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98 s10 rough idle when warm ( P0300 - random misfire )

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Old 12-31-2008, 08:57 AM   #1
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98 s10 rough idle when warm ( P0300 - random misfire )

The truck is idleing rough after it's warmed up and it begins hesitating. If I connect the ODB scanner sometimes I'll see a P0300 multiple random cylinder misfire (but check engine light doesnt actually come on all the time).

I've changed the plugs, wires, and checked one of the original ignition coils (haven't had time to check the other which I replaced from advance auto 1 year ago).

When it's cold it runs great. Also when driving (above idle RPMs) it seems OK too. Any ideas?
Old 01-02-2009, 01:45 AM   #2
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Re: 98 s10 rough idle when warm ( P0300 - random misfire )

hook it back up the the scanner, and read the sensors. Sounds like a sensor shorting out, or if the truck has a control module on it, then i would check that also.
Old 01-05-2009, 08:19 PM   #3
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Re: 98 s10 rough idle when warm ( P0300 - random misfire )

ignition knock sensor/oil psi sensor on the side of the block
Old 01-06-2009, 01:22 AM   #4
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Re: 98 s10 rough idle when warm ( P0300 - random misfire )

maybe this will help.....

A P0300 OBD code indicates a random or multiple misfire. If the last digit is a number other than zero, it corresponds to the cylinder number that is misfiring. A P0302 code, for example, would tell you cylinder number two is misfiring. Unfortunately, a P0300 doesn't tell you specifically which cylinder(s) is/are mis-firing, nor why.


i would check knock sensor like kdnybtr said.
Old 01-08-2009, 12:47 PM   #5
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Re: 98 s10 rough idle when warm ( P0300 - random misfire )

Thanks. Is there a way I can test the knock sensor or control module? Or do I just have to replace them? Is the control module the module that the ignition coils bolt onto?
Old 01-08-2009, 01:00 PM   #6
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Re: 98 s10 rough idle when warm ( P0300 - random misfire )

Any pics or diagrams where those sensors are? There are a few on the side, I'm not sure what I"m looking for...
Old 01-08-2009, 09:49 PM   #7
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Re: 98 s10 rough idle when warm ( P0300 - random misfire )

no, if you have a haynes or chilton book the call it the oil pressure sensor. its in front of the coil packs, its about the size of your finger. i had the same problem but it was pretty obvious when it was dangling from the block. the are normally a order item from advanced or auto zone and only about 25 bucks. you can check the resistance of the sensor with a multi-meter, i just dont know the scale or range it should have.
Old 01-10-2009, 12:38 PM   #8
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Re: 98 s10 rough idle when warm ( P0300 - random misfire )

Quote: Originally Posted by kdnybtr
no, if you have a haynes or chilton book the call it the oil pressure sensor. its in front of the coil packs, its about the size of your finger. i had the same problem but it was pretty obvious when it was dangling from the block. the are normally a order item from advanced or auto zone and only about 25 bucks. you can check the resistance of the sensor with a multi-meter, i just dont know the scale or range it should have.
Thanks, my (useless) Haynes book doesn't have anything in the index for the oil pressure sensor.

Is it one of the 2 connectors here?

Old 01-11-2009, 11:49 AM   #9
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Re: 98 s10 rough idle when warm ( P0300 - random misfire )

I can't tell you if its one of those connectors but I have a misfire in cylinder 4 and I have to check compression again but i believe it's a little low. I'm going to change coil packs. I bought Summit Racing's brand becuase it's made by ACCEL and it's like half the price.
Old 01-12-2009, 09:33 PM   #10
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Re: 98 s10 rough idle when warm ( P0300 - random misfire )

If it was the oil pressure sensor, wouldn't I see some fluctuation in my oil pressure gauge on the dash? When the truck hesitates, the gauge doesn't move one bit.

