2003 Gt Engine Failure

If this were my car I would make sure there weren't any vacuum leaks. Then I would have graphed RPM's, TP-MODE, and IAC duty percent. While working the throttle, check that TP-MODE goes to "C/T" (closed throttle) each time.

Next I would look at the relationship between RPM and IAC duty percent. Looking to see if the IAC duty percent is changing as this indicates that the PCM is attempting to adjust the idle.

If the PCM is attempting to adjust the idle, are the RPM's changing in response?

What are the ranges of the IAC duty percent? (Min/Max)?

The ONLY method the PCM has to adjust idle is via the IAC duty percent. So one of the things we would want to know is:

• Is the IAC duty percent between 40-60%. Values outside this range indicates a problem. Either too much or too little air.
• Is the IAC duty percent changing and is the IAC valve itself actually changing in response? Cross check this by measuring the voltage across the IAC.
• Is the PCM getting an accurate TPS closed throttle signal? Because if it's not, no further tests are possible as the TP-MODE has a major impact on how the PCM responds.
• Is the PCM getting an accurate vehicle speed? Again the PCM does take the car's speed into consideration.

The combination will give clues to where the problem really is. For example, let's say the PCM duty percent changes but the voltage at the IAC never changes. This would indicate a BAD PCM.

Let's say that the IAC duty percent never changes. To me that says a the PCM tune is corrupted.

Let's say that the IAC duty percent is ALWAYS below 20%. This says there no adjustment left in the IAC. Likely due to excessive air passing the TB.

Has the throttle body been changed on this car? Perhaps from a different model year or after market?

Did the Ford dealer perform a smoke test?

A back yard method to test for Alternator AC ripple is to measure the AC voltage at the battery. Set the VOM to read AC. Do not use DC or auto. The value should be low (<0.2).