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Poor MPG by Dotrenrew
Started on: 07-08-2013 02:16 PM
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Last post by: Dotrenrew on 07-29-2013 09:14 PM
Dotrenrew
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Report this Post07-08-2013 02:16 PM Click Here to See the Profile for DotrenrewClick Here to visit Dotrenrew's HomePageClick Here to Email DotrenrewSend a Private Message to DotrenrewEdit/Delete MessageReply w/QuoteDirect Link to This Post
I am only getting about 22 mpg with my 84 duke. I ran a paperclip diagnostic and only came up with #12 repeating itself. I have a brand new o2 sensor. I do smell it burning really rich, but I don't know why.

Any help is appreciated.

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Report this Post07-08-2013 03:35 PM Click Here to See the Profile for dobeySend a Private Message to dobeyEdit/Delete MessageReply w/QuoteDirect Link to This Post
Still has the original cat in it?
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Dotrenrew
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Report this Post07-08-2013 03:46 PM Click Here to See the Profile for DotrenrewClick Here to visit Dotrenrew's HomePageClick Here to Email DotrenrewSend a Private Message to DotrenrewEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by dobey:

Still has the original cat in it?


No cat. It was removed and never replaced.

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[This message has been edited by Dotrenrew (edited 07-08-2013).]

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Report this Post07-08-2013 04:00 PM Click Here to See the Profile for dobeySend a Private Message to dobeyEdit/Delete MessageReply w/QuoteDirect Link to This Post
No cat doesn't help with the rich smell.

Is the 22 MPG city only, highway only, or combined?

Which transmission is it?

Checked for exhaust or vacuum leaks? When was last tune-up? Any hesitation or loss of power while driving? Brake calipers sticking?
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Report this Post07-08-2013 04:02 PM Click Here to See the Profile for cebixSend a Private Message to cebixEdit/Delete MessageReply w/QuoteDirect Link to This Post
I get about that in my '85 duke auto mostly highway doing 60-70 mph. Is that poor mileage?
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Report this Post07-08-2013 04:18 PM Click Here to See the Profile for 92wastheyearClick Here to Email 92wastheyearSend a Private Message to 92wastheyearEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by cebix:

I get about that in my '85 duke auto mostly highway doing 60-70 mph. Is that poor mileage?


I got mid-high 30s with 86 SE (Duke/5 speed)....don't know what an automatic should get ....but probably should be higher than 22 regardless

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Dotrenrew
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Report this Post07-08-2013 04:20 PM Click Here to See the Profile for DotrenrewClick Here to visit Dotrenrew's HomePageClick Here to Email DotrenrewSend a Private Message to DotrenrewEdit/Delete MessageReply w/QuoteDirect Link to This Post
I have only put about 1,000 miles on it since I got it running. New plugs/wires.

I haven't taken it on an expressway yet. I don't usually drive on them. I usually stick to rural country roads. About 55-65 MPH.

I don't notice any sluggish performance. Maybe it is only supposed to get 22-25 mpg. I just figured it should be near 30 mpg.

It's a 4 speed manual. Somebody said there were two different ones based on the 5th digit in my VIN#. My 5th digit is M, but I don't know what it means.

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Report this Post07-08-2013 04:32 PM Click Here to See the Profile for dobeySend a Private Message to dobeyEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by Dotrenrew:
It's a 4 speed manual. Somebody said there were two different ones based on the 5th digit in my VIN#. My 5th digit is M, but I don't know what it means.


Check the RPO tag on top of the wheel well under the hood. You should see MY8 or M19 somewhere. That identifies the transmission you have. If it's M19, then you have the 4.10 performance transmission, which will get lower MPG than the MY8 econo trans.

Also, do you have stock wheels/tires on it? Or do you have taller or shorter tires? Having different tires than stock will affect MPG, and could cause the odometer to become slightly off, resulting in false calculations.

The 84 also wasn't the best for MPG. Sounds like you're mostly combined economy, so 22 isn't too far off. 24-25 is what is listed for combined for the 84, on fueleconomy.gov, based on transmission. So I'd just check for a small vacuum leak somewhere, or similar, that might be causing the MPG to be off. It's not a lot, so it could be that, or it could just be a different than stock tire size.
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Dotrenrew
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Report this Post07-08-2013 04:47 PM Click Here to See the Profile for DotrenrewClick Here to visit Dotrenrew's HomePageClick Here to Email DotrenrewSend a Private Message to DotrenrewEdit/Delete MessageReply w/QuoteDirect Link to This Post
Ok, I have the M19 tranny. I also put 15" GT wheels on the car.

