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Recent drop in MPG - What to troubleshoot next?
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jwendorf
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PostPosted: Fri Jan 20, 2017 7:26 pm    Post subject: Recent drop in MPG - What to troubleshoot next? Reply with quote

In the last few months the MPG in our 1989 2.1L Carat has dropped off significantly, e.g. formerly we were getting ~210 miles per 14gal fillup, recently it has been closer to 150miles.

The car is driving and idling fine, i.e. there hasn't been a sudden change in drivability.

To complicate things, about two months ago I re-did the fuel lines after which our gas mileage dropped really fast. I quickly noticed that some of the new hoses were leaking at the injectors because I hadn't tightened the hose clamps enough. That leak has been fixed and can't find any identifiable leaks, however the mileage is still low (although presumably not as bad as when the injector hose was leaking).

I've also checked the brakes to make sure nothing is dragging or stuck, but I'm pretty sure I would notice that in the drivability of the car anayway.

I last did the spark plugs, cap and wires about 2.5 years ago.

What else could account for a drop to 10mpg?
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hdenter
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PostPosted: Fri Jan 20, 2017 8:50 pm    Post subject: Re: recent drop in MPG, what to troubleshoot next? Reply with quote

Well, if you have made sure there are no leaks and that your fuel and air filter have been changed fairly recently, I would test the temp2 sensor to see if it is providing accurate info to ECU. If that checks out, I would unplug the O2 sensor and drive it around a while and see if it change anything. If no change, you can look into it's performance and check the wiring harness to it.

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0to60in6min
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PostPosted: Fri Jan 20, 2017 8:54 pm    Post subject: Re: recent drop in MPG, what to troubleshoot next? Reply with quote

Quote:
I last did the spark plugs, cap and wires about 2.5 years ago.


it's been quite a while... I'd do it again.. now...

yes.. Temp2 sensor and connector


Last edited by 0to60in6min on Fri Jan 20, 2017 8:57 pm; edited 1 time in total
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dhaavers
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PostPosted: Fri Jan 20, 2017 8:55 pm    Post subject: Re: recent drop in MPG, what to troubleshoot next? Reply with quote

Here's a few WAG's, FOC: Fuel &/or expansion tanks: vent lines, crossover
tube, grommets...??? Running rich? Suspect O2 sensor...

Pump/filter/return lines? Leaks would not be noticeable unless parked
& idling, otherwise you leave a trail of drips & never notice...

Fuel pressure regulator? Bad diaphragm could be leaking fuel to the
vacuum intake, also cause running rich...

Having said that, I'm sure a real expert will be along soon... Wink

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air-h2o-air
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PostPosted: Fri Jan 20, 2017 9:20 pm    Post subject: Re: recent drop in MPG, what to troubleshoot next? Reply with quote

every injected vehicle looses during the cold seasons....did you drop mpg as cold set in?
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hdenter
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PostPosted: Fri Jan 20, 2017 9:30 pm    Post subject: Re: recent drop in MPG, what to troubleshoot next? Reply with quote

He was barely getting 15mpg (which isn't that great) and dropped to 11mpg. Weather wouldn't do that...

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E1
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PostPosted: Fri Jan 20, 2017 9:43 pm    Post subject: Re: recent drop in MPG, what to troubleshoot next? Reply with quote

Did you replace the fuel filter?

We get up to 19.5 mpg in an overloaded Westy with a giant stump on the roof, so 10 is begging for mercy.

How many miles on air filter and plugs and such in 2.5 years?

Grounds okay?
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jwendorf
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PostPosted: Fri Jan 20, 2017 10:05 pm    Post subject: Re: recent drop in MPG, what to troubleshoot next? Reply with quote

Thanks, all good suggestions. The air filter is recent, but the I haven't done the fuel filter for several years.

I will start with new fuel filter and sparkplugs, see if that gets me anywhere.
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Ahwahnee
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PostPosted: Sat Jan 21, 2017 7:47 am    Post subject: Re: Recent drop in MPG - What to troubleshoot next? Reply with quote

Many have encountered an abrupt drop in MPG when the (green) coaxial wire to the O2 sensor shorts to ground.

This wire has a coaxial shield and a central wire - the usual failure mode is that the coax (which is grounded) comes in contact with the center wire or the female connector. Doesn't take much - just one strand of that braided coax shiel coming in contact with the copper center wire will be enough to fool the ECU into running rich.

The test is to use a multimeter and confirm the conductor wire is not grounded. The fix is to trim the coax back a bit and use shrink tube to keep it tidy.

All that said - this problem usually also creates some running issues and power loss while you say 'The car is driving and idling fine'.

Temp II sensor can be tested with an ohmmeter (graph in the Bentley or ProTraining manuals). The usual failure mode there results in the van being hard to start when the engine is hot - but perhaps in the cold weather you have not been in that situation much.
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jwendorf
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PostPosted: Sat Jan 21, 2017 10:26 am    Post subject: Re: Recent drop in MPG - What to troubleshoot next? Reply with quote

Thanks, I'll check the O2 sensor as well. I should have mentioned that I put a new O2 sensor in about a year ago.
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PostPosted: Sat Jan 21, 2017 11:52 am    Post subject: Re: Recent drop in MPG - What to troubleshoot next? Reply with quote

What awahnee said. When my mpg soiled the bed, I replaced the O2 sensor and noticed a tear in the wire insulation. Heat shrink wrapped it, tested OK, problem solved.
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PostPosted: Sat Jan 21, 2017 1:43 pm    Post subject: Re: Recent drop in MPG - What to troubleshoot next? Reply with quote

I hadn't even thought of this, but we were re-ended in late July on driver side right above where the O2 sensor is located. The "fenderbender" destoyed our fiberglass bumper, but I'm wondering if just the shock of the collision might have also damaged O2 sensor. I'm thinking I will replace that first to see if it makes a difference.
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Ahwahnee
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PostPosted: Sat Jan 21, 2017 2:12 pm    Post subject: Re: Recent drop in MPG - What to troubleshoot next? Reply with quote

That is suggestive and worth following up on. But before you replace a part - it is quite simple to test for a short to ground on the O2 lead.

This is the green wire that plugs into a double-male connector:

Image may have been reduced in size. Click image to view fullscreen.


You unplug that green wire and use an ohmmeter on it's connector. You should zero ohms to ground. If you find continuity to ground then the connection is compromised.
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pushkick
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PostPosted: Sat Jan 21, 2017 6:36 pm    Post subject: Re: Recent drop in MPG - What to troubleshoot next? Reply with quote

have you checked your cat converter ? sometimes they get blown out.
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Terry Kay
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PostPosted: Sat Jan 21, 2017 7:53 pm    Post subject: Re: Recent drop in MPG - What to troubleshoot next? Reply with quote

A "blown out" converter would cause the engine to exhaust easier and give
you a little more mpg. offering the exhaust less restriction on the way out.
A plugged converter would show low mpg.

Can you get a wiff of raw fuel at the end of the tailpipe?
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BigNick0
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PostPosted: Mon Jan 23, 2017 12:21 pm    Post subject: Re: Recent drop in MPG - What to troubleshoot next? Reply with quote

For what it's worth, my 1991 2.1L Carat gets between 300-320 miles between fill-ups. It sounds like you had some issues before your recent drop in mpg.
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