First: If you're referring to COP, (Coil On Plug) ignition, the PCM does not have the capability to fire 6 separate coils individually, and sequentially with the firing order. Fuel injectors yes, but not ignition coils. EDI is cheaper to produce than COP for obvious reasons.

Second: As I mentioned above, "For all intents and purposes, rotating the distributor on EDI systems does not have a direct affect on ignition timing." In actuality, the air gap between the rotor segment and the cap terminal increases proportionately with the amount of ignition advance being applied by the PCM. Camshaft retard adjustment provides a "benchmark" for the PCM, if it is out of spec, ignition timing is off proportionately. For example: If camshaft retard is -10 degrees, (desired spec is zero) and the PCM commands +10 degrees, the net advance being applied to the engine is +20 degrees. In this scenario, the cap terminals are retarded 10 degrees and the coil is firing 10 degrees before the "benchmark". On the other side: If cam retard is +10 degrees and the PCM applies +10 degrees, the net being applied is zero degrees. The camshaft retard adjustment aligns the camshaft position sensor with the camshaft, (hence the name) and the distributor cap terminals inherently end up where they need to be.

 

Ok, well just to simplify the explanation, it's all about the blazer 2000 LT 4.3l vortec distributor.

Ok so from what I understand now, it's as I thought, the spark gap will be lengthen if the PCM advance or retard the ignition. The base parameters are set by rotating the distributor and the PCM then will use this as a base reference to adjust timing.

So yeah by lengthening the gap, the spark has to have more voltage thus less current for the same input power! So lengthening the gap will lead to weaker spark in the end, since the spark gap at the spark plug is always the same no matter what, so it's a loss at the distributor cap gap with the rotor.

So basically, this answers my first question, a fully electronics ignition system must be much more efficient, since the operation has no moving parts, the spark gap is only at the spark plug and the timing is controlled by the PCM and switched by mosfets at the coil input.

** I re-read your explanation about cam shaft timing, is the distributor rotor geared to the camshaft? Maybe that's where I fail to understand the entire system, I'm gonna dig the net for a diagram of the entire ignition system.

 

GM doesn't like referring to it like that, but yup, that's about it. They'd prefer to tell us "Ignition timing is not adjustable", and that rotating the distributor is for adjusting camshaft retard, which it does. What they neglect to say is that when camshaft retard is adjusted correctly, base ignition timing is as well, (in a round about kind of way).


Correct on all counts It's a more expensive way of doing it: 6 coils instead of one, more programming in the PCM, more wiring but, no ignition module.


You are correct. The distributor shaft has a gear that meshes with a gear on the rear end of the camshaft. Both gears have the same number of teeth so they rotate at the same speed. The camshaft gear at the front of the engine has twice as many teeth as the crankshaft gear so the camshaft rotates at half of crankshaft rpm. It takes two complete crankshaft revolutions and one camshaft revolution for each cylinder to complete all 4 strokes, (intake, compression, power, & exhaust). This link is for a DOHC, (Dual Over Head Camshaft) engine, but it's a good "visual".
Silverlight Four Stroke Engine Demo Page - Raj Kaimal

 

Reviving the old thread, I'm back to square one! (more like square two) .. The truck is acting up once again!

Someone told me, when I got distributor cap and rotor changed that if they weren't AC delco, I would have the same problem in a couple of months. Yup, I'm back!

I'm wondering if it's this part once again, but here are my observations :

- Weak orange spark on spark plug.
- Replaced 4 spark plugs (don't have the tools to go behind the steering column, is there some magic there? I've put the socketed box on the spark plug but I have no 3/4" wrench that fit there)
- Engine light flashes when running.The truck have almost no power, it revs very high if I want, but no power.
- Code P300 random misfire
- Smell of gas inside the truck and in the muffler (most probably misfire related)
- 2Kohm resistance on spark plug wiring
- 5Kohm resistance on spark plug itself (new ones also). So I think the old ones are still good, simply by deduction, multiple misfire, and I changed 4, still no change in performance and weak spark.


