Bad Engine Miss

Brent13 · Jul 24, 2017

My 1974 has developed a really bad miss. It has happened two other times, I pulled over, and it cleared up. This third time, it would not clear up. Here is the story.

Started up the truck, no problems. Off I go, and about a mile down the road it begins to miss really bad, I pull over to hopefully clear it up. And in the past, I have stopped for a few minutes, it runs like crap for a few minutes, and then it clears up. I believe it is an electrical issue and here is why. I lose my tach signal. Truck is running, albeit very bad, but the tach needle is at zero. It bounces around a bit, 0 to maybe 200 rpm. Why am I losing the tach signal when this miss happens? Truck has a new MSD box, less than a year old. Could my MSD box be heading south??? Truck has Painless harness, installed same time as MSD box. Engine is 302 and a C4 transmission.

Is this enough info to diagnose as a strong case of electrical issue? I am all ears......

Thank You,
Brent

DJs74 · Jul 24, 2017

Brent13 said:
My 1974 has developed a really bad miss. It has happened two other times, I pulled over, and it cleared up. This third time, it would not clear up. Here is the story.

Started up the truck, no problems. Off I go, and about a mile down the road it begins to miss really bad, I pull over to hopefully clear it up. And in the past, I have stopped for a few minutes, it runs like crap for a few minutes, and then it clears up. I believe it is an electrical issue and here is why. I lose my tach signal. Truck is running, albeit very bad, but the tach needle is at zero. It bounces around a bit, 0 to maybe 200 rpm. Why am I losing the tach signal when this miss happens? Truck has a new MSD box, less than a year old. Could my MSD box be heading south??? Truck has Painless harness, installed same time as MSD box. Engine is 302 and a C4 transmission.

Is this enough info to diagnose as a strong case of electrical issue? I am all ears......

Thank You,
Brent

When this is happening, have you noticed what your voltage is doing? The MSD box needs a minimum amount of voltage to operate properly - if you have a voltage problem (something less than 12), it could be affecting your ignition box and related components like the tachometer.
Other than that, if it is an electrical issue elsewhere... it could be anywhere.

I'm sure others will chime in, there are some very good electronic gurus on here... I'm not one of them but I have experienced the low voltage problem in the past and the MSD box isn't happy without a minimal amount of voltage.

DJs74

gr8scott · Jul 24, 2017

Are you taking the tach signal off the MSD? Try disconnecting it and see what happens.
Also what DJs74 said about voltage.

Broncobowsher · Jul 24, 2017

Pull the plug wires one at a time and try and find what cylinder(s) are still firing and which are not.

Any other noises?

Quick junkyard compression test. Disable the ignition system and crank the engine. It should be an even load on the starter. r-r-r-r-r-r-r-r If the compression is real weak or gone in a cylinder there will be a moment where the starter speed picks up, often to the point where it sounds like the engine almost wants to start. More of an r-r-r-r-r-R-r-r- sound.

Bad Engine Miss

Brent13

Sr. Member

DJs74

Bronco Guru

gr8scott

Bronco Guru

Broncobowsher

Total hack

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