1997 Chevy Blazer misfire

Give us the code to help you diagnose

I will assume that the code is P0306. Here is the description and instruction to repair P0306. I will also include instruction for P0108, just in case you need it.

DTC P0306 - CYLINDER NO. 6 MISFIRE DETECTED

Circuit Description

Misfire is detected using Camshaft Position (CMP) and Crankshaft Position (CKP) sensors. When VCM senses CKP deceleration not associated with normal engine speed reduction, CMP is used to determine misfiring cylinder. VCM determines misfires on each cylinder and evaluates a random misfire and set DTC.

Conditions for setting DTC are:

No IAC DTCs set.
No CKP, CMP, MAF, TP or VSS sensor DTCs set.
ECT at least 19°F (-7°C).
Fuel tank level is greater than 10 percent.
Engine speed at least 600 RPM and less than 5600 RPM.
System voltage at least 9 volts, but less than 14 volts.
Positive throttle position change less than 4.9 percent for 100 msec.
Negative throttle position change less than 2.9 percent for 100 msec.
A misfire is detected.

Diagnostic Procedures

1. Perform ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK. Retrieve and record FREEZE FRAME and/or FAIL RECORDS data for each DTC set. Go to next step.
2. If any other DTCs are set, diagnose affected DTCs. If no other DTCs are set, go to step 4).
3. Start engine and allow it to idle. Using scan tool, read MAP sensor voltage. If voltage is greater than 4 volts, go to DTC P0108. If voltage is not greater than 4 volts, go to next step.
4. Turn ignition off, and then on. Duplicate conditions required to set DTC. If DTC P0306 sets, go to next step. If DTC does not set, go to step 9).
5. Install a spark tester on No. 6 spark plug wire. Disconnect fuel injector harness at intake manifold. Crank engine and check for spark at tester. Repeat procedure for each spark plug wire. If spark was okay on all wires, go to next step. If spark was not okay, go to step 10).
6. Remove and check spark plugs for wear, excessive air gap, cracks or fouling. If spark plugs are okay, go to next step. If spark plugs are not okay, go to step 11).
7. Perform cylinder compression test. If problem is
found, repair as necessary. After repairs, go to step 13). If compression is okay, go to next step.
8. Perform INJECTOR CIRCUIT TEST. If fuel system is okay, go to next step. If fuel system is not okay, go
to step 12).
9. Misfire is currently not active. See DIAGNOSTIC AIDS.
10. Test IGNITION SYSTEM.
11. Replace spark plug(s). After replacing spark plugs, go to step 13).
12. Repair as necessary. After repairs, go to next step.
13. Using scan tool, select DTC, CLEAR INFO function. Start engine and allow it to reach
operating temperature.

Diagnostic Aids

A misfire could be caused by excessive vibration from sources other than engine. Check following
items for possible cause of misfire: tire/wheel out of balance/round, uneven brake rotor or drum
surface, or a rough road condition. Check plug sires for proper installation and operation. Check fuel
level, condition and quality.

DTC P0108 - MAP SENSOR CIRCUIT HIGH VOLTAGE

Circuit Description

Manifold Absolute Pressure (MAP) sensor measures changes in intake manifold pressure (vacuum). A low voltage signal, 1.0-1.5 volts, is sent to VCM on signal circuit at closed throttle (high vacuum). A high voltage signal, 4.0-4.5 volts, is sent at wide open throttle (low vacuum).

Conditions for setting DTC:
No TP sensor related DTCs are set.
TP angle is not greater than 96.8 percent when engine speed is not greater than 1000 RPM.
TP angle is not greater than 89.8 percent when engine speed is greater than 1000 RPM.
MAP is greater than or equal to 4.9 volts.

Diagnostic Procedures

Perform ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK. Retrieve and record FREEZE FRAME and/or FAIL RECORDS data for each DTC set. Go to next step.
2. Correct any engine idle or vacuum problems before proceeding. Turn engine on and allow it to idle. Using scan tool, read MAP sensor voltage. If voltage is less than 4 volts, go to next step.
If voltage is not less than 4 volts, go to step 4).
3. Turn ignition off. Disconnect MAP sensor harness connector. Turn ignition on. If voltage is less than one volt, go to step 5). If voltage is not less than one volt, go to step 9).
4. DTC P0108 is intermittent. If any additional DTCs are set, diagnose affected DTCs. If no additional DTCs are set, go to DIAGNOSTIC AIDS.
5. Using a DVOM connected to ground, probe 5-volt reference circuit at MAP sensor harness connector. If voltage reading is greater than 5.2 volts, go to step 10). If voltage reading is not greater than 5.2 volts, go to next step.
6. Using a test light connected battery voltage, probe MAP sensor ground circuit at MAP sensor
harness connector. If test light illuminates, go to next step. If test light does not illuminate, go
to step 14).
7. Check MAP sensor vacuum source for restriction. If restriction is found, go to step 15). If no restriction is found, go to next step.
8. Replace MAP sensor. After replacing sensor, go to step 19).
9. Check for short to voltage in MAP sensor signal circuit. Repair as necessary. After repairs, go to step 15). If circuit is okay, go to step_18_). **Disregard *_*
10. Turn ignition off. Disconnect VCM White harness connector. Turn ignition on. Using DVOM connected to ground, check voltage on 5-volt reference circuit at VCM harness connector. If voltage reading is greater than 5.2 volts, go to next step. If voltage reading is not greater than 5.2 volts, go to step 13).
11. Disconnect EGR harness connector. Check voltage on 5-volt reference circuit at VCM harness
connector. If voltage reading is greater than 5.2 volts, go to next step. If voltage reading not
greater than 5.2 volts, go to step 16).
12. Repair short to voltage on 5-volt reference circuit. After repairs, go to step 19).
13. Using DVOM connected to ground, check voltage on VCM harness connector terminal GR12. If voltage reading is greater than 5.2 volts, go to step 17). If voltage reading is not greater than 5.2 volts, go to step_18_). **Disregard *_*.
14. Repair sensor ground circuit. After repairs, go to step 19).
15. Repair as necessary. After repairs, go to step 19).
16. Replace EGR valve. After replacing EGR valve, go to step 19).
17. Repair short to ground on 5-volt reference circuit. After repairs, go to step 19).
18. Replace VCM. Program replacement VCM using required equipment. After replacing VCM, go to next step.
19. Using scan tool, select DTC, CLEAR INFO function. Start engine and allow it to reach
operating temperature. Operate vehicle within conditions required to set DTC.

Diagnostic Aids

Check MAP sensor signal and 5-volt reference circuits for intermittent open condition. With ignition on and engine off, MAP signal is equal to atmospheric pressure with signal voltage high. This information is used by VCM as an indication of altitude. Comparison of this reading with a known good vehicle with same sensor is a way to check accuracy of suspect sensor. Readings should be within 0.4 volt of each other. Disconnect sensor from bracket and twist sensor by hand to check for intermittents. Output changes greater than 0.1 volt indicate a faulty sensor connection.