Absolutely horrible MPGs

[Kody165]

I couldnt find hardly if any similar issue elsewhere on the forum. I have an 87 XLT with a 2.9...in the neighborhood i get around 10 mpg, i never do highway because of the remote area i live in. the engine has over 400k miles on it so its not so surprising. the engine has always had this issue, even after timing adjustments,a new K7N air intake with intake vacuum hose plugged, and exhaust overhaul (free flowing dual pipes). there is also a dead spot where i have almost no torque at around 2-3k rpms and a vacuum sensor is rather noisy when i turn off the engine, sounds like somethings spinning and slowly stops, one of the sensors right where the airbox would be, this has also always been there. i was wondering if its just because the engine is elderly that im having this problem, or if its a sensor issue? My driving habits dont change the economy at all either, so thats why im thinking it may be an 02 sensor or a vacuum issue.

 

[adsm08]

Well, if you have true duals that can mess with the O2 readings since you only have one oxygen sensor. Also, if the exhaust is too free-flowing it will negate the scavenging effect produced by proper sized pipes. This will make the engine work harder to breathe.

Your buzzing noise is not a vacuum sensor. It's also not the source of your problem. Mine used to have the same buzz, it just went away one day, don't remember what it was, but I do know it's a normal condition.

I would start with a compression test. See what your numbers are like, with 400K on the clock.

 

[Kody165]

Id expect the scavenging problem, but i know the exhaust didnt change anything. the O2 sensor is in place before the H pipe ends right in front of where the would0be cats would go. besides, nothing changed after i did the mod, but thanks for your input, ill try the compression test and see what i get....what would a healthy pressure be? i know its gotta drop from the numbers in Haynes manual

 

[tmcalavy]

I bet both your tailpipes are black as the ace of spades, too. With that many miles, it will be lucky to have any compression at all...it's probably guzzling more than it burns out the pipes. I had a full-sized 69 F250 Ranger that did that if you put a four-barrel on that 390 V8...it was a pulling beast, but very, very thirsty...even when gas was .99 cents/gallon.

 

[Kody165]

i found that retarding my timing a bit solved the RPM dead spot, we'll see how it affects gas milage

 

[Road Angels]

Pull your plugs if its going thru the engine they should be black, if so you you more than likely killed your o2 sensor, if not check fuel hose from tank to filler neck for leaks, at 10 mile per there has to be black smoke out the exhaust, also does your cel work, bad cts could be keeping it in a rich condition

 

[Kody165]

there is no black smoke and i honestly cant find any leaks. at least the full roll of butcher paper undearneath and my nose couldnt find anything

 

[RonD]

Neighbor with a siphon hose?
Stop at the same Bar each night?
lol


Could odometer be off?
Different tire size, different differential
I mean if you are not burning extra gas and you are not leaking it....then maybe the measurement calculations are wrong

 

[Kody165]

maybe they have a way to siphon it, i cant get down either of my filler hoses with any sort of tube, but my MPGS are already back up to around 15, so im not worried anymore. def gonna get some locking gas doors