Mileage dropped to 16 and idle shakes the truck

[cross6]

My mileage dropped from 19 to 16 suddenly (verified by pump math AND message center) and the idle is really shaking bad.

No CEL.


Pretty sure it's NOT vacuum, since the idle is around 550rpm and doesn't bounce around.



What I've tried:


New balancer
New fan clutch
New upper intake manifold gaskets
New plug wires
New plugs
Cleaned MAF


I'm stumped.

 

[Rick]

Check your fuel pressure. The fuel pressure regulator may have went bad causing it to dump extra fuel.

 

[rhauf]

Also try cranking the engine with the key off so it doesn't start. (do this by shorting the solenoid)

this will tell you if you have for some reason lost compression in one cylinder (burnt valve for example), because the engine will crank faster during the compression stroke of that cylinder. uneven compression always shows up when cranking. if this turns out to be the problem, you can find out which cylinder it is by pulling out one plug at a time until you pull one which doesn't cause a change. ....or just do a plain old compression test. i'm just a lazy guy so i do things the easy way (as stated)

 

[Rofocowboy84]

Holy ****, I'm lucky if I even get 16 mpg, and that's if I'm gentle on the pedal (which isn't fun, so I normally don't do it...)

 

[alexk]

^^ I was thinking the same thing, someone gets more than 16? I only manage to get 16 under regular. Even driving the parkway for 140 miles I only managed 18.

 

[tv4fish]

alexk: You have the V8 -whatdya' expect I have the 4.0 V6 and in the Summer consistently get over 20 (not much, but over 20 )

 

[trucku]

I get 19 tops on highway. City I see 12.5mpg

Good luck on finding your problem Cross6, maybe I have the same issue. I have done everything you have except the fan clutch with no gain in mpg.

 

[Jayness@psn]

i have the 4.6 and average 17-19 mpg. One easy trick. . . just let the pressure off your foot when your crusing, makes a 5mpg jump. (sorry for getting off topic, plus scan gaugeII works wonders to see mpgs)

 

[tonydp]

Pretty sure it's NOT vacuum, since the idle is around 550rpm and doesn't bounce around.

There is a thread bouncing around here. Your mounty is experiencing the same as other w/possible vacum leak. If you haven't found it by later I'll link it, bed time now.

 

[joshtypesn]

Idle air control valve

 

[azpima67]

drop in mpg

I experienced mpg drops in my old EX (1993) and my new one(02). Both were apparently due to a dirty Mass Airflow sensor (MAF). The '93 had it's MAF replaced. But I got a bit smarter and just cleaned the MAF on the '02. Do a search on MAF on this or another EX board for more info. Autozone, etc sells MAF cleaner. On the '02 the MAF is on the pipe between the air filter and the engine intake. It has an electrical connector so you can't miss it. Just be carefull of the couple of wires inside the sensor. The cleaner is an no residue cleaner. Unfortunately they attached it with 4 "security" torx screws. Checker had a set of the security torx bit for $5 or $6 bucks.