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Discussion Starter · #1 ·
Hi All,

I fairly recently bought a 1996 Plymouth Voyager with 140k miles as a second vehicle. It's in great shape cosmetically and the engine and tranny purr along very smoothly. I like it alot. However, I'm getting really poor fuel economy - about 12mpg in the city and maybe 14mpg on the highway. I've replaced a bunch of items to try to fix this, but with no luck so far. Here what I've replaced:
- Spark plugs
- Ignition wires
- Air filter
- Pre-cat oxygen sensor
- Oil and oil filter
- PCV valve
In addtion, I've thoroughly cleaned out the throttle body. The fuel pressure is good - about 50psi steady with the engine running, and when I shut off the engine the pressure remains at that level indefinitely. I believe this indicates that I don't have a leaky injector or a bad fuel-pressure regulator (right?). The exhaust is a little loud - probably a minor leak somewhere, but I don't think that the cat is plugged up, given that the engine runs to well otherwise - what are the symptoms of a plugged up cat?

Any ideas, suggestions, etc? Greatly appreciated, thanks,

-Daire
 

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It might be the crankshaft position sensor going bad/maladjusted. Is it throwing codes? Not that it might be if it's ailing and not completely failed. Still it doesn't hurt to check. Using the key method is described in the manuals and the Big Box Auto Stores will scan it for free if you can get it there. I would use a MOPAR unit if I were replacing it.
Plugged cats usually act like the engine is restricted, loss of power, sluggish, etc.
Edit to add: The manifolds will glow cherry red if the cat is really plugged.
 

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my son's 94 (actually it is mine but I do not get to drive it much) was getting bad mileage. It tirns out that the fuel lines at the rear were rotten and leaking.

Have a look at those
 

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Discussion Starter · #5 ·
Thanks Mike and Webbee for the suggestions.

I don't think there are any fuel leaks. I got down under the tank and traced the lines back up to the engine area and didn't see any dampness. Also, since the fuel pressure holds steady when I shut the engine off, I think this rules out a leak. Though maybe a leak in the return line would not cause the pressure to drop off - not sure about this. I'll take another look under there with the engine running to see if anything drips...

I recently replaced the brake pads and shoes and flushed the fluid. I'm fairly sure that none of the brakes are dragging, but I'll double check...

The tranny seems to be fine - acceleration is good - but I'm not sure how to diagnose a slipping tranny - how can I check this, what symptoms to look for?

I scanned for codes with a generic OBD-II scanner - nothing. Hopefully if there is something wrong with one of the sensors it'll hurry up and degrade to the point that it'll trigger a code - I hate to replace relatively expensive parts like these just hoping for the best.

The engine doesn't feel at all sluggish or restricted, and it recently passed state emissions testing, so I'm reasonably confident that the cat is okay.

Thanks again guys. If/when I get to the bottom of this I post again in the hope of helping someone else out at some point...

-Daire
 

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Discussion Starter · #7 ·
Thanks Mike. I took it out for a spin this evening, and the tranny seems to be working fine. It does shift into 4th gear - the revs jump up when i shift from "D" to "3" while moving. Also, I couldn't detect any slippage - when I stomp on the gas the van seems to accelerate at about the same rate as the engine speed increases.

I have noticed a slight whine from the tranny (I think) that begins at about 28mph and goes away at about 40mph, in both "D" and "3". I'm guessing this is related to the lockup, and hoping it's not the start of a serious issue. I'll try changing the fluid to address this, but the fluid that's in there looks good - bright red - and presumably the correct fluid, though I can't be sure of that.

Also, none of the wheels were warm to the touch at the end of my 20 mile spin, so I don't think there's a significant brake dragging problem.

Thanks again.
 

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Yes, the first rule of tranny problems is to change the fluid to fresh ATF+4. Use a MOPAR or NAPA filter as these are known to fit/work. It could be a sticking solenoid which the new fluid should free up.
 

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The number i cause of poor fuel mileage is a slow or bad O2 sensor. It will not always throw a code because the computer is still getting a signal, but the O2 sensor is not functioning quick enough to keep up with the changes in the engine. Most often you will see a gradual decrease in engine performance until the O2 sensor quits working all together.
 

