P0300 Code. I need Help!

[bartlanz]

So before the weather got cold (probibly about 2 months ago) I did a tune up (Cap, Rotor, Wires, Plugs) on my 98 Tahoe in an attempt to not need to do it when it got cold out. WELL OF COURSE it hasn't worked. For the last month or so, my truck has been running progressively worse, and I Have been trying to figure out what it is. Today it threw out a code P0300 (Random Cylinder Misfire Detected) I did a search on the site here and came up with no real solution.

 

When I did the tune up I used the recomended AC Delco Platnum Plugs, Bosche Wires, and what ever brass contacted cap & rotor the guy recomended (suposedly a good one) all purchased from AutoZone. At the same time I did my tune up I also changed my fuel filter.

 

I replaced my Coil last night thinking that it might have a crack in it (wich it did) but apparently that wasnt the cause of the problem. I cleaned my MAF with Radio Shack Electronics Cleaner. I also have put one tank of Techron through it (two bottles of techron for my 26 Gal tank) as well as a bottle of "dry gas". *obviously not ALL of this was last night*

 

I bought the truck roughly a year ago, and have put about 20,000 miles on it. Brining its milage close to 102,000. It has a practily new fuel pump done 9 months ago by my local stealership.

 

It tends to run poorly when the weather is humid or rainy. I have checked all of the wires connections to the cap, as well as to the plugs and from the coil to the cap. I also sprayed the wires cap and coil down with "wire drier". My plan is tomorow after work to pull the cap and inspect it.

 

Sorry for such a novel of a post but I am just trying to give you guys all of the info. If there is some kind of reading or something you want to help trouble shoot this tell me, I have borrowed my buddies SnapOn scanner, and this thing reads a bunch of different sensors.

 

 

GUYS PLEASE HELP ME!!!

[covvy]

Using the scanner, what is the fuel trim. It should have long term and short term for bank one and two. If it is more than 7% on the long term then there is a vaccum leak of some kind. May be vaccum line or intake manifold leaking. Check to see if the coolant temp sensor is within proper range. At full running temp it should read about 95 celcius to 110 give or take a few. If this is all good then check your 02 sensor bounce on both sides to ensure proper engine feedback. Any other codes? I have seen once when a rotor was installed wrong and was sitting unlevel on the distributor. Just a thought. Good luck and let us know what the sensors are saying.

[sperry]

Revisit the autozone parts. The brand name equip may be ok but I've had bad rotor's and caps from them. The rotors flashed through causing problems like that. Perhaps your dilemma is similar with your high-voltage parts too.

[brownk01]

Revisit the autozone parts. The brand name equip may be ok but I've had bad rotor's and caps from them. The rotors flashed through causing problems like that. Perhaps your dilemma is similar with your high-voltage parts too.

 

<{POST_SNAPBACK}>

 

 

Check to be sure that you plug gaps are correct also. I have a 99 Yukon that was throwing a misfire code (and missing) and found it to be a plug with too wide a gap (after 3 months of trying other things). Just a thought. Good luck.

[Peter_C]

Gm plugs should not need to be gaped. Replace the cap and rotor with A/C Delco and go from there.

[sperry]

You should also check those plug wires. Unless they are wire core, you should have between 3K and 7K Ohm's per foot. Mainly, see if they are open-circuit. High voltages jump the gap in opened-circuit plug wires, gradually widening the gap as it arc's it away, until you get too weak of a spark and/or flashover. Once resistor wire arc's internally, they degrade fast. Wire conductor is a bit slower.

[sperry]

This pertains to mostly later ("above vehicles" that I didn't copy down), but MAY be all newer AC-Delco plugs that fit your truck:

 

"4.8L, 5.3L, 5.7L or 6.0L V-8 Engine (VINs V, P, T, Z, G, S, N, U -- RPOs LR4, LM4, LM7, L59, LS1, LS6, LQ9, LQ4)

 

A new spark plug has been released for use in the (above vehicles). The new spark plug has an Iridium tip instead of the current Platinum tip. Due to the different tip design, the gap of the spark plug has also changed. The new spark plug, P/N 12571164 with AC Delco P/N 41-985, is gapped to 1.01mm (0.040 inches) when the spark plug is made. The spark plug gap is set during manufacturing and should not be changed or damage to the spark plug may result. Any new spark plug found to not be properly gapped should not be used."

[budhayes3]

I agree that the first thing you should do is ditch the aftermarket cap, rotor, wires, and go with the OEM stuff. Always seems to be a problem with aftermarket tune up parts. Personally, I feel that anything with the name Bosch is junk. I've had nothing but bad luck with their stuff, and everything I've heard has been bad too.

[sperry]

I had Bosch plugs once ( a while ago however). That was enough. One (porcelin) shot out from the base. Another fell apart when I went to remove them. Three weeks after I installed them carefully. The auto parts store mgr. didn't care and gave me some lame excuse about overtorquing and distorting them. Funny how I changed other plugs when I used to race. never a problem like that!

 

And a guy at work couldn't get two of them out of his VW! They fused in or something?

[Johnny P]

So before the weather got cold (probibly about 2 months ago) I did a tune up (Cap, Rotor, Wires, Plugs) on my 98 Tahoe in an attempt to not need to do it when it got cold out. WELL OF COURSE it hasn't worked. For the last month or so, my truck has been running progressively worse, and I Have been trying to figure out what it is. Today it threw out a code P0300 (Random Cylinder Misfire Detected) I did a search on the site here and came up with no real solution.

