I pulled some recent codes from my 89 bronco 5.0. There were only two so I fixed the Throttle position sensor and the truck ran well. I then replaced the Idle Air Controller and the truck ran well when driving. The only problem I have now is when I put it in park. Once I do that I the idle will drop below 500 rpm or sometimes stall. If it does not stall the idle will come back a little bit. What do you thing it might be? When the truck is in gear and stoped or when I just start it it will idle at approximately 725 rpms. Any help will be appreciated.

Thanks.

 

I am going to assume that when you replaced the IAC you cleaned the throttle body as well. A filthy TB will momentarily starve a hot engine before the IAC can catch up and compensate, especially if the butterflies snap shut quickly.

725 rpm is actually pretty close to normal for a curb idle in my experience. The drop is the concern. Check all of the belt-driven components for free movement. A sticky A/C clutch or power steering pump can drag on the engine a bit and cause idle problems.

If neither of these appears to be the problem the ECT sensor can send incorrect readings to the ECA and still be "within established parameters" which would mean the ECA would not trigger a fault code but the ECT is still sending bad info to it. The return voltage from the ECT sensor back to the ECA when the engine is cold (less than 68 F) is approximately 3VDC. When the engine is a normal operating temp., the signal from the sensor should be about 0.5VDC. The ECA will not trigger a fault code as long as the voltage it reads from the ECT is within the 3 to 0.5VDC range. However, if the ECT is "sticking" during operation, it could be that the ECA is still receiving the 3VDC signal even after the truck has fully warmed up. This condition would cause the ECA to leave fuel/air ratio higher on the fuel side which can potentially flood the hot engine at idle.

All of these potential problems could produce a decrease in MPG. It may be slight but its possible.

 

Thanks for your detailed reply. I am going to work on it this weekend.

 

I have not finished all your recommendations but I do have one more piece of information. I thought the stalling was just happening in park but it also happens in nuetral. If it is kept in gear, reverse or others, it is fine.

Do you still recommend the same course of action?

Thanks

 

Yeah, I would still think that unless the computer is throwing new codes or a specific code that you can trace that particular isuue, proceed as I mentioned. Your truck is too old to have an E4OD in it so there is no MLPS to fail or get "eccentric" as I like to call it.