Need some help diagnosing poor city mileage (1 Viewer)

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Nov 8, 2009
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So, this has been driving me crazy for some time now. (Don't mind the pun). I've spent days trying to get the bottom of it but I'm reaching the end of my knowledge about the specifics of the engine management system. The problem is I get regular highway mileage but terrible town mileage. I know everyone says these things get bad town mileage but I've had it for six years so I know what I used to get. I'm in the 15 mpg (16L/100km) range on the highway but getting 7 or 8 mpg around town. Also, I can smell all the fuel coming out the exhaust when it's idling. It seems to me it's in limp mode and the computer isn't running the fuel the way it should. I have replaced in the last year the thermostat and the O2 sensors. I have tested the coolant temp sensor, vaf (repeatedly), tps (set up and tested repeatedly) and the rpm sender in the distributor. On to the ecm. I've been following the testing procedure in the fsm.

Here are some numbers...

The first four test (batt, mrel, igsw,+b to e1) check out.

Then things get sideways.

IDL to e2 throtle open is only 0.2 to 0.6v. It should be 9 to 14
vcc to e2 is in spec
VTA to e2 throttle closed is in spec
vta to e2 throttle fully open is less than 1v. It should be 3.2 to 4.9
vs to e2 plate fully closed is 2.25v. It should be 3.5 to 4.5
vs to e2 plate fully open is within spec.
Now when I try and measure vs to e2 while the truck is idling it either stalls or coughs and sputters like crazy because the air fuel ratio becomes super rich. (Much more that it is already.)

I haven't done the full second page of voltage tests yet but the ones I have check out. So far the issue seems to relate to the information the tps and vfa are providing the computer. The emc ground is solid, the vs wire is solid and so is the idl wire.

Should I run every voltage test possible along with the resistances before these numbers will be of any help to someone with knowledge about testing the computer? Also, I should add that I have a parts truck. I have tried another ecm, vaf, tps, all yielding the same results.

Some fresh ideas or info on the subject would be appreciated.

Thanks
 
Joined
Nov 8, 2009
Messages
31
More info. Year, mileage, what country, tire size, mods, color of paint?

Have you check your brakes for sticky brakes/calipers?
1994. 222000 miles. In Canada but it came from America originally. 235/85r16 or 32s at correct pressure. Brakes are fine. Just had a sticky caliper on the rear and replaced it. This has been going on longer than that little caliper problem. Old man emu 2 inch lift.
 

Finnyfam60

Driving miss daisy
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Sep 21, 2011
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Dixon, Ca
 
You said you used a different ecm with same results. I thought that meant you ruled out the ecm. Use a third if you must, but I don't think that is it. Maybe engine harness issues. How old is that. I think it's gonna be the injectors though. That poor of mileage and the smell of fuel out the tail pipe meant a you're getting poor burn, so too much fuel (clogged injectors will drip instead of spray, hence too much fuel). Have you done a tuneup? (Plugs, wires, etc)
 
Joined
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You said you used a different ecm with same results. I thought that meant you ruled out the ecm. Use a third if you must, but I don't think that is it. Maybe engine harness issues. How old is that. I think it's gonna be the injectors though. That poor of mileage and the smell of fuel out the tail pipe meant a you're getting poor burn, so too much fuel (clogged injectors will drip instead of spray, hence too much fuel). Have you done a tuneup? (Plugs, wires, etc)
I did use a different ecm with the same results thus most likely ruling out the ecm as the issue. But, the ecm isn't getting the information it should, therefore not doing it's job. I'm looking for that issue. I think if it was injectors I would have worse highway mileage too. It's the same as ever though. I've inspected a bunch of the wiring although certainly not all of it. I looked at the wiring by the egr, it looks fine. I've been hoping to find some ugly wires but haven't. It seems like a wiring issue to me since the sensors test in spec. I 've been checking grounds hoping to find a problem but so far all seem to be good.
 
Joined
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So, this thread is dead but I'll post this for anyone that might stumble across it. After many many hours of searching, the problem ended up being a little rust hole in the fuel filler neck. With the fuel system sealed up my fuel mileage returned to normal. I wish I could have found it a little earlier but at least I found it.
 
Joined
Dec 11, 2009
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124
Location
Los Angeles
 
 
So, this thread is dead but I'll post this for anyone that might stumble across it. After many many hours of searching, the problem ended up being a little rust hole in the fuel filler neck. With the fuel system sealed up my fuel mileage returned to normal. I wish I could have found it a little earlier but at least I found it.
How did you find the rust hole?
 
Joined
Nov 8, 2009
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Wow. How did you figure that one out???
I filled up one day. Drove a short distance and parked pointing up a pretty good hill. I think I came back out to my truck a few minutes later and caught a whiff of fuel. I followed my nose to the leak. Funny cause I had suspected a leak a couple times before but I didn't inspect the filler neck close enough. Just around the tank and the fuel lines. The hole was on the top of the pipe where it attaches to the body.
 
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Southport NC
So, this thread is dead but I'll post this for anyone that might stumble across it. After many many hours of searching, the problem ended up being a little rust hole in the fuel filler neck. With the fuel system sealed up my fuel mileage returned to normal. I wish I could have found it a little earlier but at least I found it.

Thank you for following up with the final verdict!
 
Joined
Nov 8, 2009
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How does a leak affect fuel usage?
I was told by a mechanic friend that if the fuel system isn't sealed you then have a vacuum leak. As you probably know, any un metered air getting into the engine will cause problems for air fuel mix. In this case it used extra fuel around town and I was down on torque. Mostly apparent trying to climb hills on the highway.
 
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Dec 10, 2007
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Thanks for the follow-up information. Did you get photos of the rusted out pipe, and how did you fix it?
 
Joined
Nov 8, 2009
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Thanks for the follow-up information. Did you get photos of the rusted out pipe, and how did you fix it?
No photos. I have a parts truck but I was away from home for work when I found the problem. To Jimmy it I cut out the pipe and put a piece of hydraulic hose in there. Hose clamped it to the good ends of the filler neck. I'm just putting paint on the one from my parts truck so I should be able to install that one soon.
 
Joined
Mar 27, 2016
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Austin, TX
No photos. I have a parts truck but I was away from home for work when I found the problem. To Jimmy it I cut out the pipe and put a piece of hydraulic hose in there. Hose clamped it to the good ends of the filler neck. I'm just putting paint on the one from my parts truck so I should be able to install that one soon.
What's your mileage now?
 

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