mx5cartalk.com

Phil Barnett wrote:Troubleshooting

I would attack the problem in the following order, least expensive to most expensive:
- Not the least expensive, but if you have a 1990 or early 1991, you may have a Short Nose Crank. These are known to fail and cause engine running issues. Search engine use with the words - MIATA SHORT NOSED CRANK should get you more reading than you wanted.
- Set the Base Idle to 850 rpm with TEN and GND connected at the diagnostic block.
- Set the ignition timing to factory spec. This will interact with the idle speed so you'll have to go back and forth until you get both of them correct.
- For the NA6, the factory spec for setting the dash pot is to adjust its screw until it just touches the throttle pulley flange at 2500 rpm. Lubricate it while you are there.
- Check the crossover tube for leaks and cracks.
- Check the clutch pedal blue plastic rivet is in place.
- New PCV Valve.
- If no automatic transmission, check the plug on the back of the manifold for leaks. If automatic, check the modulator for diaphragm leaks.
- Check for vacuum leaks. You can pinch off small tubing with a pair of pliers. If you can pull a vacuum hose from it's nipple with little friction, that hose needs to be replaced. You can use an unlit propane torch to locate the source of a vacuum leak. When the propane arrives at the leak, the engine will change speed.
- Check the Injectors for air leaks or replace the injector seals.
- Test the Brake Vacuum Booster for leaks from the point it attaches to the intake manifold. (use something like a Mity-Vac to see if it can hold a vacuum)
- Check that the throttle plate is returning to the stop.
- Check for a binding throttle cable.
- If you have a 94-97, clean the MAF sensor with an approved cleaner.
- Test and adjust the Throttle Position Sensor.
- Check for loose connections to all the sensors and the computer.
- Test all the sensors to see if they are actuating.
- Test that coolant is flowing through the ISCV.
- New spark plugs and wires.
- Replace the engine thermosensor.
- Clean the ISCV. Instructions are in the Miata.net Garage. New throttle body and ISCV gaskets.
- It could be an internal leak. Remove the EGR valve and test it for leaks. If you can blow through it, it's bad. This is a vacuum leak you can't hear when the engine is running.
- Replace the EGR valve.
- Replace the ISCV. If you haven't replaced them yet, new throttle body and ISCV gaskets.

And I hesitate to add, replace the engine control computer. Obviously, it would be best to do a substitute test before laying out this kind of coin.

manga_blue wrote:Any more?

techno wrote:So now here is the procedure.
1) Make sure that the engine is warm, but not hot.
2) Turn off all electrical items (fan, radio and lights)
3) Start the car and after it settles into a regular idle notice where the needle is on the tachometer. It should be just over the graduation mark below the "1". (around 850/900 rpm)
4) Bridge the TEN and GND cells
5) You should notice the idle speed change after a few seconds.
6) Have a look at the tachometer. If the ambient idle speed moved then you have discovered the problem.
7) Slowly turn the idle screw until the revs come up to just over the graduation mark on the tachometer.
Do this slowly, as there is a slight lag in screwing and response.
9) If the radiator fans come on during the process then stop adjusting and wait for it to go off.
10) When you have it idling where you think it should be remove the bridging wire. In a few seconds the ECU idle setting will kick in. If the engine remains at the same rpm then you have matched the ambient idle to the ECU setting.
11) If not, then replace the bridge and adjust the idle screw,
12) Repeat steps 10 and 11 until the idle speeds are matched.