Idle droop?

[skams]

Hi folks
I have been whatching with interest the posts on idle droop but my little problem dosen't seem to fit anything that has come up so far:
Often when I stop at a giveway or stop sign I notice that the idle speed will momentarily drop off almost the point of the engine stalling and will then bounce back to normal, say around 750-800 rpm. I have also noticed the general reluctance for the engine to run at a speed just above idle, I can sit there with the engine hot & idling and gently push down on the accelerator and the engine speed will increase momentarily then fall back to idle. I can continue pressing down on the accelerator and the same thing will happen untill I reach a point when the engine speed will suddenly leap up to about 1200-1400 rpm. If I now reduce the pressure on the accelerator, the oposite will happen but not as dramatic or not at all.
I have had the car for comming up 4 years now and it has had the same symptoms pretty much since I have owned it and as far as I know is pretty much a std 1990 model eunos (MX5).
The car otherwise performs faily much as advertised and on a trip returns arround 36 - 40 MPG ( or arround 13 -14 km/litre). The last time I checked nothing showed up on the self diagnostics
Any one got any ideas as to where I should start looking before I start tearing things appart.

Cheers Simon Senior

[DBM58]

Hi Simon

There was a discussion recently on the Sebring Supercharger Forum that (perhaps) covered a similar problem, encountered after someone had installed the supercharger kit themselves. Quotes from the discussion are below:

Initial post:
To state the problem:

1. Car run's perfectly when cold.

2. As soon as Normal Operating Temperature is reached, no matter how
hard accelerator is pressed, (in gear or in neutral), engine will not
rev above 2250 rpm.

3. To best describe the fault, imagine a young driver sitting at
traffic lights blipping the throttle repeatedly.
Brum.....Brum....Brum.....Brum!

4. Switch engine off for a few minutes (about 10 or so) and engine
will run fine, but as soon as engine warms up, the problem comes
back.

5. To fill you in on the tests I've done so far;
-I've changed all plug leads and plugs.
-Put jumper lead from GND to TEN (Makes no difference)
-Used LED in diagnostic box, (No fault codes logged)
-Disconnected all visible electrical connectors and soaked with
electrical cleaner.

First reply:
The only thing you can have disturbed is the throttle switch...

I know it makes no sense re heat but it is the only connection you've disturbed.

Get new wire and re do both ends.

Second reply:
It is the TPS. The Throttle Position Sensor is sending a signal that the
throttle is closed so any speed above 2250 will have the fuel cut. Take the
black plastic cover off of the sensor and clean it of any water. Also make
sure the wiring is correct. The sensor signal is ON for idle OFF for cruise
and ON for WOT. You can completely detach the sensor wires and the ECU will
think the engine is at cruise all the time. You can drive the car this way
if you get stuck.

Perhaps someone who knows could confirm whether this is of any help.

[Growler]

I did have a similar problem with another car years ago.
It turned out that the intake hoses were not on properly allowing more air into the system. From memory the computer reads this and tries to compensate for it by richening up the mixture. It then senses it has too much fuel and leans it out. This is where the irratic idling comes from. (Revs rising and then dropping, rising and then dropping).
It may not be the problem in your case but something else to eliminate.

Growler.

[black bess]

Quite a lot of discussion about idle droop on the UK Forum. (www.mx5ocforum.co.uk) Solutions include spraying injector cleaner into the air inlet (remove the air filter) to cleaning out the idle air control valve. A worn throttle body is another suspect, but replacement is very expensive.