Small quirk in idle set up

soxnail · 09-20-2007, 12:40 PM

Hi y'all,
I just changed the oil & filter in the car. Every time I do that I also look at the idle set up as the it can get off center, but the idle stabilization is good enough that it does not show up. Any way did the usual shorted B and C on the diagnostic plug to take the ISV out of the equation, normally I would just use the rpm gauge on the car to set the idle, but since I had a scope in the garage I used it to monitor the rpm what I found was that with a scope you can actually see the rpm go up and down in sync with the O2 sensor output. As much as +/- 50 rpms so I disconnected the O2 and the rpm stabilized, I then set the idle via the air bypass screw and released the ISV the rpm stayed dead stable. I then reconnected the O2 and every thing stayed stable and good. I don't think this much percision or care is needed in setting it I was just intrigued to see that the O2 mixture control can have that much effect without the ISV to counter it.
Yeah I know I need to get a life.

Sox

eddieo · 09-28-2007, 02:23 AM

Sox

I finally managed to cure my "limp home" problem after fitting a hew knock sensor and then a replacement TPS. The turbo now works great - consistently producing 1.7 - 1.8 bar. However my idle is now a bit rough. Its fine when cold but when warmed up it will rise to 1400rpm then fall back down to 900. is this likely to be an ISV problem?

Eddieo

soxnail · 09-28-2007, 05:14 AM

Put the ISV in "bypass" mode by shorting the pins on the diagnostic port. If the problem goes away then it was the ISV. If it does not then it is some thing else. (Though if the diagnostic port to dme circuit could be bad and the ISV is not bypassed is a possibility) start with this test.
Sox