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1987 Eldorado
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82 Posts
Discussion Starter #1
My 87 Eldo (4.1, 63,000 miles) has gotten, at best, 22 MPG on the highway. Usually it is less than that. And I get 15-16MPG around town. I am light footed too.

I see many others posting 26-28 MPG highway, and 19-20 MPG city.

I have tried to check/replace everything I have heard about on this forum, yet my mileage has not improved, other than when I finally put good plug wires on (upped city from 13 to 16 MPG city).

Only thing I haven't done is replace the catalytic converter. How do I determine if it is restricting/clogged?

Any other suggestions?

Thanks.
 

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93 & 88 Allante, 88 Eldorado
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778 Posts
Your fuel economy could be slightly low. What speeds on the highway are you driving? You'll get your best fuel economy right around 60 mph.

What type of tires are you using? Wide and low profile tires hurt fuel economy. You'll get the best using the stock 205 width tire.

What brand of plugs and wires are you using? You definitely want to use AC Delco plugs and wire on these engines.

Is your torque converter locking up? Make sure you're not getting that E39 code.

For better fuel economy, try advancing your timing a few degrees. Best fuel economy is achieved when it just barely pings under heavy throttle.

I've heard that a less restrictive CAT back exhaust system can help with fuel economy. Have an exhaust system made with 3" pipes and use a low restrictive muffler. The lowest restrictive muffler I'm away of is Thrush glasspack.

A restricted/clogged catalytic converter would also be associated with driveability problems such as delayed shifts and no power when accelerating.
 

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'87 Coupe DeVille 4.1L FWD(New engine '08) 1981 F100 '03FXDL
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879 Posts
Oxygen sensor......that is supposed to be replaced every 30k miles on that engine. MAP sensor also has a lot to do with mileage. BTW: any E codes?
 

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1987 Eldorado
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82 Posts
Discussion Starter #4
Oxygen sensor......that is supposed to be replaced every 30k miles on that engine. MAP sensor also has a lot to do with mileage. BTW: any E codes?
Replaced Oxygen sensor recently
AC Delco Plugs
Accel wires: the large ones 8 or 9 mm
Drive at 60 mph on highway with a light foot
Stock 205 tires slightly overinflated

Only code is related to EGR: PO48 Current/ EGR Fault

I replaced the EGR, cleaned the passages. I get the check engine light after running the car for about 20 minutes.

This was discussed here: http://www.cadillacforums.com/forums/ht4100-4-1-4-5-4/150175-po48-current-egr-fault.html

Not sure what to try next.
 

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'87 Coupe DeVille 4.1L FWD(New engine '08) 1981 F100 '03FXDL
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879 Posts
Check the complete vacuum system for a leak. Check the EGR solenoid. The code doesn't necessarily point only to the valve, but to the complete EGR system. Your diagnostics show 'po' codes?
I'd also do a vacuum check on the engine. Reading vacuum is a tried/true method of diagnosing engine performance.
 

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'85 Fleetwood FWD
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728 Posts
The list of factory equipment codes will show the final drive ratio.
For example, my old '85 has a listed code of F79, which is a 2.97:1 final drive ratio. They list 3.73, 2.93, 3.08, and 3.42 for this one. If the mileage has always been the same and not recently gone down, it may be the drive ratio and not some engine function.

Just an idea!!

BILL
 
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