Intermittent miss at light load.

[534BC]

Originally Posted by BlazerLT

Cpi Injector Leaking!!!

Can you explain how a fuel leak will cause a misfire at very light throttle and from 5-30 minutes of driving, yet be ok at any other throttle position and time?

[534BC]

Also I would like to know what arrangment the injectors are in and how they deliver fuel.

like is there 6 of them and are they at each intake port? Do they continuosly spray fuel? just varyingthe amount? or do they turn on and off and if so are they timed? If so when do they spray, and for how long?

[achevy4power]

i also had a 95 blazer that did the same thing it seems like a lot of them get a slight misfire i had to change the whole fuel injection assembly it was like 3oo bucks

[534BC]

I repaired a leak under plenum a number of years ago on mine, as I recollect it had a plastic line with a tiny pin hole in it and would only show up if it sat for 30 minutes-3 hrs.

My symptom is misfires between 5 - 30 minutes at very light load. When it is acting up you can see spark jumping on many of the wires. I'm tempted to put on some better insulation wires, but I'm convinced it is only a symptom of my real problem.

[BlazerLT]

Originally Posted by 534BC

A lean mix, advanced timing, higher cylinder pressure can all cause a misfire. A rich mix is easy to fire, a lean mix is much harder to fire.

Wrong, a rich mixture is harder to fire seeing there isn't enough oxygen in the intake charge to fully combust the large amount of fuel. When an engine is running lean at idle you won't really see a whole lot of a difference but when it is rich it will be sitting there puking out black smoke trying to stay alive while running very poorly.

Running lean makes the mixture OVERLY combustable seeing it will cause PREIGNITION, pinging, and detonation seeing it will ignite too early because of the increased combustion temperatures.

Originally Posted by 534BC

Setting the cap/rotor relationship at a specific advance point and then advancing or retarding spark starts to pull the spark away from the particular post. Advancing it will cause the spark to lead the post, retarding it will cause it lag behind and jump backwards towards the post.

You REALLY need to go back an re-read what you thought you have read about the ignition system. No one advances the distributor cap and then advances the distributor so who ever told you that or taught you that is full of it.

You advance the distributor ONLY, not the cap and the distributor separately.

The jumped to and from the cap posts is a little ridiculous bud, and you really need to rethink and relearn what you know about the internal combustion engine.

Originally Posted by 534BC

In extreme cases such as low compression blower apps where the range of advance is very great from low vac high rpm and a lot of advance to high boost and low rpm and not much advance causes one to set the cap/rotor relationship to the hardest to fire spot (high cylinder pressure)

Again, completely wrong. The timing is retarded under blower conditions, not the cap and rotor relationship. In a computer controlled application, the timing is retarded on the fly by an external control where as if you have a carberated system, the timing is retarded to lower combustion temperatures, richen up the combustable mixture and lessen and preignition.

Again, you need to relearn about timing and what it means.

Originally Posted by 534BC

The problem shows up mostly on adjacent cylinders causing a cross-fire and back fire with small diameter distibutor caps. My problem never backfires, and I assume it is because there is never an adjacent cyclinder with mixture to fire, it just jumps out to a ground somewhere on the engine, another wire, or within the cap itself and simply shows up as one missed spark.

NO!!!!!! Get off your ass and get the upper plenum cover off and you will notice that the poppets fro the CPI injector are plugged or the fuel pressure regulator is leaking causing a lack of fuel to go to some cylinders.

If you think it is the cap, then replace the cap and rotor, but other than that on a 1995, you can't touch the timing so don't even start to mess with it.

I own this year and engine and I know it from stem to stern and the problem is always REALLY simple.

You need to realise that you are overcomplicating this situation and ignoring the obvious and for what reason I have no clue.

If the ignition system is new then MOVE ON TO SOMETHING ELSE.

It isn't some ridiculous hypothesis about spark jumping in and out of posts, it is about one of three things.

Air, fuel, or fire.

Now that you have made sure the fire is fine, MOVE ON TO FUEL! and stop sitting there babbling on about the ignition system.

