Horrible MPG cause unknown ('99 olds intrigue)

Hello group.
I've a 1999 Olds intrigue (3.5l/94K) that's been getting awful fuel economy for some time now and nothing seems to help. For the first year
or so I owned it, it consistantly got 19-20mpg in all city driving. For the last 1.5 or so years it has dropped to 14-16mpg, usually closer to 14.
As soon as I noticed the huge difference at the pump, I started having work done but nothing's changed. Thus far I've had new platinum plugs put in, a $180 fuel system cleaning, tires rotated and balanced and I've run some chevron techron thru it. I also get regular oil changes w/mobil1 and just had the tranny fluid changed. I check tire pressure often and drive pretty conservatively.
People have suggested the next thing I should try is replacing the oxygen sensor, but both mechanics I've asked about it seem to think it would be a waste, and that without a check engine light it's probably working okay.
So really I don't know what to do, but this problem is driving me nuts! I gotta think that something is going very wrong, but not knowing autos very well, or being a DIYer I have to rely on the mechanics. Is there some kind of diagnostic machine that can find the problem?
Aside from the awful mileage problem, it's been a pretty good car. I'm just getting so frustrated with this that I'm thinking of trading it in. Thanks in advance for any suggestions!
GW
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That's quite a drop.

It's a good idea to find out what the problem is before you start throwing parts at it.

Time to find a mechanic who knows how to use a scan tool. It is possible for the O2 sensor to read on the lean side, which will cause the ECM to add more fuel, without turning on the check engine light. You should be able to see this with a scan tool.

Find someone with a good scan tool that knows how to use it. They need to observe the sensor values with the engine running. You may need to go to a dealership to find a capable scan tool but there is alot of info to look at for someone who knows how to use it.

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knowledgable regulars reporting that she has seen several older cars benefit from new O2 sensors in the absence of CELs. The most obvious improvement reported has been in throttle response speed, and 1999 really doesn't qualify as being that old. That's a big drop, too. If the engine is just running rich that much it ought to show up as poor color on the spark plugs.
I very much agree with Mike (the other Mike) about getting some better diagnostics before throwing money at the problem. I'd make one exception: if the thermostat hasn't been changed recently - accept only an OEM thermostat because the aftermarket ones are really junk - this would be a good time. A cool engine can have an amazingly bad effect on fuel economy. If the dealer offers more than one temperature, choose the higher (highest) one. Especially in modern cars, the higher temperatures are welcome to the engine and actually increase the resistance to overheating by increasing the temperature of coolant entering the radiator and causing the heat to be lost more quickly. One caveat - I prefer synthetic oil with high temperature thermostats. No data behind that, just my opinion.
Mike
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I'm going to weigh in with Mike and Mike and make it three of a kind. You really need to take the car to a mechanic that can scan it properly. I'm a bit disturbed that two mechanics told you that you probably don't need an O2 sensor, but did not recommend you bring the car in for a scan. I'd also check the engine coolant temperature sensor. That's not the same as the sender that operates your temperature gauge. If it's not functioning properly the car will remain in open loop and run rich.
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Mike Marlow wrote:

Hi...
If the temperature sensor were the culprit, I'd expect that a quick and dirty giveaway would be the engine idling much faster than normal, wouldn't it?
Take care.
Ken
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That wasn't a symptom on my truck - a 94 Chevy K1500.
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I have a 2000 GLS Intrigue, the only thing I can add is the engine is fairly advanced, being it is based off the Caddy Northstar. Doing diganostics will turn out something.
I'm not sure this is physically possible and a long shot, but an injector might be bad sending too much fuel. I've seen them go lean from build-up, but ....?
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I had a similar drop on my 93 Intrepid. Down to under 14. I changed the O2 sensors and it came up a bit, but not into the mid-20's like before. Also changed plugs and took the throttle body apart and cleaned every part I could take apart. Even pulled the intake off and put in a new EGR.
Nothing seemed to help.
It also had a slight 'surge' at highway speeds. Cruise seemed to pump on level roads just a bit, but was fine with any kind of incline.
A mechanic friend said change the wires. Did that, everything straightened out and it was up to 26 highway.
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Do yourself a couple favours and do 3 things. 1- buy yourself a service manual of some sort, even the cheap chilton's and haynes have a trouble shooting section. (which happens to list O2 sensor as a cause for rapid drop in fuel economy). 2 - Find yourself a new mechanic, as the two you took the car to have no idea what they are doing, or they know exactly what they are doing - scaming $$$ from you. and 3 - get the O2 sensor changed.
Snow...
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