 

When I did the tune up I used the recomended AC Delco Platnum Plugs, Bosche Wires, and what ever brass contacted cap & rotor the guy recomended (suposedly a good one) all purchased from AutoZone. At the same time I did my tune up I also changed my fuel filter.

 

I replaced my Coil last night thinking that it might have a crack in it (wich it did) but apparently that wasnt the cause of the problem. I cleaned my MAF with Radio Shack Electronics Cleaner. I also have put one tank of Techron through it (two bottles of techron for my 26 Gal tank) as well as a bottle of "dry gas". *obviously not ALL of this was last night*

 

I bought the truck roughly a year ago, and have put about 20,000 miles on it. Brining its milage close to 102,000. It has a practily new fuel pump done 9 months ago by my local stealership.

 

It tends to run poorly when the weather is humid or rainy. I have checked all of the wires connections to the cap, as well as to the plugs and from the coil to the cap. I also sprayed the wires cap and coil down with "wire drier". My plan is tomorow after work to pull the cap and inspect it.

 

Sorry for such a novel of a post but I am just trying to give you guys all of the info. If there is some kind of reading or something you want to help trouble shoot this tell me, I have borrowed my buddies SnapOn scanner, and this thing reads a bunch of different sensors.

 

 

GUYS PLEASE HELP ME!!!

 

<{POST_SNAPBACK}>

 

 

I could have written your email. 2000 Tahoe LT 5.3 L Same issue. Exact same issue. I have the Actron 9135 and it picks up the PO300 and sporadically EVAP, O2, HO2, and EGR sytems not ready.

 

I also noticed it was very rough in the warm weather first thing in the morning. Now that it is in the 40s here and it starts right up. Once it gets warm it begins to miss.

 

HELP!!

[budhayes3]

I could have written your email. 2000 Tahoe LT 5.3 L  Same issue. Exact same issue. I have the Actron 9135 and it picks up the PO300 and sporadically EVAP, O2, HO2,  and EGR sytems not ready.

 

I also noticed it was very rough in the warm weather first thing in the morning. Now that it is in the 40s here and it starts right up. Once it gets warm it begins to miss.

 

HELP!!

 

<{POST_SNAPBACK}>

 

 

The "EVAP, O2, HO2, and EGR sytems not ready" just means that those systems havent run their self tests yet. Just have to daignose what's causing the P0300. Since it's intermittent I'd lean more towards a plug, wire, injector, etc, as opposed to something mechanical like a valve or loss of compression issue.

[budhayes3]

Hey Johnny P, have you checked your fuel pressure regulator yet? If it's leaking fuel it'll give you hard start condition and a slight rough idle until it gets "blown out". You wouldn't notice it as much in colder weather. It's real easy to check.

 

In the center of the driver's side fuel rail, there's this circular looking piece with a thin vacuum line going to it. (The vacuum line comes from under the intake cover). That item is the fuel pressure regulator. Pull the vacuum line off of the reg and check for the presence of fuel...if fuel runs out, or even if there's a strong fuel smell, that regulator is leaking and allowing extra fuel back into the intake and richening the mixture.

 

The fp reg costs about $60-$70 from the dealer and is held in with a clip. Piece of cake.

 

Oh yea, Welcome aboard!!

[covvy]

More often than not I see P0300 as a central fault. Like possibly a cap and rotor or intake leak because it is affecting more than one cylinder. One or two bad plugs would set something like a P0301 or P0302 and up. P0300 is the entire engine at different cylinders is having problems based upon crank shaft rotation speed over 360 degrees.When a cylinder fires the crank speeds up and the PCM records this. If it doesn't speed up at the correct time then is says that a misfire has occured. Using the cam reference it knows what cylinder it was that misfired. Something like an intake leak or stuck parially open EGR valve often cause P0300. If idle vaccum is low and runs rough but runs well at 2500rpm then I'd say EGR but if it is poor all across the line than I'd look at cap and rotor or intake leak. Ensure that all plug wires are tight. I have seen this often when customers tune thier own cars and then can't figure out why the light came on after the tune up. (long story) When in doubt rule out what it isn't and go from there. Good Luck

[covvy]

About the tahoe, if its runs worse as it warms up than I'd be wondering about a fuel issue. The engine may be either rich or lean. Can your scanner give you any info about the sensors? Make sure the temp sensor is ok and the fuel trim as well as o2 sensor function. At work I do a good deal of my diag based upon those sensors. What is your millage and what do the plugs look like. Powdery black is too rich. Clean and a bit white is normal.

[Johnny P]

Thanks for the tips. I hadn't checked the FPR. I started collecting info. All indications are pointing to FPR, O2, EGR,plugs, wires injectors. Why do they give such a general code? When you look it up in Haynes they tell you to check nearly every system under the hood. I'm so relieved - I can rule out everything aft of the post cat o2 sensor.

 

I first noticed the condition in the early summer - July. In August I did the plugs, (not the wires - they all tested fine) I replaced the fuel filter, ran two bottles of injector cleaner through. How big a difference do the platinum tipped plugs make? More than one person has suggested regular spark plugs? What do you think? I am planning to begin looking into it this afternoon ... I take it you think I should check the FPR first.

 

I am glad I finally found this board. I have a Volvo 850 and a Jeep Wrangler and there are some awesome forums for those rides .... finally I found one for the "beast" !!