My 91 is running terrible, could use some help

When you pulled the Iac did you reset your idle?
Clear the EEC-IV's idle memory by disconnecting the battery for 20+ minutes.
Disconnect the Idle air bypass solonoid
Reconnect battery
Start engine and set idle speed to desired RPM with stop screw on throttle body (900 rpm works great with E303 or larger cam)
Turn off engine and reconnect the air bypass solenoid
*Optional -- set TPS to 0.90 - 0.95 volts (Do not exceed 1.0 volts MAKE SURE!) Use a digital volt meter!
Start engine and let it idle for 2 minutes with no accessories on.
Turn off engine for two minutes
Start engine again and run for 2 minutes with every accessory turned on.
Turn engine off again.

Seems like you covered the basics. So, I suggest disconnecting the MAF and see how it runs. Could be a dirty MAF or air filter. That often won't throw a code, but will cause problems.

The other thing is a possible vacuum leak. Try a vacuum gauge, and see if that's okay.
After that, do a cylinder balance test. An injector, spark plug, or wire may have gone bad.

First, of course, double check that all of the spark plug wire are on tight. They can come loose when doing work on the engine.

The other thing is that the timing may be off. After the car is warm, see if adjusting the timing makes a big difference. Likely not. But, easy to check.

Good Luck!

smoke out of the dipstick what color is it ? Have you refilled with a different octane then normal?

I'd put your old PCV valve in to rule out the variable of a bad new PCV valve. It might have a bad spring (it would still rattle and "seem" ok however).

I agree with the CBT.

anyway you can get that PVC screen out. Stab it with needle nose plyers if you have to and yank it out.

you may want to try a leak down test or Compression test. Doesnt hurt

For the CBT you can floor the throttle after your done the KOER Test. I do it 3 times. If you use you CE light to count the flashes. Say just #7 cyl is not working right. you would get 7 flashes. If all Cyl. are fine then you wiill get 9 flashes

Cylinder balance test:

Revised 21-Nov-2010 to improve readability of the cylinder balance test.

Warm the car's engine up to normal operating temperature. Use a jumper wire or paper clip to put the computer into test mode (see dumping the codes below).

Start the engine and let it go through the normal diagnostic tests, then quickly press the throttle to the floor. The engine RPM should exceed 2500 RPM's for a brief second. The engine RPM's will increase to about 1450-1600 RPM and hold steady.

The computer will then shut off each injector, one at a time. When it has sequenced through all 8 injectors, it will flash 9 for everything OK, or the number of the failing cylinder such as 2 for cylinder #2. Quickly pressing the throttle again up to 2500 RPM’s will cause the test to re-run with smaller qualifying figures. Do it a third time, and if the same cylinder shows up, the cylinder is weak and isn’t putting out power like it should.

See the Chilton’s Shop manual for the complete test procedure.

Dumping the computer trouble codes
Dump the codes and see what the computer says is wrong…Codes may be present in the computer even if the Check Engine light isn’t on.

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. You watch the flashing test lamp or Check Engine Light and count the flashes.

Be sure to turn off the A/C, and put the transmission in neutral when dumping the codes. Fail to do this and you will generate a code 67 and not be able to dump the Engine Running codes.

Dumping the Engine Running codes: The procedure is the same, you start the engine with the test jumper in place. Be sure the A/C is off and the transmission is in neutral. You'll get an 11, then a 4 and the engine will speed up to do the EGR test. After the engine speed decreases back to idle, it will dump the engine running codes.

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. You watch the flashing test lamp or Check Engine Light and count the flashes.

See Troublcodes.net Trouble Codes OBD & OBD2 Trouble Codes and Technical info & Tool Store. By BAT Auto Technical





If your car is an 86-88 stang, you'll have to use the test lamp or voltmeter method.
There is no functional check engine light on the 86-88's except possibly the Cali Mass Air cars.



The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the self test connector.

89 through 95 cars have a working Check Engine light. Watch it instead of using a test lamp.



The STI has a gray connector shell and a white/red wire. It comes from the same
bundle of wires as the self test connector.


WARNING!!! There is a single dark brown connector with a black/orange wire. It is the 12 volt power to the under the hood light. Do not jumper it to the computer test connector. If you do, you will damage the computer.

What to expect:
You should get a code 11 (two single flashes in succession). This says that the computer's internal workings are OK, and that the wiring to put the computer into diagnostic mode is good. No code 11 and you have some wiring problems.

Do a compression test on all the cylinders.
Take special note of any cylinder that shows up as weak in the cylinder balance test. Low compression on one of these cylinders rules out the injectors as being the most likely cause of the problem. Look at cylinders that fail the cylinder balance test but have good compression. These cylinders either have a bad injector, bad spark plug or spark plug wire. Move the wire and then the spark plug to another cylinder and run the cylinder balance test again. If it follows the moved wire or spark plug, you have found the problem. If the same cylinder fails the test again, the injector is bad. If different cylinders fail the cylinder balance test, you have ignition problems or wiring problems in the 10 pin black & white electrical connectors located by the EGR.

How to do a compression test:
Only use a compression tester with a screw in adapter for the spark plug hole. The other type leaks too much to get an accurate reading. Your local auto parts store may have a compression tester to rent. If you do mechanic work on your own car on a regular basis, it would be a good tool to add to your collection.

With the engine warmed up, remove all spark plugs and prop the throttle wide open, crank the engine until it the gage reading stops increasing. On a cold engine, it will be hard to tell what's good & what's not. Some of the recent posts have numbers ranging from 140-170 psi. If the compression is low, squirt some oil in the cylinder and do it again – if it comes up, the rings are worn. There should be no more than 10% difference between cylinders. Use a blow down leak test (puts compressed air inside cylinders) on cylinders that have more than 10% difference.

See the link to my site for details on how to build your own blow down type compression tester.

Fix the code 12 and the cylinder balance test will run. That will tell you more than the temp readings on the headers.

Start the engine and let it go through the normal diagnostic tests, then quickly press the throttle to the floor. The engine RPM should exceed 2500 RPM's for a brief second. Release the throttle and the engine RPM's will increase to about 1450-1600 RPM and hold steady.

IAC doesn't work: look for +12 volts at the IAC red wire. Then check for continuity between the white/lt blue wire and pin 21 on the computer. The IAC connector contacts will sometimes corrode and make the IAC not work. The red wire on the IAC is always hot with the engine in run mode. The computer provides a ground for the current for the IAC. It switches the ground on and off, making a square wave with a varying duty cycle. A normal square wave would be on for 50% of the time and off for 50% of the time. When the idle speed is low, the duty cycle increases more than 50% to open the IAC more. When the engine speed is high, it decreases the duty cycle to less than 50% to close the IAC.

Note that the IAC can only increase idle speed. The lower limit for idle speed is set by the idle screw on the throttle body.




More tests:
Disconnect the computer main connector and the IAB connector. Measure the resistance between pin 21 on the computer connector and pins 40 or 60. If you find less than 200 Ohms, which is effectively a short inside the computer, that would keep the IAB fully turned on.

An old-fashioned dwell meter can be used to check the change: I haven’t tried it personally, but it should work. In theory, it should read ½ scale of whatever range you set it on with a 50% duty cycle. An Oscilloscope is even better if you can find someone who has one and will help.