Intermittent misfire

[Antz81]

I've just started to get an Intermittent misfire on my 81.
It only seems to happen once the engine has warmed up. It happens while cruising and under acceleration, not sure about idle. While cruising (at 65mph) the mis can be felt and if you watch the tach it can drop anywhere from 50 to 500rpm before recovering. Sometimes it will go for a few miles with no issues.

I plan to check the distributor bearings and I'll try a new ignition module. Anything else i should check?

 

[toobroketoretire]

I've just started to get an Intermittent misfire on my 81.
It only seems to happen once the engine has warmed up. It happens while cruising and under acceleration, not sure about idle. While cruising (at 65mph) the miss can be felt and if you watch the tach it can drop anywhere from 50 to 500rpm before recovering. Sometimes it will go for a few miles with no issues.

I plan to check the distributor bearings and I'll try a new ignition module. Anything else I should check?

I had a similar problem several years ago and finally found it was my torque converter clutch unlocking and re-locking very rapidly. Under a heavy throttle it did it so rapidly the whole car shuddered. Try putting it under the heaviest load you can in high gear from say 60 mph to 90 mph and see if it gets worse. And if your plug gaps have reached .055" it's very possible one of them just isn't firing (I had that happen before) because .055" is about the limit of what an HEI can fire. Without the side pressure from breaker points HEI bushings last virtually forever.

If you have an automatic and it really is misfiring you should be able to make it happen by just "power braking it" on your driveway. Pull the plugs and look for one that is darker than the others. I have had carbon fiber core wires burn out a 1" section inside so while your plugs are out check your wire's ohms resistance and look for an "open".

If you have an IR gun see if one exhaust port is running a lot cooler than the others. That's how I found my #2 cylinder wasn't firing. if you don't have an IR gun smear a little grease on each port and look for one that isn't melting the grease. That would indicate a misfiring cylinder.

Take one spark plug wire off and stick a 1/4" X 3" bolt in it. See how far it'll throw a spark. A good coil will throw a 1-1/4" spark so if yours only throws a 3/8" spark you may have a very weak coil or failing module. I only buy Standard Blue Streak modules after having two Chinese modules fail with very few miles on them.

Let us know what you find.

 

[Antz81]

I had a similar problem several years ago and finally found it was my torque converter clutch unlocking and re-locking very rapidly. Under a heavy throttle it did it so rapidly the whole car shuddered. Try putting it under the heaviest load you can in high gear from say 60 mph to 90 mph and see if it gets worse. And if your plug gaps have reached .055" it's very possible one of them just isn't firing (I had that happen before) because .055" is about the limit of what an HEI can fire. Without the side pressure from breaker points HEI bushings last virtually forever.

My car is the 4 speed manual, and plugs are new and were gapped to 0.045".

I have had carbon fiber core wires burn out a 1" section inside so while your plugs are out check your wire's ohms resistance and look for an "open".

Leads were replace only a few hundred miles ago, checked them anyway. all read between 5k and 11.5k ohm depending on length so no issues there.

If you have an IR gun see if one exhaust port is running a lot cooler than the others.

All run at the same temp (or close enough). Not that I expected any different since it will run for a few miles without an issue.

Rotor, coil and cap have all also been replaced in the last 2 years. I've only done a couple of thousand miles since then.

 

[dougelam]

Are you able to monitor your closed loop ststem during these events?

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[Antz81]

Are you able to monitor your closed loop ststem during these events?

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I don't have a tech1 or anything like that if that is what you're asking.
One of my meters (not a fluke) does have quite a quick response time though. It has been able to help me diagnose problems because of this that my fluke could not.

 

[dougelam]

Just wondering if your experiencing a lean miss fire condition

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[Antz81]

Just wondering if your experiencing a lean miss fire condition

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I suppose it is possible. The meter I refered to has bar graph on it as well as the display. If set to DC but connected to AC the AC can be seen on the bar graph. Not sure what would happen when set to record.

 

[dougelam]

I believe you can check your system status while driving. When the diagnostic terminal of the test connector (ALDL) is grounded, jumper A to B, with the engine running, the computer module will enter the Field Service Mode. If the engine is running in Open Loop Mode, the CHECK ENGINE (MIL) light will flash quickly, about 2 1 / 2 times per second. When the engine is in Closed Loop Mode, the MIL will flash only about once per second. If the light stays OFF most of the time in Close Loop, the engine is running lean. If the light is ON most of the time, the engine is running rich.

QUICK AND EASY ☺


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[Antz81]

I believe you can check your system status while driving.

I'll try give this a go on Thursday or this weekend depending on the weather.
I'll still order the ignition module. If it's not the problem then I'll keep it as a spare.

 

[toobroketoretire]

I'll try give this a go on Thursday or this weekend depending on the weather.
I'll still order the ignition module. If it's not the problem then I'll keep it as a spare.

