How to diagnose a random misfire?

[Matt Gruber]

Dead spot in the TPS?

i know of guys that cut open the tps, and solder the 3 fingers and that fixes it.

[Shopboss]

I worked for Chevrolet during those few years. The 350's had this issue of constantly breaking down the wires. I truely believe it was from excessive load from the shear weight of the vehicle and torque converter clutch, of which the torque converter clutch never released when it should have. You had to really get on the gas for it to release, and if not it would load the engine hard. Those vettes shifter fast and to soon loading the engine pretty hard. They also where set up to run on the lean side for emissions and that wasn't good for the plugs or wires either. It's been a while, but I believe at the time the wires where grey in color and you could see carbon tracks on them, like a grey powder area or traces of grey powder.

Hopefully your not mis-diagnosing an engine miss for torque converter clutch shutter.

X2

I was working as shop-foreman at a Chevy dealership in the 80s. Seams like we had several of these "skips" come in each week. The converter clutch would hunt in and out and feel just like a skip.
Get it up to speed and under load so you feel it and then just touch the brake enough to cut the converter out. If it goes away then you've found the problem.

As pointed out it could also be a cracked plug. Try this, get a spray bottle full of water. With the engine at idle, mist around the plugs and plug wires. If it skips or you hear a snap then figure out which wire or plug is breaking down. If you do this at night or in a dark place you should see a spark.

Pull the plugs and check for a crack. Twist the porcelain between your fingers and the dirt and oil from your skin will imbed in the crack and make it show up.

When we have an engine on the Dyno that has a skip under load, 90% of the time replacing the plugs fixes the problem. Even a plug that looks ok and not fouled can cause this. In fact last week we chased a miss between 5000 & 6000.
We checked the plugs and they looked great. Since I didn't have another set like them we tried other things. After wasting a couple of hours we finally made a trip to the parts store for a new set of plugs the miss went away.

If you need a good cheep spark plug tester use a good old HEI dis. Hook it up with the cap and coil off and to the side so you can fire the plug from the center coil terminal. With the power on, hold a soldering gun, " not an iron", close to the pickup coil, the part with the green and white wire. Pull the trigger and the plug will fire ever time the field changes in the coil of the gun usually 60 times a second.

Keep in mind that a plug can break down under pressure but fire perfect in free air. If you want to take things a step further, rig up a pressure chamber with a sight glass so you can use regulated air pressure to simulate cylinder pressure. An old head and some thick plexiglass works great. Just be careful not to blow things up in your face. It's interesting to watch the spark diminish as the pressure increases. This will also show you why a modern HEI system workers so much better than an old points system.

Donny

[2.2=8]

I worked for Chevrolet during those few years. The 350's had this issue of constantly breaking down the wires. I truely believe it was from excessive load from the shear weight of the vehicle and torque converter clutch, of which the torque converter clutch never released when it should have. You had to really get on the gas for it to release, and if not it would load the engine hard. Those vettes shifter fast and to soon loading the engine pretty hard. They also where set up to run on the lean side for emissions and that wasn't good for the plugs or wires either. It's been a while, but I believe at the time the wires where grey in color and you could see carbon tracks on them, like a grey powder area or traces of grey powder.

Hopefully your not mis-diagnosing an engine miss for torque converter clutch shutter.

X2

I was working as shop-foreman at a Chevy dealership in the 80s. Seams like we had several of these "skips" come in each week. The converter clutch would hunt in and out and feel just like a skip.
Get it up to speed and under load so you feel it and then just touch the brake enough to cut the converter out. If it goes away then you've found the problem.

As pointed out it could also be a cracked plug. Try this, get a spray bottle full of water. With the engine at idle, mist around the plugs and plug wires. If it skips or you hear a snap then figure out which wire or plug is breaking down. If you do this at night or in a dark place you should see a spark.

Pull the plugs and check for a crack. Twist the porcelain between your fingers and the dirt and oil from your skin will imbed in the crack and make it show up.

When we have an engine on the Dyno that has a skip under load, 90% of the time replacing the plugs fixes the problem. Even a plug that looks ok and not fouled can cause this. In fact last week we chased a miss between 5000 & 6000.
We checked the plugs and they looked great. Since I didn't have another set like them we tried other things. After wasting a couple of hours we finally made a trip to the parts store for a new set of plugs the miss went away.

If you need a good cheep spark plug tester use a good old HEI dis. Hook it up with the cap and coil off and to the side so you can fire the plug from the center coil terminal. With the power on, hold a soldering gun, " not an iron", close to the pickup coil, the part with the green and white wire. Pull the trigger and the plug will fire ever time the field changes in the coil of the gun usually 60 times a second.

Keep in mind that a plug can break down under pressure but fire perfect in free air. If you want to take things a step further, rig up a pressure chamber with a sight glass so you can use regulated air pressure to simulate cylinder pressure. An old head and some thick plexiglass works great. Just be careful not to blow things up in your face. It's interesting to watch the spark diminish as the pressure increases. This will also show you why a modern HEI system workers so much better than an old points system.

Donny

X3, The water mist trick works even better if you mix up some salt water. If the running ignition system does not arch-over with salt water mist, then I would pretty much rule out a secondary igniton problem. also when doing the salt water mist test have someone load it up against the converter a little. spray it over the distributor also.

[EthylCat]

Could it be as simple as the ignition does not have enough power to fire the plug at near peak/peak torque cylinder pressures. Maybe that dip in VE in that region is making it worse with a lean A/F.

Is the new ignition module a OE GM one or aftermarket?

