OR

when I was messing with the vacuum system on the 03 to help troubleshoot the 00 I messed something up, which is also very possible.

The dme analyzes feedback from the knock sensors to detect preignition and detonation. Low octane gas in high compression motors can cause both, although detonation (aka knock) is fast more likely. These events do not throw a code per se, but instead causes the dme to retard timing in an attempt to minimize this from occurring.

Misfires, on the other hand, are caused by lack of detonation in a cylinder and are picked up by variations speed calculated from input from the crankshaft position sensor relative to what cylinder is (or should be) during at that moment.

In other words, bad gas usually doesn't cause a misfire. It just makes the engine run like **************** until the dme pulls timing to compensate and then you lose power.

I would pump out the gas, put in fresh 93, and hope for the best. If it continues to happen, double check your work. And call a priest, because yo **************** is haunted.

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Tell me about it. When it started to do it it felt like when an o2 sensor goes bad. I'll tackle it tomorrow. Just seems such a coincidence to happen right after getting fuel.

Hi

I have not read every thread, but have you done a smoke test? I tracked down an annoying vacuum leak on mine that way. Spraying easy start around did not work at all.

Berni

Sorry for jumping in a bit late. You said in your first post that when you removed the oil cap the idle changed and per your video . Did you notice any pressure when you removed your oil cap? would the crankcase ventilation have some to do with this?

Very good question I think. This is something I have seen different answers on and different results, even with my 2 986's. Everyone says that if you take the cap off idle should change. Make sense. Some talk about how it should be difficult to take the cap off cause there should be a vacuum on it, but, there aren't many conversations about this.

With my 2000 taking the new cap off, idle changes and you can hear a slight vacuum noise. The pressure doesnt feel all that great, but that could be because it is a result of whatever issue this car has.

Now, with my 2003 doing the same thing the results are the same. Idle does change, and, that vacuum noise exists but it also doesn't sound or feel any different then my 2000 which clearly has an issue with vacuum or something not creating enough vacuum.

So to answer your question, no I dont notice much of a pressure, or a vacuum when removing the cap. With other cars like my Jeep Wrangler it works different. There is pressure when removing the cap and removing the cap is a way to tell if the rings are bad by causing blowback. Done this on many cars that I have looked at buying where you would take the cap off and see if oil shoots all over the hood. This is cause of course the cap is part of the valve cover, which in the case of a 986 isn't the case directly.

Really though, I am not sure if there should be pressure....or vacuum when removing the cap. Haven't seen anyone say one way or the other with certainty only that the idle changes.

Well I am mostly old school but I am fairly sure that the crank case vent system can affect the vacuum overall.

The crankcase on these cars is supposed to have a slight vacuum.
One reason is the rings are a low tension ring so the slight vacuum helps seal the rings to the cylinder walls.

Crankcase vacuum is tested with a manometer.
There is a spec. for vacuum.
It is measured in "inches of water"
Sorry I don't remember the spec. offhand but it is here somewhere on the forum

Intake vacuum on the other hand is measured in "inches of mercury"
So each test requires its own type gauge.

It's 5 inches of water.

Most modern engines work on positive crank ventilation (pcv) in order to evacuate blow by fumes and funnel them back into the intake. As blue said, ours actually works in opposite to achieve the same effect, plus to seal the low pressure rings (although there are reports of running our crankcases just vented to atmosphere, which transfers them to a PCV system. But I digress ...)

This is a tiny vacuum. The cap should come off easily but you can feel a pull if you put your hand on it.

Slack manometer is the tool for measurement. This is one way to actually tell if your AOS is bad (AOS failure allows more vacuum pull of the crankcase)

The idle changes because the intake pulls on the aos, which pulls on the crank case, which communicates with the fill tube. Open the cap and you've essentially created a vacuum leak past the TB, letting in unmetered air and causing the idle to surge.

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5 inches of water
That's on the max end I believe
Do you know what the low end spec. is???
just so I can log the info.;)

Lol. I'm not aware of a range per se, just that number.

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There was a YouTube video I watched a couple weeks ago. I'll get it tomorrow, it talked about this exact thing.

Yep there is going to be an operational range on something like vacuum.
So a low end and a high end to the operational range.
If the Spec. was just "5 in.of water" then any thing above or below that spec. would be a faulty AOS. Thus an operational range;)

Think this is the video:
https://www.youtube.com/watch?v=wVLuC84OL5c&t=1s

Thanks:
He is saying 4-6 inches of negative vacuum.
So there is the operational range.

I know this is an old thread, but did you ever determine the cause of your problem? My 2001 S is experiencing the same symptoms.

The OP was thinking of "pushing his car over a cliff"....perhaps he did!