97 Blazer with code P0300

ok, I'm new here but I need nelp please. I have a 1997 Blazer 2rd/2wd with a 4.3 Vortec and a 5 speed. It was running perfect then I shut it off went into a store for 2 min. max came back out and started it. I took off and all of a sudden it started sputtering and backfiring. I have checked coil, camshaft position sencor, cap, rotor and all are good. If I keep it over 1500 rmps then it runs fine but once it drops below 1500 then it starts spitting and sputtering. If I am not on the gas hard then it will run great but if I push the gas to hard or to soft then it spits and sputters and backfires. Any help would be appretiated.
Thanks,
Blazin97

Ignition switch module.

Known problem on a 1997.

The replacement procedure is in the FAQ at the top of the forum.

Ignition switch module.

Known problem on a 1997.

The replacement procedure is in the FAQ at the top of the forum.

Doesn't sound like a switch problem to me (But I would replace it if it never was replaced anyway). When my switch went (and also the EEM a year later) no codes at all were present.

Symptoms sound more like a plugged EGR valve.

Doesn't sound like a switch problem to me (But I would replace it if it never was replaced anyway). When my switch went (and also the EEM a year later) no codes at all were present.

Symptoms sound more like a plugged EGR valve.


Thats what I was thinking because the switch you dont have power and I still got a lotta power.

Thats what I was thinking because the switch you dont have power and I still got a lotta power.

As both BlazerLT and I said, It doesn't hurt to replace that switch if it never was replaced. It will fail at some point. It cost <$100.00 at Autozone, and it's the same exact switch a dealer will sell you for double the price. It's good insurance.

One other thing to take a very close look at is the Distributor cap (preferably at night when you would be able to see arching). I've seen this mentioned in a few posts relating to the P0300 error code (which is just a mis-firing code, very non-specific)

Good luck, keep us posted.

Switch can cause all kinds of problems and it is not just a power, no power situation.

It can also cause weird things like transmission codes and such as well so don't discount my advice too soon.

Switch can cause all kinds of problems and it is not just a power, no power situation.

It can also cause weird things like transmission codes and such as well so don't discount my advice too soon.

I wasnt just dicounting you advice, from what all I've read on the net and what I've heard from other Blazer owners they all say they nad no power and thats how they knew the switch was bad.

I wasnt just dicounting you advice, from what all I've read on the net and what I've heard from other Blazer owners they all say they nad no power and thats how they knew the switch was bad.

Nah, much more problems can come from it.

Misfiring, tranny shifting problems, engine dying all of a sudden, fuel pump won't prime etc...

The lack of power symptom is more related to the MAF sensor.

Nah, much more problems can come from it.

Misfiring, tranny shifting problems, engine dying all of a sudden, fuel pump won't prime etc...

The lack of power symptom is more related to the MAF sensor.

Thanks for the info. I will order it today and replace it tomarrow and post if it helps.
Thanks for the help.

Its not the ignition switch. Replaced it a minute ago and it still miss fires and back fires. PLEASE HELP. Im outta ideas.:banghead:

Do you have or recently installed a K&N air filter?

PO300 - Random Cylinder Misfire

First it looks like you scanned for codes already, but did any other codes showup?

This is a tricky one to fix sometimes, as it could be, like LT said, a MAF sensor, could be injector problems (fuel problems) even bad gas can cause this.

It could be ignition problems too. Did you check for comtinous, strong spark? Using a spark tester, start truck up and snap the throttle, the tester should show strong pulses of spark, indicating a good ignition system.

If your ignition system passes, then you have a fuel problem, or emission system problem, which could lead to injectors, or MAF, just to name a few.

You need to start ruling out out some possible things first, and I would start with the ignition system first.

