Need a Tune-up or a New Cat?

[mysticv6]

Recently, my car has been running pretty poorly. I have 92,500 miles on my 95' 3.4 liter. I got it used around 62k miles, and I think that most of the parts are original.

I had been planning on changing the plugs, wires, and fuel fitler since I'm sure it needs that anyway.

Whats wierd is that it runs the roughest at low rpm's. On a wide open run from 0-100, accelerating hard, it runs pretty good still. But just trying to leave from a light calmly while driving aorund town, the engine sounds real rough, up until maybe 2500 rpm. Also, at idle, when I listen to the exhaust (stock) it sounds almost like I have a mild cam, when I know I dont. Listening up in the engine bay, I dont notice that.

My gas mileage has been way down recently (13mpg vs. 17 I used to get), and the car has also been running hotter. I attributed the heat to the electric fan potentially going bad, but I'm wondering if possibly the exhasut manifold isnt heating up more becasue of a clogged cat.

Any ideas?

[ARTFART]

ONLY do this when your car is cool , get a rubber hammer and tap your cat , if you hear a bunch of crap rattling around , your cat is shot !

[ARTFART]

HOWEVER , this could also be your plugs , wires , fuel filter , or even fuel pressure regulator . OR it might just need a good tune-up .
Have the plugs ever been changed ?
Has your fuel filter ever been changed ?

[mysticv6]

Originally posted by ARTFART:
HOWEVER , this could also be your plugs , wires , fuel filter , or even fuel pressure regulator . OR it might just need a good tune-up .
Have the plugs ever been changed ?
Has your fuel filter ever been changed ?

I got the car used, so I dont know 100% for sure, but my guess is that NO, nothing has been changed. I'm going to do the plugs, wires, and fuel filter anyway, because I think they are original, and if not they have at least 30k miles on them anyway.

Is there anything else involved in a tune-up?

If the pressure regulator was bad, would it still run ok at higher RPM's?

[Marty85]

It sounds like your just in need of a tune up. As soon as I get the money im going to give my car a tune-up (plugs, plug wires, O2 sensors, maybe even coil packs as preventive maintenance and a possibly a fuel filter). My 95 3.4 has about 123k on it now and im pretty sure it still has factory wires still on it. The plugs im not sure of but I know it needs a tune-up just because its time for one even if its only preventive maintenance.

[mysticv6]

Originally posted by ARTFART:
ONLY do this when your car is cool , get a rubber hammer and tap your cat , if you hear a bunch of crap rattling around , your cat is shot !

I didnt hear anything rattling around, nor have I ever smelled anything bad. Is there still a chance its the cat?

[Marty85]

Hmm, the two tell-tale signs of a bad cat is a bad rotten egg odor from the exhaust when the car is running and a few other that im not sure off. Also if you car has 92k on it then its probably really time to change the O2 sensors. They dont last very long, I think I saw 80k once quoted here on the board so I would invest in a pair of O2 sensors while you were at it. They shouldnt be to expensive for our 3.4's since they dont use heated O2 sensors like OBD-II cars do.

[mysticv6]

Originally posted by Marty85:
Hmm, the two tell-tale signs of a bad cat is a bad rotten egg odor from the exhaust when the car is running and a few other that im not sure off. Also if you car has 92k on it then its probably really time to change the O2 sensors. They dont last very long, I think I saw 80k once quoted here on the board so I would invest in a pair of O2 sensors while you were at it. They shouldnt be to expensive for our 3.4's since they dont use heated O2 sensors like OBD-II cars do.

I was thinking about doing the 02 sensors actually. How many do 3.4's have? I looked last night and didnt see any behind the cat. I think I might just remove it completely. With no sensor behind it, it shouldnt throw a code or anything.

[mysticv6]

I've read around, and it sounds like the 3.4's only have 2 02 sensors, like I was guessing.

But do any of these explanations really make sense? I was convinced it was the 02 sensors, but it seems worse when the motor is cold... when it would be running open loop and the 02 sensor is not used. The 02 sensor would explain the decrease in gas mileage though.

I really find it hard to believe its the plugs or wires when it runs fine from say 2500 - 5000 RPM. I would think if it was plugs or wires the opposite would be true.

A clogged cat I would think could also affect the gas mileage. But it doesnt rattle when I bang on it or smell bad.

There is a popping noise I've heard a few times, and from what I've been reading, that happens with bad 02 sensors.

Is it possible this is a combination of things?

[AaronG]

just do genral mantance, start with o2s, then plugs and wires, fuel filter. ive gone thourgh 3 used cars, thats the first thing i always do is get everything that will break with time, replaced so i know about how much time is left on it.

[SirShaun]

Does your car have a miss in it? Does the car shake at idle and low rpms? Does it feel like engine isnt fireing exactly right throwing it off balance? My 94 at 115k miles i thought i needed a tune up. Started running on 5 cyl rough as hell. CHanged plugs,wires,and ECM. Come to find out my car had a vacum leak and Computer went bad. My problem started out like yours. Started with a little miss making it sound like i had a cam and eventually got worst.

[mysticv6]

Originally posted by SirShaun:
Does your car have a miss in it? Does the car shake at idle and low rpms? Does it feel like engine isnt fireing exactly right throwing it off balance? My 94 at 115k miles i thought i needed a tune up. Started running on 5 cyl rough as hell. CHanged plugs,wires,and ECM. Come to find out my car had a vacum leak and Computer went bad. My problem started out like yours. Started with a little miss making it sound like i had a cam and eventually got worst.

It does shake at idle sometimes. Being a V-6, its always been a little bit rough, but recently its been a little rougher. I really notice it most when I load the engine and start trying to move at low RPM's in first gear. I dont think its running on less than 6 cylinders, becasue it revs just as high as it used to, and pulls pretty hard up top still.

It does sound like it has a cam almost when you listen to the exhasut at idle. But listening to the engine, under the hood, it seems to be running fairly smooth at idle still sometimes.