Random Cylinder Misfire

Cannot source problem on a 1997 Chevy C-1500 p/u with 4.3 vortec motor. New Plugs, Wires, Cap, Rotor, IAC, EGR Valve, Injector Spider, Fuel Filter, Only thing not new is Coil. Computer Diagnosis calls out a “Random Cylinder Misfire/PO 300”. Seems to isolate #4 but gets no more specific. Check for broken plug-no change. Check Engine Light comes on and sometimes blinks. Cylinder Balance test ok, Fuel pressures are ok. No apparent change in Tachometer when miss occurs. Has newly rebuilt transmission which was returned to re-check for vacuum leak/etc. checks out ok. Been checked by small shop and dealer shop. Short of spending hundreds of $ to locate a possible harness short or other kind of problem would there be a suggestion?

The random misfire might be caused from carbon buildup on top of the pistons and/or intake valves. So you might want to try a decarbonizing of the engine. And this can be done with a can of SeaFoam Motor Treatment.

Get the engine up to operating temperature and shut the engine off.

Disconnect the brake booster vacuum hose at the brake booster.

Adapt a hose that fits into the end of the brake booster hose and can be inserted into the can of Seafoam.

Take a pair of pliers and pinch off the adapter hose.

Have somebody start the engine, and get the idle speed up to about 2,000 RPM’s.

Slowly open the pliers and let some of the SeaFoam be drawn into the engine. When the engine speed starts coming down, pinch off the hose again and let the speed come back up. Repeat this until all of the SeaFoam is gone from the can, and immediatly shut the engine off.

Reconnect the brake booster hose.

Wait about a half hour and restart the engine, and bring the idle back up to about 2,000 RPM’s until the smoke clears.

Oh yeah. All the while you’re doing this, there’s going to be a large amount of smoke coming out of the tail pipe. But that’s just the SeaFoam doing its thing.

Try this and see if it gets rid of the misfire code.


Man, that sounds awful simple and too good to be true. Going to give it a try though! Thanks - will report back.

There are still parts that could be thrown at the problem. You could try another distributor from a salvage yard. The distributor comprises the ignition coil, the pick up coil, and the ignition module, and (?). I have to wonder if Mr. Haynes, or Mr. Chilton’s, or Mr. Service Manual were consulted for testing the various components? They provide such consulting for a modest fee, they should be used.
Also, a good OBD II scanner can, in the proper hands, yield a lot of useful information. (Did anybody use terms like: freeze frame, or sensor trace?). Those words should be in any mechanic’s vocabulary.

Seafoam application did not solve the problem. Went ahead and put in new coil too - no change. BTW, shop also had done cylinder compression tests which were all within normal limits so my suspicion of a burned valve is starting to look faulty. Noteworthy that while Seafoam was doing its incredbly smoky thing we noticed a leak @ manifold/y-pipe donut. Wondering if Catylitic Converter may be clogged/blocked/faulty and if there may be some errant signal being sent back to the computer to cause the miss? Don’t have access to OBD II but may have to look into seeking one out.

A random misfire is often caused by this item. Not that I would install a used one though; unless it were free or a new one was obscenely overpriced.


The point is: there are so many things that can cause misfire. Sometimes, it’s two, or more items which are marginal, which act together to cause a problem. The item ok4460 shows a link to is a CPS (Crankshaft Position Sensor). Click on this Auto Zone page for replacement and testing instruction of the CPS: http://www.autozone.com/servlet/UiBroker?ForwardPage=az/cds/en_us/0900823d/80/08/6e/99/0900823d80086e99.jsp Carefully note that the voltmeter setting is for AC voltage for testing the CPS.

Crankshaft Position Sensor was tested and found to be ok.

What are the definitions of “freeze frame” & “sensor trace”?

Random misfire and good compression check. It could be a weak exhaust valve spring too. It is just one of many possible causes. If the exhaust valve sometimes hangs open, it can ruin the intake cycle so that you aren’t drawing enough of the fuel/air mixture into the cylinder. You might also have large deposits on that valve that make it slow to close. The valve rotates during each lift. This won’t register during a compression check. Sometimes the valve stem gets gunked up and causes the same problem. If so; you might have success by putting something in the oil that frees stuck valves.

Check this web page http://www.aa1car.com/random-misfire/. It will give you more detail about some of the causes, and how to do the tests, for random misfire. The Haynes, or Chilton’s, will have further details on what to look for when using a vacuum gauge to troubleshoot an engine’s intake vacuum.
Here’s another possibility: http://www.obd-codes.com/forums/2812-obd-codes-1998-chevy-c-1500-p0300-code.html

A static check of a crank sensor is not always accurate. This is true of many electronic parts and in fact, many times a factory manual may even specify something to the affect; “If a part tests fine but is suspect it should be replaced”.
(paraphrased a bit there)

Granted, that’s not a real scientific statement but it’s often true.

Quite true. I recently had a 4 cylinder engine with an intermittent misfire on #3 cylinder (DTC P0303. Thank goodness for OBD II !). It had an ignition coil for a pair of cylinders (two towers per ignition coil). With a multimeter, the ignition coils checked good. I switched the ignition coil electrical control wires around, and I switched the spark plug wires to different cylinders. A test drive revealed a different cylinder as misfiring. This is good news. Really! It showed that one tower on a two tower ignition coil can misfire. The other tower (which used the same ignition coil) did not misfire! I changed the ignition coil, and all was light and happiness!

With backfire, we need to know what we’re dealing with, before we can figure out how to fix it. We start with an understanding of what backfire is, which is: the rapid burning of a combustible fuel/air mixture in the intake manifold. It’s the result of two things being available in the intake manifold at the same time: a combustible mixture and a flame. The combustible mixture comes from the fuel injectors and the air intake tube. The flame can reach the mixture when an intake valve is partially open. The flame front leaves the cylinder, through the opening, and ignites the mixture in the intake manifold.
An intake valve can be partially open if: the spark occurred too early (overly advanced timing),or if there are hot spots in the cylinder causing pre-ignition, and the flame front comes through the still open intake valve; the valve spring is weak, and closes the valve too slowly; an accumulation on the valve delays valve closing; there is a crack in the valve; the valve face to valve seat leaves an opening (debris, warpage).
A compression check will reveal if the intake valve isn’t closing completely, or is leaking. A vacuum test will show that and several other things. A timing check will indicate if timing of spark is too early. A valve spring tension check will reveal weak valve springs. Valve action can be seen with a timing light. It’s strobe action can “stop” the action of the valve stem ends and valve springs, and reveal erratic motion.
Sometimes, in troubleshooting an engine, inspiration counts as much as perspiration.

Thanks for the input. Maybe I’m just too inexperienced to understand but for all of the time and money spent on parts and diagnosis already I have a difficult time understanding how something like this would not have already been picked-up on. What’s the possibility that the onboard computer is having a brain overload and simply not processing information correctly and causing the misfire ~ and if so, shouldn’t that be picked up by the dealer’s diagnosis also?

There are many grades of parts. What brand parts were used, especially the plugs.

The dealer’s mechanics haven’t been successful in troubleshooting this misfire problem. So, seek out an independent shop/mechanic. You can print out these pages showing the problem, actions taken, and the advice of those on this forum. Let us know the results, ok?

What octane of gasoline have you been using. If you use the wrong octane, it can cause the engine to misfire. If you’re using the recommended octane, try a higher octane. If no change, go back to the recommended octane.

Does the truck have an aftermarket security system? If it does, or did, Chevrolet has TSBs about them causing multiple misfires.