Heres some info directly from a diagnosis flow chart I have;

DTC PO3OO WILL SET WHEN

No Throttle Position (TP) sensor DTCs.
No Manifold Absolute Pressure (MAP) sensor DTCs.
No Engine Coolant Temperature (ECT) sensor DTCs.
Rough road is not detected.
ECT is between -6.75°C (19.85°F) and 12O°C (248°F).
Engine speed between 250 RPM and 5500 RPM.
System voltage between 9 volts and 16 volts.
+Throttle position change is less than 6.25% for 100 msec.
-Throttle position change is less than 1.5% for 100 msec.
Misfire is detected.ACTION TAKEN (VCM DEFAULTS)
If the VCM determines that the misfire is significant enough to have a negative impact on emissions, the Malfunction Indicator Lamp (MIL) will be turned "ON" after the misfire has been detected on two nonconsecutive trips under the same operating conditions. If the misfire is severe enough that catalytic converter damage could result, the MIL will flash while the misfire is present.

DTC P0300 WILL CLEAR WHEN
The VCM will turn the MIL "OFF" after three consecutive trips without a fault condition present. A history DTC will be cleared if no fault conditions have been detected for forty warm-up cycles [coolant temperature has risen 22°C (40°F) from start-up coolant temperature and engine coolant temperature exceeds 71°C (160°F) that same ignition cycle] or the Tech 1 clearing feature has been used.

DTC CHART TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.

If DTCs P0337 (Crankshaft Position Sensor Circuit - Low Input) or P0338 (Crankshaft Position Sensor Circuit - High Input) are set, this could result in a misfire condition.
In order to duplicate the conditions under which the misfire occurred, it may be necessary to drive the vehicle and monitor the Tech 1 "DTC Set This Ignition Cycle" variable.
When instaling distributor cap and rotor, make certain the cap is properly sealed on the distributor. When checking spark at the spark plug wires, the spark should be consistent. A few sparks then nothing is no spark.
At this point, the ignition system is OK and the problem may he in the fuel system. Chart A-6 "Fuel System Diagnosis" must be performed to determine the cause of the problem.
This test win determine if the fuel injectors are functioning properly.
This test will determine if the poppet nozzles are functioning properly.
At this point, both the ignition system and fuel system are OK. The problem is caused by a mechanical problem in the engine.DIAGNOSTIC AIDS
An intermittent misfire could be caused by many different conditions.

SPARK PLUG WIRES/COIL WIRE
Ensure that the spark plug wires are securely attached to the spark plugs and the distributor cap. Check the wire routing to ensure that crossfiring is not occurring. If the misfire occurs when the weather is damp, the problem could be due to worn plug wires. To test for this condition, spray the wires with water and with the engine running, watch for spark to jump from the wires. If a spark is visible, replace the wires.

CONTAMINATED/LOW FUEL LEVEL
Check fuel condition and quality. Dirty or contaminated fuel could cause a misfire condition. If the fuel level is low, contaminants in the bottom of the fuel tankcould be introduced into the fuel metering system.

MISINSTALLED (LOOSE, COCKED) CRANKSHAFT POSITION SENSOR
A misinstalled crankshaft position sensor maybe detected while observing engine speed on a scan tool. A large change (twice) in engine RPM while the throttle is held steady may indicate an improperly installed crankshaft position sensor.

Do you have or recently installed a K&N air filter?
yes I have a K&N, it has been on there since before I bought the Blazer 2ys ago. I oiled it about 3 months ago and dried it completely with a shop vac.

As Mike2004tct said above, take a real close look at your Distributor Cap. I had the same problem on my 99, and it was a hairline crack on the feed between two plugs. This causes a random multiple misfire, it is a fairly inexpensive replacement, but definately buy a good quality cap to replace it. If it doesn't make any difference you can take the cap back and get a refund, but this is quite frequently the problem.

yes I have a K&N, it has been on there since before I bought the Blazer 2ys ago. I oiled it about 3 months ago and dried it completely with a shop vac.

This could be your problem.

You could have oil on your MAF sensor which can be causing the fuel system to mess up which will cause the misfire.

This could be your problem.

