98 4.0 SOHC Idle

[98XLT4]

I've got a '98 XLT with the 4.0 SOHC (2WD) with a strange idle problem. When the truck is cold, the idle is fine at around 700 or so RPM. When the truck warms up to operating temps, the the idle slowly increases as the temp increases - up to a max of about 1000 RPM. After the idle increases, the transmission shifting becomes erratic and seems to "stick" in gear. For instance, at 50 MPH in 5th gear if I take my foot off the gas, the RPM's don't immediately drop but tend to stay up as if I still had my foot lightly on the gas cruising. Any thoughts as to what's causing this problem? I've replaced the IAC already.

 

[neverhit]

Probably your thermostat is going bad. It's not opening and therefore not cooling the engine as well, so the engine thinks it's still cold. As a result the computer keeps the idle higher. I had the same issue, especially noticed that my temp gauge was higher than it should have been. When I replaced the thermostat, temp went to ~1/2 up the gauge and has never gotten hot and now idles at ~600

 

[98XLT4]

Thanks for the quick response. I'm not familiar at all with the Ford electronics - but just for curiosity sake, doesn't the computer know the temp of the engine regardless of whether the thermostat is opening fully or not? I would think if the engine was running hotter the computer would recognize this and adjust fuel/timing accordingly. Regardless, the truck has been running hotter than previously so I think you're right that a new thermostat is needed. Hopefully that will take care of it - thanks for the help!

 

[98XLT4]

I should also mention the temp gauge is in the lower middle to middle - even when the engine has run the hottest, the gauge has never gotten above 3/4 to the "H". When the engine's cool, the problems aren't noticeable. It's when the temp gauge reaches the middle of the "symbol" on the gauge that the idle increases - first to maybe 900 RPM. Then normal driving - stop at a stop light and the engine will idle around 1000 RPM. Same cycle until the engine is eventually in the 1200 RPM range at idle, but with no codes thrown.

 

[gijoecam]

Any time the engine gets above normal operating temperature (due to a bad thermostat, for example) the ECU raises the idle and adjusts the shift points to maximize cooling. It's very possible that's what's happening. There's no reason the idle should rise, unless you have a vacuum leak that's allowing unmetered air into the intake downstream of the throttle body.

-Joe

 

[98XLT4]

Thanks to all for the responses and the explanation - learn something new everyday. I'm definitely gonna focus on the thermostat - thanks again!