[ericn1300]

after fixing the CPI i was still having a missfire on my 92 4.3. turned out to be a bad distributor bearing. i suggest tearing down the CPI and inspecting the spider assm. and checking the distributor at the same time since it will be easier to get to with the vortec plenum removed. here's a good post with lots of pics: http://www.automotiveforums.com/vbul...ghlight=spider

[534BC]

Thanks, I may just do that. Was it perhaps the top bushing/bearing in the distributor housing? I broke my housing the last time from overtightening the screws and may even replace it. That is also probably what the shop was referring to when he said "worn out dist gear"

[BlazerLT]

Originally Posted by 534BC

Thanks, I may just do that. Was it perhaps the top bushing/bearing in the distributor housing? I broke my housing the last time from overtightening the screws and may even replace it. That is also probably what the shop was referring to when he said "worn out dist gear"

No, the gear is on the bottom.

You don't have aproblem with your distributor, you have a problem with either the cap not being installed properly or the fuel system.

[534BC]

A worn gear can cause end thrust in the shaft and some backlash throwing that rotor/post relationship off. I don't think the cap can be on wrong (other than me overtightening screws)

[BlazerLT]

And what if you breaking the screw points is not allowing the cap to seat properly and therefore is allowing moisture into the cap and causing the misfiring.

This is why after 5-10 minutes it stops as the heat from the engine drives off the excess moisture.

A worn gear would cause a misfire all the time. Not just 5-20 minutes after starting the truck.

[534BC]

My cap WAS bolted down as good as a new, but I will check it to make sure is still tight. A good idea about cap and moisture, that is helpful. I've ran enough water over all secondary ignition components to make an off-roader jealous and just a short drive clears up any moisture missing. I attempted to duplicate problem like that, but could not.

My problem is completely undetectable until the tork convertor locks up. That is how I assume it is going to closed loop and the very second it does is when it starts missing (as long as the vac is between maybe 10-20")

It gradually gets better until about 30 minute or so and does not nessecarily have to be driven the whole time, but after the time expires the problem cannot be detected.

I do have a theory about the leak in the intake, but will get set-up with a pc to verify all values to be sure. I kinda want to watch stuff oncreen while it is acting up.

[BlazerLT]

Originally Posted by 534BC

My cap WAS bolted down as good as a new, but I will check it to make sure is still tight. A good idea about cap and moisture, that is helpful. I've ran enough water over all secondary ignition components to make an off-roader jealous and just a short drive clears up any moisture missing. I attempted to duplicate problem like that, but could not.

My problem is completely undetectable until the tork convertor locks up. That is how I assume it is going to closed loop and the very second it does is when it starts missing (as long as the vac is between maybe 10-20")

It gradually gets better until about 30 minute or so and does not nessecarily have to be driven the whole time, but after the time expires the problem cannot be detected.

I do have a theory about the leak in the intake, but will get set-up with a pc to verify all values to be sure. I kinda want to watch stuff oncreen while it is acting up.

I would take this to the dealer and get them to attach a Tech 2 to it and take it for a drive, they will be able to pin-point the exact problem.

Don't just throw random parts and hypothesis at it, get some actual testing done.

Still, have you chanecked your CPI injector yet?

[BlazerLT]

Originally Posted by 534BC

My cap WAS bolted down as good as a new, but I will check it to make sure is still tight. A good idea about cap and moisture, that is helpful. I've ran enough water over all secondary ignition components to make an off-roader jealous and just a short drive clears up any moisture missing. I attempted to duplicate problem like that, but could not.

My problem is completely undetectable until the tork convertor locks up. That is how I assume it is going to closed loop and the very second it does is when it starts missing (as long as the vac is between maybe 10-20")

It gradually gets better until about 30 minute or so and does not nessecarily have to be driven the whole time, but after the time expires the problem cannot be detected.

I do have a theory about the leak in the intake, but will get set-up with a pc to verify all values to be sure. I kinda want to watch stuff oncreen while it is acting up.

I would take this to the dealer and get them to attach a Tech 2 to it and take it for a drive, they will be able to pin-point the exact problem.

Don't just throw random parts and hypothesis at it, get some actual testing done.

Still, have you checked your CPI injector yet?

[ericn1300]

Originally Posted by BlazerLT

A worn gear would cause a misfire all the time. Not just 5-20 minutes after starting the truck.

Actualy my distributor had a bad upper shaft bearing that caused a cold start misfire that would go away after a few mins until it got worse. The engine was warm and the mechanic miss diagnosid and replaced the CPI spider and fuel lines and test drove it. Almost $900 and it did the exact same thing the next day when it got cold again.

[BlazerLT]

Hope you didn't pay for it.

You don't have to pay for them doing a trial and error shot on your truck.

If you paid, you got screwed.