97 Blazer with code P0300

[blazin97]

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

[BlazerLT]

Ignition switch module.

Known problem on a 1997.

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

[mike2004tct]

Originally Posted by BlazerLT

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.

[blazin97]

Originally Posted by mike2004tct

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.

[mike2004tct]

Originally Posted by blazin97

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.

[BlazerLT]

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.

[blazin97]

Originally Posted by BlazerLT

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.

[BlazerLT]

Originally Posted by blazin97

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.

[blazin97]

Originally Posted by BlazerLT

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.

[blazin97]

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

[BlazerLT]

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

[blazes9395]

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.

1. 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.
2. 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.
3. 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.
4. 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.
5. This test win determine if the fuel injectors are functioning properly.
6. This test will determine if the poppet nozzles are functioning properly.
7. 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.

[blazin97]

Originally Posted by BlazerLT

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.

[billibong]

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.

[BlazerLT]

Originally Posted by blazin97

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.