Intemittent Idle Droop

mobymidge · Post by **mobymidge** » Sat May 14, 2011 5:01 pm

I hope some one can help with this issue I am having with 1991 NA 1.6 manual.

I get intermittent Idle Droop when coming to a stop. I have cleaned the throttle body, replaced the throttle body and ISC valve, checked the idle speed, checked the TPS, replaced spark plugs and leads, nothing has helped. When the issue occurs, I can jump out and bridge pin 10 and grd and the idle instantly steadies out.

The issue has only started since I had a flat battery (battery has been checked out as well). Any help would be greatly appreciated, this is driving me nuts!!!

WideOpenThrottle · Post by **WideOpenThrottle** » Sun May 15, 2011 8:42 am

Hows your air filter? Is it clogged?
I would also be checking that there are no leaks or cracks in the tubing from the air flow meter to the throttle body.

Does the idle droop below the normal idle rpm or hesitate above it then drop down to normal rpm?

Crank angle sensor?

Maybe check for vacumm leaks to the brake booster & charcoal cannister.Its always aprocess of elimination but maybe someone else can nail it!

WOT

Peter Stevens · Post by **Peter Stevens** » Sun May 15, 2011 12:25 pm

Hi
I had much the same problem with my 2002 NB vs manual after much trial
& error & 3 trips to so called experts,one who thought it was the mass air
sensor which he
took out to clean it ,while doing so he broke the hot wire off, stuffing
it completely it had to be replaced,was not the problem, the throttle--body
was removed and cleaned-
still no joy-eventually the cam shat position sensor was replaced,problem
nearly fixed. while fiddling with every thing the factory throttle butterfly
stop got moved,now does
any know how to reset this to its original factory setting

Peter

[Posted via external email]

brownie · Post by **brownie** » Sun May 15, 2011 10:08 pm

O2 sensor maybe?
Mine does it as well,Normally on a hot day,usually settles down after a bit,quite pronounced after fitting the light fly wheel, so much it would stall the car at intersections when the idle was doing it's thing.All i did to fix that was adjust the throttle damper.
i have read else where similar problems and seems to go back to the O2 sensor.
I keep meaning to change mine with another ...but always runs fine until a hot day comes along and then who the hell wants to go under the bonnet on a hot day? and the sensor at work,i'm home.....ahh bugger it worry about it another day

WideOpenThrottle · Post by **WideOpenThrottle** » Mon May 16, 2011 5:26 pm

I ran my 89mx which has the single wire O2 disconnected for a while until i got my wideband so maybe you should disconnect yours & see if the problem goes away.
I was told they burn after 80thou k & the single wire o2 does stuff-all anyway!

mobymidge · Post by **mobymidge** » Sat Jun 11, 2011 6:15 pm

Okay, I'll update what I have done so far.

I've replaced all the vacuum hoses. Still had the problem
I've changed the spark plugs. Still had the problem
I've confirmed the ISCV is working
I've reset the engine timing. Still had the problem
I've reset the TPS. Still had the problem
I've replaced all the electrolytic capacitors in the ECU (after reading this on a US forum). Still had the problem

I have just unplugged the O2 sensor. Still has the problem.

The O2 sensor was replaced about 12 months ago with a 4 wire type.

I have run it without the air filter on and still had the problem so can rule out a blocked filter.

I have found that I can trigger the issue by gently opening the throttle very slightly from under the bonnet. The engine revs will immediately drop and it then goes in to its cycle of revving high and then nearly stalling. I can do this with GND and TEN bridged and also without it bridged. Does this mean I can rule out a sensor issue? Have checked for error codes and none are present.

I replaced the plug leads about 6 months ago (didn't coincide with the issue starting) with a set of Champion leads from Repco when the mx developed a miss fire while out shopping after I found one broken lead. I see that most websites recommend the NGK leads so could this be my problem?

Thank you for all the advice so far.

sprsta · Post by **sprsta** » Sat Jun 11, 2011 6:52 pm

i have a spare ecu if you want to try plugging that in

mobymidge · Post by **mobymidge** » Sat Jun 11, 2011 8:51 pm

Thanks sprsta, I have just boiled my air valve and will see if that has made any difference. If that hasn't worked then I will take you up on your offer.

