Surging RPM
 

Hello to all. I have a 1997 F150 XLT Lariate 4X4 S/C 4.6L O/D. I'm the original owner. 180,000 miles. Always serviced. Tranny fluide is up and clean. When driving 50 to 60 mph, 1500 RPM, the truck for no reason jumps to 1900 RPM and then goes back down to 1500 RPM. It will do this on and off for my hole drive, about 40 mins. The next day it will be fine. Then it will do it all over again. Got mad and drove the **** out of it the other day and now it seems fine. When I drive, it is just the RPM going up and down. The Tranny is not slipping or shifting out of O/D. Thanks for your help. Dave.

I can buy a '98 F150 4.6L triton with the same problem. It is the only F150 triton I can find on the philippines, but I am hesitating to buy, because I dont know what causses this problem.

Did you find a solution for this problem?

4maT

Sorry sounds like its shifting out of OD or at least the lockup converter is cycling. Has there been any check engine light (CEL) activity? Guess there is the possibility of an engine miss that just hasn't thrown a code.

Thanks for the reply,

The check engine light is burning, but I am not able to read the code. There is no Ford service station on the Island.

Anyone else had this problem?

Same Opinion:
 

:thumbsup: When the change in indicated rpm occurs, do you hear the engine changing speed as well? If you do, then I concur with Bill's observation: Torque converter clutch cycling in and out.

If the tran were cycling in and out of OD, and you say it is not, the rpm change would be greater with very pronounced shifts.

If only the Tachometer reading is changing, then something in the PCM is not happy.

Try locking out OD, run in 3rd and see what the thing does.

If the CEL is illuminated, then a scan of the system is necessary to see what is causing the error code. There are simply too many variables possible to guess at the cause....unless someone has had the exact experience and effected a fix. :unsure:

Does it only do it driving, or does it do it at idle once it begins the surging? A vaccum leak can show up after engine is warmed up, if it is a small one

Something Completely Different:
 

:unsure: Another candidate for the trouble-maker is the fluid temperature sensor in the tranny. The TC clutch will not engage until the fluid reaches a certain temperature. If the sensor is faulty or the wiring to it makes intermittent contact, the TC clutch will engage or disengage contingent upon the condition.:unsure: