Base idle

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Base idle

Postby graeme » Wed Mar 25, 2009 9:17 pm

Something I've noticed over the last couple of days. The car runs fine, but when I pull up to lights/roundabouts/junctions, the base idle sits at 1500 rpm for a few seconds then drops to 8-900 where it should be. I've heard of this happening but can't remember what the reason is. Is is something to do with the AFM?

Did I do enough in that question to make it sound like I knew what I was on about? :lol:
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Postby Dot and Chas » Wed Mar 25, 2009 9:32 pm

Graeme,hopefully Sarahlc will be along shortly! :wink:
Chas............Fire up the Z-Sport!

Get Busy Living!!!!
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Postby sarahlc » Wed Mar 25, 2009 10:19 pm

Thanks Chas, no pressure then :wink:

Check that you don't have a vacuum leak from anywhere and that no Air is being drawn in after the the MAF meter (1.8's don't have an AFM they have an Mass airflow meter) from say a problem with the PCV valve on the Cam cover.
Also check that the Throttle cable is not sticking.
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Postby Dot and Chas » Wed Mar 25, 2009 10:38 pm

wot she said!!! :lol:
Chas
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Postby Richardn » Thu Mar 26, 2009 9:11 am

need to diag it also as it could be a temp sensor, or the ISC playing up.
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Sexist

Postby Davy » Thu Mar 26, 2009 1:43 pm

I don't want to sound sexist here, but I think the women know more about the workings of the MX5's than the men....

Now I know who to ask :wink:

If I get stuck..... :?
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Postby Dot and Chas » Thu Mar 26, 2009 3:31 pm

:lol: :lol: :lol: :lol: :lol: :lol:
Chas............Fire up the Z-Sport!

Get Busy Living!!!!
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Postby Dot and Chas » Thu Mar 26, 2009 3:40 pm

:lol: :lol: :lol: :lol: :lol: :lol:

OOPS DOUBLE POST :oops:
Chas............Fire up the Z-Sport!

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Postby graeme » Thu Mar 26, 2009 9:29 pm

Was any of that in English? :shock:

So, do I a: take it to a garage
or b: ignore it?

I am a big fan of "b" whenever possible I might add :wink:
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Postby Richardn » Fri Mar 27, 2009 9:00 am

diag is done with the diode plugged into the little black box. That tells you if a sensor has gone.

check for air leaks by moving hoses and listening for a slight whistle
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Postby mgrays » Fri Mar 27, 2009 12:00 pm

The way I would approach is;

1. start and allow to warm up
2. find the throttle body on LHS of engine
3. look under to right of throttle for a green or blue electrical connector
4. push in the little tag on this connector and pull it off.

a. does engine rpm rise = base idle is not right
... air leak.. go look at the back of the inlet manifold beside bulkhead for the little black cap (about 10mm OD, 25mm long) that points backwards.. these do fall off sometimes.. make sure all the hoses are on every pipe on the inlet manifold .. and they have not cracked etc by wobbling them and listening for the rpm change. Basically push and pull everything on LHS of engine but not your plug leads.
... someone messed with car years ago and wrecked base idle (sure it was OK last time I looked but that was nearly a decade ago :wink: )
... temp sensor duff so the Choke is out in old speak! Need to do the thing with the diodes to find this.. maybe garage or find a techie 5 bod! Fuel consumption will have gone up and it will not feel as crisp an engine. Damage will occur with this.. Cat will die, bores will wear, oil will be diluted so more wear etc.

b. engine rpm does not rise
.. your Idle Speed Control valve is gummed up and has got sticky .. happens with age .. really needs pulled apart and cleaned up but that does mean breaking into cooling water so job for the garage.. just make sure they do not mess with throttle body setting .. ISC valve is what you just pulled the plug off BTW :wink: It sits under the throttle and controls idle using the ECU from 2000 rpm and lower

c. engine idle erratic with/without plug on
.. the throttle switch is broken/out of adjustment .. garage job.. but does not sound like that. (Switch that tells ECU the throttle is closed and so it should be controlling idle with the ISC)

If it is messed with car .. and at end of process.. set the base idle using the big screw down the hole that points upwards on the left of the throttle. If really lucky this will have a plastic cap over it still but most lost them years ago. DO NOT ADJUST THE THROTTLE STOP (as 50% of useless mechanics will do) as it will wreck the throttle body lining, your throttle will jam wide open and generally you will start to fear stopping at junctions. Throttle stop is the screw that the throttle rests against when the accelerator is at idle.


Now your original option "b" to ignore.. well.. as long as it is not the temp sensor (ie driving with choke on).. it will not harm... but expect it to go the other way some day so it dies at a junction and you will then start to lose faith.. but I guess it should get better as the temps rise if it is ISC valve but then it will get worse in as the temps fall.. My money is on the ISC valve being gummed up ..

Now you are super confused! :lol:
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Re: Base idle

Postby BigChris » Fri Jun 11, 2010 2:24 pm

I've got what sounds like the same problem on a Mk1 I bought a couple of months ago:

Uses a ridiculous amount of fuel, even compared to my 1994 1.6 Audi 80.

I can almost watch the needle drop on general driving.

Sometimes on start up engine races at about 1400 rpm instead of the usual 950 or so, other times it doesn't do this and acts normally.

The guy at mx5 parts thinks could be water temp sensor?

Does anyone have a diagnostic tool to plug into the little back box to check for a failed sensor? or know a place who would look at this for me without:
a) ripping me off hugely
b) replacing parts that are working fine
c) blundering about in the dark and causing new problems
;-)

That would be great, thanks guys and girls and hope to meet some of you at the next tech day

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Re: Base idle

Postby Richardn » Fri Jun 11, 2010 3:43 pm

Diag tool is a led and a piece of jumper wire/paperclip pennies at tandy or http://www.mx5parts.co.uk/product_info. ... ucts_id/35
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Re: Base idle

Postby Bobjasper » Tue Apr 28, 2015 10:33 pm

Hi there, I have been following this chat and I have a similar problem with my mx5, I hope you can help. My car is a mk 2.5 I believe, 2002 in the year 1.6. My problem is the car idle's at 1400 rpm but when I depress the clutch it reduces by 400/500 rpm when cold, 1200 rpm again when clutch depressed it reduces by 200/300 rpm when warm.
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Re: Base idle

Postby drumtochty » Tue Apr 28, 2015 11:29 pm

Where are you based and in your case go on to ebay and buy a cheap code reader anything about £15 max.

It is plugged in just below the fusebox at your right knee when sitting in the divers side seat.

That will display and fault codes.

Do not start the car just put the ignition on to read the codes.

Why did you post rather than just follow Malcoms simple procedure.
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