So maybe the combo of the transmission, wheels and no cat is keeping it mpg down to 23.

Thanks for the input. Now I have to figure out how to get rid of the rich exhaust smell. It's really bad with the windows down and sunroof open. I wonder if there is an exhaust leak. I'm not familiar on what the motor should sound like. I had a similar issue with an old VW jetta. Exhaust leak and stinky passenger compartment.

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Report this Post07-08-2013 04:59 PM Click Here to See the Profile for dobeySend a Private Message to dobeyEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by Dotrenrew:
Thanks for the input. Now I have to figure out how to get rid of the rich exhaust smell. It's really bad with the windows down and sunroof open. I wonder if there is an exhaust leak. I'm not familiar on what the motor should sound like. I had a similar issue with an old VW jetta. Exhaust leak and stinky passenger compartment.


Does it smell like exhaust, or more like gas? You might want to check the fuel filter and lines, as well. If they're corroded enough, gas can start seeping through, even if it's not a full blown leak, and it will smell pretty strong.

Also, depending on how bad it is, just installing the cat again, might fix it; as the purpose of the cat is to "burn" any fuel that wasn't burned in the cylinder, in a way that also cleans up the emissions a bit. Without a cat, those droplets of fuel will most likely not get burned, and will stick in the expelled gas, causing the exhaust to smell extra rich.
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Report this Post07-08-2013 05:08 PM Click Here to See the Profile for DotrenrewClick Here to visit Dotrenrew's HomePageClick Here to Email DotrenrewSend a Private Message to DotrenrewEdit/Delete MessageReply w/QuoteDirect Link to This Post
It smells like rich exhaust. I don't smell any gasoline.

Maybe I'll check for an exhaust leak and if that isn't the problem, I'll work on getting a cat installed.

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Report this Post07-08-2013 05:43 PM Click Here to See the Profile for FFIEROFREDClick Here to Email FFIEROFREDSend a Private Message to FFIEROFREDEdit/Delete MessageReply w/QuoteDirect Link to This Post
IF you have a lower than stock T-STAT in it you will have poor mpg. If the TQ converter is not locking up it will have poor mpg.
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Report this Post07-08-2013 06:58 PM Click Here to See the Profile for dobeySend a Private Message to dobeyEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by FFIEROFRED:

IF you have a lower than stock T-STAT in it you will have poor mpg. If the TQ converter is not locking up it will have poor mpg.


His MPG is on par for what the EPA estimates are. The difference from that to 17 that he's seeing could be something as simple as driving style. Changing the stat, or even running without one, will make no discernible difference to MPG. I was running without a stat at all in my 87 GT, and a blown head gasket, right before I let the state inspection lapse, and put her away in the garage, to prep for the drivetrain swap. And I was still getting 17-19 MPG mostly city then (Getrag car).
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Dotrenrew
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Report this Post07-08-2013 07:06 PM Click Here to See the Profile for DotrenrewClick Here to visit Dotrenrew's HomePageClick Here to Email DotrenrewSend a Private Message to DotrenrewEdit/Delete MessageReply w/QuoteDirect Link to This Post
Besides swapping out for an econo-transmission and driving like Ms. Daisy, is there any way to get better mpg with my current set-up?

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Report this Post07-08-2013 07:21 PM Click Here to See the Profile for dobeySend a Private Message to dobeyEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by Dotrenrew:

Besides swapping out for an econo-transmission and driving like Ms. Daisy, is there any way to get better mpg with my current set-up?



Most of the little things you can do, are just that, very little. Some of them are even a bit of work for little to no gain. Some are a bit costly and won't really gain you much either. I'd start with checking your exhaust system for leaks and getting a new cat put in place where it's supposed to be. A compression test would be good to do too. If any pistons are getting blow by, or valves are sticking a little, you could be losing a little there. And if an exhaust valve is open enough to lose any compression, you could be getting some unburned fuel mixture pushed out and into the exhaust as well, contributing to the really rich smell.

I don't know how many miles are on your car, or what service life it's seen, but an overhaul to get everything sealed back up proper like it was new wouldn't hurt it, I'm sure. Unless it's been done recently of course. But again, it's an expensive thing to do, and won't really gain you MPG in your driving conditions. You're already right at the EPA estimate for the most part, so I don't think there's really anything you can do to get more than 1-2 MPG increase. And you might be able to see that just changing driving style a bit and trying hard to get that.
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Report this Post07-08-2013 09:13 PM Click Here to See the Profile for HudiniClick Here to Email HudiniSend a Private Message to HudiniEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by FFIEROFRED:

IF you have a lower than stock T-STAT in it you will have poor mpg. If the TQ converter is not locking up it will have poor mpg.



