QuestionQUESTION: 1991 Mazda Miata will start and run fine as long as the car is driven down the road, but stalls and won't idle when at a stop. The fuel system has been flushed and the throttle body, idle air control valve, and tps have been replaced. It has new plugs and wires. The MAF has been replaced with another just to see it if that was the problem and still no luck. Any suggestions?
ANSWER: Get a vacuum reading. (have someone maintain 1000 RPM while you check these two)
Check your timing.
Check all air connection points between the throttle body and the MAS.
TPS units seldom fail.
Have you ever had water on the floor board passenger side?
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QUESTION: I believe that there has been. I know the top leaks and there has been water
on the drivers side. It would be safe to say that there has been on the
passenger side. Is there something important on the floor board of the
passenger side?
ANSWER: Yes, the ECU, the cars computer control unit is mounted under the carpet under an access panel where the passengers foot well is. Not the greatest location for and ECU, subsequent years they were relocated. Water of 1-2"+ can have an affect on the ECU, dampness retained in the jute padding can present moisture. The ECU is a last course of action however, when trouble shooting.
You can remove the rocker sill retainer and peel back the carpet to expose the access panel which is held in by three 10mm head fasteners. If water has been there you'll see corrosion. If you see corrosion unbolt the ECU and try unplugging the two connectors (they are not easy to remove and require patience, do not pull on the wires). Look for corrosion on the plugs and contacts, use a dielectric cleaning spray to clean both the ECU contacts and the contacts inside the plug. A short or poor connection at the ECU can cause a wide variety of seemingly unexplainable faults. In some cases I have manipulated the plugs with the engine running and noticed changes in the way the engine runs. Do not disconnect while it is on though.
Now after reading this, don't go straight to the ECU, again it is last on the flow chart to check. Get the timing numbers and get the vacuum readings first. ECU's are not cheap so let's not assume it's the worst case until the simple stuff has passed inspection. Most issues with a Miata are simple and revolve heavily around preventative maintenance items.
Horse before the cart!
service@miatashop.com
"If your good at what you do you should be well paid"
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QUESTION: What should the vacuum reading and timing be? If a coil pack or cam
position sensor was bad would it cause this same problem? That is great info
about the ECU. I probably would have never found it.
AnswerTiming should be 10-14 degrees BTDC at 750 to 900 RPM. You will need to shunt the ECU before being able to get an accurate reading.
Vacuum should be 15-17 Hg. steady with throttle closed 750-900 RPM.
Coil pack; you would have a dead misfire on two cylinders
CAS crank angle sensor, is two phase so the same would apply, you would have a dead misfire on two cylinders.
If you have spark on 1&4, and 2&3 then it is doubtful you have primary ignition issues and no signal issue exists if spark is visible on all four.
If your not sure when the wires and plugs were changed then take this opportunity to get a new set of NGK wires and plugs and replace them, consider it normal preventative maintenance. Primary ignition break down is a common source of most driveability issues with a Miata. Spark energy can be restricted and absorbed within bad wires or can alternate path if there is some issue with a spark plug and it's insulator.