Do you have a spare AFM to try as well?

KSCRIM · Post by **KSCRIM** » Sat Jun 11, 2011 11:28 pm

For Peter;- I had a problem where the throttle was sticking shut. I found that the stop screw was in the wrong place, allowing the throttle butterfly plate to touch the body and stick. I reset the stop to just hold the throttle open enough to not stick. The adjustment was very fine to make, about 5 degrees between sticking and free.

With the engine stopped, back the stop screw out until the throttle is sticking, then screw it in until it just touches the stop lever, then in another 5 degrees. I used the allen key with the short length in the screw and the long length to turn the screw accurately.
Then reset the idle mixture with the air bleed screw to get the right idle RPM.
If it still tends to hunt, adjust the stop screw in another 5 degrees and reset the RPM's with the air bleed screw.

There is probably a setting procedure used in the factory, to insert a fine wire in between the throttle body and the butterfly edge as a feeler gauge, and then set the stop. but I don't now what the diameter of the wire would be.
I do remember reading about the procedure in some car manual somewhere, sometime.
I would think it would be about 0.5mm diameter or so.

Peter Stevens · Post by **Peter Stevens** » Sun Jun 12, 2011 9:12 am

Hi , Thanks KSCRIM for the info ,I got a tip from a mates son who was a
techo with the McLaren works team which fixed the problem,
I put a cigarette paper in the butterfly and adjusted the stop screw
until I could just pull the paper out. The main cause of the original fault
was the camshaft position censer which was found after much trial
and error(mostly error) during which the throttle factory set was
played with upsetting the idling ,now after resetting as mentioned
,the engine is now running like a new one.

Cheers Peter

-----Original Message-----
From: KSCRIM
Sent: Saturday, June 11, 2011 11:28 PM
To: electrical@mx5forum.co.nz
Subject: [MX5Elect] Re: Intemittent Idle Droop

For Peter;- I had a problem where the throttle was sticking shut. I found
that the stop screw was in the wrong place, allowing the throttle butterfly
plate to touch the body and stick. I reset the stop to just hold the
throttle open enough to not stick. The adjustment was very fine to make,
about 5 degrees between sticking and free.

With the engine stopped, back the stop screw out until the throttle is
sticking, then screw it in until it just touches the stop lever, then in
another 5 degrees. I used the allen key with the short length in the screw
and the long length to turn the screw accurately.
Then reset the idle mixture with the air bleed screw to get the right idle
RPM.
If it still tends to hunt, adjust the stop screw in another 5 degrees and
reset the RPM's with the air bleed screw.

There is probably a setting procedure used in the factory, to insert a fine
wire in between the throttle body and the butterfly edge as a feeler gauge,
and then set the stop. but I don't now what the diameter of the wire would
be.
I do remember reading about the procedure in some car manual somewhere,
sometime.
I would think it would be about 0.5mm diameter or so.

------------------------
Kevin 98NB6C

---------------------------------------------

Sent from an mx5forum.co.nz topic here:
http://www.mx5forum.co.nz/viewtopic.php?p=46117#46117

---------------------------------------------

[Posted via external email]

sprsta · Post by **sprsta** » Sun Jun 12, 2011 10:17 am

nah dont sorry i gave mine away

mobymidge · Post by **mobymidge** » Sat Jun 18, 2011 7:58 pm

Well I think I have finally got it sorted.

I found on a US forum details concerning an adjustment screw on the AFM. It is hidden under a metal cap, the cap requires drilling out. Once exposed, turn the screw anti clockwise (count how many turns it takes, mine took 1 1/4). Now turn it 4 turns clockwise. Hey presto, no idle droop.
The screw apparently alters the amount air that bypasses the vain on the AFM, therefore leaning off the mixture.

WideOpenThrottle · Post by **WideOpenThrottle** » Sun Jun 19, 2011 9:28 am

Sounds a bit drastic? got any photo's or links to thee site?

mobymidge · Post by **mobymidge** » Sun Jun 19, 2011 12:40 pm

Here is the link to the forum;

http://forum.miata.net/vb/showthread.ph ... actory+set