Odd question for you guys, during my drive too work at 2pm im going 70mph on the highway at 1800rpm, on the way home about 1130pm however on the drive home, same route same speed the engines running at 70mph 2000rpm . For the life of me i dont why. The only difference is temperature outside from 80ish to 70ish, and driving with the lights on. But every vehicle ive ever owned or driven running the lights has had no effect on my engine rpm before, at idle the rpm is the same, 1000rpm in park, 850ish in drive with my foot off the pedals. Its only while actually driving at night that the rpm runs higher by a full 200rpm. Does anyone have any ideas?

 

Is there much of a grade difference in the two trips? If you're got a slight downgrade by day, and a slight upgrade at night, that could be normal slippage through the torque converter.

If somehow related to the lights, that should be easy enough to confirm. Switch them on during the day and see if the RPM changes.

 

I had meant to run it with the lights on today on my way into work, but forgot, as for hills ive got ups and downs on the trip, but im taking my reading from the flat streaches between.
Aside from lights ive got no idea what would potentially cause that, and still why would my lights cause a full 200 rpm higher run, you know?
Ill make sure and run the lights on my in to work tommorrow.

 

Maybe the lights are causing some electrical change/interference that is throwing off the tach, and the actual engine speed is no different.

 

If you have good hearing, you should be able to hear how fast the engine is turning, and carefully compare that going both ways. If it sounds the same, then the tach is lying. If it doesn't sound the same, then something is slipping.

 

Well i ran the lights on the way in today, same rom range as my drive in yeasterday without the lights on, engine sounds the same going both ways, i had the radio turned down on my drive going last night and way in today, maybe it is just lying to me then. Im not sure.

 

I installed a factory tach in my dash. Usually it works fine, but a little while ago the guage would give off readings and I could tell since I have a manual and the rpm was different day to day while cruising at 100kmh. Sometime it even bounced around and just went nuts. I never found the issue. It's recently been working well though. Maybe you have the same issue.

 

This thread might be of assistance: https://www.ford-trucks.com/forums/7...dywhompus.html

 

Im running an aftermarket tach as this truck did not come with one from the factory, ive got it grounded inside the cab where the the brake controller is grounded, ill take it off,clean up where it grounds and the connections on the brake controller/tach/chassis and see if that fixes it, but i doubt it will as the problem is only at night. Even when running the lights during day time. But that link did help a bit gave me the idea to check that as it may not be making the beat connection.