specific problem. But I could easily be convinced otherwise with data.



Also, I've e heard that generic Bosch O2 sensors are available for the car for about 50 bucks, by I called the only auto parts store in town that carries Bosch parts, and they want $93! A little high for something I have doubts will help.



Oh, and I pulled out of this weekend's DE. It just doesn't sound smart to spend two days driving at the limit with a car that's running so crappy.

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[Cloud9...68]

My first clue! I watched the engine run tonight in the dark, and nothing happened for awhile. Then, after about 5 minutes (I had driven the car about 45 minutes before, so it the engine was still quite warm), I started seeing small flickering sparks around the terminals of the coil. Then after a few more minutes, I started seeing faint sparks along several of the plug wires. This is consistent with the fact that the stumble gets worse after driving the car for about 20 minutes. The sparks became more visible when I pushed on the throttle cable abruptly to rev the engine. In fact, when pushing on the cable abruptly, I think I could actually get it to stumble, so I've made a connection between the stumble and the sparking.



The wires are Nology's, which were installed by the previous owner. I've heard from others that it's not recommended to use anything but stock Porsche wires. I can't explain the sparks around the coil (grounding issue?), but at least I've finally got a lead after months of no progress. I suppose I should replace the plug wires with stock Porsche ones, but I'd appreciate any other inputs anyone may have. Thanks.



By the way, I also tried disconnecting the Variocam solenoid, and that had no impact on the hesistation.

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[Greimann]

Great progress!



One other thing to consider is the grounding of the plugs in the heads. They should be installed with a conductive antisieze. If there is a grounding issue at the plug threads, then the high voltage will try and find ground any way it can, including leaking around the coil and plug wire boots.

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[flash]

i thought you did the "watch it in the dark" thing already?



very cool - good luck

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[flash]

another thing though - no impact onthe hesitation after disconnecting the solenoid?



doesn't that mean it isn't working? if things changed for the worse, then i would say it was working - if it didn't change, it sounds like it isn't



that does not mean you don't ALSO have a problem with your wires, so keep going there too

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[Cloud9...68]

[quote name='Greimann' date='Jun 30 2005, 09:55 PM']Great progress!



One other thing to consider is the grounding of the plugs in the heads. They should be installed with a conductive antisieze. If there is a grounding issue at the plug threads, then the high voltage will try and find ground any way it can, including leaking around the coil and plug wire boots.

[right][post="6635"]<{POST_SNAPBACK}>[/post][/right][/quote]



Thanks for the moral support. And that's good input about the grounding of the plugs in the heads. I did use anti-seize, but I don't know how conductive the stuff is. It's called Versachem Type 13, but there's no indication on the tube as to its conductivity. The chassis ground near the coil looks a little rough; the first thing I'm going to do is clean it up. Should I just go ahead and replace the plug wires, or is there more troubleshooting I need to do to pinpoint the source of the sparking? I'll answer my own question by checking as many of the grounds as I can get to...

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[Cloud9...68]

[quote name='flash' date='Jul 1 2005, 11:48 AM']another thing though - no impact onthe hesitation after disconnecting the solenoid?



doesn't that mean it isn't working?  if things changed for the worse, then i would say it was working - if it didn't change, it sounds like it isn't

[right][post="6673"]<{POST_SNAPBACK}>[/post][/right][/quote]



I'm still struggling to understand the ramifications of disconnecting the variocam solenoid. My understanding is that the solenoid actuates a shuttle valve that allows oil to act on the piston which moves up and down, changing the length of the variocam chain run. This is what allows the intake cam to move in relation to the exhaust cam, changing the timing of the intake valves. If the solenoid is disconnected, presumably the shuttle valve would remain shut, not allowing oil to flow into the the variocam mechanism, which would keep the intake valve timing fixed, and more advanced at low to mid-range rpm than would be ideal for optimal torque. Dave G set me straight on this; the variocam summary by Ugo Manfredi and Derek Holliday had it backwards.



I know I'm being wishful here, but given that there was no change in the hesitation symptom with or without the variocam solenoid engaged, I would think that this would tend to rule out the variocam. At least that's what I very much want to believe, because if the car needs a new variocam mechanism, it's going to keep stumbling and hesitating for many a day, because I just had to buy a new car after the engine seized up on my other one. That wasn't an altogether bad thing, because that car (a '94 Altima) needed to be put out of its misery anyway, but the timing of it put a big dent in my 968 repair budget...

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[flash]

my thought is the exact opposite



based on what dave is saying, with the variocam disconnected, the timing remains in the "retarded" state of low rpms (idle -1500) - this would cause a severe power problem in the mids (1500-5000) - then over 5000 it would smooth out again, because the timing is again where it should be



since you only have the problem when it's warmed up, you may have a bad seal or a bad component in there that fails when warm



i would still fix the wire problems - you should have no sparks floating around in there



you could easily have both coincidental problems

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[wmuldowney]

I know this is an old post but it fits a current issue to the letter. Was there any resolution to this problem? O2, or blue sensor or other?

My car sat for several months (4-5) without being ran started. Did not have problem before and now does.
New DME, new ECU (don't jump start off another car!), new crank position sensor.

Another issue is starter makes sick noise, starts fine and without a lot of drag, just sound odd.

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[rl968]

What kind of fuel filter did you put in the car? I replaced my filter, which was the original one, a few weeks ago with a Mann filter that is the correct one for the car and had a significant problem. The problem was so bad that the car would not start and I wasn't getting any fuel to the rail. I replaced the Mann with a Porsche filter and it was back to normal. Worth a check.

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[Lear35A]

I too am glad for the thread bump. What was the final resolution to the hesitation problem??? Inquiring minds want to know.

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[Cloud9...68]

If you guys are inquiring about my hesitation problem, which was what prompted this thread five years (!) ago, it was 100%, with zero doubt whatsoever, the plug wires. I replaced the Nology's with stock 968 wires, and the problem completely vanished. It seems like bad plug wires have been the root cause of several problems with these cars, but I guess that shouldn't be a surprise, given their age.

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[wmuldowney]

OK I know this is a old post but I am now experiencing the hesitation/stumble at low RPM / loads, I am having same problem and driving me nuts. No error codes. I didn't see where you found problem or figured it out?

I did try all but 2 suggestions but to no avail. Did not replace "blue sensor or O2".



Sorry for reviving old post but this one most closely addresses my issues.

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[tamathumper]

The post just before yours states that, unequivocally, it was the plug wires.

If you guys are inquiring about my hesitation problem, which was what prompted this thread five years (!) ago, it was 100%, with zero doubt whatsoever, the plug wires.

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[DaveN]

I had this in my Toyo pickup (yea, not a hi performace machine. Ran great unless you tried to accelerate and then it would sputter and cough. I checked everything even the plugs and wires. Took it to my mechanic friend and lo, it was the only connection I didn't remove and check; the wire at the coil end. PLUG WIRES!

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[wmuldowney]

Didnt see the plug wires before my post. Crap. I check those.

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[wmuldowney]

Wow what a difference the new wires made! Problems solved and some I didn't know I had. Amazing how much better it runs, espicially at idle.

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[Cloud9...68]

Interesting... Did you have stock wires before, or some sort of aftermarket? The Nology's the <acronym title='previous owner'>PO</acronym> put on my car were absolute crap - fell apart in my hands as I was pulling them off the plugs.

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[wmuldowney]

Stock wires and from what I can tell, original. Just turned 100k miles. They looked fine from visual insepection while in car but once removed, not so much.

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