Time to change plug wires?

[cheezwizard]

1999/3.5GX/95K

So my '99 has 95K on it and I'm thinking about changing the wires. I've been dealing with awful fuel economy for the longest time now. The car used to get a solid 19mpg/city and then pretty much overnight it dropped to 14mpg. One fillup was normal and the next one the pump just kept pumping long after it should've stopped, based on the miles driven on the trip odo.

I've had new GM platinum plugs put in, had some 'fuel system cleaning,' new PCV valve, filters and fuel pump checked out okay and probably some stuff I'm forgetting. I've changed oil regularly, and the car has always run very well, idle is normal.

Most recently I had the O2 sensor replaced and was feeling like this would have to do the trick. The car seemed to perform a bit better right away and that gave me some hope. Right after having the sensor replaced I filled the tank. I thought it was looking good when the fuel needle was still at the full line after 20 city miles. Unfortunately, it's now been 60 city miles and it's down to the 3/4 tank line, just like always.

So yeah, it's a bummer. During my online search for a solution (I've posted about it here and elsewheres in the past), people have mentioned new plug wires a good few times.

I don't really mind having the O2 replaced, since it was due for a change anyway, and I'm thinking after 95K and 9 years of age the wires are probably due as well.

I'm just not sure if I should go ahead and have new wires put in next, or try to locate the right garage with the right scanners/diagnostic stuff and have them try to locate the problem. I don't have a great deal of faith that they'll be able to based on past experience, and of course don't wanna be wasting money on some wild goose chase!

I'm not particularly knowledgeable about the cars, so I have all the work done by a garage. I'm not even sure where the plug wires are, and doubt I'd be able to tell by looking at them whether they're in bad shape.

Thanks a bunch for any suggestions, gang. If anyone cares to recommend a particular brand of plug wires, a place online to get'em and what not, that'd be cool too!

Word.

[99intriguePA]

cheezwizard, If I read your post correctly, you have the 3.5L, which doesnt actually have plug wires. The plugs are connected directly to the coils with a rubber boot. I doubt the boots would be bad unless you have a large oil leak or whoever changed the plugs messed them up somehow.
My '99' 3.5 has 150,000 miles with the plugs changed once and no problems.
How long ago were the plugs put in?
Is the check engine light on?
Lots of things can affect fuel mileage, from plugs and air filter right down to tire pressure, so you may have to start checking out possible causes one by one.

[cheezwizard]

Thanks, yes I have the 3.5 and I think I've looked at all the simple possibilities (or had them looked at) and more - tires, filters etc. I've had the fuel consumption problem for maybe two years now and it's been in for many things. I've only put 10K on the car in the 3 years I've owned it and have never had a check engine light come on. The new plugs were the first thing I had done after the mpg went to crud, so the plug replacement job wasn't the cause but didn't fix it either. I haven't had the problem with oil consumption so I think I'm okay there.

The ignition wires I was looking at getting are on the page below (bottom of the page), but from what you say perhaps I should be looking elsewheres for the problem.

http://replacement.car-stuff.com/par...howdc=true#top

[LittleHoov]

What brand of O2 sensor was it replaced with? Do you know?

[Rolm]

Assuming you are correct on your mileage dropping so dramatically, I would look at mechanical causes such as seized brake calibers, seized calibers can place a real drag on the car other then that if your engine is running ok and your exhaust is not emitting smoke, then I would say you better re-check your mileage

[kmohr3]

Maybe for some reason the auto trans overdrive isn't working properly. I've heard from my GM trans guy that sometimes the shaft that runs 4th gear can strip out the splines and result in the loss of 4th gear (overdrive). I'd think you would have noticed this, though. Just a thought..........

[cheezwizard]

Thanks all,

It's a walker brand sensor, got it from oxygensensors dot com - $50. When I started the car after it was installed, the engine went up to 2K rpms for an extra second or so longer than normal, then went to idle speed. I thought maybe that was like, the computer aquainting itself to the new sensor's readings. The performance seems a bit improved so hopefully it's good.

I had a brake inspection not too long ago when I had the tranny fluid changed. They checked out okay. I occasionally get an ABS light and/or a grinding feeling thru the pedal when going slow around corners. This may be due to a corroded harness, forgot the part # but after having the right hub assembly replaced almost three times (twice under warranty) due to ABS code indicating a speed sensor failure, the GM shop found it was this corroded harness causing the failure. I don't know where or what this harness is, or if it may involve other systems.

As for the tranny, it seems good as ever, no changes. On the highway when the cruise control kicks in I get a bit of g-force going on.

[99intriguePA]

I looked at the site you posted for the plug wires and they do have wires listed for the 3.5, but I can guarantee you the ones in the pic aren't for the 3.5. The pics are probably just generic, so I assume they would provide you with the boots.
I'm not saying that the boots can't be bad, but if all other remedies have failed it might be worth removing the coil covers and taking a look or having someone check it for you.
I wonder if a weak or failing coil could cause bad gas mileage without setting a misfire code?
I think that if your 02 sensor wasn't working right it would set a code/check engine light.
Are you using the same type of gas?

[LittleHoov]

Your 02 sensor can be extremely sluggish, almost to the point of failure long before it throws a code, they typically only throw codes when they arent working at all. I would be kinda leary of replacing it with a no-name brand like that.

What sort of fuel mileage numbers are you actually getting? I never saw you mention the actual mpg you figured. You just said that the gauge was dropping quickly.

[cheezwizard]

99/3.5/95K

I've kept track of mileage at every fill up. I always fill the tank and reset the trip odo to track mpgs. For about the first year, it was a solid 19 mpgs at every fillup. If I had 100 miles on the trip, I could count on the pump to stop around 5 gallons. The first time I noticed the pump kept going was shortly after a trip up north, about 700 miles there and back, though I'm not sure how long after the trip this fillup was.

An odd thing is, on trips since then to the same place/miles I actually got around 26 mpgs going 75-85 in light/no traffic. That was another reason I thought the O2 was the culprit, I read somewhere that they don't operate at higher speeds, but I'm not sure.

Anyway, calculations since the economy went bad (now I use a calculator to do the math, dividing miles driven by gallons put in) have ranged from 14-17 mpgs, mostly towards the low end in almost all residential suburb type driving. It's pretty rare I get on the highway.

I'd give anything for the check engine light to come on and tell me what's wrong! I'd really like to keep the car for a long time, but this really nags at me even though I don't drive it much. I'm even considering going back to regular oil. I've used mobil 1 since I bought the olds and the mpgs went bad around 4K miles of ownership or less. I know it's probably totally irrelevant, but I'm at a loss to solve this.

[cheezwizard]

Oh, sorry '99intriguepa, back when I bought the car I would usually buy 89 octane, not loyal to a particular brand but it was mostly exxon/mobil. When gas prices started going up about two years ago, I started buying regular, not from any particular station.

[LittleHoov]

Well any mechanic worth their salt should be able to hook the car up to a scantool or diagnostic machine and tell you if your O2 sensor is getting a little sluggish or not.

Do you smell gas at all when outside the car? Or at any time? Theres always the possibility of a fuel leak somwhere, which would of course cut down on your gas mileage.

[Jimmy Olsen]

Just my 2 cents, but perhaps there is a restriction in the exhaust system.

Another possibility is a severely blocked air filter.

[LittleHoov]

Originally Posted by Jimmy Olsen

Just my 2 cents, but perhaps there is a restriction in the exhaust system.

Another possibility is a severely blocked air filter.

You would think that both of those would cause noticeable performance losses.

But just for the heck of it check your air filter, then bang on the catalytic converter with something solid like a hammer or jack handle. Any rattling in there?