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1999 Seville STS 4.6L V8
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51 Posts
Discussion Starter · #1 ·
So my 99 Seville started felling sluggish yesterday. When I got gas the pump sounded kind of funny. But the car was fine for about half a tank of gas. Then it just started to fell funny sluggish and not as responsive.

I checked the codes and got PCM P0300 - Engine Misfire Detected

The car just had all new plugs and new wires almost a month ago and it has ran amazing since then. I still went back and checked them. So I am thinking maybe bad gas?

I went to autozone and got some sea foam and put it in there and filled up again. Still kind of sluggish..

Do you think it's just bad gas and what can I do to fix it other then sea foam? Thats about the only thing I can think of or that really changed.
 

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Super Moderator
2010 DTS
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88,148 Posts
Bad gas is unlikely. Putting that snake oil in the tank can do more harm than good. Check your fuel pressure.
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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73,446 Posts
You either rent or buy a fuel pressure test gauge with hose and fitting (not expensive) and attach it to the Schrader valve under the black plastic cap on the fuel rail - just like pumping up a tire.
 

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1999 Seville STS 4.6L V8
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51 Posts
Discussion Starter · #5 ·
I went to auto zone they hooked up to the car said the mass air flow sensor was bad and said that could cause a misfire and make it run a little bad like it is.

Going to start with that..
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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73,446 Posts
I went to auto zone they hooked up to the car said the mass air flow sensor was bad and said that could cause a misfire and make it run a little bad like it is.
What car ? What engine ? What year ? The same 99 Seville as in Post #1 ? Update your profile - it sure makes it easier for continuity.

Your car has a perfectly functional built-in code reader/scanner/memory. Study the sticky post ^^^ "How to pull codes" if you have a Cadillac in some year after about 1991 or so............

What if you spend $255 for a new MAF at AZ and that doesn't solve the problem ?? FWIW, a MAF is nearly bulletproof if you don't mechanically screw with it.

What plugs ?? What wires ?? Is everything connected exactly as shown in the diagram ?

I know this all sounds sort of snarky, but there's precious little info to go on.
 

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24 Posts
I'm no expert, but i just had the p300 misfire code for the last month. tried everything. cleaned throttle boby, changed gas stations, ckecked wires , plugs( changed them both last fall). still couldn't figure it out. :hmm: so this is what i did.
started car and pulled each wire out of engine,by the boot, to "see and hear" if the wires were firing. u would see an arc to the engine block or just by pulling wire away from sparkplug you can hear the popping noise. sure enough i got to clyinder 8 and nothing. dead. (remember new plugs ,wires). so then i traced the wires to the coils near the fire wall. 4 of them each with 2 wires connected to it. pulled the 8 off the coil and nothing, no arc. no. 3 is on that coil . pulled that one to. nothing. (make sure u put the wires back as u go). so i decided to pull no 1 wire off the that coil and sure enough there was an arc . also got pretty good shock too . make sure u use your right hand! lol. (next time i would shut off car , pull the wire off coil. place it close enough so it will arc to the male connection ,then start and look ).
so went to store, picked up coil and before i installed it , switched coils around and to see if it was coil . it was. installed new one and my car is now fixed.
sorry for the winded explanation. i'm not on here to often so wanted to explain in detail.
oh part was 50 bucks!
 

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1999 Seville STS 4.6L V8
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51 Posts
Discussion Starter · #9 ·
I'm no expert, but i just had the p300 misfire code for the last month. tried everything. cleaned throttle boby, changed gas stations, ckecked wires , plugs( changed them both last fall). still couldn't figure it out. :hmm: so this is what i did.
started car and pulled each wire out of engine,by the boot, to "see and hear" if the wires were firing. u would see an arc to the engine block or just by pulling wire away from sparkplug you can hear the popping noise. sure enough i got to clyinder 8 and nothing. dead. (remember new plugs ,wires). so then i traced the wires to the coils near the fire wall. 4 of them each with 2 wires connected to it. pulled the 8 off the coil and nothing, no arc. no. 3 is on that coil . pulled that one to. nothing. (make sure u put the wires back as u go). so i decided to pull no 1 wire off the that coil and sure enough there was an arc . also got pretty good shock too . make sure u use your right hand! lol. (next time i would shut off car , pull the wire off coil. place it close enough so it will arc to the male connection ,then start and look ).
so went to store, picked up coil and before i installed it , switched coils around and to see if it was coil . it was. installed new one and my car is now fixed.
sorry for the winded explanation. i'm not on here to often so wanted to explain in detail.
oh part was 50 bucks!

That might very well be my issue, I did plugs and wires but I did them one at a time not to mix them up, but one might not be sparking. That makes sense. :)
 

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1999 Seville STS 4.6L V8
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51 Posts
Discussion Starter · #10 ·
What car ? What engine ? What year ? The same 99 Seville as in Post #1 ? Update your profile - it sure makes it easier for continuity.