I have seen where the bias voltage on the pickup coil input is higher than a factory one and it causes the module to turn the coil on late and off early weakening spark. (decreasing dwell)

Just a thought

[Shopboss]

I worked for Chevrolet during those few years. The 350's had this issue of constantly breaking down the wires. I truely believe it was from excessive load from the shear weight of the vehicle and torque converter clutch, of which the torque converter clutch never released when it should have. You had to really get on the gas for it to release, and if not it would load the engine hard. Those vettes shifter fast and to soon loading the engine pretty hard. They also where set up to run on the lean side for emissions and that wasn't good for the plugs or wires either. It's been a while, but I believe at the time the wires where grey in color and you could see carbon tracks on them, like a grey powder area or traces of grey powder.

Hopefully your not mis-diagnosing an engine miss for torque converter clutch shutter.

X2

I was working as shop-foreman at a Chevy dealership in the 80s. Seams like we had several of these "skips" come in each week. The converter clutch would hunt in and out and feel just like a skip.
Get it up to speed and under load so you feel it and then just touch the brake enough to cut the converter out. If it goes away then you've found the problem.

As pointed out it could also be a cracked plug. Try this, get a spray bottle full of water. With the engine at idle, mist around the plugs and plug wires. If it skips or you hear a snap then figure out which wire or plug is breaking down. If you do this at night or in a dark place you should see a spark.

Pull the plugs and check for a crack. Twist the porcelain between your fingers and the dirt and oil from your skin will imbed in the crack and make it show up.

When we have an engine on the Dyno that has a skip under load, 90% of the time replacing the plugs fixes the problem. Even a plug that looks ok and not fouled can cause this. In fact last week we chased a miss between 5000 & 6000.
We checked the plugs and they looked great. Since I didn't have another set like them we tried other things. After wasting a couple of hours we finally made a trip to the parts store for a new set of plugs the miss went away.

If you need a good cheep spark plug tester use a good old HEI dis. Hook it up with the cap and coil off and to the side so you can fire the plug from the center coil terminal. With the power on, hold a soldering gun, " not an iron", close to the pickup coil, the part with the green and white wire. Pull the trigger and the plug will fire ever time the field changes in the coil of the gun usually 60 times a second.

Keep in mind that a plug can break down under pressure but fire perfect in free air. If you want to take things a step further, rig up a pressure chamber with a sight glass so you can use regulated air pressure to simulate cylinder pressure. An old head and some thick plexiglass works great. Just be careful not to blow things up in your face. It's interesting to watch the spark diminish as the pressure increases. This will also show you why a modern HEI system workers so much better than an old points system.

Donny

X3, The water mist trick works even better if you mix up some salt water. If the running ignition system does not arch-over with salt water mist, then I would pretty much rule out a secondary igniton problem. also when doing the salt water mist test have someone load it up against the converter a little. spray it over the distributor also.

Also if you throw on some suntan lotion it helps. Don't do squat for the spark but the smell puts you in a better mood. Try some Beach Boy's tunes as well

Donny

[MadBill]

An injector balance test would also be prudent.

[headshop57]

Whatt I use for diagnose a spark miss fire, is a timing lite with a RPM counter I hook avery cable and check at rpm that start the problem, and note the RPM count on every cylinder, the one missing will have lower count, since rpm counter counts impulse every time spark plug fires, if there is no spark there is no impulse and there is no count, it could be the spark plug, the wire, distribuitor and on a Corvette I'll look for a bad ground.

[MadBill]

But it can't know where the energy goes, so jumping to ground other than across the gap would surely still register?

[headshop57]

IF ground is not found, there is no discharge, a misfire will get register, if is a random problem with distribuitor, there is no energy going thru wire, a misfire will get register, if ground is found before point of signal pickup, a misfire will get register.If is a Corvette I definitely look for bad ground or not enough ground, I'll check transmission shift cable geting hot or some other way that the energy is finding ground, ground has always been a problem with vettes.

[Shopboss]

Any one mention a leaking valve yet. Even a loose or tight guide can cause this.

Donny

[Schurkey]

You should run it on a scope and check for bad wires, available voltage etc.

^^^ Winner.

That would be the first thing I'd do if the vehicle were in MY garage.

[c4mo]

The engine is fresh, just had it out to put new cam in, when it was out new valve guides and a valve job was done to the heads. Also i had this same exact problem rpm/map area before i had the engine out. Therefore i made sure to double check all grounds on the harness when the engine was out. The pickup is a bwd brand, i replaced it before the engine was out, and it made no change. I have a otc injector tester that measures the pulses per second, ive used it on the car and they seemed to be all equal, the injectors also all ohm the same. Also looking at the datalog, no where am i losing voltage to the fuel pump, at the least it is 13.9 volts at any given map or rpm. Next big question where to find a scope, my father was a mechanic and had one, which i still have, but it will not turn on, so i understand how they work, but do not have one per say. Would most mechanics have one, or where could i get one?

[BrazilianZ28Camaro]

What timing you set at the 3500-3800 rpm?

What fuel? What about the piston to head clearance?

Are you sure your engine isn't detonating at pk torque rpm?

[c4mo]

Timing in that area is highlighted in the photo. Marathon 93 octane. Quench is .039. 0 deck, .039 gasket. It shows some detonation, at 4000-4400 rpm in the 40-90 map kpa area, specifically i plan on reducing that in the calibration a little to see if that helps. However i want to try one thing at a time, that is a good point however.

timing2.jpg

timing.jpg

[MadBill]

...I have a otc injector tester that measures the pulses per second, ive used it on the car and they seemed to be all equal, the injectors also all ohm the same...

The injector test I referred to is the the one that pulses each injector once or several times with the engine shut off and you compare the resulting residual fuel pressure after each. If for example, the pressure drops by 15 lb. after each of five are pulsed but only 13, 12 & 10 for the last three, they are partially clogged and could cause lean misfires especially at higher but still closed-loop MAP values.