Altima idle, PCV valve and overrevving (again!)

Sorry to keep beating what might be a dead horse by now, but I've been thinking about the issue with my Altima again where the revs suddenly jump from around 1100 to 2000 as I am easing into the throttle... and I might have an explanation for it.

It's a 99 Altima, 5 speed, with 117,000 km.

When I got the car it was idling around 1000 RPM even when warmed up completely. I was able to adjust the idle, but only down to about 750rpm with the TPS disconnected, and the service manual says it should be 650 rpm,

+/- 50.

However, with the TPS connected normally, the revs settle around 750, which is within spec (700 +/- 50 IIRC). What I realized today, is that what is probably happening is the ECU is closing the idle control valve further than normal in order to maintain the target idle speed.

With that in mind, I suspect the ECU is programmed to more or less match the opening of the throttle by opening the idle control valve as much as it can so that when I take my foot off the gas, the engine doesn't immediately spool down, which can be annoying when coasting.

Now, given that the IACV is closed further than normal at idle, and assuming that the ECU is programmed to open the IACV with the throttle (I suspect it's probably a linear map of TPS angle to IACV opening?), then it makes sense that as I reach the beginning of where the IACV starts to open, it jumps to what would *normally* be only slightly more opening than at idle, but in my case because it's more closed at idle, there's an abrupt change instead of a smooth increase in speed.

Does that make sense to anyone else?

If that's what's going on, then where is the extra air coming from that's causing my idle to be too fast? Is there anywhere else metered air might get into the engine? I suspect maybe I have a PCV issue because the revs will drop to 400 rpm when I am in idle adjust mode with the TPS unplugged if I pull and plug the PCV breather tube between the valve cover and the intake. There's a fair bit of vacuum at the valve cover fitting at idle which is adding to the airflow through the MAF at that point.

I'm hoping I can find where the line from the PCV valve to the intake manifold is connected, and disconnect it to see what effect that has on everything... too bad the PCV valve is so well buried on these cars!

Reply to
JM
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OK how about this hypothesis:

You bought the car used, correct?

If so, then maybe the previous owner did something in which he wound up getting, or putting an ECU for a 99 Altima automatic in that 99 Altima 5-speed manual of yours. With an automatic ECU, the ECU is expecting to communicate with and automatic tranny computer, which your car doesn't have. Because of this, the ECU doesn't know when the car is in neutral at all, hence it will push the idle up because it always "thinks" its in D or 2 or 1 etc.

Just one of many thoughts.

CD

Reply to
codifus

Well, that thought did cross my mind.. and I suppose it would be easy enough to check. But I would expect that unless he did some serious hacking with that ECU it would be complaining about not having a trans computer to talk to.

Reply to
JM

"JM" wrote in news:5ZdTh.23511$ snipped-for-privacy@ursa-nb00s0.nbnet.nb.ca:

i would expect a CEL with that misapplication..........kjun

Reply to
Kjun

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