+1

Use the stock 195* T-stat. Edit: saw you have the 4 speed. Confused your post with another.

[This message has been edited by Hudini (edited 07-09-2013).]

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Report this Post07-09-2013 11:13 AM Click Here to See the Profile for armosSend a Private Message to armosEdit/Delete MessageReply w/QuoteDirect Link to This Post
Your mileage is on par with a V6. My old 84 auto was also about the same, but that was an auto, and the engine was in bad condition. I'd expect yours to do better.
However, roads and driving patterns are different for everybody and that can throw the numbers around quite a bit. If rural country roads are the typical environment then I'm assuming reasonably long trips and not a lot of stop and go. 55-65mph is a good range for mileage also, as compared to 70+ that people usually drive on interstates.

What coolant temperature are you getting most of the time? Do you have good compression? Have you checked ignition timing?
The 15" wheels might be hurting in stop and go traffic, but they aren't overly huge. They probably help in steady cruising as they reduce cruising RPM some.
The odometer will show around 2-4% fewer miles than it should, depending what the original and current rear tire size are.

Since it seems rich, it might be worth looking for an exhaust leak at the manifold. Since the O2 sensor is right at the manifold I don't suppose a leak anywhere downstream from that would affect it.
Look for any vacuum leaks. Pay attention to the lines that connect to the MAP sensor.
It wouldn't hurt to also check the condition of the coolant temperature sensor (the one that talks to the ECM, not the gauge). This is done by measuring resistance between the pins, and comparing that with a resistance vs temperature chart.

Assuming the car gets into closed loop, anything that would cause the car to run rich should be picked up by the O2 sensor, and ultimately compensated. If the ECM still can't achieve stoich then it would give a "rich exhaust" error code, which you're not getting. However, relying on the O2 feedback doesn't make things run as efficiently as if everything works correctly.
If it really is consistently rich, then either it's not going into closed loop, or for some reason the O2 sensor isn't seeing it. When you replaced the O2 sensor, you didn't use dialectric grease on the connector did you? The connector needs to breathe outside air, grease messes that up.

If you run the engine while the ALDL A-B pins are jumpered, the "check engine" light will flash in response to the O2 feedback. The light will be on when the O2 reports rich, and off when it reports lean. I guess you could try doing that to see what the O2 is saying. In a happy car it should blink steadily. But keep in mind this mode also disables the electronic spark advance, so it will be running retarded.

If the injector is in bad shape it could affect combustion efficiency. I don't know if the TBI injectors have many issues, or how expensive it is to get them properly refurbished.
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Report this Post07-09-2013 12:56 PM Click Here to See the Profile for edfieroClick Here to Email edfieroSend a Private Message to edfieroEdit/Delete MessageReply w/QuoteDirect Link to This Post
No one picked up on the fact that the tires were changed to 15". Isn't that going to change the speedometer and odometer ?
has that descrepancy been accounted for in calculating the mileage?

I have an 84 Duke 4 speed and get 28mpg, driving 60mph, and about 85% highway.
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Report this Post07-09-2013 01:13 PM Click Here to See the Profile for dobeySend a Private Message to dobeyEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by edfiero:

No one picked up on the fact that the tires were changed to 15". Isn't that going to change the speedometer and odometer ?
has that descrepancy been accounted for in calculating the mileage?

I have an 84 Duke 4 speed and get 28mpg, driving 60mph, and about 85% highway.


Yes. If you read the thread, you'll see that I specifically mentioned tires. The difference in size between the 13" and 15" tires at stock sizes isn't that big though. They are almost exactly the same diameter. Highway and country roads are not the same, country roads do have stop signs and intersections, as well as rougher roads. Highways are smoother and typically no stopping.

Do you have the MY8 or M19 trans? There are two different 4 speeds on the 84s. The econo trans will definitely get better MPG, especially at 85% highway compared to combined rating with no highway, but mostly country roads.

[This message has been edited by dobey (edited 07-09-2013).]

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Report this Post07-09-2013 01:35 PM Click Here to See the Profile for edfieroClick Here to Email edfieroSend a Private Message to edfieroEdit/Delete MessageReply w/QuoteDirect Link to This Post
I have the 4.10 rear.
Spin just under 3 grand at 60 mph.
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Report this Post07-09-2013 07:13 PM Click Here to See the Profile for armosSend a Private Message to armosEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by edfiero:
No one picked up on the fact that the tires were changed to 15". Isn't that going to change the speedometer and odometer ?
has that descrepancy been accounted for in calculating the mileage?

