Pothole leads to misfire...

myk

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Hello everyone. I browsed through all of the misfire threads but didn't see anything that related to my issue. In any case, my '98 GT was running great as it always does, when I suddenly hit a pothole pretty damn hard on the way home. At the next light, the car was very rough at idle and wanted to die. Shortly after, the CEL came on. I got home, put on my code reader and got a PO 306, as well as being told that the EVAP sensors "weren't ready for testing."

Being it was PO 306, I did some preliminary checks on cylinder six:
-made sure the plug wire was on the plug tightly
-made sure the plug wire was on the coil pack tightly
-made sure that the injector connector was on properly
-did a resistance check on the primary and secondary terminals on the passenger side coil pack. The primary side was fine, under 1.0 ohms, but the secondary side read high at 13K ohms when it's supposed to be 11K or less, between cylinders 1 and 6.

At that point I figured great, it's the coil pack, but then I checked the driver's side coil pack and it had the same resistance and readings on the primary and secondary terminals as the other one. If the coil packs were bad, wouldn't there be misfiring all over the engine? In any case, I don't have time to work on it now, but I'm figuring my next steps will be to test the coil pack to see if it's actually firing, the injector to see if it's actually pulsing, and the spark plug wire to see if it's broken somewhere inside of itself.

Do you guys have any suggestions on what I could look at next? Could the EVAP monitor "not being ready," according to my scanner, mean some other problem other than ignition and spark possibilities causing the misfire? I've never had to dig this deep into the car in this way before, so how do you guys go about testing the coil packs, injectors and spark plug wires?

By the way the car is bone stock and has about 115K miles on it. Thanks in advance...
 

KillNThrill24

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I don't know a lot on the NPI coil pack setup, but just from what you said... If my car hit a pot hole and suddenly was misfiring, I'd suspect something got knocked loose. If you have a CAI and it's down in the fender well, make sure the dam MAF didn't fall off (I've been there before lmao).

You said you checked to make sure your wires and coil packs are tight and I'd say that's a great start. Make sure your plugs themselves are still tight in the head as well.

You'd be surprised how much shit can go wrong just from a shock like that to your car.
 
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myk

myk

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Thanks for the response. I might go ahead and replace the coils and wires because I don't remember when I did them last; actually the coils might be OEM. I bought the car brand new but I don't remember ever changing out the coils. Even if they're not the problem I'm sure they've been on there for a while and I guess it couldn't hurt...
 

ttocs

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it almost sounds like you might have a bad connection somewhere, those can be a complete pain in the ass to track down. You can do a LOT of electrical troubleshooting with no tools and just getting stupid. First start the car and start with technical taps and just tap on each of the coils, sensors, or anything you can see that is electrical. Don't be afraid to give it a good wrap with you knuckles and see if the idle changes at all. If you find an area that makes a change then look a little deeper. If you do not find any problems with this then get stupid and just grab every harness you can get your hands on and push/pull/tug/twist and just generally dick with every bit or wiring and harness you can see. Again if it starts to change how it idles after you dick with something, look closer. I have found my harnesses are starting to wear out and it they tend to have more problems once the motor is hot so don't be afraid to let it get up to temp before doing all of this
 
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Makoto

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I was having some weird stuff happening with my motor so i replaced all the coils and it solved a lot of issues. if its a maintenance thing i've never touched and suspect i generally do it if for nothing more than its one less thing to worry about.
 

Notthenow

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Check the evap solenoid, they can be flaky. It sits on the passenger side strut structure under the a/c dryer. Maybe it bouncing off the structure finished it off or affected the connector. They usually have a foam insulator around them. Could also have cracked the evap canister. It's in the passenger wheelwell against the firewall.

Can't imagine why it would only code as a #6 misfire, but since it says evap not ready....
 
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myk

myk

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Wait...So the evap system can cause misfires? How is that?
 
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myk

myk

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it creates and intake vacuum leak which causes the car to go lean and then will misfire...

Yep, unless I am mistaken, evap is unmetered.

And here I am, on the verge of ordering coils, wires, injectors; when it might not even be any of those things. I'll take a look at the evap system too. I mean, why else would the evap system be showing up as "not ready" on the OBD reader? Thanks guys..
 

Notthenow

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Just start checking what you can. As toccs said, all electrical connections should be checked. Also, disconnecting the battery is said to put the evap in check mode for a while.
 

Metal Head

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Check the battery hold-down & terminal connections. A large hit might have knocked the battery loose, and loose terminal connections can show up with all sorts of intermittent issues.
 
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myk

myk

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Check the battery hold-down & terminal connections. A large hit might have knocked the battery loose, and loose terminal connections can show up with all sorts of intermittent issues.
Thanks for the suggestion. I looked at the battery and made sure the clamps were tight. I also poked around the engine bay a bit and didn't see electrical connectors loose or unplugged. I will keep looking.
 

ttocs

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It can be very hard to see if a harness has a bad connection seriously start the car and just go dick with all the wire harnesses you can see. Don't be afraid to get a little rough with it as any harness should be able to with stand a solid pull/tug and not have an issue.
 
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myk

myk

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Just an update, I pulled, reattached and examined all electrical connections, plugs, harnesses, etc, just to see if a difference in idle quality could be found. I dug into the bay and crawled underneath the car looking at all of the EVAP stuff that I could see, but I didn't experience any changes, so I figured that with #6 misfiring the problem had to be in there. I bought myself a noid light kit to test injector pulses, and got ready to test the coils and wires with a spark plug gap tester. I also had an itch about the spark plug wires so I bought a set of Motorcraft wires too.

This afternoon I swapped out the #6 wire with the a new wire as I figured it'd be the easiest thing to test and the rough idle disappeared, mostly. I swapped the rest of the wires out and the idle is now as smooth as silk; I was reminded that my '98 GT is the smoothest idling and running car I have. The old Motorcraft wires aren't that old, but the OHM values were pretty high, at least twice as much resistance compared to the new Motorcraft wires, so I guess it helped. I guess the pothole was just a coincidence? I guess I'll never know, but this is what I get for driving around an ancient car.

I want to thank everyone in here for helping me sort this out, otherwise I would've just started throwing EVAP and other random parts at it. I figure I can probably use the noid light kit and spark plug tester some other time or on one of the other cars. I do have another question though: even though the car is running well for now, should I go ahead and swap out the coils too? I'm not sure, but I think they're from the factory because I don't ever remember changing them out since I bought the car new. The 'spec for resistance on those things is supposed to be 11K OHMS, and mine measure out to be about 13-14K OHMS, but I read somewhere that shouldn't be a problem. I'll buy Motorcraft coils if I have to, but do you guys think I should bother with them? Thanks again everyone...
 

Notthenow

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Good job man, congrats. I swapped to cops, so can't help. I'm sure toccs or one of the other experts have thoughts on the coils.
 

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