1.8 idle

Post here if you require help or have information that may assist others with their MX-5.

1.8 idle

Postby rustyarchs » Fri Oct 02, 2015 12:54 am

Right guys i'm about pulling my hair out here and needing some advice!

Car is a 1.8 na 1998 reg

From cold the car starts and within a couple of minutes it has as very slight misfire but that's not really what my problems is just thought I'd add that in so all details are included, any way the cars I'dle starts around 1000rpm them quickly rises to 1200, stick the car in gear and the revs stabilise around 900 drive around for a bit and stop in gear idles at 900ish till again as soon as neutral is seected revs idle around 1400 drive about again stop in gear alls fine at 900 but as soon as its in neutral its now climbed again to 1600 and this scenario just keeps continuing till its at around 1800 and I'm now home,

I'm a bit miffed here as the revs climb after every stop so surely can't be the base idle?

Revs are as steady as a rock when stopped in gear at 900ish but as soon as neutral is selected it climbs and seem to go up every time depending on length of journey so I'm guessing temp related? But after some guidance on possibilities before i start ordering more random things lol

Nothing is showing up through the diagnostic port

List of part changed so far

Fuel filter

Parts checked for resistance with an ammeter and reading fine


Contact cleaned the maf and pretty much every plug and earth in the bonnet bay, fired fuel treatment through the tank, also used combustion chamber cleaner through the intake which certainly seemed to clean everything up and make a lovely smoke screen for 10mins

Can't find any obvious air leaks and I've disconnected the battery numourous times

Prob one or two other things I've done but can't remember as it's taken me a few weeks to get this far lol

Posts: 7
Joined: Thu Sep 03, 2015 2:15 am

Re: 1.8 idle

Postby drumtochty » Fri Oct 02, 2015 9:37 am

There is a switch on the gear selector that makes contact when the car is out of gear and that sends a signal to the ecu to say do not let the revs drop too far.

So it appears the ecu signal to stabilse the idle is getting confused.

Are you saying that you have an led and a shorting link that have been connected to the diagnostic port.

Have you set the base idle, google for the procedure.

Have you checked the throttle position sensor again google for that.

Then possibly a case of cleaning the idle control valve biu you need to buy some paper gaskets before you do that.
99 Mk2 1.8 IS Silver
08 Mk3 2.0 RC Sport Copper Red Sold
18 Mk4 2.0 GT+
User avatar
OC Member
Eddie Cairns
MX-5 OC Club Member
Posts: 2685
Joined: Thu Jun 18, 2009 1:42 pm
Location: Drumtochty Glen, Auchenblae

Return to Technical

Who is online

Users browsing this forum: No registered users and 3 guests