Intermittent high idle problem- need diagnosis help!

[bensale]

The problem is that for no reason when my 1990 na6 is warmed up it will idle at somewhere around 1500 rpm for no reason. When the car is cold there does not seem to be a problem. However, sometimes the car will idle perfectly irrespective of temperature.

After reading this tread I decided to replace my idle speed controller. I purchased a new (second hand) throttle body off the mx5 centre only to find that I still have the same problem. However, the only difference now being that when the car is idling normally and the A/C is turned on the idle speed will drop around 400 rpm instead of increasing to 1100 rpm. Previously the idle speed only dropped when the car was idling at 1500 rpm.

I checked the resistance of my old idle speed controller and found it was 13.5 ohms, indicating to me that the ISC is not actually at fault...

So what do i try next? Help!

[devolutio]

i have the exact same problem. but i'm lazy so i haven't really looked into it other than reading the internest.

[Uncle Arthur]

perhaps its your temp sensor (green top) - mine was recently doing exactly the same thing - revving high and over revving - basically pumping the engine so full of fuel that it would flood and stall. Given the temp sensor controls idle (by way of controlling the electronic choke) it makes sense.

[bensale]

Thanks Uncle Arthur, I thought it may be a sensor of some kind, that makes a lot of sense. I'm working at the moment but will check it out when i get home.

The sensor sounds like a PITA to get to... Did you have to remove the coil pack to get to yours? Sounds like I may have to make myself one of those wizz bang ECU code reading lights and check it out that way?

[Uncle Arthur]

Well - it wasn't hard to get to on mine because I've moved all the sensors to the front of the engine with a coolant re-route....

But yes - it's a bit fiddly, but still is a pretty easy job if you remove the coil packs (3 bolts) and/or the CAS - loosen two bolts and remove another two and make sure you mark it before you pull it.

I didn't bother testing that sensor - I just replaced it. Based on what was happening (the car running way rich and over revving) it just made sense to change that sensor.

[bensale]

I'm not certain if my car is running rich though which confuses me. I keep a mental log of my fuel consumption and its been constant at around 8.5-9l per 100 kms even since the problem has occured. Although if its not the ISC then it would make sense if it was that sensor. Where did you buy the new sensor from?

Nice mod by the way, I can see how that would be very handy! I dropped by a dick smith on the way home, absolutly useless... No electrical stuff what so ever, just tv's and mobile phones... So i'll have to try anouther one I know has somewhat of a range or go to jaycar when I can find the time. It doesn't sound too hard to change from what you're saying (excpet the CAS- I hate removing that) so I hope that is the problem.

[Rotary]

It may still be in idle control. As Uncle said when water temp sensor is faulty it will run very rich, the computer thinks engine is cold and runs cold fuel enrichment, so won’t be that if it’s not running rich

According to a net sourced 1990 workshop Manual, there’s a Coolant operated Air Valve which could be clogged up so intermittent malfunctioning, try the tests on F99 below.

http://users.tpg.com.au/mattgaz/WS%20Ma ... 90-F97.gif

http://users.tpg.com.au/mattgaz/WS%20Ma ... 90-F98.gif

http://users.tpg.com.au/mattgaz/WS%20Ma ... 90-F99.gif


With new TB does the idle drop from 900 to 500 with A/C on? If so, its possible that the ISC in that isn't working, but worked fine in the old one, unless there’s an electical problem, which would be my next check after the Air Valve.

[bensale]

Ah excellent! Thanks for that Rotary! I'll give that a go on the weekend. I really appreciate those links

Confusingly initially with the new TB the idle did drop from 900rpm to 500 with the A/C on. However, today it didn't drop, it went up as it should...

I'll also do the engine diagnostics check to see if it shows any fault codes. I got the bits to make one of them today

I'm scared that it may be electrical... I figure mechanical issues are fixable, essentially a part is broken its just a case of finding which one. I'm scared that chasing down an electrical problem will be like finding a needle in a haystack...

[devolutio]

sorry for hijacking your thread, but i just scientifically guestimated that my car is using around 11 litres per 100km. that combined with a idle that moves around 1200rpm.... i should probably check that sensor. will post results.

[Juffa]

Hey Ben. Do the revs change if the clutch pedal is pushed down, and the car is in gear..and foot off the gas pedal?

Could be the small white switch mounted on the clutch. In the USA this stops people from starting the car unless the clutch pedal is pushed in. In our cars the switch senses when the clutch is in or out, and combined with the gearbox neuteral switch it feeds info to the ECU. The switch on mine broke. With the car in gear, the clutch pedal in and gas pedal not depressed the engine revs sat at around 1500 Rpm. A small spring in the clutch switch had broken. I went to Jaycar. Purchased a $2 swith ripped the spring out and used it in the Mazda switch.

J

[Rotary]

I'll also do the engine diagnostics check to see if it shows any fault codes. I got the bits to make one of them today

I'm scared that it may be electrical... I figure mechanical issues are fixable, essentially a part is broken its just a case of finding which one. I'm scared that chasing down an electrical problem will be like finding a needle in a haystack...

Sounds like you have some electrical knowledge, which will help,

Electrical problems aren’t too hard mostly involving voltage checks at the start and finish (i.e. ECU and ISC) and checks for continuity with a good WS Man to guide which points and paths to check.

The difficulty would come when its intermittent, then we need to establish what the voltages are in normal operation (either by WS Man guide or by testing in operation)
then what’s going on with voltages when the fault is occurring at ECU and ISC

As pointed out by Juffa, there’s often a few parts that can give similar problems, I would start by checking what's easiest in the most likely category.

[bensale]

Juffa, 'Manga blue' recomended that to me in anouther thread, I checked it out and it seems that is not the problem. The idle problem is irrespecive of the car being in gear, out of gear, clutch in or out.

Rotary, I'm begining to lean towards it being an electrical issue of some kind. I simply haven't had the time to do any real diagnosis as I've just taken on a massive amount of work... Hopefully I'll have some time tomorrow.

It has really been confusing me for the last few days. The idle has been better, more often than not it will eventually settle to the correct idle speed. Sometimes when coming to a stop it will idle at 1500rpm for a few seconds and then drop to 850. Othertimes it will stay at around 1500rpm for a while longer but it can be prompted to idle correctly by placing electrical load on the car (ie holding the window switches down or similar) However, it has at no stage settled to the correct idle immeditatly as the car used to.

Also thank you so much for the PM, I just downloaded the PDF

[bensale]

I managed to get the time to get the engine diagnoistics tests done... I'm even more confused... It flashed 6 times? I can't find any mention of what that means...

[physco88au]

i have exactly, i mean exactly the same issues....very interested in the results

[Juffa]

Ben,

Do you have access to any MX-5 workshop manuals....I have a Enthusiasts guide and a Haynes if you want to borrow them?

The only catch is you have to come on my 'First Run of Summer' run this Sunday.

J