Poor MPG follow on

AbnMike

Well-known member
Apr 6, 2016
1,218
117
Western Slope, CO
Interesting development.

Drove to an MX race pulling my trailer, started with 11 miles on the ODO (city driving) and headed East:

154 miles.
14 gallons
11 MPG (but that's pulling the trailer)

LTFT was at 2.3 for a majority of the way, then dropped to 1.6

Way back:

109.7 miles
9.5 gallons
11.54 MPG

LTFT dropped to 0.8 for 3/4 of the trip.

LTFT is at 0.8 this morning.

I've done nothing to the truck but complain about it on the internet.
 

AbnMike

Well-known member
Apr 6, 2016
1,218
117
Western Slope, CO
Anyone? Wondering why it would simply eventually just drop to 0.8. Noticed it's back up to 1.6 after a couple days of in-city driving.

What causes them to fluctuate? Or is it only mine that the LTFT fluctuates like this?
 

Tugela

Well-known member
May 21, 2007
4,754
561
Seattle
I don't have any suggestions for you, but following your observations I was curious to see if your consumption figures matched my own. On a full tank I drove 75 miles, evenly split between city and highway. 5.1 gallons = a hair shy of 15mpg. Didn't plug in my reader so no idea what fuel trim stats were. Mods are slightly larger-than-stock AT tires, small suspension lift, roofrack. I'll have to compare with consumption when the tank is 1/4 full.
 

AbnMike

Well-known member
Apr 6, 2016
1,218
117
Western Slope, CO
Thanks, Tugela.

I'm trying not to be a pest/annoying on this issue, but I hate not fully understanding something. And these wild fluctuations are frustrating as hell, especially when they occur after not making any changes.

It almost seems as if the truck ran better when running harder (pulling a trailer), which reduced the LTFT down to an almost perfect 0.8, which doesn't make sense to me.
 

salvvia

Well-known member
May 28, 2005
990
39
BIG WHEEL ROVN IN KNOXVEGAS TN.
Food for thought run your truck hard keeping your rpms higher than normal even if you have to shift manually just dont drop out your trans mission on the road sometinmes the granny driving is detrimental and our engines need a little rough love
So give it a little hell at least through a full tank maybe even add a bottle of lucas injector cleaner take some notes and report back
By the way my 5spd only gets about 14mpg . Drive it like you stole it
 

Manofspacebob

Active member
Aug 11, 2009
35
0
North East
Could this be an old fuel pressure regulator? I bought stu454's Discovery 1 back in October and had a very similar issue. My LTFT values were starting to get really high and the mpg dropped. At first I tried replacing spark plugs, wires, fuel filter and eventually the fuel pump. Went to a local shop and had it smoke tested and they found zero leaks. Nothing seemed to change after each step so I put the original fuel pump back in and thought why not try to drop a new fuel pressure regulator into it? It ended up solving my issue.

If you already replaced the fuel pressure regulator and I didn't read it; sorry lol.
 

AbnMike

Well-known member
Apr 6, 2016
1,218
117
Western Slope, CO
It's to check the condition of the diaphragm in the FPR. If gas is present in hose or coming out of regulator, then the FPR is bad.

You're pressure jumping above 100psi can't be good.

I don't recall there being fuel when I removed the vac hose.

What should it do when I clamp the return line?
 

kk88rrc

Well-known member
I do not know exact number but last time I tested mine it was 40 ignition on and 50 at idle with return line clamped. Also, the fact that your pressure drops from ignition on at 38psi to running at 2500rpm with 25psi doesn't look good. Age of fuel pump & filter?
 

AbnMike

Well-known member
Apr 6, 2016
1,218
117
Western Slope, CO
I do not know exact number but last time I tested mine it was 40 ignition on and 50 at idle with return line clamped. Also, the fact that your pressure drops from ignition on at 38psi to running at 2500rpm with 25psi doesn't look good. Age of fuel pump & filter?

Both are brand new.
 

mearstrae

Well-known member
Mar 15, 2017
143
18
Pennsylvania
Start it up and pull the vacuum line off and plug it, then wait a few minutes (it may take while), and see if gas drips from the regulator. if so, it's shot and needs replaced (about a $50 item).
 

AbnMike

Well-known member
Apr 6, 2016
1,218
117
Western Slope, CO
Start it up and pull the vacuum line off and plug it, then wait a few minutes (it may take while), and see if gas drips from the regulator. if so, it's shot and needs replaced (about a $50 item).

I let it sit with the line off and plugged after a drive - and it probably sat for 5-10 mins. No leaks I could see.