I had a Chinese module begin failing a year ago and it started by intermittent misfiring of cylinders and progressed to the point where the engine died. After cooling off for 10 minutes it would run again but then die again. It was the failure of ignition modules that prompted me to run a 1/2" hose from my evaporator to my HEI cap to provide a steady flow of air thru the cap. I am also using RTV under the module because I found RTV transfers heat very well.

 

[toobroketoretire]

An intermittent misfire wouldn't show up on the tachometer unless several cylinders began misfiring at the same time. Could be a failing module or it could be a fuel pump giving up and lately I have been hearing about a LOT of failing fuel pumps. Sadly none of the C3's came with pressure taps in the pump-to-carb fuel line so you can check the fuel delivery under actual running conditions. Several years ago I was having an intermittent low power problem and after installing a pressure tap in my fuel line I found my NEW fuel pump was only delivering 2 psi at 3000 rpm and 0 psi at 4000 rpm. Rather than risking getting another weak pump I went the electric fuel pump route.

When I tested my fuel pump at idle it always showed 5 psi and it showed 5 psi when cruising at 60 mph. But under a full throttle it fell on it's face. The best way to test the output is to tape a pressure gauge to the outside of your windshield then see what it shows under full power on several full power runs. One run may show normal but another run could show almost no output if the valves in the pump aren't sealing.

 

[dougelam]

I had a Chinese module begin failing a year ago and it started by intermittent misfiring of cylinders and progressed to the point where the engine died. After cooling off for 10 minutes it would run again but then die again. It was the failure of ignition modules that prompted me to run a 1/2" hose from my evaporator to my HEI cap to provide a steady flow of air thru the cap. I am also using RTV under the module because I found RTV transfers heat very well.

Ant,
PLEASE
PLEASE
PLEASE

DON'T USE RTV
The dielectric grease IS NOT for heat transfer!

DON'T USE AFTERMARKET MODULE
They only cause problems, like toobroke found out

DON'T PUT HOLES IN YOUR DISTRIBUTIOR CAP
Your asking for trouble when moisture gets in

If you can't get one local post the online Cost and I'll compare it to what I can get one for and mail it to you



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[toobroketoretire]

The HEI modules get VERY hot in use and need to transfer their heat to the HEI housing via the white stuff. But I am experimenting with Permatex Ultra Black RTV because it transfers heat very well.

I used to use AC Delco modules all the time until they began failing rather early so I am now using Standard modules. Always avoid CHEAP modules because they are made in China using inferior components.

I have been running a vented HEI cap to help prevent module failures and so far it appears to be helping. No different than using fans in an electrical cabinet to cool transformers and such. As my blower is on HIGH almost all of the time when my A/C is on there is a significant amount of cool air that circulates thru my HEI cap.

 

[dougelam]

The HEI modules get VERY hot in use and need to transfer their heat to the HEI housing via the white stuff.

If that was the case why not bolt it on directly without anything?
That would give you 100% heat transfer

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[Antz81]

If that was the case why not bolt it on directly without anything?
That would give you 100% heat transfer

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Actually if they really wanted heat transfer there would be thermal paste. And certainly not RTV.

I'll let you know later on about the module. Just need to call the local guy once he opens.

 

[dougelam]

Actually if they really wanted heat transfer there would be thermal paste. And certainly not RTV.

I'll let you know later on about the module. Just need to call the local guy once he opens.

Correct, the grease is to insulate it from the heat not to transfer it.
His module is running hot because he thinks that he needs to transfer it's heat to the distributor that is in fact a lot hotter, around 400° be his standards.

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[toobroketoretire]

Actually if they really wanted heat transfer there would be thermal paste. And certainly not RTV.

I'll let you know later on about the module. Just need to call the local guy once he opens.

As I remember RTV didn't appear until the mid 1970's and was intended for gasket sealing. I'm using it as an experiment and so far I haven't had any module failures. The HEI housing is the heat sink for the module to transfer it's heat into and the white "grease" is the medium used to transfer that heat. You can certainly use anything you want. As far as the cool air flowing thru the cap I can't see how it could harm anything as the air flows right over the top of the HOT module on it's way out................taking heat with it.

 

[Antz81]

If you can't get one local post the online Cost and I'll compare it to what I can get one for and mail it to you

Ok. The local guy only has the Standard ones. (And is out of stock for the 81 anyway)
Online price for one is $66.11

 

[Antz81]

As I remember RTV didn't appear until the mid 1970's and was intended for gasket sealing. I'm using it as an experiment and so far I haven't had any module failures. The HEI housing is the heat sink for the module to transfer it's heat into and the white "grease" is the medium used to transfer that heat.

If it was intended to be a gasket sealant, why use it for something totally different. Especially when there are better products out there.
The is a big difference between the dielectric grease and themal paste. Both look the same. But thermal paste is used to promote heat transfer. It is used in high power electronics to to help transfer heat between components and the heatsink.

 

[Antz81]

It was the failure of ignition modules that prompted me to run a 1/2" hose from my evaporator to my HEI cap to provide a steady flow of air thru the cap.

How does this work in winter when you are using you heater? Or have you fitted it between the blower and the heater core?