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1994 Mitsubishi 3000gt VR-4
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Discussion Starter #1
So, I think I know what the issue is but I kind of want to make sure.

So...last week I replaced Wires and plugs. (Along with other maintenance items.) The car drove fine the next few days until two days ago. I initially noticed it because the car sounded off and drove suspect.

Got home and immediately started pulling the wires from the coil pack one by one while the car was started. Every cylinder is getting spark save for cylinder 4.

Now I know these old cars have issues with coil packs and sometimes they need to be replaced. But I don't want to jump the gun.

So I know the coil pack produces the spark through the wires and if one of the coils isn't firing....typically means thats the issue.

So I guess my question is...am I correct? Is it the wire? Or should I just replace the coil pack and the PTU?
 

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93 NA ATX 3000gt DOHC
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...am I correct? Is it the wire? Or should I just replace the coil pack and the PTU?
I have to say maybe to your guess, reason being the coil that fires #4 cylinder also fires #1 cylinder. So if you had good spark on #1, I’d expect you to have good spark on #4. For that reason I wouldn’t suspect a problem with the PTU because the same transistor there fires the 1 & 4 coil. Which also makes it unlikely (but possible) that the coil would be the problem.

This leads to a question about what you mean when you say “Every cylinder is getting spark save for cylinder 4”, does that mean there is no physical spark jumping from coil socked to plug wire? Or does that mean there’s no change in idle speed or roughness when wire is pulled? With wire pulled do you see spark jump from coil socket to coil bracket?
 

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93 NA ATX 3000gt DOHC
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no physical spark jumping from coil socked to plug wire?
This to be exact. Sorry for the lack of clarification.
If you still have one of the old wires, plug it into #4 socket and hold plug end near some metal point on engine to see if get spark to jump there or put old spark plug into it to check for spark. If you have spark with old wire to ground, that might indicate new wire on #4 cylinder is bad not allowing a path to ground. If still no spark there may be a problem with the coil, you can buy a individual coil to replace without buying the entire coil pack.
 

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93 NA ATX 3000gt DOHC
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Interesting. Thank you I will try these two methods.
Also if you have a multi-meter, you could unplug both #1 & #4 wires from coil sockets and take ohm reading between the two sockets to verify coil secondary resistance. Don't have any power turned on.

Resistance should be between 11.3 to 15.3 KiloOhms (11300 to 15300 ohms) @ ~ 68 degrees, higher or lower temp might change it a little but not much.
 

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1991 Dodge Stealth ES 5sp
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I had some misfiring issues recently.

To identify if it is the coil pack: unplug each wire from the top of each coil one at a time and listen for the "spark" while the car is running and plug back in. If you hear it, the coil pack is fine.

To identify if it is the ignition wire: unplug each wire at the spark plug each time and listed for the spark as well (the ticking from the spark arching)

Also if you decide to check the multi-meter way to see if your wires are ok - the spec is typically per foot. So each ignition wire will have a different resistance.
 

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I had a similar issue with my previous VR4.

Did plugs and wires and shortly after noticed a stumble. I had upgraded to larger wires with decreased resistance just prior to the stumble.

Pulled the wires from top of coils and noticed only one wire wasn’t firing when both were plugged in. It’d only fire one cylinder at a time when the other wire was pulled off completely the firing cylinder would switch.

When both were plugged in it was the longest wire on that coil that wojldnt so assumed electricity follows the shortest path of least resistance and most of the voltage was running to the other wire. Sparks two wires with one firing.

Bought only new coils and rebuilt my coil pack and replaced the PTU with a brand new one. Fixed the issue. New coils had noticeable difference in resistance reading. Seemed the cars idle and low range was improved too. Bought a coil Hotwire kit to but this rebuild of coils and new PTU solved my issue.

Tested the resistance between the two coil poles on old coil and it was slightly higher then the spec range. Assumed the drop in wire resistance was enough to make the coil the weak point.

I cheated and hooked up a timing light to the different plug wires to detect any signal. That’s how I noticed only one firing when both were plugged in.
 

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1994 Mitsubishi 3000gt VR-4
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Discussion Starter #9
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Sorry for the delay guys. So I think I'm pretty much narrowed it down to one coil. In the picture below the second from the top is not firing. Even tested with a multimeter. Nilch.(excuse the messiness of the wires as I've been in trial an error trying to figure this out.)

Also one thing I did notice that was odd. Is when the car is running I'm able to see the light from the spark in the top one. When I remove it, it starts arcing towards the right. Again the second from the top has no spark.

Going to have to put in an order for a new coil pack assembly. Lol
 

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93 NA ATX 3000gt DOHC
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In the picture below the second from the top is not firing. Even tested with a multimeter.
That would be #4, you can confirm by number stamped on top edge of coil bracket. What ohm reading did you get from socket to socket? Not sure what Nilch means. Edit: never mind had to lookup what Nilch means (I’m old), no ohm reading would indicate open secondary winding most likely near #4 socket which obviously will still allow discharge out #1 side of coil. Interesting…. never thought about that possibility.

Also one thing I did notice that was odd. Is when the car is running I'm able to see the light from the spark in the top one. When I remove it, it starts arcing towards the right. Again the second from the top has no spark.
With engine running and wire removed it’s often normal for spark arc to travel toward bracket to the right. But does sound like you’re not getting anything out of #4 which should do same as #1 since it’s the same coil. Your choice whether to change entire coil pack or just that one coil.
 

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1994 Mitsubishi 3000gt VR-4
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Discussion Starter #11
Sorry about the delay. So the issue was as I initially stated. The coil packs and the ptu were the problem. Namely the coil for cylinder 4. Upon further inspection of the coil it was indeed caked up with corrosion. After swapping coil pack assembly and ptu misfiring is no more.
 
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