that little poof and plop stumble on startup - Jeep Cherokee Forum
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that little poof and plop stumble on startup

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Old 05-16-2018, 07:55 AM   #1  
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Default that little poof and plop stumble on startup

So as you regulars know, I replaced my 200k injectors with some $45 Chinese 4 holes from fleabay. Initial results were great..overhead mileage indicator went up 2 tenths of a mile so far..but on startup it ran rough for about 30 seconds and was getting worse. Tried resetting the pcm, sama ting. It straightens out fine after about 30 seconds, though.
So this morning I pulled the distributor cap on a hunch. Cap still fine, rotor...on the copper end, not the button end, it's all black. Thought maybe, this must be paint from the manufacturer, no, I can see copper where the spark was hitting..no wait...it's carbon, ..and the spark is hitting half way down and on the top side...A bit of cleaning with the tip of a screwdriver reveals mushy carbon buildup. It's only..wait...4 years old? So cleaned it all up to shiny copper and popped it back in.
Starts right up smooth. No stumble. Easy to hear each cylinder going off. Nice. We shall see.
I have posted supposed fixes before that have turned out not so good.
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Old 05-16-2018, 09:00 AM   #2  
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If the electrodes in the cap are aluminum, buy a premium cap with copper electrodes. The aluminum wears out quickly and makes a bigger gap which causes loss of spark voltage at the plugs.
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Old 05-16-2018, 10:10 AM   #3  
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If the electrodes in the cap are aluminum, buy a premium cap with copper electrodes. The aluminum wears out quickly and makes a bigger gap which causes loss of spark voltage at the plugs.
Which causes more chance of the spark jumping off erratically, even under the cap.. Had one but I cheaped out.
Kind of makes for a good argument for distibuterless ignition, does it not? Which they started in what...99?
Makes me go back to the days of the Accel electronic replacement for my buddy's 67 lemans with the 389 hi compression. Oh the Edelbrock high rise, small Holley 2bbl, Hooker headers and Thrush sidepipes were cool too I guess. But I digress.

I wonder if there is an aftermarket Accel for the 4.0...
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Old 05-16-2018, 08:42 PM   #4  
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Yeah, I had a Mallory unit on my 392 Hemi and I got hit with a stray spark which just about knocked me on my butt! The GM HEI distributor was an excellent unit also, BUT that's where I learned about aluminum contacts! That one would drill holes thru the center contact of the rotor and ground out on the advance plate.


Older engines only needed around 30k to 40k volts to fire cleanly. High compression engines need around 60k to fire cleanly and that puts a strain on distributor caps and rotors. Most people don't realize there is a spark generated between the rotor and cap and aluminum melts at a lower temp than brass or copper. That "carbon" is transferred metal particles and oxides that are conductive.

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Old 05-17-2018, 04:53 AM   #5  
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I guess since my 98 will never be a 'coil on plug' I'm stuck with a dizzy, might as well make the best of it. Those copper caps n rotors are not cheap! Was 99 when they started something different? I have heard of coil packs, how does this work?
As an electrician, I do know that actually physically connecting copper to AL can cause problems unless you use a connector that is specifically rated for such. AL wiring in houses was permitted during the Vietnam era to save on copper as it was needed for ..well, bullets.
Then they found out the AL was too soft for small wires and would just squish under screws on receptacles, which loosened the connection and started many a fire. If you look at a standard type receptacle, it says "CU only"

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Old 05-17-2018, 06:35 AM   #6  
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As an electrician, you are aware that a transformer on DC just builds a magnetic field, correct? When the supply power is removed the magnetic field breaks down inducing a current in the windings. This is the principle of how a spark coil works. This is more for the other listeners that you, BTW. "Dwell time", the time the coil is energized, is important to output voltage. That's why a high rpm's, spark voltage drops.

Having said that, the coil pack on a 4.0 (99-04) contains 3 coils that fire 6 plugs, 2 at a time. (waste spark on exhaust) They are all energized all the time and are grounded thru drivers in the PCM. When called upon to fire, the PCM opens the ground on that coil and fires that pair of plugs. The plug pairs are internally wired inside the coil pack. Which pair fire is determined by the cam sensor and timing is computed by the crank sensor, MAP sensor, and TPS sensor inputs to the PCM. The PCM also takes into account vehicle speed and other data to determine spark timing. Fuel mixture is controlled by injector "on" time.

It would be tough to figure out a way to get the spark advance right mechanically rather than using a computer.

About that aluminum wire. The stupidest use of aluminum wire I ever saw was in the handlebar harness in several years of Harley Davidson motorcycles. With constant flexing, they cracked causing the kill switch to shut off the engine or the starter button to not work. D'oh!

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Old 05-17-2018, 08:05 AM   #7  
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As an electrician, you are aware that a transformer on DC just builds a magnetic field, correct? When the supply power is removed the magnetic field breaks down inducing a current in the windings. This is the principle of how a spark coil works. This is more for the other listeners that you, BTW. "Dwell time", the time the coil is energized, is important to output voltage. That's why a high rpm's, spark voltage drops.

