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My Car is Hosed v1: CEL comes back on after 30 miles

Ns1

Ns1

No Lifer
So I need to get a smog check but can't because my CEL is on

I go to get it repaired. It comes back, drives fine, after 30 or so miles the CEL comes back on. No noticeable change in driving experience. Take it to the car shop, misfire in cylinder 4. Replaces plug wire, everything OK CEL is off.

After 30 miles, CEL comes back on. No change in driving experience.



Shop is closed until tomorrow, any idea what might be the source of the problem?
 
Wait, the shop replaced one plug wire?

Spark plug wires come in sets and proper maintenance dictates that all wires be changed at the same time. Also, when tracing a misfire, the first action should be replacing the plugs themselves, not the plug wires.

If replacing the plugs and the plug wires doesn't help, then the next step is to replace the distributor cap and rotor. If the car does not have a distributor, you'll likely need to replace the coil pack(s).

ZV
 
problems like this are often quite common to a specific vehicle, I'd start by googling the model of your car plus cylinder 4 misfire and see what you get. Most likely it's a plug, wire or coil, but it could be something totally bizarre, the coolant temperature sensor in my car can cause a random cylinder misfire code when it goes bad.
 
^ Plugs, plug wires, then coils. Unless your car is a weak COP design like mine (Lincoln LS - COPs are known to go bad on earlier models).

Also, getting the code pulled at AutoZone / Advanced Auto would be a good idea.
 
re above^^

Shop replaced cap/plugs and whatnot as a normal tuneup procedure but did not replace plug wires. They replaced one wire because that was where the misfire occured (cylinder 4 IIRC)
 
Ns1 said:
re above^^

Shop replaced cap/plugs and whatnot as a normal tuneup procedure but did not replace plug wires. They replaced one wire because that was where the misfire occurred (cylinder 4 IIRC)
Click to expand...

Plug wires are sold in sets though and if one wire is old enough to need replacing, then they all do. Replacing only one plug wire is a bit like replacing only one tire because the other three are just barely better. I wouldn't give my business to a shop which promoted such half-arsed practices. Since the wires are only sold in sets, I'm curious about what the shop did with the other wires they didn't need.

It just seems very odd.

If everything else is verifiably good, then it's time to start looking at the injector for the misfiring cylinder. If it's clogged, that one cylinder may be running a little lean.

ZV
 
Zenmervolt said:
Plug wires are sold in sets though and if one wire is old enough to need replacing, then they all do. Replacing only one plug wire is a bit like replacing only one tire because the other three are just barely better. I wouldn't give my business to a shop which promoted such half-arsed practices. Since the wires are only sold in sets, I'm curious about what the shop did with the other wires they didn't need.

It just seems very odd.

If everything else is verifiably good, then it's time to start looking at the injector for the misfiring cylinder. If it's clogged, that one cylinder may be running a little lean.

ZV
Click to expand...

they swapped it with a wire that they had. told me to come back if the CEL stays off so that i can buy a full set. Bringing it back tmr, will update then.
 
Ns1 said:
they swapped it with a wire that they had. told me to come back if the CEL stays off so that i can buy a full set. Bringing it back tmr, will update then.
Click to expand...

Interesting. I'm still not sure I like the idea of any shop just using some random plug wire they had laying around, but I'm a bit obsessive about maintenance.
 
Zenmervolt said:
Plug wires are sold in sets though and if one wire is old enough to need replacing, then they all do. Replacing only one plug wire is a bit like replacing only one tire because the other three are just barely better. I wouldn't give my business to a shop which promoted such half-arsed practices. Since the wires are only sold in sets, I'm curious about what the shop did with the other wires they didn't need.

It just seems very odd.

If everything else is verifiably good, then it's time to start looking at the injector for the misfiring cylinder. If it's clogged, that one cylinder may be running a little lean.

ZV
Click to expand...

Nope, you can buy wires in singles. I know AZ did it when i had my shop. Some old timers shops had a wall of just plug wires. I think on soem older carb motors the wires would fall out of the holder and burn on the exhust. So just replace that one and keep going.

But most are sold in sets today and most don;t need to be replaced as often. My dad has over 200k on his 99 GMC truck.


Also OP what is your car. year, make, model?
 
Marlin1975 said:
Nope, you can buy wires in singles. I know AZ did it when i had my shop. Some old timers shops had a wall of just plug wires. I think on soem older carb motors the wires would fall out of the holder and burn on the exhust. So just replace that one and keep going.
Click to expand...

Cool. Did not realise that; I've only ever seen them sold in sets.

Zargon said:
you can buy just the boots from MSD and lenghts of wire too......
Click to expand...

True, but I don't see a shop doing that given the extra time and effort over just ordering a full set that are pre-made.

ZV
 
Zenmervolt said:
True, but I don't see a shop doing that given the extra time and effort over just ordering a full set that are pre-made.

ZV
Click to expand...


I totally agree.

you do those in sets unless something odd happens, like my buddy who left some unhooked and it got shut in the hood and had bare wires, since they were pretty new still I had one made up for him instead for a few bucks(re used the ends)

HarryLui said:
CEL could be hundreds of things. If you want possible solution, post the actual code.
Click to expand...

yeah.
 
They replaced a faulty o2 sensor. Evidently my car has 2 o2 censors, one on the front and one on the back of the cat. They replaced the one on the front.

Long term story is even though they replaced the first sensor, 2nd sensor always runs rich. They suspect that after the first o2 sensor failed that the cat is now oversaturated and causes the 2nd sensor to register as rich.

Anyway, CEL is now off and I’m gonna run a few dozen miles and hopefully pass smog.
 
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