That's the 2-4% I alluded to. The difference in odometer readings from P185/80R13 to P215/60R15 is about 2%. The difference from P215/60R14 to P215/60R15 is about 4%. So if his car originally came with the 14" wheels then an apparent 22mpg is really 23mpg. If they were originally 13" then the difference is less (those tires have a bigger diameter than the 14s due to the 80 profile).

 
quote
dobey
Highway and country roads are not the same, country roads do have stop signs and intersections, as well as rougher roads. Highways are smoother and typically no stopping.

Rural roads normally don't stop very much (few intersections and most side streets yield) and the cruising speed is lower. I get at least as good, often better mileage on those roads due to the speed difference. I'm sure they chew up my tires more though.
But every road is different, and nobody really knows the driving patterns in detail except the OP. I agree it's a significant variable.

Driving behavior also makes a big difference. If he's flooring it at green lights/stop signs then that would kill the mileage he's otherwise making. So there might not be anything hugely wrong, but lots of things can be checked over, especially since this car was apparently a recent resurrection.
I'd definitely be curious about the rich exhaust smell. I've never run a car without a cat so if everything else is working, I don't know if it should smell like that or not. My understanding has been that ideally, if everything is in good tune then the cat shouldn't have all that much to do.
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Report this Post07-09-2013 08:08 PM Click Here to See the Profile for cebixSend a Private Message to cebixEdit/Delete MessageReply w/QuoteDirect Link to This Post
I don't want to steal the thread but I guess keeping this stuff in one will make it easier to find in the future.

Referring to my previous post: '85 auto duke, new coolant sensor for the computer, TCC working, stock temp t-stat, new plugs and wires, new fuel filter, stock wheels with 195/70R14 tires, no codes being thrown, speedo is pretty much bang on (measured with a GPS) so I guess the odometer is counting all right. 22 mpg, 24 on a good day with a very light foot highway doing 60-70 MPH. Brakes not dragging I guess (I can push the car around easily and the wheels never get hot after a drive). How can I check the o2 sensor? Is it possible it may be failing without throwing a code? Just putting the ALDL into diagnostic mode and watching the light should tell me it's alright?

[This message has been edited by cebix (edited 07-09-2013).]

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Report this Post07-10-2013 08:30 AM Click Here to See the Profile for armosSend a Private Message to armosEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by cebix:How can I check the o2 sensor? Is it possible it may be failing without throwing a code? Just putting the ALDL into diagnostic mode and watching the light should tell me it's alright?

The O2 sensor is supposed to switch between high and low voltage, indicating a rich or lean mixture. As they age, my understanding is that they begin to react more slowly. I suppose it might be possible to judge their condition by looking at the "rich/lean counter" in WinALDL. This counts how many times the signal from the O2 sensor has switched. On a good sensor it should switch more quickly than on a worn sensor, but I don't know how much difference would be seen. I don't know how reliable this test would be, I'm just guessing it could be valid.

Other than that, I don't know of any other way to judge their condition. They won't cause an error code unless they completely die. If you don't know how old it is then I'd replace it. Denso is generally recommended, not Bosch. RockAuto sells them for something like $15 shipped. Don't use dialectric grease on the connector, it's supposed to breathe through there as a reference.
Replacing mine (unknown age, maybe original, no error code) improved my mileage by about 10%. It may or may not help you, but they're so cheap it's something I would try.

[This message has been edited by armos (edited 07-10-2013).]

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Report this Post07-10-2013 01:30 PM Click Here to See the Profile for cebixSend a Private Message to cebixEdit/Delete MessageReply w/QuoteDirect Link to This Post
Yeah I don't know how old the o2 sensor is. One more thing I forgot, the EGR valve is blocked off and so is the vacuum for it. Could that harm the MPG much?

[This message has been edited by cebix (edited 07-10-2013).]

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Report this Post07-29-2013 09:14 PM Click Here to See the Profile for DotrenrewClick Here to visit Dotrenrew's HomePageClick Here to Email DotrenrewSend a Private Message to DotrenrewEdit/Delete MessageReply w/QuoteDirect Link to This Post
UPDATE

At my latest fill-up, I found that I got near 29mpg. I drove a lot of back roads and put nearly 200 miles on the car in one day. I never got over 60 mph and I never "got on it"
at stop signs.

I think I can do better than that too because I still smell rich exhaust when I have the sunroof and windows open while I am slowing down. Also, I smell gasoline when the
car is just sitting and not running.

Those are only a couple things on my "fiero-do-list" next week.

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