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Spark plugs still misfiring after replacing them

I have a 2005 Cadillac CTS and just replaced the spark plugs and boots and for some reason I’m still getting misfires. The misfires sound a lottle different and feel different now but they’re still there. I also made sure that the spark plus were gapped and everything. Is there anything else that can cause this?

Misfires can be caused by bad plugs, wires, coils, injectors, low compression, bad gas, etc…

You don’t just replace parts when a misfire occurs.

You determine what’s causing the misfire.


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Is there any way I would be able to determine it? I know it can’t be the spark plugs or boots but is there any clear signs in can look at in the engine?

It depends on how many cylinders are misfiring and their locations.

A bad head gasket can cause misfires.


Its 5 out of 6 could a head gasket cause that many?

5 out of 6 cylinders misfiring?

Timing chains/guides.


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If the timing chain theory doesn’t pan out, I’d start by

  • visual inspection of vacuum hoses, electrical connectors, battery terminals, etc
  • reading out all t he diagnostic codes
  • verifying the input voltage to the ignition system is correct
  • the crank sensor is working correctly
  • fuel pressure is correct

I’d be surprised if something didn’t turn up as the cause by that point.

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Is there any way i can find out or anything I can look at to know for certain its the timing belt?

Use a strobe light that fires along with cylinder #1’s spark plug. Aim it at the timing marks and see if the timing is right on, or erratic. Try it at various RPMs.

Also, are the plugs you put in the ones specified by the carmaker - not necessarily the same as the ones that were in there? That spec may be in the owners manual or on a sticker under the hood.

3.6 or 2.8 motor? 2.8 has belt and is fairly stable till it breaks. 3.6 has chain and it usually runs poorly when the chain/guides wear.

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To find out if the problem is with the timing chains/guides/tensioners, remove a valve cover to expose the valves.

Have someone get on the crank bolt with socket/breaker bar, and turn the engine in the normal direction while watching the valves.

Now have that person turn the engine in the opposite direction.

If the crank can be turned more than 5 degrees in the opposite direction before you see any valve action, points to the mechanical timing system.


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Thank you guys for the help. The spark plugs are the ones autozone told me to get. I wi definitely try the strobe light tonorrow. Its also making a VERY loud noise when driving till i get up to 50 mph. Like a clicking noise, could this be cause from a poor timing belt?

It’s a 3.6 and when i get to higher rpms there’s less of a clicking noise