[ds968]

The car is mostly a weekend driver so it typically sits for five days before I start it on weekends - when I first do that it does idle perfectly right away, but if I give it some gas while still sitting and/or trying to drive it out in the first 30 sec to 1 min after start, it hesitates and stumbles pretty badly. However, returning to idle it's once again as smooth as can be. So I'm guessing it could not be the Idle Stabilizer Valve, or it would have a problem idling as well [img]style_emoticons/<#EMO_DIR#>/huh.gif[/img] The MAF is also clean, so I'm ruling that out.. Spark plugs are firmly in place, as are the wires, the distributor cap and rotor bits and pieces are smooth and shiny, so I have no clue where to look next. BTW, after a minute or so of letting it warm up, the symptom disappears entirely and does not manifest itself again on any other starts during the weekend drives, or at any other times and conditions while driving... until the following weekend when I start it cold again [img]style_emoticons/<#EMO_DIR#>/dry.gif[/img]

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[flash]

how old is your O2 sensor?

also, free flowing exhausts do not allow the O2 sensor or the cat to heat up as quickly or as well, so you could just be seeing the delay as a result

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[ds968]

I haven't changed the O2 sensor in seven years, and have no idea if it was ever changed before then. However, if it was bad wouldn't it 1) throw a fault code and 2) also cause issues with the smog test ?
As for the second thought.. I kinda' doubt it, since I've had the free flow exhaust for the same six years, and the same O2 sensor, so this symptom would have surely happened before.. [img]style_emoticons/<#EMO_DIR#>/huh.gif[/img]

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[flash]

not at all - the O2 sensor is an item that gets contaminated over time - as it begins to fail, it begins to show symptoms - it will generally be a gradual process - it will not throw a code until it gets really bad, but it will definitely show signs in operation

they should generally be changed about every 60k

a high flow exhaust allows the sensor to cool more quickly - this can lead to improper operation of that unit - it also can lead to more rapid contamination

that being said, i tend to think it's the MAF dirty again, but that's easy enough to find out, since it's so easy to clean

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[ds968]

nah, I cleaned the MAF around a month ago, so could be then the O2 is just at the early stages of acting up..

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[PorscheG96]

I don't think the O2 sensor is part of the fueling system until it reaches operating temp and at that point all it does is make fine adjustments. The DME cold start map is dumping loads of fuel into the engine at the point in time that Dan is describing these symptoms and in fact, my 968 engine hesitates a little around 2k RPM's the first or second time that I accelerate it cold after not driving it for a week or longer. Engine is completely stock including exhaust and I don't really know that it's a problem.

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[flash]

i'll look at that map tomorrow

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[williamoss]

As many of you are aware I bought Pete's car (RS BARN). He just called me to tell me of a couple of quicks of the car, This hesitation being one of them. His opinion is that the software that was written in the 1990's was slow and cumbersome. That being said, if the computer does not see any coolant temperature at all or low oil temperature the computer tries to send the car into a safety mode which limits the amount of fuel to the injectors. Pete said that by letting the car warm up at the beginning for about 2-3 minutes before driving the car it will eliminate the bucking or coughing aspect. I always let my cars warm up anyway, to allow the oil a chance to circulate throughout the engine.

William Moss

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[flash]

interesting side note: the M3 does EXACTLY the same thing, and has been checked twice for that - according to BMW it is NORMAL

that being said, i suspect a fuel filter or check valve issue on dan's car - it idles fine, but as soon as you touch the throttle, and need more fuel, it balks - however, it does not do it unless it sits for a few days - if the check valve were letting the fuel go back to the tank, or not letting enough go forward, it might not have enough for the first 20-30 seconds to run under any load - the same could be said of a partially clogged filter that had gunk settling into the wrong spot when allowed to sit for days

i suggested a DME relay bypass test - bypass the relay so the pump comes on with the key - let the fuel system charge in position 2 for 30 seconds or so (however long the problem seems to exist) - then start the car and see if it persists - if the problem goes away, you have found the culprit

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[ds968]

software would not be the issue here, nor would it be "normal " as is the case with BMW, since this problem has never manifested prior to just a month ago.. so it has to be something which is starting to act up for whatever reason.. so the fuel
filter or check valve, or some other component seem to be more plausible culprits

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[flash]

agreed

it makes me crazy that the M3 goes through this nutty cycling stumbling thing for a few seconds before it settles down - it's not a big deal, and it's over by the time the car hits the end of the driveway, but still it bugs me - i checked 2 others cars, and they did the same thing - wacky

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[Jamie '92 968]

Dan,
My car also does exactly what you describe, but only when cold, and only on occasion.
I mostly experience the problem in the evening (in the winter only) when I start the car up for my commute home from work.
It has only been sitting for the day. It goes away within a minute of starting the car.

In the past year I've cleaned the MAF, replaced the plug wires, cap and rotor, cleaned the injectors, and replaced the fuel filter, but I still experienced occasional stumbling until the temperatures started to rise over the last few months.

I've always suspected the fuel pump. The fuel pressure tests normal, but I've never had it tested when cold and after sitting for a long time. Could the fuel pump be malfunctioning intermittantly?

I've experienced a fuel pump failure on another car in the past, and it felt a lot like this.
Also, my car seems to run noticably better with a full tank of gas. Could this be a clue?

Jamie

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[flash]

there is a difference between pressure and volume - you can have pressure, but still not have volume - in fact, as is the case with an adjustable regulator, you can increase the pressure, but the volume correspondingly decreases (just like squeezing off your garden hose)

to test a pump, you need to do a volume test

but, i tend to think it is more simple than that - i suspect either a sticky check valve, or leaky injector seals which swell back up when warm, or dirty injectors which run fine once things get flowing - any one of these could lead to an intermittent condition like this

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[Jamie '92 968]

OK, I haven't played the clueless card in a while...

Where do I find the check valve, and how do I check it (or replace it)?
Don't tell me it's in the tank with the pump.

Jamie

EDIT...

Sorry Flash, I read back through the post and found your procedure for checking the valve.
Though I don't know how to do the relay trick you describe.
Also, since it's so intermittant for me the test would not be very conclusive.

Maybe it's easier to just swap it out if it's not too difficult or expensive.

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[ds968]

started it today after its usual one week rest, expecting the same symptom but it did not happen - no problem revving it up right away, no hesitation. so this problem does not manifest itself with consistency.

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