QuestionQUESTION: My 1990 miata until recently would run fine but would not idle. I finally got the
car to stay idling but idles very rough and has a miss to it. Also when I rev
up the engine and let it come back to idle, the engine will stop at about 1500
rpm stay there for about 30 to 45 sec (maybe a minute) then drop to about
800 rpm and idle very rough. Could this be a possible coil pack going bad or
something else?
ANSWER: The symptoms you describe could be as simple as ignition break down (ie; a bad wire(s) or a spark plug(s)) It could also be a stuck or non functioning air valve or gum/varnish build up in the throttle body. Mis-adjustment of base timing and idle could also be to blame.
Start simple. As most issues with early Miatas are simple and revolve around common preventative maintenance items.
Check for fire on 1&4 and then 2&3 use a positive grounding test plug since a CDI system like the Miata needs to find a good ground to avoid primary winding overload. A lack of fire on any pair indicates a coil or a crank angle sensor signal, lack of fire on a single lead is a primary wire issue.
If you haven't in awhile get a new set of NGK wires and spark plugs as a first step. See if that resolves your issue, 90% of the time it does. Then move to clean the throttle body, ICS (idle control solenoid) and the PCV while your at it.
The next logical step would be to check the air valve for movement over it's temperature range. If you have let the coolant go and corrosion is present in the system this could be a good indicator that the air valve (a thermostatic device) similar to a thermostat in operation could be stuck or non-operable. It is a cold start device that adjusts bypass air during warm up (a choke of sorts). This is a worst case item so try the basic preventative items first.
service@miatashop.com
"if your good at what you do you should get well paid"
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QUESTION: Other people have worked on this car and have changed plugs and wires. So just to make sure they don't have any wires crossed which is the number one plug? Is it the first one at the front of the car....1,2,3,4 towards the back of the car? Also does this car have an egr valve? If so where is it located?
ANSWER: Yes the fire order is 1,2,3,4 from front to back and the coil pack is (from driver's side to passenger side) 4,1,2,3. There is no EGR on 90-93 models.
After you check the firing order and are sure you have good ignition and wires you'll need to establish a base line timing and idle. Depending upon your elevation 10 to 14 degrees BTDC at 850 to 900 RPMS. The ECU will need to be shunted during setting of the base line.
There are two devices that need to be checked for proper operation afterwards. The ISC (idle speed control) valve and the Air Valve. If either one has failed the car will not recover and hold an idle properly.
service@miatashop.com
---------- FOLLOW-UP ----------
QUESTION: If I understand correctly the air valve and idle speed control valve both need
coolant flow to work properly. Is this correct? Today I just discovered that I
have little to no coolant flow thru either valve. So I am assuming that I have a
blockage somewhere.
AnswerRight, coolant flows via a bypass line into the passenger side of the ISC out the driver's side up to the air valve then back to the thermostat housing. The air valve only functions during warm up. The ISC functions throughout the entire engine speed range. Coolant should flow freely at all times.
To check the ISC. Engine off, key in the "Run" position. Un-plug the ISC valve (green connector front). You should here a click as the solenoid engages and disengages as you plug and unplug the device. This will help you to determine if the ISC is working electrically.
It's purpose is to control intake air by regulating the amount of air allowed to bypass the throttle valve. It comes into play during warm up, when either the AC is on, when the power steering is activated and/or when the regulator demands power from the alternator. Each device listed absorbs varying degrees of horsepower from the engine thus resulting is reduced RPM at idle. The ISC compensates via the ECU to help stabilize the idle.
IE; the AC can use 8-10 HP, at idle, with out the ISC the engine speed would drop significantly and possibly result in a stall out. With the ISC, the idle is automatically adjusted upwards to compensate for the demand on the engine.
You've been educated in Miata Idle Controls 101. Your degree is "in the mail"...
Hope it helps you figure it out.
service@miatashop.com
"if your good at what your do you should be well paid"
