The engine idles fine when cold then drops to about 800 and I have no problems at all except maybe inconsistent idle (goes up and down but not by much) although when I turn off the engine and then start it again while it is still warm I get the following symptoms:

  • Right after I start the engine the RPM drops low and the engine vibrates for a second or two then it picks back up to about 800
  • When I give it gas and then release the throttle the same thing occurs

Those symptoms stay for about 2-3 minutes then go away except the slightly inconsistent idle speed. Could the ECU be adjusting the AFR to fix this issue after it gets enough readings from the pistons reaching TDC?

As of my understanding right now this engine uses two sensors/valves to adjust idle speed, Idle Air Control Valve and Idle Speed Control Valve (ISCV). But I do not know exactly where the problem might be located. I tested the ISCV using a multimeter and all the readings seemed to be correct. I also have no Check Engine Light. One thing that I do not know if it is related is that my spark plugs have those black watery deposits on them. Could that be because the AFR (air-fuel ratio) is rich because of the adjustments made by the ECU? Or are they not related?

EDIT: After doing more research, apparently ISCV and IACV are the same component just different acronyms (should have been obvious). I think the problem could also be a vacuum leak? since the multi-meter readings on the valve are proper should I eliminate the valve being an issue?

EDIT #2: I found something pretty big in my engine, I was using see foam to clean my iac and there seemed to be smoke coming out of my exhaust manifold. When I put my hand over the muffler tip the engine did not stall as expected and the smoke increased coming out of the manifold, so I have a pretty big exhaust leak. Can someone please give me resolution, can such leak cause such problem? It also explains why the car have been smelling

Update 2/13: I changed the headers on the car and although it is a lot quieter and smoother the problem is still there. I am going to visit the iac valve and see if it needs cleaning

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    Say more about the new ECU. Is "afr" air-fuel ratio?
    – dlu
    Commented Nov 18, 2016 at 18:08
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    Yeah I meant by that the new air fuel ratio made by the ecu. But I do not even know if the ecu is actually doing anything I just took a guess
    – method
    Commented Nov 18, 2016 at 18:09
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    I'm wondering how/if the IAC valve works. For example, if it is set up to let air in and isn't opening (or opening enough) on a warm start then the mixture might be a bit too rich on warm starts.
    – dlu
    Commented Nov 18, 2016 at 18:18
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    Checking the vacuum lines is a good idea, the car is old enough for the lines to be starting to fail. McMaster-Carr carries some nice silicone line if you decide to replace it. Also, have you checked for dirt or something else that might be restricting the IAC valve from moving?
    – dlu
    Commented Nov 20, 2016 at 0:44
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    @RobertS.Barnes, I believe that I have fixed the problem. I will do one more test tomorrow to confirm and then post an answer. It seemed to be an issue with a piece called throttle damping valve used to "soften" the closing of the throttle plate when released only found in the 1.6 miata engines I believe
    – method
    Commented Mar 7, 2017 at 9:09

2 Answers 2


list of things to check and some questions

  • the IAC valve might be giving the correct readings but is it stiff or sticking?
  • does any of this happen when it is warm outside as in during summer?
  • when you blip the throttle the car will stumble because of the sudden rush of air and the injectors firing to make up then suddenly no air and a too rich scenario. but it should recover quickly afterwards and not waver.
  • can you take a picture of all the spark plugs lined up from the front of the engine to the back? This is so we can see if they are shiny black, wet, dull black, white, or some mix of those. Shiney black we want to avoid as it means burning oil, wet black usually means burning lots of oil, white I do not suspect since you have a manifold leak, dull black would be the oxygen sensor picking up a lean condition due to the manifold leak introducing clean air and tricking it. This will tell the computer to run richer.
  • how often do you need to add a quart of oil?
  • are you warming up and running for a while then restarting right away as in seconds or less of off time? or is this more like grabbing a coffee and returning off time?
  • can you take an ohm reading off your oxygen sensor? this will tell us if it sees a lean or rich state. specifically when the engine stumbles after starting.

