Rpm fluxuation

ford

#1

concering my 2005 ford five hundred 3.0 dohc v-6 auto trans w/114,000 miles. at cruising speeds around 65-70 with rpms at 1800-2000 there is rpm fluxuation(increase and decrease) and eng surge(jerking) with no noticeable speed change. This problem only occures at the above rpms/speed. idle is fine, acceleration is fine, start up and shutdown is fine. spark plugs new…changed after is problem started with no change in above symptoms.


#2

Your engine is under a lot of load at highway speeds, yet does not have the rich mixture that it has when accelerating, and that may be why it’s only showing up there.

Since it’s accelerating well I’m inclined to eliminate the fuel pump and regulator as suspects. As well as a vacuum leak. Doesn’t mean they’re impossible, just low probablility. Since it’s otherwise operating well the ignition system seems low as a probablility also.

But get the computer checked for stored codes anyway. There’s little to work with here and a code would be a real help.

If you have no codes…then I’d suspect the torque converter lockup clutch. By the way, when was the last time you checked your tranny fluid? How’d it look? Did it smell odd?


#3

no codes at all. All fluids checked last oil change 120 miles ago. i did not do the maintanence so I do not know about any smell or discoloration. I will have to check that. Thank You


#4

As I read the post I was thinking torque converter lockup clutch as well. These can do some counter-intuitive things. E.g. an abrupt change in throttle position will unlock it and cause an rpm surge. This makes sense if you floor it, but you can also pull your foot from the throttle and watch the rpms jump - I’d call that counter-intuitive, but it actually means it is working - at least for some designs under some conditions.


#5

I’d have it looked at. Your ignition system can be looked at on a scope (traces can tell lots of tales that aren’t otherwise obvious) and a tranny shop can possibly find something that did not trip a code. In my mind both the engine and the tranny are both equal possible causes. Both would normally trip a code (the tranny something in the P700+ range), but without a code I think an assessment of both is in order.