Intermintent P0300 random misfire

**eteller** · 01-21-2013, 04:03 AM

Ok I've got a 2000 T/A 56K mi. with new fuel filter, MSD plug wires, PCV valve, new PCM with tune from Frost. Car runs great but started to develop a really bad miss on the way home in the slush the other day. Pulled the code P0300 Random Misfire. Searched some threads that said to try and reset the PCM. Well I just got a new one in from Frost so I swapped them, no difference. Only happens at low rpm or idle, maybe twice a day. Car has no power and shakes really bad when it happens, otherwise it runs great. Any ideas? Checked all the vacuum connections and electrical connections, cleaned MAF, ran two cans of B12 throught tank in the last few months.

**2002ssslp** · 01-21-2013, 05:12 AM

How is your charging system and battery ? Recheck the intake for leaks with carb cleaner. Also, how are your spark plugs ? I see you did the wires but do you have fouled plugs or this the gap too large because they are worn ? Let me know how you make out.

**eteller** · 01-21-2013, 05:17 AM

Originally Posted by 2002ssslp

How is your charging system and battery ? Recheck the intake for leaks with carb cleaner. Also, how are your spark plugs ? I see you did the wires but do you have fouled plugs or this the gap too large because they are worn ? Let me know how you make out.

Battery is pretty new, charging system ok according to stock gauges (13.8v). Did hunt for vacuum leaks a while back while figuring out if all the AIR stuff was removed. I do need to change the plugs, when I bought the car a few months ago I pulled a few and they looked fine. If I'm gonna check them I'm going to change them as it's a royal PITA. Hoping for a little warmer weather to do it. Under a car on a wet driveway at 35F is no fun! Even with no AIR or EGR #8 is impossible from the top.

**cammed goat** · 01-21-2013, 03:19 PM

Go over your plug wires and make sure they are ALL snug. If you hear a wire or two click, then you've repaired your issue.

**cammed goat** · 01-21-2013, 03:23 PM

Check your TPS as well. You might have to check the voltage from that as well if it's never been replaced.

**eteller** · 01-29-2013, 07:05 AM

Update! Over the weekend I finally got some lift time at a friend's shop. We replaced all the plugs (and changed all the fluids). We found one Autolite double platinum with a bent electrode on #6 that had a gap off the scale. Putting in new NGK TR55's seems to fixed the issue.

**35th-ANV-SS** · 01-29-2013, 12:15 PM

Glad it was an easy fix. NGK are the way to go.

**eteller** · 01-30-2013, 03:30 AM

Originally Posted by 35th-ANV-SS

Glad it was an easy fix. NGK are the way to go.

Arrrgh!!!!!!!!!!!!!!!!!!!! I spoke too soon it's back, almost like a bog. If I nail it, it clears up.

**cammed goat** · 01-30-2013, 07:37 AM

I would pull the number 6 plug and see if the gap is off. You could have a mechanical issue.

**eteller** · 02-02-2013, 04:39 PM

I doubt something mechanical would widen the gap, plus the miss is intermentent. Took it on a little trip this weekend mostly interstate and opened it up a few times. It's now missing on the top end when I floor it, I'm thinking O2 sensors. Whats the current go to O2 sensor as bosch 13111's seem to have fallen from grace as good long tube sensors?

**35th-ANV-SS** · 02-02-2013, 04:49 PM

WOT when you know your car is misfiring is a very bad idea.

**rossterman** · 02-03-2013, 09:59 AM

Had similar issue with 2001 SS w/ 45k miles. All stock except lid. Got the p300 code the other day and it ran horrible. Checked each cylinder firing by water spray on the exhaust port area. My luck as #8 wasn't firing. Weird thing was that i Was getting a check engine light that came one for a couple of minutes then went away (while still missing horribly on the way back to the house. Figured was a coil so pulled everything and checked the coil resistance, plugs, and wire resistance. Everything was OK. Decided to put in new plugs and wires (while swapping the #8 coil for #2 so would be easy to replace if indeed a bad coil. Put it all back together and is firing on all 8 now although check engine came on and stayed on while warming up in the garage. Reset the p300 code again and took it out and ran great! The plugs and wires looked fine so somewhat baffled as to what fixed it???

**pajeff02** · 02-03-2013, 12:27 PM

Check all of your plug wires. You should hear/feel one "click" on each plug and two "clicks" on each coil pack.

**35th-ANV-SS** · 02-03-2013, 09:15 PM

Originally Posted by eteller

Whats the current go to O2 sensor as bosch 13111's seem to have fallen from grace as good long tube sensors?

As far as I know, those are still the ones to get.

**pajeff02** · 02-04-2013, 03:37 AM

^^

**eteller** · 02-04-2013, 03:43 AM

Originally Posted by 35th-ANV-SS

As far as I know, those are still the ones to get.

I had thought those had fallen from grace and Denso's were the current go to sensors? I'm leaning towards sensors as mine are spliced (no codes though) and the problem originally started when it was wet. Plug wires are on tight.

**eteller** · 02-04-2013, 02:43 PM

Was a slow day at work so I spent some time scouring threads. Found the PCV lines might be the issue. I didn't have enough 3/8" line to replace the whole shebang but I found the part that holds the pcv valve was very loose. The line to the manifold got replaced and I put a hose clamp on the part that holds the valve (new style valve). Got a new Y connector just need some more 3/8" line and I'll rebuild the whole assembly. The rest including the stock Y feel ok.

**cammed goat** · 02-04-2013, 03:29 PM

Spliced? Are they soldered together and wrapped in heat-shrink tubing? If you have any exposed wiring, add water and shit goes wrong.

**eteller** · 02-04-2013, 05:28 PM

O2 wires look bundled in electrcal tape. As we all know splicing o2 wires are a no no so the sensors will get replaced next. Funny thing is now there are no codes. Car runs great 99% of the time, its intermentent.

**eteller** · 02-07-2013, 03:46 AM

Ok so I got a pair of Bosch 13111's on order via Amazon. In the mean time my car looked like it had been dipped in salt so I decided to wash it, including the engine. Well the miss got MUCH worse. Yesterday morning it bucked like a bronco starting at about 100F and did not quit till the temp gauge hit it's normal operating temp. If I increase the throttle to about a third it 'kicks' in abruptly and seems normal, like a switch was flipped. So last night, figuring I got some thing wet cleaned all the contacts to the PCM, IAT, MAF, TPS, main coil connectors, the MAF itself with MAF cleaner. I also checked all the plug wires. I ran out of daylight and did not check the individual coil connectors. Same issue this a.m., problem occurs a few minutes after start up, problem goes away around the time it hits 195F, problem disappears if it hit's around 1/3 throttle. This sounds like the TPS to me, but would the problem be temperature dependent? I checked the TPS with a volt meter and it seems ok, I do not have a scan tool, just a code reader. BTW car runs no better with MAF disconnected.