The sudden dropping low after starting could be another reason to suspect the exhaust leak. once the car starts it will have higher revs. This creates more exhaust flow. Once it starts dropping to idle the flow of exhaust will create a vacuum pulling clean air in. This trips the computer to go rich causing the engine to stumble for a little bit. So far this is what it is sounding like.

Well the constant building speed can create internal pressure on the manifold which would push exhaust gasses out rather than suck clean air in. This would give the oxygen sensor a proper reading. This would cause the computer to give the proper amount of fuel to achieve balanced stoichiometry.

Once you drop the gas to idle the exhaust gasses that were leaving at a high rate of speed suddenly are not being pushed any longer, you can think of how a toilet flushes, this creates a vacuum which is pulling clean air into the manifold. this tricks the oxygen sensor into thinking you are lean telling the computer this causes the computer to richen up the fuel charge. This causes the extra long recovery after suddenly letting off the gas.

You can see this for yourself with a smoke machine. Blow the smoke at the leak and increase the RPMs steadily and you should see the smoke moving away from the leak. Letting off suddenly should jerk the smoke back towards the leak.

it looks like the 2 left sparkplugs were the new changed ones. about how many miles ago? the middle right one is dull and a little sooty looking which may be rich but not burning oil. The far right one is oil burning.

whenever looking at spark plugs you really want them to all come out a brown. This means there is some carbon there and some lean-ness but it is sitting right on the edge of both.

General rule is:

  • white = lean
  • dull black = rich
  • shiny black = oil burning
  • brown = happy and balanced

The sucking sound you hear near the throttle body is probably air trying to get around a closed or just cracked open throttle valve. Is the throttle body sitting before or after the mass airflow sensor? If it is before a leak shouldn't matter but if it is after a leak can cause a lean condition. if you want to check for leaks around there take a torch but don't light it just open its valve and point it near things and see if the RPMs rise. BE VERY careful this can cause fires and explosions use common sense or you will be sorry also wear protection and have a fire extinguisher ready. This does have the potential of catching your car on fire!

the o2 sensor should read all over the place between .9 and .1 volts. you sure your volt meter is working properly? If it is not grounded out it will read 0 telling the engine you are permanently too lean so it will go full rich. If grounded out it will still not work because they need lots of heat for it to produce voltage. here is more information on O2 sensors. The part you are going to be most interested in is.

How it Works

The O2 sensor works like a miniature generator and produces its own voltage when it gets hot. Inside the vented cover on the end of the sensor that screws into the exhaust manifold is a zirconium ceramic bulb. The bulb is coated on the outside with a porous layer of platinum. Inside the bulb are two strips of platinum that serve as electrodes or contacts.

The outside of the bulb is exposed to the hot gases in the exhaust while the inside of the bulb is vented internally through the sensor body to the outside atmosphere. Older style oxygen sensors actually have a small hole in the body shell so air can enter the sensor, but newer style O2 sensors "breathe" through their wire connectors and have no vent hole. It's hard to believe, but the tiny amount of space between the insulation and wire provides enough room for air to seep into the sensor (for this reason, grease should never be used on O2 sensor connectors because it can block the flow of air). Venting the sensor through the wires rather than with a hole in the body reduces the risk of dirt or water contamination that could foul the sensor from the inside and cause it to fail. The difference in oxygen levels between the exhaust and outside air within the sensor causes voltage to flow through the ceramic bulb. The greater the difference, the higher the voltage reading.

An oxygen sensor will typically generate up to about 0.9 volts when the fuel mixture is rich and there is little unburned oxygen in the exhaust. When the mixture is lean, the sensor's output voltage will drop down to about 0.1 volts. When the air/fuel mixture is balanced or at the equilibrium point of about 14.7 to 1, the sensor will read around 0.45 volts.

When the computer receives a rich signal (high voltage) from the O2 sensor, it leans the fuel mixture to reduce the sensor's reading. When the O2 sensor reading goes lean (low voltage), the computer reverses again making the fuel mixture go rich. This constant flip-flopping back and forth of the fuel mixture occurs with different speeds depending on the fuel system. The transition rate is slowest on engines with feedback carburetors, typically once per second at 2500 rpm. Engines with throttle body injection are somewhat faster (2 to 3 times per second at 2500 rpm), while engines with multiport injection are the fastest (5 to 7 times per second at 2500 rpm).

