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Erratic idle

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Erratic idle

Postby Pops » Sun, 09 Apr 2017 1:15 +0000

Possible fix for erratic idle after disconnecting battery.
I had the erratic idle problem after disconnecting the battery, tried all suggestions to fix but nothing seemed to work, but as time went by the problem seemed to be getting better (but very slowly )but still a right nuisance in traffic with the revs fluctuating from about 400 to about 1200 on idle.
The reason for disconnecting the battery....:I had been trying various things to cure what I felt was a lack of power and heavy on fuel ,including new fuel pump,temp sensor etc, (after doing the timing belt the power has returned) it was suggested that I disconnect the battery and press the brake pedal to reset the ecu. It was after this that the idle problem erupted , I tried all suggestions to no avail , so started backtracking what I had done in the past. Somehow I decided to start by refitting the old fuel pump, when I released the pipe from pump TO engine there was massive pressure in it,( no noticeable pressure in the return pipe),so much that it sprayed fuel about 10feet behind the car . So without disconnecting the battery I refitted the pump, started the car and have had no problem since , I believe the cure was releasing the fuel pressure and letting it reset itself . Might be worth trying for anyone who has the same issue.
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Re: Erratic idle

Postby Real Rayzor » Sun, 09 Apr 2017 5:00 +0000

That's very interesting Pops... as you know I've the same problem and have tried everything as well. Where did you release the pipe.. At the engine side or at the pump.
2002 MK2.5 Sunlight Silver (sold)

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Re: Erratic idle

Postby Pops » Sun, 09 Apr 2017 5:28 +0000

Real Rayzor wrote:That's very interesting Pops... as you know I've the same problem and have tried everything as well. Where did you release the pipe.. At the engine side or at the pump.


Ya, at the pump ( was changing it anyway),pressure was high enough to spray fuel about 10feet behind the car.
I really don't know if it's a cure or not but I haven't had the problem since so it has to be worth trying.
John
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Re: Erratic idle

Postby TOM 5252 » Sun, 09 Apr 2017 8:09 +0000

Hi Guys
There are instructions in the Haynes manual for releasing this fuel pressure. It is slightly different for different years of MX-5,but essentially says to start the engine and then remove the fuel pump relay and wait for the engine to stall. Then switch off the ignition. So you don't have to open any pipe joint to release the fuel pressure to try a reset.
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Re: Erratic idle

Postby Real Rayzor » Mon, 10 Apr 2017 12:53 +0000

Thanks Tom.. will give that a go.. TBH... I was was having visions of petrol flying all over the place. Lol
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Re: Erratic idle

Postby Steve » Mon, 10 Apr 2017 7:32 +0000

fuse is 2nd from front in the engine bay fusebox (30A).
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Re: Erratic idle

Postby Pops » Fri, 14 Apr 2017 3:31 +0000

Did you try it yet Ray ? I've had no problems with the idle ever since without doing anything else.
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Re: Erratic idle

Postby Aerofoil » Sun, 05 Nov 2017 12:32 +0000

Hi guys, lots of interesting comments re the fuel pump. Thanks to all.
Can anybody help me with a problem setting the idle speed on my mk2.5. It varies from 500 rpm to 1200 rpm. Whenever i try to set it, (on the throttle body) it returns to the previous setting. I read somewhere that its controlled by the ECM, so how can i change it.
Would be grateful for any info.

Phill
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Re: Erratic idle

Postby Pops » Sun, 05 Nov 2017 4:33 +0000

Hi Phill, have you checked for vacuum leaks also splits in the intake pipe ( you’ll have to remove it for proper inspection), it’s possible the air flow meter might be faulty, try disconnecting it and running for a while, there should be a bit of a difference in the way it runs connected/disconnected
Unfortunately when things go wrong there’s many reasons why, faulty parts such as idle control valve, air flow meter,cam angle sensor, ecu etc.
I know this link is for a mk1 but worth a try
https://www.miata.net/garage/KnowYourCar/S7_Idle.html
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Re: Erratic idle

Postby Aerofoil » Mon, 06 Nov 2017 12:03 +0000

Thanks for the info Pops. Trouble is i dont know where to find some of the items you have mentioned, despite having a Haynes manual! I have an appointment for my NCT test in a couple of weeks so have to find where the problem is. I haven't cleaned the throttle body yet, or any of the other items you mentioned, so I'll have to get my ass in gear. I have already fitted a new cam belt , valve stem oil seals, oxygen sensors, plugs ect and spent €1000 at garages with no results.
Many thanks for your reply.
Phill
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Re: Erratic idle

Postby Pops » Mon, 06 Nov 2017 1:19 +0000

Try doing the ecu reset by disconnecting the battery for 10min ( don’t know if you need a radio code)and keep the brake pedal pressed for a few mins while it’s disconnected, you never know, it might help
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Re: Erratic idle

Postby Westcoast » Thu, 09 Nov 2017 2:28 +0000

Be careful cleaning the throttle body. You don't want to use anything too abrasive else you could allow too much air past the throttle butterfly (when closed) and make things worse. I have read reports that the inside of the throttle body is coated with a plastic coating or something, to make it smooth and create a good seal for the throttle butterfly, but I don't know whether that's true or a myth. But just in case, don't use anything too aggressive.

Idle control is done by the ECU using the IAC (Idle Air Control valve) which is located next to the throttle body where the plastic intake pipe meets it. The ECU uses the IAC to allow a varying amount of air to bypass the throttle butterfly. Idle problems could indicate a problematic IAC or of course an electrical problem such as bad connection or even bad battery or alternator which is causing the voltage to fluctuate too much.

The US Haynes manual I have for my mk1 1.8 is decent for what it does cover, but the 'Rod Grainger' workshop manual is a lot better in terms of detail.
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Re: Erratic idle

Postby Aerofoil » Sat, 11 Nov 2017 12:34 +0000

Thanks Westcoat , will center my checking around the items you mentioned. Seems to me I would be better off selling the mk2.5 and buying a mk1. There's a lot more info on this car than the later model's, especially on the net. Anything you alter on the later model's are corrected by the ECU.
So your back to square one.

Anyone want to buy a Mk 2.5!!
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Re: Erratic idle

Postby Westcoast » Wed, 15 Nov 2017 9:54 +0000

I'd suspect that the idle speed variation is due to the ECU trying to compensate (perhaps using the IAC) for some other problem. In addition to what's been suggested, I would also check basics like...
- spark plugs: check for condition and correct spark gap
- spark plug leads (or does your mk2.5 have coil-on-plugs?)
- base ignition timing: Advancing the ignition timing a touch is a common, basic mod on MX-5s, or at least on mk1s and mk2s anyway - I must admit I'm not sure about the mk2.5

Oh, and when trying to troubleshoot an idle speed problem, try to make sure as much electrical stuff is switched off as possible - lights, stereo, fan, heated rear window, etc. Even a failing battery can cause high load on the alternator. If the ECU can modulate the idle speed to compensate it may run at a fast idle. If it can't, the high load on the alternator and the engine may cause a low idle.
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