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Bob, Auto Tech
Category: Chevy
Satisfied Customers: 4087
Experience:  Plus 40 years GM, over 30 certified with Chevrolet, ASE certified Master Tech. (Expired, Retired)
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96 chevy blazer: v6..4.3l that misfires at cylinder #4 what should I

Customer Question

I have a 96 chevy blazer v6 4.3l that misfires at cylinder #4 what should I do? I already changed the spark plugs, wires, rotor, distributor cap
Submitted: 8 years ago.
Category: Chevy
Expert:  Bob replied 8 years ago.
The best way to find the problem isf you have already changed the wire and the plug, cap, rotor etc would be to check it for intake leaks next. Use a can of spray carb cleaner and spray around the base of the intake or anyplace else that the motor could suck air into. If you hit a spot that is leaking vacuum., you will see an immediate change in the way the motor runs. If that happens you have found your problem. Replace the defective gasket, etc. If you find nothing, then try pinching off any vacuum hoses as close to their supply connection as you can. If that changes the way it runs, either the hose or the component the hose connects to is leaking vacuum. If you find no vacuum leaks, the next step would be to do a compression test on the motor to see if you have any internal parts that are causing the problem. Rings, valves etc. If the compression test checks good that leaves you with nothing left except the injectors, so it then would be time to run a balance test on them. Good luck and thanks
Bob and 2 other Chevy Specialists are ready to help you
Customer: replied 8 years ago.
So what is the compression specs on that truck be I checked the #4cylinder and the one next to it #2cylinder they were both 90psi
Expert:  Bob replied 8 years ago.
First let me say thanks for the accept, it is appreciated. 90 psi is on the low side. The problem is that the factory does not give us a number and say it should be with in a certain percentage of that number. What we look for is uniformity and how close they all are to being the same. To test 2 adjoining cylinders is not really the best way to test it. If say, it has a bad head gasket between number 2 & 4 cylinders, then they could both read low and equal. Which is what you have now. You really need to take a reading on all 6 cylinders and then squirt some light weight oil in the cylinders and take another reading. Then compare the two readings. If the second reading is a lot higher than the first, by say 20% or more, then you know that the rings are worn as the oil seals up the compression leaking around the rings. I would expect a good running but well worn motor to have compression of under 100 psi, but would also expect them to all be equal. If you have 2 at 90 psi and the rest are at 110 or 120 or so, then you have a problem where the two 90 psi readings are. If you have 5 cylinders that are over 110 and one that was at 90 then we know the 90 again is a problem. Or if all of the cylinders are at 90 and one reads 60, then we look at the one that has 60. If it has low compression and still runs good and smooth, that just means the motor is getting tired and will need a rebuild soon. To find a mechanical reason for the miss, we should notice one cylinder that has a lot less compression than the rest of them. When you get one like that, then the next step would be to pressure test the cylinder and find out where the compression has gone. We would bring that cylinder to top dead center and then inject air into it and see where the air leaks out. If we hear air going out the tail pipe, then we know it has a bad exhaust valve in that cylinder. If the air escapes out through the intake manifold or carb or throttle body, then we know it is a problem with an intake valve. If we get a lot of air out of the valve covers or the oil fill or dipstick tube, then we know the air is going past a piston or the rings. Etc. If all of the cylinders are with in 10 or 15 psi of each other, then your problem is maybe not mechanical or maybe related to an injector. For testing them, you need some special equipment, etc. But on the compression test, you are looking more for a large discrepancy on one cylinder from the rest of them. Unless the other 4 read real high compared to the two that are now 90. Let me know. Thanks
Customer: replied 8 years ago.
I know I have to wait for the truck to get to normal runnig temp before I do compression test and that come to my next question. Why does my fan comes on before my car reaches normal temp as soon as I crank my car the fan comes on and the defrost is not on and also my temp guage is not working.
Expert:  Bob replied 8 years ago.
Are we talking about the fan for the heater and defroster? If so, I would suspect maybe a bad relay. If a relay shorts out, the fan will usually come on any time you turn the key on. The only other thing I could think of off hand would be if the switch on the heater control panel was bad. The temp gage could be a blown gage fuse, a bad temperature sending unit on the motor or a bad gage, or even a broken wire.
Customer: replied 8 years ago.
I'm talking about the radiator fan and also if my pcm is bad will it create all this problem even the miss fire because when I also turn my car on the check engine suppose to come on it doesn't. Let me know what you think thank you
Expert:  Bob replied 8 years ago.
Big difference on the fan. Your pcm could cause all of those problems, fan coming on, temp gage not working and even the check engine light, but I would be surprised if it was causing the miss. The first thing to check is all of the fuses, to make sure one of them is not blown and causing some sort of back feed that is affecting the pcm and that system. Use a test light or a volt meter to check both sides of each fuse and make sure they are ok. Let me know what you find.
Customer: replied 8 years ago.
I checked all the fuses two or three times before and they was all good, so how do I check if the pcm is creating all this problem.
Expert:  Bob replied 8 years ago.
You really need to get a hold of a class 2 scan tool and see if it will power up and read codes or not.
Expert:  Bob replied 8 years ago.