The oxygen sensor must be hot (about 600 degrees or higher) before it will start to generate a voltage signal, so many oxygen sensors have a small heating element inside to help them reach operating temperature more quickly. The heating element can also prevent the sensor from cooling off too much during prolonged idle, which would cause the system to revert to open loop.

Heated O2 sensors are used mostly in newer vehicles and typically have 3 or 4 wires. Older single wire O2 sensors do not have heaters. When replacing an O2 sensor, make sure it is the same type as the original (heated or unheated).

  • -I do not think the iac is sticking or stiff but is there a way to know? -I think it did occur during the summer although I am a new owner so I am not completely sure. -If I give it a small amount of gas and keep building up the engine does not seem to struggle at all the issue happens once the peddle is released. -I will upload pictures in a few hours. -I found an oil leak which I am not completely sure where it is coming from but I am sure this is not related to my problem as it happened before oil started leaking. -More like grabbing coffee. -I will try to do so soon. Thanks
    – method
    Commented Nov 21, 2016 at 1:20
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    @method updated my answer with more info. I will be gone till about midnight pst but will be back on briefly before going to sleep.
    – Cc Dd
    Commented Nov 21, 2016 at 1:40
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    Here is an album of the spark plugs, they do not seem as bad as last time I checked them but the two of them have been replaced recently, also not as black as I thought they were: imgur.com/a/ptFnR
    – method
    Commented Nov 21, 2016 at 2:48
  • Your explanation makes a lot of sense, I will get the o2 readings when I can and check with a ciggerate or something the air flow around the manifold.
    – method
    Commented Nov 21, 2016 at 2:50
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    @method are you measuring voltage? if so what range is it set to. if it was .7 volts your a little rich. .9 is full rich. Also how long and hard are these taps on the gas? all engines will dip if you quickly blip the pedal. this is because they do not have time to react then they spray too much fuel because they detected the throttle opening fast then it suddenly closes effectively flooding the engine for a split second.
    – Cc Dd
    Commented Nov 23, 2016 at 3:17

No codes, poor idle, and those spark plug pictures indicate that your car needs a serious tune-up. The fuel system, combustion chambers, valves, EGR, catalytic converter are probably all very dirty. I have a '94 Taurus that had the same problems and here is how I fixed them:

Start by changing the motor oil and filter with a top synthetic oil like Mobil 1. Substitute one quart of oil with Marvel Mystery Oil (or automatic transmission fluid).

If the car has a PCV valve and EGR valve, make sure both have been replaced and the tubing cleaned with carburetor cleaner. Replace the air filter and spark plugs. Check the intake manifold and vacuum hoses for air leaks.

Add Marvel Mystery Oil (or automatic transmission fluid) to your gas tank at fill-up at a treat rate of 1 oz. per gallon. Drive the car immediately in stop-and-go traffic for 10 minutes, shut off engine for 10 minutes, then drive on the highway at 70 MPH for 10 minutes. Repeat this cycle multiple times, for at least an hour, and allow the mixture to penetrate at least two days.

When your tank drops to 10 gallons of gas, add one can (15 oz.) of Berryman B12 Chemtool and one can catalytic converter cleaner (or simply 2-3 ounces of Berryman's for every gallon of gas). Immediately drive the car on the highway for 30-60 minutes, then "floor" the car at wide-open-throttle (WOT) from 50-80 MPH. Repeat the full throttle blasts at least six times, but up to 30 times if the cat. & sensors are very dirty. Be sure the cooling system and transmission are flushed and working properly beforehand. Turn the heater and fan on maximum, with windows open, when performing the WOT blasts.

The Berryman's and cat. cleaner evaporate out of the gas tank after about 24 hours, so be sure to run the car hard right away. The Marvel Mystery Oil never fully evaporates. The two products – Marvel Mystery Oil & Berryman”s B12 Chemtool -- work together and will never damage any components of the car.

When the problems are resolved and you refill the tank, continue adding Marvel Mystery Oil at a treat rate of 4 oz. for every 10 gallons of gas. It will continue to clean deposits. Every fourth fill-up, add one can of Berryman's and drive at highway speeds.

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