Hi

When I push the RPM above ~2500, my 2003 Volvo s40 shudders and the CEL flickers on but goes off when I ease up on the gas. Otherwise the car runs normally.

O’Reilly Auto Parts person pulled P0304, and recommended replacing the spark plug ($9) and the coil ($25). At the prices they quoted, maybe it makes sense to just go ahead and replace both, but when I looked online for nicer parts, I’m finding that coils are more like $70 (like this one at iPd).

Is it safe to use the cheaper coils and plugs?

Would you try replacing the plug first or the coil first?

Thanks.

 

If you don't know the history of the plugs in the car, and assuming it's got at least 70-100,000 miles, it would make a lot of sense to just swap the plugs as a basic (cheap) maintenance item. Plugs are cheap, and gas isn't, so even a tiny improvement in efficiency will pay for those plugs in short order.

And while you're doing that, you can swap the coil from the suspect cylinder to another one. Then if you still have the CEL and code after the plugs (and you may not!) you will be able to see if the problem followed the coil (in which case it should be replaced) or not (in which case you should start looking at other issues, including injectors, or just connections / plugs / cables, etc.).

Personally I'd trust a used Volvo coil more than I would an aftermarket coil (which is the reason I suggest that people DON'T swap out all the coils as a troubleshooting method). Of course, a new Volvo coil is even better, but at the price, I'd go used every time. Then again, I'm kind of cheap and stubborn. ;-)

 

Thanks for the response -- what you suggest makes sense.

The car has 125k miles and all the plugs were replaced at 98k miles (by the previous owner). They are pretty cheap, as you note (~$10 each?). So, should I still replace them?

I'll try swapping the coils and see if the misfire follows the bad coil.

Where do you buy your used Volvo parts?

Thanks.

 

most likely its the coil/wires - plugs should be changed every 60-75K depending on model but if you swap the coil, might as well pull the plug to give it a look/check the gap etc. Also check the plug type - don't assume the prior replacement got it right. For most Volvos (particularly turbos) more expensive plugs are not necessarily better - so if you decide to swap plugs, a good choice is genuine Volvo. Note that the Gen1 S40s also are known for issues with the wiring harness that feeds the coils. Inspect the connectors for cracks and consider replacing the harness or repairing cracks in the connectors with RTV or similar.

As to where to buy parts, for me it really depends on what I need. For genuine Volvo, I use Tasca Parts (a dealer web store here in New England). For Volvo OEM and quality aftermarket, I use FCP Euro (also here in CT) as well as IPD USA (west coast). I also use Amazon as other dealers and Volvo friendly stores sell there. Finally I do use Autozone, NAPA and Advanced Autoparts for other stuff. Last time I did coils on my S40 I did genuine Volvo via Tasca as I also did the wire harness, but used Autozone with good results on my VW CC 2.0T when it ate a coil.

 

With less than 30,000 miles on those plugs, there isn't a whole lot to be gained, though it's still a low-$ / high return maintenance item. I guess if I could find appropriate plugs in the $4-6 range, I'd probably just swap them out "just because". Like mt6127 mentioned, I'd make sure that the plugs that were installed were the right ones.

My go-to for "odd car parts" is normally eBay, where it's pretty painless (and usually pretty cheap) to buy used parts from vetted sellers. FCP is certainly a good choice, as is IPD - doubt you'd have to worry about aftermarket parts from either of them (though they're probably a lot more $$$ than used eBay coils, which I'd expect to last virtually forever).

 

I'm a very lazy shadetree mechanic, and I found a new coil for just $20 on Amazon by Beck Arnley... so I'll just try swapping out the coil and see if that cures the problem.

I think all that is required is to take the engine cover off w/ a T30 wrench, disconnect the coil at the connector, then pull it straight out, right?

Thank you all!!!

 

yes the coils are just held in by a couple of screws and have a short plug wire to the second cylinder. not nearly as big of a pain as swapping a coil on my VW 2.0T where you need to lift the whole row without breaking the loom :-)
As noted by multiple posts, its not uncommon for the wire harness connectors to crack so best to inspect that a swell.