*** Resolved, 9/29/11. With 100% certainty, a failing CKPS (Crankshaft Position Sensor); probably not allowing coil enough time to build up enough current before firing a particular cylinder. ***

** (Also please note this is the very first thing zman suggested. How 'bout that guy, huh? **

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, determined by screwdriver-to-within-1/4"-of-ground method.

Cylinder:

1: Steady spark, but somewhat weak
3: Very strong and steady (actually somewhat lethal)
5: Weak and erratic
7: Weak and erratic
2: Very strong and steady
4: Weak and erratic
6: Weak and erratic
8: Very strong and steady

I have a guess, but I'd like to get some unbiased opinions on what might be going on.

NAPA closes in 50 minutes, so any timely opinions would be greatly appreciated!

Thanks, John D
Canton, CT

 

Crank or cam sensor going bad maybe. Or the plenum is shot and the cat is clogged. Try unscrewing the pre cat o2 to see if it runs any better.

 

Thanks zman, those were my two suspects - CPS and CKPS.

I'm very convinced that it's a spark/ignition problem as opposed to a clogged cat; the engine revs very nicely, with no hint of an exhaust restriction - and I can't think why bad 02 sensor readings would cause a misfire on only the same particular cylinders. Then again, what the hell do I know.

I think I'll just go get the two suspects and try them one by one.

Still open to suggestions from anyone. Thanks.

 

You still need to address the plenum and cat issue, before you get cracked heads(which you may already have). Revving the motor with no load on it , is not a test. And unscrew the o2 like I said.

 

How does a clogged cat cause weak spark?

 

Sensors won't cause weak spark, all they are doing is telling the PCM when to fire what. If you get consistent fat spark from the coil wire, then the problem is downstream from there. Test that first.

 

"How does a clogged cat cause weak spark?"

Thank you Nova.

"And unscrew the o2 like I said".

Having never had the honor to serve in uniform, I'm unaccustomed to taking orders. Just so you know.

Brian - Focused like a laser beam as usual. After I determine that the coil is putting out consistent sparks, I'm going to pull and examine the distributor. My NAPA owner said he thinks this year 318 has a metal drive gear instead of a plastic one, but it still might be bad. Maybe the teeth are worn enough to pass the spark along at the wrong time to certain cylinder, but not so worn that it loses time altogether. Also going to examine the reluctor vanes (or whatever the hell they're called); can't see how they'd get worn from an air gap, but I think that could theoretically cause this problem.

NAPA talked me out of throwing money at the CPS/CKPS since they consider them go/no go items. has anyone actually seen them go just slightly bad, or is it all-or-nothing with them?

Thanks, John D

 

they have so many different failure modes, it actually IS funny...... wet/dry/hot/cold/phase of the phriggin' moon..... But, since you are getting spark, I think the crank sensor is just fine. PCM just opens and closes a switch to ground/or not the coil, to fire it. Really simple circuit. Might have a look at PCM grounds.....

I really suspect either cap, or plug wires. The distributor drive also drives the oil pump, so there hadn't better be any plastic there......

If your weak sparks are continuously weak AT the plug, I would more suspect the wires...... or cap/rotor.

 

I didn't say it would! But I do know that a "screwdriver" is not going to tell you anything. This is not the 60's. Whatever, good luck, I hope you get it figured out.

 

The have the cool nifty neato spark checkers at the parts store. They are actually kinda useful.