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Discussion Starter · #1 ·
So searching this site I've found that the following list can cause the engine misfire error codes on our cars:

Mass Air Flow Sensor
Coils
Plugs / Wires
O2 sensors
Vacuum leaks
Aftermarket parts
Bad gas / water in the gas
head gasket
fuel injectors

With so many possibilities causing a single error code, how is one supposed to diagnose the cause of a misfire code? What does the Lexus dealership do in these cases? Certainly they don't just replace everything. I ask because I was getting a cylinder 4 misfire code. Over the weekend I changed my plugs (even though I'm only at 73k miles) to see if that was it. My CEL came back on, and although I haven't had a chance to get the code re-read, I suspect that it's the same thing. I'm just having a hard time trying to figure out what to look at next. Is there any way to narrow down the list? i.e. do certain issues cause noticeable problems with the way the vehicle performs? There's been no noticeable affect on my car since the CEL first came on.
 

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As a technician this is how i diagnose it.

1)Take plugs out look at plugs, check to c if there is oil on it, if it looks like it burns too much or too little fuel. (if there is oil or antifreeze on the plug it's a HG problem or something to that effect)

2) compression check the cylinder and make sure it is within tolerance of the other cylinders around it. (if it is out of tolerance it is an HG problem or engine problem)

3) swap the coil from a cylinder that doesn't have a misfire. Clear the light and tell the customer to come back when the light comes back. When the light comes back check to see if it's the same cylinder (if it's the same cylinder go to step 4, if the cylinder has moved replace coil) For an distributor or a non COP system i just stick a screwdriver in the plug wire and check for spark

4) So we have spark and compression so only one thing left, fuel. Take a noid light and make sure the injector is getting a signal. If it's not gettin signal start tracing wires. If it does get a signal replace injector

Finish

BUN
 

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Well said, the only thing I would change is try cleaning the injector before swaping it out. Unless you have more injectors on hand.
 

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Discussion Starter · #5 ·
As a technician this is how i diagnose it.

1)Take plugs out look at plugs, check to c if there is oil on it, if it looks like it burns too much or too little fuel. (if there is oil or antifreeze on the plug it's a HG problem or something to that effect)

2) compression check the cylinder and make sure it is within tolerance of the other cylinders around it. (if it is out of tolerance it is an HG problem or engine problem)

3) swap the coil from a cylinder that doesn't have a misfire. Clear the light and tell the customer to come back when the light comes back. When the light comes back check to see if it's the same cylinder (if it's the same cylinder go to step 4, if the cylinder has moved replace coil) For an distributor or a non COP system i just stick a screwdriver in the plug wire and check for spark

4) So we have spark and compression so only one thing left, fuel. Take a noid light and make sure the injector is getting a signal. If it's not gettin signal start tracing wires. If it does get a signal replace injector

Finish

BUN
how do you check the compression? Is that something that's done via a diagnostic reader or something? I can't see how this would work with a manual compression guage, since we have to take apart the engine to access the plugs.
 

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No it works the same one. just take the one plug out and crank the motor. Just a cranking compression test is needed, not really a running. A manual gauge works bro. Just becareful of backfiring flames out the TB. I damned near burnt my eyebrows off one time like that.

BUN
 

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Discussion Starter · #7 ·
No it works the same one. just take the one plug out and crank the motor. Just a cranking compression test is needed, not really a running. A manual gauge works bro. Just becareful of backfiring flames out the TB. I damned near burnt my eyebrows off one time like that.

BUN
Thanks for the information. This is really good to know. How do I crank the engine if I have the throttle body disconected/taken apart?
 

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Just turn the key, it'll still crank, I've cranked engines with half the intake manifold off. But seriously take out the fuse for the EFI or you will get burned.

BUN
 

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Discussion Starter · #9 ·
Just turn the key, it'll still crank, I've cranked engines with half the intake manifold off. But seriously take out the fuse for the EFI or you will get burned.

BUN
Thanks for all the info. This has been helpful. I'm going to try to check this out to see if I can figure out my problem.

+rep for you
 
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