Vacuum gauge question

[jnichol]

Hi, I've been working out the bugs on my 1986 F150 with FI 302 engine, it has a new timing chain and a full tune up. The truck is performing poorly on fuel mileage at about 15. I've connected up a vacuum gauge and ran some checks that are non conclusive, it does show a slight vacuum loss during slow acceleration (19 at idle dropping to 17 at times and up to 20 when engine smooths out). I checked the voltage on my TPS but my meter is not accurate down to 10ths of a volt but it appears linear. My question is has anyone had experience using a vacuum gauge while driving down the road? Most of the trouble shooting guides are for diagnosing while stationery. While driving on the road my vacuum tends to run at about 10 when up to speed on level, lower uphill and higher on the down grades. Any ideas are appreciated.

 

[DNFXDLI]

For the most part, vacuum will do just what you described...lower under acceleration and higher under no acceleration. As the butterflies open, there is a loss in manifold vacuum.

 

[SuperCab]

what transmission is in it, what are the axle ratios...

depending on above items, 15 may be all you'll see out of a 302...

do you do mainly city or highway driving?

 

[jnichol]

The transmission is an AOD with .67 overdrive, the differentials are 3.55. The tires are 31/10.5R15 which is 7% over stock and the smog pump has been removed. The only other alteration is it has true dual exhaust with glass packs. There is a slight ruff exhaust sound when slowly increasing RPM then it smooths out, when it is running ruff the vacuum decreases. Occasionally when driving I can feel a slight loss of power when slowly increasing speed. I only get 15 mpg on a good day when driving carefully on a mostly highway trip. I'm only turning 1550 RPM at 60 mph.

 

[jnichol]

Decided to change out the throttle position sensor, I had read a previous post about built up deposits inside the throttle body. He wasn't kidding about the black carbon build up inside, about .25 thousands completely covering the back side of butterflies and inside housing. It took about an extra hour of cleaning time to remove the build up with degreaser. The end result was smoother acceleration. I will be driving across Mt. Hood this week (about 150 miles) for work and I should have some fuel mileage results. Hoping for the best...