You could have oil on your MAF sensor which can be causing the fuel system to mess up which will cause the misfire.

But if it was that then it would send a code for MAF sensor wouldnt it, plus I already cleaned it(1st thing I did).
MORE INFO. Well I decided to drive it today(the 1st time since it started messing up) and I realized that it only missfires in the lower rpm's. If I keep it above 1500 rpm's then it runs like a dream but once it hits below 1500 then it runs like crap.

But if it was that then it would send a code for MAF sensor wouldnt it, plus I already cleaned it(1st thing I did).
MORE INFO. Well I decided to drive it today(the 1st time since it started messing up) and I realized that it only missfires in the lower rpm's. If I keep it above 1500 rpm's then it runs like a dream but once it hits below 1500 then it runs like crap.

Yip, sounds like the MAF then. Or the Thottle position sensor.

It is not getting the proper airflow reading at lower rpms. It won't always trip a MAF code.

Go and get a replacement at Autozone and take it for a test drive. If it solves your problem, great, if not, take it back.

Yip, sounds like the MAF then. Or the Thottle position sensor.

It is not getting the proper airflow reading at lower rpms. It won't always trip a MAF code.

Go and get a replacement at Autozone and take it for a test drive. If it solves your problem, great, if not, take it back.

It sounds like you need to get it on a scanner and see what the readings are for these parts in question, plus other parts/the whole system in general. I don't know of a parts place that lets you "test" out parts and take them back, especially electrical parts, once you buy them they are yours.

Buying parts and not knowing the exact problem is the old shot gunning method of repair, and it can get costly, especailly if you guess wrong.

I am telling him to test it, and I have heard of several people that have bought one and gone back and said it was the wrong part and they took it back.

Takes 5 minutes to test it and they will take it back if you bring it back within an hour or so.

It straight I think I still have a warranty on the MAF sensor. I unpluged it and it didn't change how it ran so I think BlazerLT is right.

its not the maf sensor. replaced it today and its still doing the same thing.

its not the maf sensor. replaced it today and its still doing the same thing.

What about the MAP sensor (manifold absolute pressure)

See this current thread which has similar symptoms as yours, but no error codes.
http://www.automotiveforums.com/vbulletin/showthread.php?t=594369

I tested the map sensor and it tested fine.

I replaced the cap, fuel filter and I put some fuel stabelizer in it and now it runs like a champ.
Thanks for all the help,
Chris

I replaced the cap, fuel filter and I put some fuel stabelizer in it and now it runs like a champ.
Thanks for all the help,
Chris

Trial and error, it's a pain. But glad to hear you solved the problem.

:thumbsup:

I replaced the cap, fuel filter and I put some fuel stabelizer in it and now it runs like a champ.
Thanks for all the help,
Chris

Remember, you replace the cap and rotor as a pair.

I have a 2000 blazer with approx. 96k mi. I also have a PO300 code showing, it's the only code showing up on the Autozone reader.... About 3mos ago did intake gasket set, new cap, rotor, wires, thermostat, AC plat plugs & fuel filter. A couple weeks ago it started having a slight random miss seemed to be at lower rpm. Went out a couple mornings ago for work & it ran like hell! that's when I went to Autozone to have the codes checked. Now it's missing BAD & popping either in the cat. or the muffler( havn't crawled under yet) & check eng light is flashing. Also running very rich. Having read the replies above, I'm leaning more to the injector setup ie. o-rings leaking etc. Does this make sence?
I'm gonna start checking the MAP ,MAF etc. tonight or tomorrow also.
thanks for any help or ideas. (I did try a new cap & rotor & it made no difference)

This could be your problem.

You could have oil on your MAF sensor which can be causing the fuel system to mess up which will cause the misfire.
Right after I bought my '99 used which already had the K&N filter on it I kept getting the same problems went through a lot of trouble before I found out the guy that owned it before me put to much oil on the filter, cleaned it, oiled it with K&N oil, cleaned the MAF sensor and no more codes.