Well I have a 95 F150 with the 302 and about 177,000 miles. I just bought the truck a few weeks ago and I did all the normal new to me kind of stuff, like new plugs, wires, oil change, fuel filter, Air filter, PCV, seafoamed the upper intake. After all that I had the computer and engine ran on a diagnostics machine and no codes or issues. However I still have a rough idle below 2000 seems to fade away around 1500, but is completely gone by 2000. Sometimes the the lope is more pronounced at times. I have completely gone through the vacuum lines and replaced most of the broken plastic lines. Also my gas mileage is horrible, on the high side its at 12mpg. Anyone got any ideas or thing to look at. I have my shop manual but I can get lost quick with that ranging from vacuum lines to blowin head gasket, my wallet would be empty if I kept throwin parts at it. Driving it doesn't seem to bad, she will get up and haul the mail but, just the idle seems to be the issue from what I can tell.

 

i had that same issue. mine ended up bein an intake gasket problem. i found that by starting my truck and spraying starter fluid around the head gasket and intake and if there is an air leak there, the motor will jump in rpms

 

Maybe there is a vacuum leak you haven't found yet. Sometimes you can hear them hiss, or like Duct Tape said, use a spray to help pin point the any possible problems. A member on here has suggested removing the fan, so you can hear what's going on with out it screaming in your ear. That my help you.

I would also clean out the throttle body if you haven't done so, and if you have MAF, check the MAF senor filament. It may need cleaning.

 

Check your spark plug wire routing and make sure that you don't have crossfiring going on. There should be a diagram underneath the hood showing you how to route the wires.

 

Ok so tried the starting fluid trick and no difference in rpm's also have check the wiring routing till I'm blue in the face. Also check the vacuum on the EGR valve and its working as advertised. Hopefully in the next week I should be able to check fuel pressure and timing. I am at a loss here and don't know what else it could be!

 

A dirty or faulty idle air valve can give that problem. If you remove it, it can be sprayed & soaked in carb cleaner or electrical cleaner to remove varnish & carbon.
Best not to spray it in place because of the risk of damaging the coating inside the throttlebody.
If the valve is your problem & this doesn't help, it will need replacing.

I have the same symptoms on my 88 right now, but only when the tanks are low - lopey idle etc - but fine with more than 1/4 full tanks (front or rear tank - makes no difference). Still trying to figure that out

 

And as for your terrible mpg....

Welcome to EFI Fords....


There all gas hoggs!


My 4.9L Straight 6 gets 11mpg MAX!

 

The idle valve you speak of is that the idle air control valve? And as far as EFI trucks go I get worse mpg than my old 82 F250 and my parents have the same truck and get 16-17 mixed driving conditions! 60 miles on a half a tank is just wrong!

 

Yep, when EFI's came out, MPG acually went down, but performance and power went up.


Older carbs got better mpg than Fuel injected ones, imagine that eh?

I guess Horse Power sells the truck better than MPG.

But what's the point in having a fast truck if your passing everything else but gas stations?

 

ha if anyone owns a truck and bitches about gas mileage, i slap them. buy a prius if you wanna save on gas.

 

You're right!

But when your 6 cyl F150 gets worst mpg than your 11,000 pound motorhome, you start asking your self some pretty interesting questions... lol

 

While I'm not a fan of complainers either, I have never followed the train of thought were to have to drive a compact if you cared about gas milage.

No matter what I drive, I try to use a little gas a possible(within reason.)

 

Yes, IAC valve.

My carbed 1983 F250 also gives me 1mpg better mileage than the 1988 F150, despite being 1/2 ton heavier, but doesn't have the performance of the F150.
Both 351W, but the 88 does have a C6

 

one fuel injected engines the throttle body needs to come off and be completely cleaned .
it is part of maintenance after lots of miles .
heres why , the idol is set buy the computer and the body will get carboned up , the idol control motor will lock up and another thing the butter fly will also get carbon around the sides and that will restrict air flow at idol , it will act like it is getting chocked !!

 

Until I had an LT1 all of the carb models got better highway mileage than a comparable EFI. On my I-6s performance and cold weather starting improved . My 300s have averaged 17 Mpg and its low end torque helps in stop and go driving.

The IACs can become sticky from deposits and it functions off idle as well. Fordfuelinjection.com has a good description of the valves function and lots of good info on the entire EFI systems.

Don't forget to check the brake booster and the vacuum motors for the heating/AC system for leaks.

regards
rikard