Tip & How-To about Cadillac STS

How to diagnose a cylinder misfire

First do a compression test on that cylinder to verify there is no internal engine problems and compression on the misfiring cylinders is the sameas the other cylinders. If compression is good verify spark on that cylinder is as good as the spark on the other cylinders if not replace the spark plug and wire, if spark is still weak replace coil. If problem is still no or weak spark then you have a wiring problem on that cylinder. If spark is good use a LED testlight to verify the circuit has good power and ground pulsating on the injector circuit, if not you have a wiring problem if the circuit is working like it should replace the injector. If everything is good up to this point you likely have a leaking intake manifold gasket.

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It run badly when cold once warm it runs ok. I have replace coils. Plugs and fuel fliter. The codes are. P300 p301 p302 p303 p304 misfire help please

Is it a 4 cylinder engine, and all four cylinders are misfiring?

Misfires will set a code when the computer senses a lag in power output from a cylinder, based on the input from the crank position sensor. Misfires are usually caused from a loss or a problem with gas, spark, or compression-one of those three is wrong for a misfire to occur. New coils and plugs, so probably not a spark problem-unless the spark timing is wrong-let's leave that for now. You should have the fuel pressure tested, and see if pressure is as specified. If pressure is low, and you have new filter, probably a weak pump.

Check cylinder compression as a last resort. Your lowest reading cylinder should be within 75% of the highest reading cylinder. A compression test is a good indicator of the internal health of the engine, the higher the compression, the more power output from that cylinder. But it should be even and balanced, lowest should be within 75% of the highest reading.

So, gas, spark, or compression?

Mar 10, 2015 | Cars & Trucks

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reasons for cylinder misfires

Ford coil on plug systems are multi strike ignition (spark) under 1300 rpm then go to single strike a single cylinder misfire can be created by several things . #1 Test engine compression on a cranking engine compression it should be around 150 lbs but if gauge reads different COMPARE to other cylinders that work fine #2 check engine running compression this will be around 120 lbs then rev over 2000 rpm and check pressure drop again COMPARE to other cylinders that work fine. any drop would indicate valve train problems. #3 All internal engine is fine then external problems are present vacuum leaks and or fuel injection volume is low.

Jul 23, 2014 | 2003 Ford Thunderbird

1 Answer

Cylinder #1 continues to Misfire

What DTC code(s) are you getting?
You performed a compression test on cylinder #1; did you test cylinder #2?
Compare the readings, if there is a significant difference in compression readings between the two, you may have a bad head gasket, a bad valve, etc.
The bottom line, the problem is internal to the engine.

Dec 09, 2013 | 2002 Buick LeSabre

1 Answer

99 pontiac grand prix that is still mis firing even after we changed plugs and wires, not sure what to do now please help

Misfires can be caused by several things including all the parts you have already replaced as well as faulty ignition coils, faulty injectors, vacuum leaks and engine mechanical problems such as low compression.
With the proper equipment, injector operation can be verified by performing injector balance tests, where the operation of the injectors in the misfiring cylinders is compared to the other cylinders that are not misfiring.
Also, engine mechanical failures such as bad piston rings or leaking/burnt valves can be diagnosed with a simple compression test.
An easy (and very inexpensive ) way to verify ignition coil operation is to swap the coil from the misfiring cylinder(s) with the coil(s) from the cylinder(s) that is(are) not misfiring. Then clear the fault codes and drive the vehicle. If the same cylinders are still misfiring, the coil is not the problem. If the misfire moves to the cylinder that you swapped with - replace the coil.
Sometimes, injectors can also be tested this way, but on most GM V-6 or V-8 engines, this is not practical due to the labor involved in "swapping" injectors.

May 10, 2011 | 1999 Pontiac Grand Prix

1 Answer

ce lights on code reads #3 cylinder misfire

I assume this 'complete' tune up was plugs, wires and plennum (intake surge tank) gasket?
If so, recommend you first check to see if you are getting an injector pulse to the #3 injector (a noid light is normally used to verify the injector is receiving the 'pulse' from the ECM. The noid light is a very useful tool in finding out if you have problems with your fuel injector. It is cheap (around 10 dollars per piece), easy to use (if the light blinks, your injector is receiving the electrical pulse to open). If after you verify the injector pulse is present, the next step is to determine if the injector or cylinder is the problem. A simple verification process requires swapping the #3 injector with another to see if the misfire moves to the swapped cylinder.(.i.e, move #3 injector to #2 cylinder & the #2 injector to #3 cylinder.).. Then run engine to see if the P0303 changes to P0302. This would VERIFY the injector is in some way defective. If the misfire STAYS at #3 cylinder, you'll want to perform a compression test of all the cylinders. If after this test you find #3 cylinder is more than 10% lower than all the other cylinders, do a 'Leak-Down' test to determine if the low compression is due to leaky valves (intake or exhaust) or whether the piston/rings are the problem. NOTE: If all the cylinders are withing 10% of each other during the compression test, the possible causes are, injector O-ring leak, vacuum leak at the intake manifold at #3 cylinder, or the spark plug not firing under compression.

Mar 15, 2009 | 2000 Hyundai Sonata

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