First off, thanks for reading.
1999 manual drives well most of the time, but occasionally, a few times a week, while driving at constant speed, the rpm will cycle. What I mean is... say I'm driving at 30mph with 2200 RPM, when the cycling occurs, the RPM jump up to 3000 then back down to 1500 then back up again and this continues until I shut the engine off. When I come to a stop, the RPM level out at idle speed for a second or two, then drop way down to under 500. The truck rarely stalls at a stop, but RPM stay low for another second or two and then pick back up to normal idle speed.

I have checked my vacuum lines, no leaks.

I am about to dive into checking sensors, but i'm wondering if you all have a suggestion of where to start or if I am even going down the right path.


additionally, and I don't know if this is related or not, but the instrument cluster died this week too. The speedometer & tach read zero & the odometer reads " - - - - - " . This has happened before when the engine ran fine, so I doubt it's related.

 

As far as the instrument cluster goes, it sounds like it may have come unplugged. Does the odometer actually read ------- or is it blank? If it's not reading anything I would say maybe try and check your fuses.

Ane for the engine........ I would double check those vacuum lines. It really sounds like a vacuum issue to me. It could also possibly be something like an O2 sensor, but I doubt it. What engine does it have in it?

 

Odometer actually reads "- - - - -". it is not blank.

Engine is stock V6. When I checked the vacuum lines, I only did a visual inspection, looking for big cracks or tears in hosing. Is there another way to do it, or any handy gadget that can help inspect?

 

Well I really don't know much about the V6. My suggestion would be to take it down to your local parts store and see if they have a scan tool that you could borrow and see if it's pulling any codes. I'm pretty good friends with my local guy so he lets me, I'm not sure if other stores are like that though.

 

yeah try getting a code reader now they are going for cheap i got mine for $145, it helps for like your odometer, and for your engine i'm having the same problem on my truck so what i might do is check the timing because the rpm's are dropping

 

You have the same problem eh?, let me know if you find anything out. Still no progress on this end.

 

Shot trouble codes last night.
P1131 - Lack of Upstream Heated Oxygen Sensor Switch - Sensor Indicates Lean - Bank No. 1
P1151 - Lack of Upstream Heated Oxygen Sensor Switch - Sensor Indicates Lean - Bank No. 2

I'm wondering if there are any indicators that would tell me the sensors both need replacing or would the fact that both failed go back to a common vacuum line that would might be leaking, or a bad common ground? Any Ideas?

 

Here is the fix for the odometer. http://www.f150online.com/forums/ele...er-repair.html

As for the unstable idle and surging and lean codes I would recommend cleaning the EGR passages in the intake.

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My 2000 F150 4X4 with 5.4L is kind of doing the same thing..every now and then the truck will start shaking and seems like a surge but mine is also throwing a P1131 code for the sensor. I ordered the sensor that Ford calls for the f150's and it ended up being wrong... You can take a look at yours and tell me if maybe you need the O2 sensor that i mistakingly bought. When you crawl underneath the truck..Like at where the exaust manifold connects to the exaust pipe..Your upstream sensors are located right there. The O2 sensor should be there with a black plastic coating and should have a gray pigtail that conects with the wiring harness..Let me know if it is and i'll cut you a deal on the sensor. Hope this helps