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Bad gas mileage on 92 Seville


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My caddy usually gets 20 mpg in town, but for the last month we've only been getting about 12-15 mpg. I filled it before a highway trip (about 60 miles one way) and used half a tank on the round trip. That shouldn't be!!!

My husband changed the plugs and wires about two weeks ago because it was running a bit rough (found a damaged wire) but that didn't change the mpg. He changed the oil and filter, no mpg change. The tires are good and inflated as per manual. There are no codes coming up. About two months ago we had a new catalytic converter installed, could that be the problem? I heard they sometimes get congested or something. The exhaust is clean and doesn't smell like gas or anything.

Any suggestions? Wisconsin gas isn't cheap, I'd like my mileage back!

April

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My caddy usually gets 20 mpg in town, but for the last month we've only been getting about 12-15 mpg. I filled it before a highway trip (about 60 miles one way) and used half a tank on the round trip. That shouldn't be!!!

My husband changed the plugs and wires about two weeks ago because it was running a bit rough (found a damaged wire) but that didn't change the mpg. He changed the oil and filter, no mpg change. The tires are good and inflated as per manual. There are no codes coming up. About two months ago we had a new catalytic converter installed, could that be the problem? I heard they sometimes get congested or something. The exhaust is clean and doesn't smell like gas or anything.

Any suggestions? Wisconsin gas isn't cheap, I'd like my mileage back!

April

April, you did an excellent job diagnosing and communicating the problem and your attempts to solve it!

The first thing I would do at this point is to check to see if you have any DTC codes set by your on board diagnostic computer. Go to the top of this page, click HOW TO, then DTC codes and then follow the the directions for your year. Post the codes here, they will help us 'see' what is going on and we can help your further.

Mike

Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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Whoops, I see you said NO CODES...

Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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OK, sorry about missing the NO CODES statement you said. Who replaced the CAT? Why was it replaced?, rattling noise? How is the engine performing? Does it seem that you lost engine power? Its NOT easy to replace the original cat, its a double walled pipe, I had a heck of a time replacing mine.

I would test your exhaust system at this point to see if you are experiencing back pressure, there is a back pressure tester that screws into the O2 port. Its possible that debris from your old CAT is clogging your exhaust system.

Also check to make sure that your tranny is going into OD. Drive the car at say 55, keep your foot steadily on the gas, watch your TACH, and gently tap your brake pedal with your left foot, your RPMS should jump about 250 RPMs. Let us know what you find

Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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OK, sorry about missing the NO CODES statement you said. Who replaced the CAT? Why was it replaced?, rattling noise? How is the engine performing? Does it seem that you lost engine power? Its NOT easy to replace the original cat, its a double walled pipe, I had a heck of a time replacing mine.

I would test your exhaust system at this point to see if you are experiencing back pressure, there is a back pressure tester that screws into the O2 port. Its possible that debris from your old CAT is clogging your exhaust system.

Also check to make sure that your tranny is going into OD. Drive the car at say 55, keep your foot steadily on the gas, watch your TACH, and gently tap your brake pedal with your left foot, your RPMS should jump about 250 RPMs. Let us know what you find

The CAT was replaced because the other one was making a rattling noise. We had it replaced by a local mechanic, who has been very reliable, though the part was considerably cheaper than what the dealer was going to charge. My husband would normally do work like that himself, but he saw that it was more complicated than other exhaust systems he's worked on, so we thought it better to take it in.

The engine was running a tad rough for a few weeks. We had a cold spell up here, so I thought it was water inthe gas and put some Heet in the tank, but that wasn't the problem, then my husband changed the plugs and wires and it seems smooth now.

I'm conservative with the power, so I haven't noticed if it has lost any. My husband would have said something last night when he passed a car at 80 mph, so the power must be fine. :D

I'll try the overdrive today and tell my husband about the back pressure tester, he's always looking for new toys.

What would we do if it was debris clogging the exhaust?

Thanks for the quick reply!

April

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I am not sure what could be done if you had debris or a clogged muffler, but it does not sound like you have that type of problem given the 80 mph pass last night.

The more I think about this, your impression that you were getting 20 mpg around town is optimistic, and the 13 to 15 sounds more reasonable, its possible that you dont have any problem at all.

Check your mileage again, reset all parameters in you DIC then check MPG for mixed driving and check your mileage on a long trip at highway speeds and report back. Check the mileage on your DIC and do it manually (the old way)

Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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I am not sure what could be done if you had debris or a clogged muffler, but it does not sound like you have that type of problem given the 80 mph pass last night.

The more I think about this, your impression that you were getting 20 mpg around town is optimistic, and the 13 to 15 sounds more reasonable, its possible that you dont have any problem at all.

Check your mileage again, reset all parameters in you DIC then check MPG for mixed driving and check your mileage on a long trip at highway speeds and report back. Check the mileage on your DIC and do it manually (the old way)

So, getting 300-400 mpg per tank was abormal? Cuz now we have only been getting 250 at the most. I do ALOT of in town driving, so we didn't really notice the bad gas mileage until the last few weeks when we've been going out of town alot. I've never had 1/2 tank burn in 120 miles before. It was usually half that. But is that the norm...16 miles per gallon?

I didn't get a chance to check the overdrive today due to unexpected company. But I'll be doing that soon.

I reset the computer this morning, I remembered reading somewhere that the computer stores your mpg usage and uses the fuel accordingly. Not sure if that's true or not, but I thought I'd give it a shot. So, we'll see what happens.

Thanks again BodybyFisher!

April

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An FPR is the fuel pressure regulator. It is on the drivers side under the beauty cover behind the throttle body near the fuel rail. It looks like a little canister with a vacuum hose on it. Pull the vacuum hose and turn the key on (engine off) to pressurize the lines, if fuel comes out the FPR where the hose attaches then it is bad. On the 4.9 it is a bear to change due to the position of the screws that hold it on.

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I found a review that has the 92 Seville fuel consumption from 16 mpg in the city to 25 mpg on the highway

If most of your driving as you say is IN TOWN you will get 16 mpg and less if its stop and go, with an 18.8 gallon tank that equates to 300 miles per tank, but you don't run the tank out till you run out of gas either

If you are looking at your DIC to determine RANGE, you MUST reset it, especially the Average Miles per Gallon. When reset, the RANGE is affected by your immediate driving habits once it stabilizes with sufficient sampling, if you are in the city, the RANGE will be reflective of 16 mph if you are on the highway doing 60, the RANGE will be refective of the 25 mph. All other types of driving are a blend of the two but a HIGH AVERAGE influenced by highway driving will over estimate range if the current driving style is city type.

Keep in mind that LOW SPEED stop and GO driving KILLS MPG and you will get less than 16 mpg if you drive in that type of traffic.

I did not suggest that you needed a FPR because you are not complaining of FPR issues (hard starting, rich mixture at exhaust, black smoke at exhaust).

I dont think you have any problems at all, I think you are misinterpreting the DIC. Pull out your owners manual and review the FUEL indicators on the DIC (Range, Avg Mpg, Miles, etc)

Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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You might look into replacing the injectors. If its not atomizing the fuel correctly, gas mileage would suffer. Also, OEM injectors on 4.9's have been known to be problematic. You might look into replacing with Ford 5.0/4.6 injectors. Others have reported success using these injectors which are known to be more reliable.

Edited by Ed Hall
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