MK1 Eunos Roadster 1991 - High Idle and stalling when coming to a stop....except when AC is on

Technical Area Engine, Transmission, Exhaust etc
1

Morning all!

I’ve had my Eunos Roadster for a few months now, and recently it’s start having some behavior problems.

When starting the car up cold, it idles high at around 1,500rpm, when up to temp it’ll sit around 1,100 to 1,200rpm, it has done this since I purchased it a few months ago.

As of a month ago, it also started to have another problem, when blipping the throttle in neutral, the car will dip in RPM below it’s normally higher rpm, and feel like it’s almost going to stall, usually this would stop happening after 5 minutes more warming up, so I didn’t think much of it, until recently, as now it’s stalling whenever I was coming to a stop, the car would drive spot on, but then as soon as I get to lower RPM’s coming to a standstill it’ll drop to zero rpm and need to be started again, once started, it’ll pull off no problem. This problem does not happen when the AC is running funnily enough.

In the last three months, the car has had the below replaced::

  • Oil and it’s Filter
  • Spark plugs
  • Coolant
  • Timing belt and Water pump
  • Gearbox Oil
  • Fuel Filter (Fitted yesterday)

Does anybody have any suggestions of what could be causing this? So far from what I’ve read it could be:

  • Earthing points being corroded and needing a clean up (Did clean one but haven’t done the other few yet)
  • That I should put through some Fuel Injector cleaner
  • Need to re-set the base idle of the car

Just looking for opinions on the above incase I’ve missed anything! Fairly new to DIY maintenance so always like a 2nd opinion.

  1. My model of MX-5 is: 1991 Eunos Roadster 1.6L
  2. I’m based near: Northamptonshire
  3. I’m looking for technical help or recommendations on: High Idle and stalling at low revs when coming to a stop.
2

With those symptoms it’s most likely air leak(s) and/or throttle stop and base idle adjustment.

See the numerous threads in the Forum about these and the correct way to set Idle speed…

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3

So atleast I was somewhat heading in the right direction! I’ll have a crack at it after work today and report back.

4

Just to check @RichardFX does the below sound accurate for adjusting the throttle stop adjustment? Alot of what I find say’s not to touch it unless replacing the Throttle body, so wanted to double check:

With the car off after being warmed up, find the tiny throttle stop set screw. Unscrew it until it doesn’t make contact with the stop, tighten until it just touches, then a half turn more. Lock in place. Jump the GND and TEN terminals in the diagnostic port, start the car with all accessories off, and adjust base idle to 850rpm.

5

Correct. The throttle stop should only be adjusted when all else has been fixed.

However there are numerous plastic and rubber hoses all going to the manifold and air pipes AFTER the AFM (which measures the air flow).
Any one of those pipes or joins leaking will raise the tickover!
And stalling as you describe is a classic symptom of air leaks.

Moving the pipes gently one at a time at likely spots might vary the leak and change the engine note.
Leaks can often be heard as a faint hiss.
Sometimes a bit of small-bore plastic tube (fish-tank bubbler?) can be used to extend your ear to the point of a leak safely, without burying your head in the engine!

I’ve always used a mechanic’s stethoscope to listen to taps, grumbles and rattles, and take the diaphragm assembly off the sensing end of the plastic tube to listen for airborne noises. The earpieces are a bit uncomfortable and I modified those on this new one.

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6

Just an update. Checked all tubing after the AFM and couldn’t find any leaks or splits on any of the pipes, took them all out and off and inspected them all, any that were slightly loose I put back with new clamps as a precaution.

I did adjust the base idle, when initially bridging TEN and GND the call stalls, but after abit of adjustment first managed to get it going, after some playing around it now idles between 950 and 1050 RPM, and no longer stalls when driving at stops, is quieter at idle, and generally feels a tad more responsive when driving (Could just be the joy of it going and not stalling overtaking me!).

When lowering the RPM below this value to the standard 900ish, it flutters abit, but I’m wondering if the “AliExpress Grade” Mushroom filter the previous owner installed is adding restriction where it shouldn’t be, so next step is installing a stock airbox I picked up with a new filter and adjusting down where I can. Wouldn’t suprise me if they had some issues themselves and messed with the idle valve already, which I’m now fixing.

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