Oil temp and pressure gauges seem to be reporting good values.
Old 01-12-2009, 10:37 PM   #11
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Re: 98 s10 rough idle when warm ( P0300 - random misfire )

I changed my coil pack and made sure the plug ends were on there REAL GOOD. All of my codes are clear now. I was actually getting a code for the Fuel Composition Sensor ($400) but it's all good. Since you only CHECKED the coil pack I'd go ahead and replace them. Could always return it if money's tight or that's not the problem.
Old 03-18-2009, 11:18 PM   #12
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Re: 98 s10 rough idle when warm ( P0300 - random misfire )

Replaced both ignition coils and the ignition module. Same problem. Also tried new plugs wires. I had another set of used plugs that I also swapped in/out.

Since I recently replaced the fuel pump with a used one, I also checked the new fuel filter to see if I had some debris that was clogging it. Nope it's all clear.

What next to check?
Old 03-19-2009, 01:54 AM   #13
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Re: 98 s10 rough idle when warm ( P0300 - random misfire )

get fuel pressure gauge to check on the fuel at idle at cold and warm
Old 03-27-2009, 08:19 AM   #14
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Re: 98 s10 rough idle when warm ( P0300 - random misfire )

I'm going to pick up a fuel pressure gauge today.

The truck also just threw a P0442 today. I'm not sure if it is related or just another separate issue. (it's not the gas cap either)
Old 03-27-2009, 07:12 PM   #15
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Re: 98 s10 rough idle when warm ( P0300 - random misfire )

my missing and missfire code was from my EGR valve. mine is the electronic one. this is one thing you will need to test with a scan tool etc, you don't want to spend $200 for it for nothing.
after a while a code for the egr valve came on so I replaced it, and the missfire problem went away with it.

so how do you know the 0442 isn't the gas cap?

Last edited by serpentracer; 03-27-2009 at 07:17 PM.
Old 03-29-2009, 12:51 PM   #16
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Re: 98 s10 rough idle when warm ( P0300 - random misfire )

nicks10:

Any luck with your misfiring problem? I'm still struggling with mine.

Tom
Old 03-29-2009, 05:30 PM   #17
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Re: 98 s10 rough idle when warm ( P0300 - random misfire )

Quote: Originally Posted by serpentracer
my missing and missfire code was from my EGR valve. mine is the electronic one. this is one thing you will need to test with a scan tool etc, you don't want to spend $200 for it for nothing.
after a while a code for the egr valve came on so I replaced it, and the missfire problem went away with it.

so how do you know the 0442 isn't the gas cap?
I checked the gas cap and it was on fine.

I bought a fuel pressure test kit from local Harbor Freight, but honestly I don't know how to test the fuel pressure. It could be the wrong gauge since I just noticed it said not for use with GM throttle body vehicles? It is also not very clear in the Haynes repair manual that I have on how to test the fuel pressure (and the pics are worthless).

So I'm still at a loss, I'll see if i can figure out how to test it.
Old 04-02-2009, 06:55 PM   #18
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Re: 98 s10 rough idle when warm ( P0300 - random misfire )

Quote: Originally Posted by nicks10
I checked the gas cap and it was on fine.

I bought a fuel pressure test kit from local Harbor Freight, but honestly I don't know how to test the fuel pressure. It could be the wrong gauge since I just noticed it said not for use with GM throttle body vehicles? It is also not very clear in the Haynes repair manual that I have on how to test the fuel pressure (and the pics are worthless).

So I'm still at a loss, I'll see if i can figure out how to test it.
that doesn't mean it's good. my 02 threw that code every now and then. I replaced the cap with the one from my 97 and the code has never come back. so I bought a new one

my 97 you have to have special fitting to put in line at the fuel filter.

Last edited by serpentracer; 04-02-2009 at 07:03 PM.
Old 04-02-2009, 07:10 PM   #19
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Re: 98 s10 rough idle when warm ( P0300 - random misfire )

Gas cap itself can be bad, when the last time you replaced it? I usually replace mine 2-3 years
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