This applies to GM’s 4.3 and 5.7 engines with a check engine light, or CEL on and the following P0300 code for random cylinder misfire.
If your ‘90s GM vehicle has a distributor but NOT TBI (throttle body injection) this may apply to you.
Typical symptoms are an easy start but misfire above idle. When cold it can backfire through the intake, and changes to a backfire through the exhaust when hot. It can accelerate strong when the accelerator is floored but bucks, misfires and hesitates from 1,000 RPM up to about ¾ throttle. Typically worse at about 2,000 to 4,000 RPM. Impossible to maintain freeway speeds without symptoms. It can also affect the shifting of the automatic transmission.
After first ruling out spark plug wires, distributor cap and rotor, you may have a fairly common problem not easily discovered without much diagnostic work. OBDII engines are not easily diagnosed as older carburetor engines. A scanner is needed to view codes and some scanners can monitor the data stream. Even some professional scanners like the Snap-On scanners cannot pick this up unless the tech knows what to look for.
You can do a couple of very easy checks with a timing light to see if this applies to your issue.
Connect the timing light to any sparkplug wire when the engine is fully warmed up. Start the engine and watch the flashing strobe. Increase the engine speed until the engine starts to misfire. Is the light still flashing consistently? Is it “loosing” spark; the light flashes intermittently? This means that the spark is not reaching the sparkplug every time it needs to fire. This causes unburned fuel to flow out the exhaust port and will ignite when it reaches the hot catalytic converter, backfiring in the exhaust.
If you find that the spark disappears, the next step is to connect your timing light to the coil wire and repeat the test. If you have a quality timing light it will follow the firing pretty good. If it’s a cheaper timing light it may not keep up with the firing and start to “show” a misfire when there is not. It’s just that the timing light can’t keep up with the firing. Connecting the timing light to the coil wire means it’s getting 6 or 8 times the pulses it would normally get at the plug wire. So a misfire at 3000 PRM at the plug wire would mean 18,000 impulses at the coil wire at the same RPM on a 6 cylinder engine. Most timing lights cannot keep up and will loose sparks, or flashes.
If you get a strong following of flashes at the coil wire, it means that the coil is delivering the spark, but it’s getting lost in the distributor before it gets to the plug wires. Now remove the distributor cap and give the rotor a back and forth twist to check the backlash. It should not be more than about 3/16 of an inch at the tip of the rotor. The vehicle I encountered had about ½ inch. More than 3/16 of an inch and the rotor will be pointed to some place between the spark plug wire contacts in the cap. This is the cause of misfire. The spark will jump between 2 cylinders or not to either. This is what is happening when you lose the spark at the plug wire.
The solution is to replace the distributor drive gear. This gear is about $75 at a dealer. The gear will wear sooner if regular oil changes are neglected. The vehicle I was working on had 220,000 kms.
To replace the gear requires that the engine first be placed at top dead center, or TDC, for number 1 cylinder. With the cap off, you can see to what cylinder the rotor is pointing. The cap should have cylinder numbers on it. Number 1 cylinder is the front cylinder on the driver’s side. It connects to the driver’s side of the distributor cap and crosses to the other side of the distributor inside the cap. If you look carefully at the mounting surface of the distributor body where the cap contacts, there is a small point indicating number 1 position. Under the engine, to the driver’s side of the crankshaft pulley is a “knife edge” casting in the timing cover. This will align with the timing groove in the harmonic balancer when number 1 piston is at TDC. With these 2 marks positioned, you can now remove the distributor. Disconnect the wire plug at the rear, and then remove the clamp & bolt. The distributor will now lift out. The rotor will twist counter clockwise as you lift the distributor. The engine must NOT be rotated with the distributor out as the oil pump drive will no longer be lined up. Once out, look at the drive gear. The teeth should only have a small shiny area where the gear has contacted the camshaft gear. The teeth should NOT be worn their entire surface. See the photo below for what a worn gear looks like.
Support the gear by laying it on it’s side on the partially open jaws of a vise. With the gear supported you can drive out the roll pin with a hammer and small punch and slide the gear off. Slide the new gear on and tap the roll pin back in.
When installing the distributor, you need to engage the camshaft gear and the oil pump drive at the same time. If the engine has not been turned it should go together with little fuss. Slide the distributor into the hole and carefully position it above the installed position. Position the rotor a bit counter clockwise of the desired position. Lower it down and wiggle it a bit to align the gears. It will drop down with little effort when aligned. Now install the clamp bolt and the wire plug at the back. Your rotor should only mover back and forth about 3/16 of an inch now. Install the cap & wires.
Note that the distributor body is plastic and the cap hold down screw are self tapping. If these are over tightened the hole will strip or the body crack. See the photo below.
I suggest tightening the screws slowly and when they begin to get tighter and then check the cap to see if it wiggles. Keep tightening only until the cap no longer wiggles.
Also a stretched timing chain can cause the same effect as a worn distributor drive gear as the indexed relationship between the crankshaft & camshaft is not correct. To check for a stretched timing chain, put a wrench on the pulley nut on the end crankshaft. Turn the crank back & forth feeling for the slack. This slack is the crankshaft turning back & forth without turning the camshaft. If this exceeds more than about 1" on the timing marks the chain & gears need to be replaced.
This repair not only fixed the engine misfire, but the transmission now shifted properly into 2nd gear.
