O2 Sensor Confusion (Chasing Idle Dip)

Cus · Postby **Cus** » Sat Sep 27, 2014 2:29 pm

Quick terminology check: is the idle cushion also known as the dashpot?

gslender · Postby **gslender** » Sat Sep 27, 2014 3:31 pm

project.r.racing · Postby **project.r.racing** » Sat Sep 27, 2014 8:37 pm

Also check the IAC.

Cus · Postby **Cus** » Mon Sep 29, 2014 3:11 pm

I can confirm there's almost no difference in idle speed with GND->TEN bridged. There is a small, small, small drop in idle after doing the bridge, but the tacho appears to be in the same place. (It's a really small drop in RPM!)

I don't think I've been fans-on hot yet (or, just haven't noticed) but the temp gets just before halfway and stays there, so I'm guessing theyre working - we went for a little run through some hills and corners after a wheel alignment this morning, then drove across town, then checked it, so it was as hot as it'll get before i start doing track days and properly wringing it's little neck.

Onwards and upwards with the TPS / IAC and the other bit (ICSV?)

Cus · Postby **Cus** » Tue Sep 30, 2014 2:54 pm

TPS is confirmed working and giving readings at idle and WOT, however it reads as still being idle beyond the 0.7mm "i'm not idling anymore" point - not sure by how much, the feeler gauges only went to 0.8mm (and it was still measuring as idle) I was going to adjust it, but then discovered I either need screwdriver fingers (Like Johnny-Five), or to remove the TB and fix it out of the car... ...it does get around to no longer triggering "i'm at idle" eventually though.

I knocked up a bit of kit so I could see the RPMs in the engine bay (using an arduino mega, an LCD display, a resistor and a LED) it may or may not be accurate but it reports between 700 and 900 at idle. (and I don't have to teleport to see the tacho from the engine bay)

The dashpot is doing "things" and comes into contact with the throttle pulley around 2500RPM as reported by the above tool, and recommended in the workshop manual.

It's almost time to pull Lil Red apart and clean intake/throttle bits, so we'll be shelving this project until I'm confident doing exactly that - remember, I'm not a mechanic yet! (and dad has way better tools than me, so it'll be a weekend at dad's when we've both got time)

As an aside, I have been fans-on hot, they were switching on and off while I was fluffing around with the tacho reader.

gslender · Postby **gslender** » Tue Sep 30, 2014 3:57 pm

The idle air valve below the TB is the next likely culprit - on two that I've pulled apart the little o-ring was perished....

david_syd_au · Postby **david_syd_au** » Tue Sep 30, 2014 4:31 pm

Cus wrote:Greeting humans!
...
The person that sold me the car advised the extractors were the factory ones (They didn't look right, but at the time I'd had about an hour worth of MX5 experience) - but there is no heat shield, and the (i assume) O2 sensor is near #4.

(See attached photo)
..

That picture in the first post shows the O2 sensor near exhaust port 4.
I had thought it was bad for the longevity of the sensor to have it that close.
Anyone comment?

gslender · Postby **gslender** » Tue Sep 30, 2014 5:13 pm

That's wideband sensors. I don't think narrow bands have that problem

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david_syd_au · Postby **david_syd_au** » Tue Sep 30, 2014 9:47 pm

gslender wrote:That's wideband sensors. I don't think narrow bands have that problem
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Ahh understood. Thanks G

Cus · Postby **Cus** » Tue Oct 07, 2014 10:13 pm

This post has taken me ages to write: I have a question, I type it out, I research up an answer, I change my question to a statement. Press "Preview". Rinse. Repeat.

Red = Parts I'm pulling off and cleaning / adjusting / etc
Green = Gaskets I'm getting (1 & 2 & 3 -- but possibly from somewhere more local, haven't looked yet)

Will there be any more Gaskets / O-Rings / Dooverlackies that I need while removing / adjusting / cleaning the TB / TPS / ICSV / AV?

gslender · Postby **gslender** » Wed Oct 08, 2014 6:54 am

Having done this myself a few times, I can confidently confirm nope!

Cus · Postby **Cus** » Wed Oct 08, 2014 6:34 pm

Cheers G!

Cus · Postby **Cus** » Fri Oct 24, 2014 9:16 pm

Good news Everyone!

I got this sorted today - pulled the red things off and cleaned them, replaced the green gaskets, cleaned the first finger-length of the intake manifold (it was oily as a teenager's face) and - most importantly - adjusted the TPS so it was reporting closed when the throttle was actually closed.

The whole issue was that the TPS wasn't adjusted correctly, so it was reporting to the ECU that the throttle was closed when in fact you were anything up to about 1500RPM - so actually closing the throttle caused the ECU to wonder what's going on with it's mixtures, then the revs dip, splutter, fart, recover.

So yeah, no actual faulty bits (Aside from the O2), but did clean out a mile of oily gunge, and sat down for 15 minutes with the TB, screwdriver, multimeter, feeler guages, and adjusted away merrily until it reported closed / open / wot correctly.