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1995 Deville 4.9
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11 Posts
Discussion Starter #1
Hello CF, I have a 4.9l 95 deville and I'm getting terrible gas mileage, the engine has 60k. I've changed plugs,wires,O2 sensors, 8 injectors, computer, engine coolant sensor, the whole distributor,FPR, idle air control valve, map sensor, throttle body, EGR valve and control solenoid motor, resonator, PCV, fuel filter and the milage sucks. Did I miss anything or any suggestions
 

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Administrator
2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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67,924 Posts
Please define "poor gas mileage" - in town, suburban, open highway, long runs.
 

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Registered
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8 Posts
OUCH..... I recently got a 2003 deville and mine runs between 19.3 and 25.4............ I'm happy. I doubt if the automobiles are comparable...
 

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Super Moderator
White Diamond '03 DHS (with DTS floor shift)
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86,579 Posts
Might have been easier to list the things you HAVEN'T changed.
17 highway is WAY low.
Have you checked for a vacuum leak? Maybe a dragging brake caliper? Partially clogged CAT (how's the performance?).
Have you pulled the DTC's?
 

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2004 Deville
2004 Deville, Silver, Gone, but not forgotten, 1972 Eldorado, 1974 eldorado, 1975 CDV
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645 Posts
Also make sure your tire pressure is where it should be, and that it doesn't need an alignment.
 

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1999 Deville base
95 Eldorado, 99 Deville, 05 CTS
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223 Posts
I agree with Ranger, a partially clogged cat. I have a 99 Deville, everything is good on mine, xcept the cat, which I know is partially clogged, because I took it off and tried to clean it. Wouldn't hardly run at all before I unclogged most of it. Runs great now, but like Ranger said, performance is down, which drops mpg.
 

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92 Fleetwood 2dr cpe - FWD, 96 Seville SLS, 02 Seville
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827 Posts
When mine was running poor mpg, it was in constant open loop due to low water temp. Check diagnostics while running to see what state it is in.
 

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1995 Deville 4.9
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11 Posts
Discussion Starter #11
Hello I had the cat checked, and he said was good, no rattling but the resonator had a hole in it so I replaced that. Should I change the cat anyway? It's the original
I agree with Ranger, a partially clogged cat. I have a 99 Deville, everything is good on mine, xcept the cat, which I know is partially clogged, because I took it off and tried to clean it. Wouldn't hardly run at all before I unclogged most of it. Runs great now, but like Ranger said, performance is down, which drops mpg.
Did you use an after market cat? I was told aftermarkets triggers a check engine light. Found an new cat on eBay, he wants $400+shipping. Is that about right
When mine was running poor mpg, it was in constant open loop due to low water temp. Check diagnostics while running to see what state it is in.
SC
 

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1999 Deville base
95 Eldorado, 99 Deville, 05 CTS
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223 Posts
How did he check the cat? The only way I know of is by putting a gauge in one of the front O2 sensor ports and check for back pressure at idle and at 2000 rpm. I took mine off, and I could actually see that the honeycomb was clogged up.
 

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1995 Deville 4.9
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11 Posts
Discussion Starter #13
How did he check the cat? The only way I know of is by putting a gauge in one of the front O2 sensor ports and check for back pressure at idle and at 2000 rpm. I took mine off, and I could actually see that the honeycomb was clogged up.
Didn't watch how he did it,,it was a muffler shop out in the country. Any recommendations as to where I should take it to be tested?
 

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1999 Deville base
95 Eldorado, 99 Deville, 05 CTS
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223 Posts
Also, how does it run when you put it to it? If it runs like a bat out of egypt, and revs right on out till it scares you, then the cats probably ok. Make sure you have no vacuum leaks anywhere at all. Look and listen real good. I use a piece of vacuum tubing in one one ear, like a stethoscope, and stick it all down around the intake manifold and everywhere I can find that looks suspicious.
 

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1999 Deville base
95 Eldorado, 99 Deville, 05 CTS
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223 Posts
If you did all that work yourself, you are adept enough to find a gauge and check the cat yourself.
 

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Registered
70 Deville 78 Seville 92 Deville 03 Deville
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713 Posts
I would think any muffler shop can test a cat and would try to sell you one if they could. If your muffler shop said it's good I would be inclined to agree with them. I would be verifying vacuum very early in the diagnosis anyway.
 

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1995 Deville 4.9
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11 Posts
Discussion Starter #18
Thanks I'll take it to Midas for another test, I know they'll put on an after market which I was told would trigger a check engine, might have to spend $400 for a new GM converter I saw on ebay
 
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