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What's killing my mileage?

brtb

NAXJA Forum User
Location
Orlando, FL
Ever since I got this thing it's been getting mileage that I considered pretty amazing from a 1988 with a big engine and 117k miles: 300-350 miles on 17 gallons of the cheap stuff. Just on this last tank it's dropped significantly... it's reading 1/2 tank, I might have gone 75-100 miles so far, and nothing's changed in the driving pattern. Ideas on anything that'd suddenly change the mileage?

I was thinking the O2 sensor might be suspect, the timer thingy's still going so I assume they were both replaced sometime before but maybe not. Dunno if it'd just go suddenly either, instead of slowly degrading with time. Any other sensors to check? No codes of course, got the usual Renix setup. Do have a DMM and an analog one if it'd help anything :D

It did have good regular maintenance done on it otherwise, got the records from the previous owner... last trans-fluid replacement was at 75k or so (pretty sure it's Dexron, bright red), same for the diff, regular 3-4k mile oil changes with 10w30 non-synth.
 
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Have no hard data yet to support it, but a recent O2 sensor change on my 160K 1989 model seems to have significantly increased my mileage.

Some of the possible culprits:
Air Filter
Distr. cap and rotor, spark plugs, wires
Throttle body and intake oil gunk from a clogged CCV system I think this can ruin your O2 sensor too.
Leaky fuel injectors- both external-you would smell gas- or leaking at the nozzle ( long crank times)
Fuel line or injector o-ring leaks.
Shifting pipes in the OEM muffler if beaten up or rusty.
Vac leak on the tube feeding the MAP sensor- this would cause a driveability problem.

Hows it running?
 
Runs fine; it's got a rev-synced rattle that I haven't traced any further than "it gets louder as the engine warms up," and just a slight lack of the pick-up factor that without any other XJ experience to compare to, I can't be sure of anyway.

Anyway, moving down the list...

Air Filter: Purolator, within 1000 miles of new.
Distr. cap and rotor, spark plugs, wires: Cap looks new-ish, though I've never taken it off so who knows about the rotor. Do those "go" suddenly? Plugs/wires replaced February, Champion standard plugs and store-brand wires (been working fine until now...)

Throttle body and intake oil gunk from a clogged CCV system: Back (thin) hose clear, front hose has slight oil residue but isn't exactly airtight (broken, taped back together for now). Both freeflowing though... could the front one being bad cause this?

Leaky fuel injectors- both external-you would smell gas- or leaking at the nozzle ( long crank times), fuel line or injector o-ring leaks: No gas smell at all if that means anything...

Shifting pipes in the OEM muffler if beaten up or rusty: New universal-fit model February, you'd think it would still be okay. As for the cat I have no clue as to the age, but it's solid outside and it's not rattling any.

Vac leak on the tube feeding the MAP sensor - this would be the sensor on the firewall behind the engine? Whatever sensor that is has plenty of vacuum, and the engine hates it when it gets disconnected :)
 
That's a good suggestion Paul. Also, these older Renix equipped XJs have what they call a Thermostatic Air Cleaner that, when not working correctly, can cause poor performance. It's a valve of sorts that's situated in front-most part of the air box. When the engine is cold this valve opens so that only exhaust manifold air is allowed into the intake. Look for the non-rigid exhaust tube (about 2" diameter IIRC) that runs from the exhaust manifold to the air box. Now, as the engine warms up, this valve closes to the exhaust air and opens the pathway to the fresh, outside air. My '89 developed a problem that kept the valve stuck so that only exhaust air entered the intake. I lost power and suffered from poor gas mileage.

Just drawing from another personal experience....do you live in a dusty environment? I do and have found I must replace the air filter quite often. I replaced the filter about two weeks ago. On my way home from Moab on October 11th (about 9 days after replacing the filter) I noticed a considerable performance loss on the way home the following week that got progressively worse over the next several days, including a decrease in MPG. I replaced the air filter and the engine can breathe properly again--a noticeable performance difference--and gas mileage returned to normal. Of course Moab is quite dusty and that was an extreme example. I find under normal conditions (where I live) I must replace the air filter about every three months.

Are you required to get an emissions check where you live? If so, how long ago and how were the results? Cap & rotor are cheap, so you may want to replace those parts. Just a couple of things to consider.

Tom
 
No emissions check in Orlando, FL. Not really dusty either, the humidity pretty much takes care of that, even though the state's one big sand dune.

Didn't think to check that front hose to the exhaust manifold; the main air intake path was clear so I kinda ignored it. Last time I looked I seem to remember that hose being slightly restricted by the transmission->radiator connection. Maybe I'm thinking of the wrong thing but I thought I've seen that referred to as the Idle Air Controller with a vacuum-controlled stepper motor... whatever it is, disconnecting the vacuum line didn't do much. I'll actually check the hose and see what's going on next time I'm in there. While I'm messing with the whole intake side of things I should probably take a look at the throttle body too...
 
The exhaust "hose" I'm referring to comes off the exhaust manifold down pipe and comes up to the air box. It's a larger hose, maybe about 2" (inches) in diameter. Attached to the front of the airbox is the Termostatic Air Cleaner, which is vacuum actuated and controlled by engine temp. Think of it as a flapper door that when in one position (when engine is cold) lets warm exhaust air into the intake via the air box to --> throttle body. When in the other position (engine reaches normal operating temp) it lets fresh outside air in by closing the pathway from the exhaust manifold. make sense?

Tom
 
Hey Tom, didn't know you tracked that down as your power robber......I ran mine disabled for years (killed the vacuum so the door is in the always open position, drawing fresh air instead of exhaust heated air) for several years before I completely eliminated the lower section of the box.

One way to eliminate this as a variable, and I never saw any drivability issues with this door in the "always open" position, even in the cold winter months of No Utah.
 
The thermostatic air cleaner was a problem on my '89. The HO engines don't have this equipment. The issue I had with my '92 was fixed with new HP OEM spark plug wires and cap & rotor, even though the items replaced were not very old.

Tom
 
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