I have a 1998 f150 4X4 4.6L with auto trans. and 45K miles.

I have just noticed that with the engine running and transmisssion in Park the engine idle rpm does not seem constant it sounds like the engine rpm slows then speeds up, it completes one of these cycles every 5-10 seconds. It is a very slight change because the tach hardly moves. I don't think this happens when in gear and stopped at a traffic light.

Could this be my radiator fan loading and unloading the engine slightly. It wouldn't seen so because the frequency of the cycle seems to fast.

Any help would be appeciated.

 

A quick listen in front of the truck will let you know if its the fan. They are pretty noisy (air turbulence noise) when they are clutched into their near direct drive state. If its the fan, you'll hear it cycle.

 

It very possibly could be the fan. With your truck in 'drive' the engine compensates for this with a slightly higher RPM cycle. This might be why you only notice it when in 'Park'.

I have noticed a similar situation with my truck, which is also very hard to notice. It is not the rough idle that many other 99-2000 owners are reporting...it is no where near that bad. It does not cycle as frequently as yours...however I have noticed it.

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2000 F-150 XLT, 4x2, 5.4L Supercab, Styleside, Black with silver two-tone, 3.55, class III towing package with heavy duty cooling package, 4 wheel disk ABS, overhead console, sliding rear window, keyless entry, dark graphite interior, in dash CD, Pendaliner bedliner, K&N air filter, rubberized undercoating, cabin filtration system, Bugflector II, 5W-30 Mobil 1, engine build 10/4/99



[This message has been edited by Dustoff (edited 02-21-2000).]

 

Yea, and the AC cycles in the heat/defrost mode as well.

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Mine: FOR SALE SOON...1997 F150,XLT,SC,Oxford white/maroon 60/40 cloth,4x4 ORP,4.6L,3.55LS,17" ORP wheels w/GY AT/S LT265/70/17,auto,6 CD,Towing package,alarm,Rhino grill guard w/PICCA lights,dual exhaust. Bought new June '96

Wife: '99Explorer Limited,pearl white/tan leather,4.0L,4:10LS,CD,sunroof,towing pkg.,alarm.

Next: 2001 F150 XLT SuperCrew, 5.4L, 4x4,3:55LS,tow pkg. etc.......

 

Could also be your air conditioning compressor kicking on and off. This is very noticeable on my wife's 6 cylinder Windstar.

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2000 F-150 XLT Sport
Silver
4 door Supercab, Short Bed (Only way to fit in the garage)
17" Sport Wheels with BFG tires
4.6l V8, Auto Transmission (First one in 20 years)
In-Dash Single CD Player

Hopes and Dreams:
Spray-in Bedliner
K&N Filter
WMS Velocity Tube
FlowMaster Exhaust
Nerf Bars