Your car has a perfectly functional built-in code reader/scanner/memory. Study the sticky post ^^^ "How to pull codes" if you have a Cadillac in some year after about 1991 or so............

What if you spend $255 for a new MAF at AZ and that doesn't solve the problem ?? FWIW, a MAF is nearly bulletproof if you don't mechanically screw with it.

What plugs ?? What wires ?? Is everything connected exactly as shown in the diagram ?

I know this all sounds sort of snarky, but there's precious little info to go on.

I will update my profile. :) Still a little new here sorry.. But it's a 1999 Seville STS 4.6 I think..


I just did the plugs on it about a month ago, so it could be that but I was getting a MAF sensor error as well.. I pulled the codes from my dash thats how I got the P0300 code, I forgot the number for the MAF sensor...

I found a MAF at a junk yard going to toss it on and see if that goes away and re check all my plugs. I hope that solves the issue. The plugs make sensor since I just did them though the MAF sensor was only $20 so no big loss if it's not really bad. LoL
 

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1996 Eldorado
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2,814 Posts
Pulling the wire is not a very good way to check for spark. The wire will deliver a strong spark yet can ground out to the plug base when installed if here is carbon tracking inside the boot & down the plug porcelien.

Plugs can be clean off but plug boots are toast if the carbon has tatooed a track inside.

 

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1999 Seville STS 4.6L V8
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51 Posts
Discussion Starter · #12 ·
Pulling the wire is not a very good way to check for spark. The wire will deliver a strong spark yet can ground out to the plug base when installed if here is carbon tracking inside the boot & down the plug porcelien.

Plugs can be clean off but plug boots are toast if the carbon has tatooed a track inside.

Well the wires are new and the plugs are new. I am not going to pull them while it's on I don't want to get zaped.. LoL I am just going to pull them and reset them and see if that helps and check each wire.

If there a safe good way to check them them to be sure it's not the coil?
 

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1996 Eldorado
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2,814 Posts
I just posted pictures of what to look for. If you changed the plugs & wires at the same time they should be good. One bad wire/plug/coil will cause it to run like crap.

If you put old wires on new plugs or vice-versa you have a probable chance of this re-occuring.

If you had an old school inductive timing light you could hook that up to each wire and see which is skipping a beat.
 

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1999 Seville STS 4.6L V8
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51 Posts
Discussion Starter · #14 ·
I just posted pictures of what to look for. If you changed the plugs & wires at the same time they should be good. One bad wire/plug/coil will cause it to run like crap.

If you put old wires on new plugs or vice-versa you have a probable chance of this re-occuring.

If you had an old school inductive timing light you could hook that up to each wire and see which is skipping a beat.
Yea I did them both plugs and wires at the same time, it ran great and smooth for a month, then just started acting up out the blue.

I did find one plug about a week ago had a little chunk taken out of it ( no idea how ) I replaced that one with a new wire but did not solve the issue.

So going to try a few other things and see if i can find a o/s timing light or a shop with one that can check it out maybe, was just trying to save a few bucks.
 

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2010 DTS
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88,148 Posts
Have you checked the coils? Borrow a known good one from another car and replace yours with it one by one. If the misfire goes away with one of them, then you know you have a bad coil and which one to replace. I took one from my wifes car to do that years ago when I had a misfire on my '97 Deville. It proved all 4 coils good.
 

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just try my"safe method". lol. have the engine off and pull one wire off the coil. place the wire bout an inch from the male connection. then start. you will hear the popping sound once you get out . if you see or hear the arcing it wont be your plugs or wires. 2 less things to worry about. once you see the arcing shut it off. if u don't then you can move in closer . do one at a time .
i was running on 6 clyinders for about a month and a half. what a difference .
let me know if you try it.
later
 

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1999 Seville STS 4.6L V8
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51 Posts
Discussion Starter · #17 ·
just try my"safe method". lol. have the engine off and pull one wire off the coil. place the wire bout an inch from the male connection. then start. you will hear the popping sound once you get out . if you see or hear the arcing it wont be your plugs or wires. 2 less things to worry about. once you see the arcing shut it off. if u don't then you can move in closer . do one at a time .
i was running on 6 clyinders for about a month and a half. what a difference .
let me know if you try it.
later
Thats kind of what mine fells like a v6.. lol
 

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1998 deville base
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1 Posts
Thanks guys, mine was running on and off on 7 cylinders. I have been screwing with this problem for about a year now, finally got ir to push a 0300 code and with that and your comments the beast is back in good form. On a good note the engine has been thoroughly tuned up due to this problem, shouldn't have anymore issues with ignition or power, just in time for summer. Let the good times roll!!!!!!!!!!!!!!!!!!
 
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