Looking For Help On Surging Idle. Went Through Check List, But Still Surging.

Let's to some sleuthing an see if you have any dead or weak cylinders.


Cylinder balance test: use this to find dead or weak cylinders:

Revised 25 March 2012 to add necessity allowing the KOEO tests to finish before starting the engine and the need for a properly functioning IAB/IAC to run the cylinder balance test.

The computer has a cylinder balance test that helps locate cylinders with low power output. You’ll need to dump the codes out of the computer and make sure that you have the A/C off, clutch depressed to the floor and the transmission in neutral. Fail to do this and you can’t do the engine running dump codes test that allows you to do the cylinder balance test.

Here's the way 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, have the clutch depressed to the floor, 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.


Here's how 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.





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. This is crucial: the same wire that provides the ground to dump the codes provides signal ground for the TPS, EGR, ACT and Map/Baro sensors. If it fails, you will have poor performance, economy and drivability problems

Some codes have different answers if the engine is running from the answers that it has when the engine isn't running. It helps a lot to know if you had the engine running when you ran the test.

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, clutch depressed to the floor 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.

Trouble codes are either 2 digit or 3 digit, there are no cars that use both 2 digit codes and 3 digit codes.

Cylinder balance test

If you have idle or IAC/IAB problems and the engine will not idle on its own without mechanically adjusting the base idle speed above 625-750 RPM, this test will fail with random cylinders pointed out every time it runs. The IAC/IAB must be capable of controlling the engine speed to run in the 1400-1600 RPM range. Playing with the base idle speed by adjusting it upwards will not work, the computer has to be able to control the engine speed using the IAC/IAB.

Warm the car's engine up to normal operating temperature. Use a jumper wire or paper clip to put the computer into test mode. Let it finish the Key On Engine Off (KOEO) code dump. Start the engine and let it go through the normal diagnostic tests, then quickly press the throttle to the floor. Remember to keep the clutch pedal (5 speed) depressed to the floor during the test. 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 engine will shut off power to 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


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/loan. 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 with a plastic screwdriver handle between the throttle butterfly and the throttle housing. 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.

I generally use a big screwdriver handle stuck in the TB between the butterfly and the TB to prop the throttle open. The plastic is soft enough that it won't damage anything and won't get sucked down the intake either.

A battery charger (not the trickle type) is a good thing to have if you haven't driven the car lately or if you have any doubts about the battery's health. Connect it up while you are cranking the engine and it will help keep the starter cranking at a consistent speed from the first cylinder tested to the last cylinder.

cobranger said:

Thanks. I'll try to run the cylinder balance test tomorrow. I should have good compression - the engine is a fresh rebuild, but I'll check it if necessary. I have a screw-in type compression tester, as well as a battery charger. I assume the quick stab of the throttle to the floor is done after the tests are completed but before the codes are dumped?

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I usually wait until it has finished dumping the engine running codes.

cobranger said:

When the engine cooled down enough to get my hands in there, I did a cylinder compression test. Results as follows:
1 - 155psi
2 - 160
3 - 155
4 - 155
5 - 160
6 - 160
7 - 155
8 - 145 #*%@! I added a little oil and it came up to 150. Is that a major problem in itself? I only have maybe 100 miles on the engine since rebuild. Maybe I just didn't hone that cylinder enough.
Now, I can understand how this would possibly cause a rough idle or running, but not running super rich and surging or hunting idle. So, Mr. Richker, what's next? I just rebuilt the engine and thought I was being very careful to get all the rings correctly spaced and right side up, but it seems I must have done something wrong. I still think there's something else going on in the EEC system.

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Disconnect the IAC and remove the SPOUT jumper and see if there are any changes in the idle. That will help eliminate the computer and sensors as a possible problem source.

The compression tests & cylinder balance tests look good. Your compression test will probably even out as you get more break in mileage on the engine.

The cylinder balance test got a 9 so all the cylinders are firing OK. If you had gotten something other than a 9, I would have recommended replacing the spark plug or wire for the affected cylinder.

Remember that the cylinder balance test doesn't find dead injectors, since it shuts off the ground for each injector as part of the test. An engine stethoscope to listen for each injector as it clicks while running is the only simple way to check the injectors for operation. A noid light also works, but you have to pull the upper intake manifold to get to all the injector wiring. If you have access to an oscilloscope, or a graphing DVM, you can backprobe the 10 pin connector to watch the injector pulse.. This is probably the best option, but not everyone can get access to an oscilloscope.





