[Cloud9...68]

Has anybody hooked a vacuum gauge up to their engine recently? Mine reads between about 14.5 and 16 " Hg at idle. It reads the same whether I tap into the brake booster line, or the fuel pressure regulator line. It's reasonably steady; just kind of hunts a little, mostly between 15 and 16". When I rev it suddenly, as expected, the vacuum drops abruptly to zero. When I increase rpm gradually, the vacuum reading climbs slowly, hitting a max of about 18 " Hg. I found an earlier post on 968.net that agreed with my idle reading, but I'd like to get a little wider consensus, if possible. In particular, Dave G., what does your console-mounted vacuum gauge read at idle?



I found a generic-looking page in the shop manual of an old car of mine (a 1994 Nissan Altima) which says the readings should be steady, in the 17-22 " Hg range. Mine is well below that, but again, if that previous post is representative, this might be normal. I'm doing this as part of the troubleshooting of my hesitation under load problem, so if the reading I'm getting is normal, it rules out several possible causes of my engine stumble. Thanks.

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[Greimann]

Be sure to check the vacuum when the engine is fully warmed to operating temperature and all accessories off. At that point I get about 18" . At cold temperatures, the idle stabilization valve opens more and the readings go down to about 16".

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[flash]

mine runs solid and about 17 at idle



is the hunting slow, fast, rhymic, or varied?



a slow hunt could be plugs too closely gapped



the bad news - the low reading though is likely either late valve timing or leaking rings



you need to do a leakdown now



the good news - a vacuum leak should have the needle bouncing all over the place rapidly (anybody know if there is a vacuum storage control device that would be switching on and off to explain the floating and rhythmic needle?)



however, squirt carb cleaner all over the intake manifold gaskets and such, looking for an idle change - if youhave a leak there, it will go up or down, depending on how bad the leak



more good news - a blown head gasket would also show a wide swinging of the needle - you can stop worrying about that

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[Cloud9...68]

I drove the car around enough to make sure it was fully warmed up, and repeated the test. I got the same anemic (compared to Dave G's and Flash's engines) 14.5 - 15.5" Hg. The fluctuation is over a range of 0.5", and has a period of just over 1 second. It's pretty constant and rhythmic. My book says this reading is indicative of "late ignition or valve timing, low compression (gasp!), stuck throttle valve (huh? mine shows no symptoms of sticking), or leaking manifold gasket."



So, this could mean a bad cam or crank position sensor, worn or broken rings (say it ain't so... the car was recently dyno'd at 213 rwhp, and pulls like a muscle car when it isn't stumbling), or possibly something with the variocam.



On this last note, according the my description of the variocam operation, below 1500 rpm, the mechanism retards the intake valve timing by 15 degrees. So, for the vacuum test at idle, a normally function variocam is retarding the timing. Presumably, a "failed" variocam would advance the timing at idle, which is contrary the the vacuum gauge reading. However, in my phone conversation with Flash, he was right, above 1500 rpm, the variocam advances the timing (this still doesn't seem right to me, but that's what the description said). This implies that a faulty variocam could indeed be the cause of my stumble, because it always happens in the 1500-5500 rpm range. Hmmm...



In any event, I'll spray some carb cleaner around the intake manifold gasket first, then drive the car with the variocam solenoid disconnected, and then have a leakdown test done on the engine.

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[Greimann]

In terms of the Variocam, think of it as increasing and decreasing valve overlap. The exhaust side is constant and the intake side is the vario part.



At 1500 RPM and below, there is less overlap to help burn the mixture more completely. This is an emission setting because I don't think anyone drives around at 1500 rpm.



At 1500 to 5500, there is more overlap to help cylinder filling. They call this the torque setting.



At 5500 an above, the overlap is reduced for, as porsche puts it, "optimum efficiency power". Optimum efficiency power? I guess that means fuel economy.





Also, the variocam has certain triggers that change the shift points. One of them is oil temperature. Not exactly sure what the effect is, but I know that from my oil temperature gauge, it take about 20 minutes of driving to get the oil up to full temperature. Perhaps there is a correlation.

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[Cloud9...68]

I'm sure you're right. But my understanding of variable valve timing systems in general is that they allow the use of a very "hot" cam (high lift, large overlap), which would normally compromise low and mid range power, by retarding the valve timing in the low and mid range. This allows you to have the hot cam for the top end, while maintaining acceptable punch in the low and mid range. I was very surprised to learn that the variocam actually advances the valve timing (intake only) in the 1500-5500 rpm range. Seems backwards, but it appears to work well, because before stumble problem started, my car definitely had a nice, strong mid-range.



Interesting that you see the oil temp stabilize after about 20 minutes (your gauge package is definitely on my list of wants for the car). I'm beginning to suspect my problem may be caused by the variocam. Oh, great, one of the most expensive components in the engine. But I'll continue to hold out hope that it's something else.



Speaking of which, I just browsed through the ignition system part of the manual, and didn't see it explicitly stated how the DME determines how much spark advance to provide. I assume it works through a combination of signals from different sensors (knock, engine temp, speed signal, etc.). I wonder how one would go about troubleshooting an ignition timing issue... A System Tester 9288 sounds like a handy device to have; probably costs as much as a back-up car, but both appear to be borderline necessities when it comes to these cars, at least based on my experience <img src="/forum/images/smilies/968/blink.gif" class="smilie" alt="" />

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[Greimann]

[quote name='Cloud9...68' date='Jun 27 2005, 04:36 PM']I'm sure you're right.  But my understanding of variable valve timing systems in general is that they allow the use of a very "hot" cam (high lift, large overlap), which would normally compromise low and mid range power, by retarding the valve timing in the low and mid range.  This allows you to have the hot cam for the top end, while maintaining acceptable punch in the low and mid range.  I was very surprised to learn that the variocam actually advances the valve timing (intake only) in the 1500-5500 rpm range.  Seems backwards, but it appears to work well, because before stumble problem started, my car definitely had a nice, strong mid-range. .....



[right][post="6427"]<{POST_SNAPBACK}>[/post][/right][/quote]

Check your sources again, the variocam operates opposite of what you state above. Below is a page from the small green sales book.

968_variocam_function.pdf Size: 419.11 KB  Downloads: 29

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[Cloud9...68]

Aha! This makes a lot more sense, and is in line with how I've always understood these systems to work. The document by Derek Holliday and Ugo Manfredi has the description of when the system advances/retards the intake valve timing backwards. I'm sure just an editing issue, as the rest of the document is excellent. Thanks for clearing this up.



I'm struggling a bit with how a faulty variocam could lead to the symptoms I'm experiencing. I suppose if the mechanism is sticking, it could be failing to achieve the degree of advance required by the particular load conditon, leading to a hesitation. So, it's still on the list, unfortunately...

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[Cloud9...68]

Well, I tried spraying carb cleaner all around the intake manifold where it connects to the head, and nothing happened. No unusual sound, no change in idle speed, etc., so I can scratch this off my list. It's narrowing down to compression loss, retarded ignition timing, or retarded valve timing, though of course it could still be something completely different.

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