"I didn't say it would!"

You obviously implied there was a connection when you offered it as a possible solution to my problem.

"But I do know that a "screwdriver" is not going to tell you anything".

It is said that a wise man knows what he does and does not know. Electrons haven't changed much since the 60's. If they will consistently and agressively jump a 1/4" gap on cylinders 2, 3, 7 and 8, and do it only weakly and intermittently on 1, 4, 5 and 6, I think that tells me something.

***

Brian -

I have to rule out wires, cap, and rotor since I just replaced them all today, and the same cylinders were showing a weak spark both before and after they were replaced.

Good info on the possibility that the CPS is acting up, and isn't just a good/no good item. I'm going to replace it on Monday; what the hell, so I'll have a spare.

Lastly - the dizzy is in the hizzy. Turns out it doesn't have the prongs/tabs/vanes I was expecting; in fact, I can't figure out how it distributes the spark at all.

Here's a picture of it - can you or anyone tell me how it does its job? Thanks.

 

OK, your right. Like I said, Hope you get it figured out.

 

I fail to see a blown plenum will crack the heads. The heads crack by themselves and usually doesn't hurt anything (on these engines).

The cam sensor doesn't dictate timing, the crank sensor does. Combined with the cam sensor it will tell the PCM which stroke the #1 cylinder is on and can get the job done from there.

 

Not the blown plenum itself, the clogged cat that is the result of the blown plenum will.

 

The cat clogs, the combustion chambers overheat, the heads crack. Usually on the #8, sometimes also #7, and sometimes also #6 cylinders.

 

Distributor is back in, and SG 23900 in-line spark tester confirms findings of Mark 1 Eyeball and #2 phillips screwdriver. Weak and strong sparks on same cylinders.

Suspecting bad ECU, or bad ground somewhere. Still will probably put in a new CPS, but couldn't tell you why.

Has anyone had this type of intermittent spark problem? It's most pronounced at idle, but is present throughout all rpm's. Thanks.

 

Trouble is, the ground path is the same, regardless of which cylinder is firing. So, a bad ground/pcm would have an all or nothing type effect.(maybe) Replacing the crank sensor isn't going to make any difference at all.... all it does is tell the pcm when to fire the coil. You could check the connector at the PCM, (black wire with gray trace, cavity 19) and make sure it is clean.... aside from that..... since you replaced everything else, and the symptoms did not change..... wonder if the transistor in the pcm is getting tired? But that seems kinda odd too.....

Grabbing a PCM from the junkyard to test with would be interesting....

 

If you say so. I'm betting there are plenty of magnums with cracked heads that have never had clogged cats. IMO it's simply a problem of poor castings and heat cycling over time.




Just because the cap, rotor and wires are new doesn't mean they can't be bad. Sounds like the only explanation to me, as others have said, the crank and cam sensors have nothing to do with spark strength, only when the coil fires. Have you replaced the coil?

 

"Just because the cap, rotor and wires are new doesn't mean they can't be bad. Sounds like the only explanation to me, as others have said, the crank and cam sensors have nothing to do with spark strength, only when the coil fires. Have you replaced the coil?"

My original post:

Hi all -

OK, here's the deal: '94 B2500 318.

I just replaced:

Cap, rotor, wires, coil.
Wires are routed according to TSB 18-48-98.
Coil was definitely weak, and engine will at least run now, but as I suspected, I have another problem.

Both before and after coil, cap, wire and rotor replacement, I have a weak and intermittent spark on several cylinders ....

****

There is *******ly [why can't I say v-i-r-t-u-a-l-l-y??] no chance it's the wires. A wire switched from a good cylinder to a bad cylinder = still a bad cylinder. A wire switched from a bad cylinder to a good cylinder = still a good cylinder. Replacing the cap, wires, and rotor didn't change a thing as far as which cylinders were missing.

I still suspect the ECU/CPM, and need to understand more about how the ignition system decides when to fire each cylinder. Can anyone tell me exactly how the ignition system works?

Does to CKPS read only TDC, or is there a pickup that it reads for each cylinder that it reads, each 45 degrees apart? If it only reads TDC, how does the computer know when to fire each cylinder?

And I'm still not clear on how the CPS interacts with the computer. It seems to just send a signal for about 180 degrees of a distributor revolution, which I don't understand.

 

The cranks sensor has 8 falling edges, it is mainly responsible for ignition. But it can't do it alone, it can tell when each cylinder is at TDC but it can't tell if it's the exhaust stroke or compression stroke, so it needs the cam sensor to determine.

FWIW, here's a scope of the stock Crank and cam sensor. Yellow is crank sensor, cam is blue. You can see there are 8 crank signals for every cam signal. This is how the computer knows which stroke the cylinders are on.