See the graphic for the 10 pin connector circuit layout.

cobranger said:

Warmed up engine, pulled IAC connector and engine died. I fired it up again, removed spout connector, and idle dropped from about 800 to 550 or 600. Pulled the IAC and it died again. I forgot to listen to the injectors yet with the stethoscope, so I'll do that tonight.

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You are seeing the symptoms of the base mechanical idle not being correctly set, or a vacuum leak.

Recommended procedure for cleaning the IAC/IAB:
Conventional cleaning methods like throttle body cleaner aren’t very effective. The best method is a soak type cleaner used for carburetors. If you are into fixing motorcycles, jet skis, snowmobiles or anything else with a small carburetor, you probably have used the one gallon soak cleaners like Gunk or Berryman. One of the two should be available at your local auto parts store for $22-$29. Take the solenoid off the body and set it aside: the carb cleaner will damage some types of plastic parts. Soak the metal body in the carb cleaner overnight. There is a basket to set the parts in while they are soaking. When you finish soaking overnight, twist the stem of the IAB/IAC that sticks out while the blocker valve is seated. This removes any leftover deposits from the blocker valve seat. Rinse the part off with water and blow it dry with compressed air. The IAC/IAB should seal up nicely now. Once it has dried, try blowing through the bottom hole and it should block the air flow. If it doesn't block the airflow, there is still something that is gumming up the works. Reassemble and reinstall to check it out. Reassemble and reinstall to check it out.

Gunk Dip type carb & parts soaker:


Setting the base idle speed:
First of all, the idle needs to be adjusted to where the speed is at or below 600 RPM with the IAC disconnected. If you have a wild cam, you may have to raise this figure 100-150 RPM or so. Then the electrical signal through the IAC can vary the airflow through it under computer control. Remember that the IAC can only add air to increase the base idle speed set by the mechanical adjustment. The 600 RPM base idle speed is what you have after the mechanical adjustment. The IAC increases that speed by supplying more air under computer control to raise the RPM’s to 650-725 RPM’s. This figure will increase if you have a wild cam, and may end up between 800-950 RPM

Remember that changing the mechanical idle speed adjustment changes the TPS setting too.

This isn't the method Ford uses, but it does work. Do not attempt to set the idle speed until you have fixed all the codes and are sure that there are no vacuum leaks.

Disconnect the battery negative terminal and turn the headlights on. Leave the battery negative terminal disconnected for 5 minutes or so. Then turn the headlights off and reconnect the battery. This erases the computer settings that may affect idle performance.

Warm the engine up to operating temperature, place the transmission in neutral, and set the parking brake. Turn off lights, A/C, all unnecessary electrical loads. Disconnect the IAC electrical connector. Remove the SPOUT plug. This will lock the ignition timing so that the computer won't change the spark advance, which changes the idle speed. Note the engine RPM: use the mechanical adjustment screw under the throttle body to raise or lower the RPM until you get the 600 RPM mark +/- 25 RPM. A wild cam may make it necessary to increase the 600 RPM figure to 700 RPM or possibly a little more to get a stable idle speed.
Changing the mechanical adjustment changes the TPS, so you will need to set it.

When you are satisfied with the results, turn off the engine, and re-install the SPOUT and reconnect the IAC. The engine should idle with the range of 650-750 RPM without the A/C on or extra electrical loads. A wild cam may make this figure somewhat higher.

codes 94 and 44. This is going to sound dumb, but its something to check. I have long tubes on a lowered 5.0 and I hit the rails on a railroad crossing pretty hard. jarred my midpipe and created leaks at the header/midpipe section. car started running like . the 02 sensors are in bungs in the long tubes UPSTREAM of the joint, but it still sucks in air at idle, especially with the cam (also an e303). its crazy, was NOT expecting it to solve my issues, but it cleared my codes and the car ran flawlessly afterwards.

modulistic said:

codes 94 and 44. This is going to sound dumb, but its something to check. I have long tubes on a lowered 5.0 and I hit the rails on a railroad crossing pretty hard. jarred my midpipe and created leaks at the header/midpipe section. car started running like ****. the 02 sensors are in bungs in the long tubes UPSTREAM of the joint, but it still sucks in air at idle, especially with the cam (also an e303). its crazy, was NOT expecting it to solve my issues, but it cleared my codes and the car ran flawlessly afterwards.

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I haven't quite figured out how an exhaust system can suck air since it's primary function is to pump spent hot gases out the tail pipe. If exhaust systems sucked air into the piping, turbochargers would never work. A manometer would show the fact that while the engine is running, there is always pressure that is higher that the atmospheric pressure outside the exhaust pipes.