Erratic idle when car is warm

spacepants_fb · Postby **spacepants_fb** » Tue Aug 24, 2010 12:32 pm

Hi everyone. New to mx5s so I just wanted to ask if any one has heard of this issue. When the car's engine is cold, idle seems to be perfect. I drive around for approximately 10 minutes or so and what then happens is that the idle seems to drop from around the 1k rev mark to 500 and sometime below when the car is stationary (eg. waiting at the traffic lights). Has not stalled yet but it has come dangerously close to doing so at one stage last night whilst driving. Intermittent and there will be periods where all is ok, and other times where it is worse than normal.

Had a read of some of the posts on here and the other forums, and there appears to be a range of reasons which may cause this...but normally it occurs when the engine has not warmed up. In my case it seems to happen when the engine has already warmed up.

The previous owner has just had the leads and sparkplugs replaced (MSD leads and NGK iridium spark plugs), fuel filter has been replaced, battery etc. So not too sure where to start - 02 sensor or ICV? From what I can tell the car runs fine and is a joy to drive, except when I stop at the light, I'm worried that it might stall. Any ideas? Thanks everyone.

taminga16 · Postby **taminga16** » Tue Aug 24, 2010 12:56 pm

Check the PCV. If it does not rattle it is probably blocked. They can be cleaned with a little spray type engine cleaner.

spacepants_fb · Postby **spacepants_fb** » Tue Aug 24, 2010 1:28 pm

Thanks for the quick reply. I'll have a look at that on Thursday with a mate. We'll see if we can give it a good clean and hopefully that fixes the issue. Fingers crossed I suppose!

sliq · Postby **sliq** » Tue Aug 24, 2010 2:06 pm

which year model is it mate? i had a problem like this the first year i had my car, seemed to have gone away on its own

spacepants_fb · Postby **spacepants_fb** » Tue Aug 24, 2010 2:25 pm

Sorry I should have included that. It is a 90 model na6. I came across this on www.miata.net:

http://www.miata.net/garage/Engine_Diagnostics.html

I'll give it a try some time in the week to see if I can isolate the issue. Car's done 205k kms and from what my friend (who sold me the car) told me, a major service was done recently. The mechanic however could not determine what the issue is. Hopefully the diagnostic tool shown on miata.net will help me fill out the blanks.

93_Clubman · Postby **93_Clubman** » Tue Aug 24, 2010 2:30 pm

1990 NA6
this used to be referred to as 'idle dip' - you might have seen some of the following threads, but there's some good stuff in them:
search.php?keywords=idle+dip&terms=all&author=&sc=1&sf=all&sk=t&sd=d&sr=topics&st=0&ch=0&t=0&submit=Search

good to hear plugs & leads are new, but worth checking the ignition timimg if it hasn't already been done. however first try disconnecting the battery to reset the ecu.

spacepants_fb · Postby **spacepants_fb** » Tue Aug 24, 2010 3:28 pm

Thank you very much for that link. I've seen some of those threads but I didn't use "Idle Dip" as the search criteria. I'll have a quick read of them tonight!

93_Clubman wrote:1990 NA6
this used to be referred to as 'idle dip' - you might have seen some of the following threads, but there's some good stuff in them:
search.php?keywords=idle+dip&terms=all&author=&sc=1&sf=all&sk=t&sd=d&sr=topics&st=0&ch=0&t=0&submit=Search

good to hear plugs & leads are new, but worth checking the ignition timimg if it hasn't already been done. however first try disconnecting the battery to reset the ecu.

spacepants_fb · Postby **spacepants_fb** » Thu Aug 26, 2010 11:04 pm

Whilst trying to troubleshoot the cause of my erratic idle (cleaning the maf, cleaning the throttle body, resetting the ecu and resetting the manual idle) the coolant inlets near the thermostat cast iron unit snapped clean off! Had coolant spitting everywhere when I turned the engine on.

Good news is the actual part only costs $30. The bad news is that to install it requires the timing belt, rocker cover, etc. to be removed. MX5Mania have told me that it will cos all up $200 and that they won't be available for another 2 weeks. No zoom-zoom for 14 days. Bummer.

93_Clubman · Postby **93_Clubman** » Fri Aug 27, 2010 11:27 am

sounds like one of the ends the heater hoses connect to - haven't heard of them breaking, but corrosion of the ends of similar pipe connections happens. i guess the upside is at least it didn't happen on a long drive from home.

spacepants_fb · Postby **spacepants_fb** » Fri Aug 27, 2010 11:54 am

It's the little metal outlet I have circled. There is an inlet coming from the bottom and this is the nipple that has snapped:

93_Clubman · Postby **93_Clubman** » Fri Aug 27, 2010 1:55 pm

yeah, corrosion i've seen has been on the pipes from the bottom of throttle bodies. can depend on type of coolant used, or not used (i.e. water), & frequency of changes.

spacepants_fb · Postby **spacepants_fb** » Fri Aug 27, 2010 1:57 pm

93_Clubman wrote:yeah, corrosion i've seen has been on the pipes from the bottom of throttle bodies. can depend on type of coolant used, or not used (i.e. water), & frequency of changes.

I hope there are not too many corroded areas, but I guess it comes as part of the territory when purchasing an older car. Like you said though, I'm definitely grateful that it did not happen whilst I was on a long trip... :shock: