New Idle Problem (I Searched First)


New Member
Oct 27, 2003
Been having this problem since I put the engine back together over the winter.

I can reset my idle, set TPS voltage, unplug the battery for an hour, clock my Mass Air Meter, check for vacuum leaks for months on end, retighten the 10-pin connectors, and nothing fixes this problem.

I've replaced every sensor in the car except the Mass Air Meter, not thinking it could affect idle this much. But now I'm thinking differently... please help. Here's the problem-

I can set the idle to 900rpms, just fine. But after about 30 seconds (and usually at EVERY stop light) the idle drops to around 500rpm or below, causing the car to die sometimes. Then after driving, or revving sometimes it will begin to idle correctly. I can reset my idle to 2000rpm, and will still do the same thing. The faster I set the idle, the faster it drops to 500rpm, and then will climb back up to where I set the idle again. The idle will drop even if the IAB valve is unplugged.

Could it be my Mass Air Meter causing this? I'm out of ideas, and the Mass Air Meter is like the most expensive sensor to replace just to find out it didn't help. :(

The only codes I get are for my emmsions stuff being gone...

Things new, or that I've tried -

2 New TPS Sensors (one was probed to death), new IAB valve, new ECT sensor, cleaned the ACT sensor, clocking Mass Air Meter, completely cleaned the upper intake/throttle body, cleaned Mass Air sensor, replaced injectors, replaced Fuel Pressure Regulator (stock), cleaned/tightened 10-pin connectors, replaced A3M computer with A9L

I have A9L, 19# Injectors, STOCK Mass Air Meter, Cobra Upper/Lower, stock heads.

Please help if you have ideas, I searched for a similiar problem, and could not find anyone else thats been through this.
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Dump the codes and see what the computer says is wrong…

Here's the link to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great.

Since your car is an 88 stang, you'll have to use the test lamp or voltmeter method. There is no functional check engine light on the 88's except possibly the Cali Mass Air cars.

See for more descriptive help
MAF problems will usually set code 66. here is the test path for code 66...

Code 66 MAF below minimum test voltage.
Insufficient or no voltage from MAF. Dirty MAF element, bad MAF, bad MAF wiring, missing power to MAF. Check for missing +12 volts on this circuit. Check the two links for a wiring diagram to help you find the red wire for computer power relay switched +12 volts. Check for 12 volts between the red and black wires on the MAF (usually pins A & B).

The MAF element is secured by 2 screws & has 1 wiring connector. To clean the element, remove it from the MAF housing and spray it down with electronic parts cleaner or non-inflammable brake parts cleaner (same stuff in a bigger can and cheaper too).

There are three parts in a MAF: the heater, the sensor element and the amplifier. The heater heats the MAF sensor element casing the resistance to increase. The amplifier buffers the MAF output signal and has a resistor that is laser trimmed to provide an output range compatible with the computer's load tables.

The MAF output varies with RPM which causes the airflow to increase or decease. The increase of air across the MAF sensor element causes it to cool, allowing more voltage to pass and telling the computer to increase the fuel flow. A decrease in airflow causes the MAF sensor element to get warmer, decreasing the voltage and reducing the fuel flow. Measure the MAF output at pins C & D on the MAF connector (dark blue/orange and tan/light blue) or at pins 50 & 9 on the computer.

At idle = approximately .6 volt
20 MPH = approximately 1.10 volt
40 MPH = approximately 1.70 volt
60 MPH = approximately 2.10 volt

Check the resistance of the MAF signal wiring. Pin D on the MAF and pin 50 on the computer (dark blue/orange wire) should be less than 2 ohms. Pin C on the MAF and pin 9 on the computer (tan/light blue wire) should be less than 2 ohms.

There should be a minimum of 10K ohms between either pin C or D on the MAF and ground.

See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host)
Probably stupid questions here, but combing the internet for info -

1.) I no longer have A/C, and the old relay was fried, so I unplugged it totally. - No way this could be part of the problem... right?

2.) Would the Vehicle Speed Sensors have anytihng to do with this? I have yet to replace those sensors, and had to run 2 wires to them from the EEC for my Mass Air coversion...
You may have tried this, but maybe you didnt think of it. Unplug the MAF meter and drive the car that way. If the symtoms go away, or change, you most likely do have a problem in that circuit. Jrichker gave you some great info on troubleshooting that circuit. Just dont plug and unplug sensors while they have power going to them, this can be bad for sensors.

Since you say it throws no codes except for emissions stuff, my next move would be pull the throttle body and give it a good cleaning. Also the clean the EGR and IAB real good. This is the root of many mustang rolling idle probs. I found a chunk of my rubber air tube jammed inside my IAB once, no wonder it was idling at 2500rpm.
Ah, the speed sensor. IIRC the speed sensor relays the vehicle's speed via the speedometer gear. Both T5 and AOD ecu's increase airflow via the IAC when the speed drops below a certain speed. It could mess up the idle, but not in the way you are describing I don't think. How are you setting your idle? Are you unplugging the IAC? It sounds really weird that the idle will drop with IAC unplugged, which points to something other than idle air control problems, since the iac is what the car uses to adjust idle to fit load being asked of the engine. How is your electrical/charging system? Grounds? EGR/Thottle body gaskets? Fuel system/filter, etc? Maybe it's time to look elsewhere? (before you give up and trade the stang in for a WRX)