So to me it points to the distributor once again.
Is there a way I can clean up the contacts to make it work again, and then put some dielectric grease on the contact points? Would that revive the distributor cap!??

I thought about sanding the contacts..


**Edit:
I'm also thinking that I could have a clogged cat, it's a far shot, but the truck also smell strange, like burning up something, really not the same smell as fuel or a cooking engine, like it's cooking up grease or something..

Thanks

 

Provided the battery is fully charged, it depends where the orange spark is. If it's at the coil output, the coil is weak. If it's at the spark plug end of a plug wire, it's either the wire or the cap/rotor. If camshaft retard is off too far, it can cause an orange spark at the plug wire as well. It doesn't matter what quality of cap you have, incorrect camshaft retard can shorten the life of it

The stink could be oil or coolant burning off of something... cylinder head, exhaust manifold, exhaust "Y" pipe or catalytic converter. Engine misfires can cause an increase in temperature in each one of those.

 

I see, I will test the spark at the coil with a metal object.

How do you adjust camshaft retard? By rotating the distributor cap I guess?

Also, what would explain that after about 5 minutes the truck starts to run pretty normally? It looks like after the CEL flashes a couple of times, the truck starts to run fine.

Also, I'll have to double check, but 1 of the O2 sensors reported something like 30% adjustment, or was it the fuel trim #2 ? I will confirm that later, but something is definitely wrong, and it may not be the ignition system.

 

Camshaft retard is adjusted by rotating the distributor while viewing the streaming data on a capable scan tool. Entry level and "enhanced" scan tools are not capable. There are some software programs for laptops etc that can access it, other than that, it requires a "high end" scan tool. The $500 ones that the auto parts stores have can't do it.

The 5 minute time frame may have something to do with the PCM switching from open to closed loop operation.

Fuel trim indicates what percentage the PCM is adjusting injector pulse width to correct the fuel mixture. Negative fuel trim indicates a rich mixture and the PCM is trying to lean it out. The PCM relies entirely on accurate oxygen sensor data to provide accurate fuel trim data. Fuel trim, along with other data, can be used to determine if a misfire is being caused by insufficent spark, or excessive or insufficient fuel.

 

Thanks,

Ok so I tested spark directly on the coil, the spark is blueish with a distance of about 3 to 4mm, but bigger than that and it weakens to orange.

I plugged the cable the goes from the coil to the distributor cap, and at the end of that cable I tested spark. It's orange for the same distance (3-4mm).

To me, this sounds like a weak coil. But the resistance measure of the coil seemed good when I tested it before, and on the HV side it is currently about 5kohms.


** Btw, there's no misfires on idle and when I rev up the engine without moving. Everything is fine and it starts right up without a hint of a problem. It's really under load that there's a significant problem, and where I find the problem different, is that it's reving up easily even when limping along. I can rev it up to 5k if I wanted, it does stumble a lot tho, like if it wasn't in sync (or like misfiring) but it's acting differently than when I had to change the distributor cap before. So that may point to a misadjustment of the cam retard like you said and maybe a weak coil as well..

So I need to go to a GM dealership/garage to fix this issue using their computer if I understand correctly? Isn't it possible to adjust the camshaft retard approximately to at least pinpoint the problem? I mean, I can make a mark of where it's at and play with it until I see a difference..

 

Resistance tests alone on a coil, are not always conclussive. Like you said, it only acts up under a load. You can't simulate a load when checking resistance. 3mm to 4mm isn't much of a gap, a good coil will produce 40K+ volts, enough to jump 20mm or more. A weak coil could cause what you're describing. Really need to have camshaft retard checked/adjusted at a GM dealer or a shop with a capable scan tool & technician. Some shops might give you the "deer in the headlights look" when you mention camshaft retard. If they do, that's your clue to find a different shop. Camshaft retard can be off as much as ~27 degrees and not set a DTC in memory or turn the SES light on. Trying to adjust cam retard without a scan tool, would be like trying to set a clock by ear

 

Let's try a new coil then

Aftermarket would do, or is it a better choice to go OEM for that?