This is what they give us on the light. Almost impossible without a scan tool. The blue high lights wont open for you.



Expert:  Bob replied 8 years ago.

Malfunction Indicator Lamp Circuit Check







Important: Before clearing the DTCs, use the scan tool Capture Info to save the Freeze Frame and the Failure Records for reference. The control module's data will be deleted once the Clear Info function is used.

Did you perform the Powertrain On-Board Diagnostic (OBD) System Check?


Go to Step 2

Go to Powertrain On Board Diagnostic (OBD) System Check


  1. Turn OFF the ignition for 15 seconds.
  2. Turn ON the ignition leaving the engine OFF.

Is the MIL ON?


Go to Step 3

Go to Step 5


Command the MIL OFF and ON using a scan tool.

Does the MIL turn OFF and ON when commanded?


Go to Diagnostic Aids

Go to Step 4


  1. Turn OFF the ignition .
  2. Disconnect the VCM C4 connector.
  3. Turn ON the ignition leaving the engine OFF.

Is the MIL OFF?


Go to Step 18

Go to Step 12


Check for an open in the Gauge, the Battery Positive voltage or the Ignition Positive voltage fuses.

Are any of the fuses OPEN?


Go to Step 13

Go to Step 6


  1. Turn OFF the ignition.
  2. Disconnect the VCM C4 connector.
  3. Turn ON the ignition leaving the engine OFF.
  4. Probe the MIL control circuit at the VCM C4 connector with a fused jumper connected to a ground.

Is the MIL ON?


Go to Step 15

Go to Step 7


Is the fuse in the fused jumper open?


Go to Step 14

Go to Step 8


Check for an open in the MIL control circuit. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you find a problem?


Go to Step 19

Go to Step 9


Check for an open between the MIL and the Gauge fuse. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you find a problem?


Go to Step 19

Go to Step 10


Check the Instrument Panel (IP) cluster harness connections. Repair as necessary. Refer to Connector Repairs in Wiring Systems.

Did you find a problem?


Go to Step 19

Go to Step 11


Replace the Instrument Panel (IP) cluster. Refer to Instrument Cluster Replacement in Instrument Panel, Gauges and Console.

Is the action complete?


Go to Step 19


Expert:  Bob replied 8 years ago.


Malfunction Indicator Lamp Circuit Check

Refer to UBEC Power, Grounding, Generator, And MIL

Circuit Description

When the ignition switch is initially turned to the run position, the malfunction indicator lamp (MIL) will be ON steady. When the engine is running and there are no active type A or B DTCs stored, refer to Diagnostic Trouble Code (DTC) List (Definitions) , the Vehicle Control Module (VCM) turns OFF the MIL. The ignition switch supplies battery voltage directly to the MIL through the gauge fuse. The VCM controls the MIL by providing a ground path through the MIL control circuit in order to turn ON the MIL.

Diagnostic Aids

An open gauge fuse will cause the entire IP cluster to be inoperative.

An intermittent may be caused by any of the following conditions:

A poor connection

Rubbed through wire insulation

A broken wire inside the insulation

Thoroughly check any circuitry that is suspected of causing the intermittent complaint. Refer to Intermittents and Poor Connections Diagnosis in Wiring Systems.

If a repair is necessary, refer to Wiring Repairs or Connector Repairs in Wiring Systems.

Test Description

Number(s) below refer to step number(s) on the Diagnostic Table.

  1. A MIL that responds correctly to the scan tool command indicates that the condition is not present at this time.
  2. The MIL should be OFF. This step checks for a short to ground in the MIL control circuit by removing the VCM from the circuit.
  3. This step checks the MIL control circuit for a short to B+ or a problem with the VCM.
  4. Locate and repair any shorts that may have caused the fuse to open BEFORE replacing the fuse.


Customer: replied 8 years ago.
With me having a low compression on cylinder #4, how long do you thing the engine going to last and also would it help a little bit if I use a thicker weight oil or not. Thank you
Expert:  Bob replied 8 years ago.
Some times they fool you and run for a long time that way. There is really no way of knowing for sure, but you can bet it will give you plenty of warning before it lets go completely. It will most likely just get worse and worse as you go, until you either decide to park it or fix it. Using a heavier oil won't hurt it and may even help it stay lubricated better. But if you start hearing strange noises from the motor, then it is time to stop driving it. Thanks