Having said that, the coil pack on a 4.0 (99-04) contains 3 coils that fire 6 plugs, 2 at a time. (waste spark on exhaust) They are all energized all the time and are grounded thru drivers in the PCM. When called upon to fire, the PCM opens the ground on that coil and fires that pair of plugs. The plug pairs are internally wired inside the coil pack. Which pair fire is determined by the cam sensor and timing is computed by the crank sensor, MAP sensor, and TPS sensor inputs to the PCM. The PCM also takes into account vehicle speed and other data to determine spark timing. Fuel mixture is controlled by injector "on" time.

It would be tough to figure out a way to get the spark advance right mechanically rather than using a computer.

About that aluminum wire. The stupidest use of aluminum wire I ever saw was in the handlebar harness in several years of Harley Davidson motorcycles. With constant flexing, they cracked causing the kill switch to shut off the engine or the starter button to not work. D'oh!
LOL handlebar harness?! DOH!

Well the electrician gig is actually not my only background, altho I been licensed for 23 years now. ..I have an AAS and a BS in electrical engineering. Your analysis is basically correct. Of course the 'coil' in a car is a "step up" 2 winding iron core auto transformer..
when the field collapses from the primary the flux increases exponentially with the voltage across it, that flux cuts across the secondary which has "more" windings than the primary (many more for a car coil) thus amplifying the voltage to thousands of volts needed for the spark plugs. Pretty close altho hardly complete explanation. Lol
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Old 05-17-2018, 12:12 PM   #8  
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Okay Dude! I was trying to keep it simple for the laymen here. In the standard breaker-point ignition system, the condenser is there to absorb the reverse flux from the collapsing primary. IDK how they do it for the digital drivers in the PCM. All I know is drivers are like electronic relays using digital level signals to control large amperage loads. In my digital electronics class, the instructor ganged several drivers together to actually light a light bulb for a few seconds before the whole thing smoked!

I'm a toolmaker with a little millwright experience handling 480 3-phase and hydraulic machine repair. 480 is some nasty s*** if you're not careful, then again you're dead if you're not careful! I know about motor starters and control circuit transformers. My buddy was an electrician for the company and printed out enough pages from the Standards Manual so I could wire my swimming pool myself and pass the Underwriters Inspection.

My dad used to call a BS degree a B*** S*** degree! When he worked for Pratt&Whitney, he had more experience with the Wasp engine than any of the engineers. In 1946, they were trying to improve the performance and reliability of the radial engine. He already knew how far you could push it before to came apart!

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Old 05-17-2018, 12:45 PM   #9  
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Okay Dude! I was trying to keep it simple for the laymen here. In the standard breaker-point ignition system, the condenser is there to absorb the reverse flux from the collapsing primary. IDK how they do it for the digital drivers in the PCM. All I know is drivers are like electronic relays using digital level signals to control large amperage loads. In my digital electronics class, the instructor ganged several drivers together to actually light a light bulb for a few seconds before the whole thing smoked!

I'm a toolmaker with a little millwright experience handling 480 3-phase and hydraulic machine repair. 480 is some nasty s*** if you're not careful, then again you're dead if you're not careful! I know about motor starters and control circuit transformers. My buddy was an electrician for the company and printed out enough pages from the Standards Manual so I could wire my swimming pool myself and pass the Underwriters Inspection.

My dad used to call a BS degree a B*** S*** degree! When he worked for Pratt&Whitney, he had more experience with the Wasp engine than any of the engineers. In 1946, they were trying to improve the performance and reliability of the radial engine. He already knew how far you could push it before to came apart!
Two things I've found in my career, everybody knows how to wire a house, and everybody is smarter than the engineer.
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Old 05-17-2018, 03:02 PM   #10  
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Well it may be everyone knows how to wire at house, but is it safe enough to not catch fire someday? I doubt it. There's a couple of outlets in this house that I had to replace because the previous owner used the ones where you just push the wires into the back. My wife was complaining about a floor lamp that was flickering so I went to pull the plug out and the outlet was hot! I turned off the power and when I took the coverplate off and loosened the mounting screws, it just about fell out of the wall! This was in an addition he had put on and every outlet in that room was the same. I think they were Eagle brand, if you know what I mean. I worked for Pass&Seymour for awhile so I got some of ours. Ours had the push-in, but you had to tighten the screws!

The previous owner was a lighting engineer for Niagara Mohawk! Not an electrician, obviously. The scary part is the house was built in 1925 and in 1963 he rewired the whole house! 100 amp service on glass fuses. I had it redone in 1995 by a qualified electrician with all breakers.

This has gotten WAY off topic, hasn't it?

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Old 05-17-2018, 05:09 PM   #11  
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LMAO yes it has. Dave...you're good stuff.
I have seen way too many Eagle drugstore brand receptacles "backwired". This is one of the great mysteries of my profession, it is allowed by the manufacturer to do it that way, yet it is inherently dangerous as heck. I think it has to do with the fact that initially they made them that way, then the parts lobbiests in congress kept it that way$$.
NEVER backwire a receptacle! In addition, wiring "through" the device is a NEC violation for the grounds, at least, and I think for the hot and neutral. So the right way is to pigtail a 6" piece of wire in to the other 2 wires, twist all 3, cut the end square, then put a nice tan colored wire nut on it and twist it tight. THEN run just the pigtail to the side screw and check it tight with an old fashioned screwdriver. Screwguns are great tho for taking them out and putting them in.
So I recommend redoing those with P&S which is the brand I use.
We could move this to the MikeHolt.com [email protected]!

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