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Discussion Starter · #10 ·
Thanks Chet. I've already changed the pre-cat O2 sensor, based on advice offered to others on this forum in the past. My understanding is that only the pre-cat sensor affects engine performance and fuel economy, and that the sensor downstream of the cat is only used to monitor the performance of the cat itself. Does this sound right?
 

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Chet42 said:
The number i cause of poor fuel mileage is a slow or bad O2 sensor. It will not always throw a code because the computer is still getting a signal, but the O2 sensor is not functioning quick enough to keep up with the changes in the engine. Most often you will see a gradual decrease in engine performance until the O2 sensor quits working all together.
Any chance this could also cause a rough idle. Just did a head job on my 3.0.#4 cyl. had bad ex. vavle causing a bad idle. Now eng. runs strong though you can still feel some roughness while in gear and not moving.
Sad that it's not smoother :dunno:
 

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I don't know if this helps, but I had a '92 Voyager with a 3.3L with this problem. It ran great, had plenty of power, but poor gas mialage. It turned out to be the fuel filter. I ended up changing that 4 times in 5 years.
 

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Discussion Starter · #13 ·
Thanks for the suggestion xswinnyx. Intuitively I wouldn't have thought the fuel filter could cause this problem, but I'm no expert... : )

I'll give it a shot. Thanks!
 

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Hi monkeywrench06,
I just thought of something:
If your coolant temperature is being read as running cold, it would run good but give the motor too rich fuel mixture. Does the temp gage read normally?
I don't know about the '96 minivan (mine is a '95 OBDI), but my '96 Ram 1500 has 2 coolant temp senders, one for the gage and one for the computer.
My $.02
Scott
 

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96

Are you getting any codes using the keydance method and counting the flashes in the MIL ? Previous poster is correct if the stat is stuck open and the engine is taking too long to get up to operating temperature this can waste a lot gas . An engine that is taking too long to warm up should throw a code 17 .
 

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Discussion Starter · #16 ·
Just to follow up - problem appears to be solved. I got about 22.5 MPG from my last tank of gas, which is a huge improvement and a number I can live with. This was about 75% highway and 25% city motoring.

Looks like xswinnyx was correct - changing the fuel filter seems to have been the key. I never would have guessed it, and I don't really understand why, but I'm grateful to be getting respectable fuel economy out it now.

I replaced a good number of other parts before the fuel filter, all of which seemed to have no effect, but all were wear-and-tear items anyway, so not really wasted on it.

Many thanks to all for the suggestions.

-Daire
 

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I had the same problem 12 city 14 highway

After numerous mechanics I read this post installed a new fuel filter my average mileage has jumped 6 miles per gallon in 10 miles. Cant wait to see mileage after the next tank of gas.

Thanks for the great information and great forum. :rck: .

Thanks Don
BTW my vehicle is a 2000 Grand Voyager SE
 

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There's just something about changing a fuel filter when you buy a used vehicle, especially if you know that it sat for any length of time. I have had to 2 recent dodge's (MY 88 d100-my avatar, I sold it, and my 89 GC) Both of which sat for extended periods of time before being run daily again. My d100 was really bad. You would put your foot on the accel pedal and it would go really slow, then all of a sudden give you whiplash!!!! Gas mileage was ok, but once I changed the fuel filter, gas mileage improved and the whiplash effect was removed too. Same thing with my GC(although to a much lesser degree with accel) but it was also leaking fuel from both rubber hoses on either side of the fuel filter, so I replaced the hoses and the filter at the same time. Much better now..
 

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Changing mine this week due to this thread. Great info. Thanks. I'm getting 18mpg per tank. Seems like it should be better. Been getting that for years tho.
 

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donandsandra said:
After numerous mechanics I read this post installed a new fuel filter my average mileage has jumped 6 miles per gallon in 10 miles. Cant wait to see mileage after the next tank of gas.

Thanks for the great information and great forum. :rck: .

Thanks Don
BTW my vehicle is a 2000 Grand Voyager SE
update

well went on a long trip only got 18 highway and 14 city my next change will be the front Oxygen sensor.. I just knew the filter was the whole problem... While I was driving if I kept my RPMs below 2500 I recieved my best mileage that includes taking off... Its still not right with no codes so O2 sensor is the next logical steep... Wish me luck... Gvoyager has 108000 miles on it.

Thanks Don
 
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