How to test a coil on a 3.8

[bu-bye]

Hey ya guys, I have a bad coil but I'm not sure which one it is. I don't know how to test it for the life of me. I did read a post about a guy testing his 3.4 and something about a paper-clip but it did not make sense to me. Could someone please walk me through testing the coils. My misfires only come under load and when its hot. I do have a nice meter to read just about anything but don't know what to set it to and what numbers are normal. Please help. thanks

[ December 07, 2003: Message edited by: bu-bye ]</p>

[silvachris1]

If you're getting an SES light, take it to Autozone or somewhere and have it scanned, the code will tell you which cylinder the missfire is on. If it's a coil, it'll prolly show two cylinders missfiring at once. Just replace that coil. 20 bux for a new one (not AC DELCO) at Autozone, not too bad. When mine started acting up, I just pulled the wires loose from the coils one at a time and then when and started the car and listened for it to arc while having a friend watch to see if it arcs over or not. The dead coil had almost no spark at all. The good one made all kinds of racket (clack clack clack clack). Who knows what you really need to do, but that's what I did.

[bu-bye]

yeah I called 4 different parts stores autozone, kragen.... and all of them would not scan my car. My town must just suck a55! I'll try the arc thing tomorrow. thanks

[Ted]

I tested mine, what you do is you take them out. And i tested the resistance on them by adding the leads of the multimeter to the two top posts (where the spark plug wires connect). I'm not sure on the exact resistance they should be, I can check for you when I get home. But i'd check to make sure that they are all the same resistance that way you can get an idea what they should be around. I know sometimes I have problems with my outer most one because it's directly under the cowl. Sometimes rain gets on it and rusts the posts a little bit. I heard of people also taking the spark plugs off the two posts and running the car. You will see an arch between the 2 posts and depending on the color of the arc tells you how good it is. I never used this method because i'd be afraid of doing it. Also some food for thought. If it were me, i wouldn't replace just one coil. Because say your other ones have too much resistance or too little and they're worn down. You put 1 new one in and your going to have a better spark on 2 of the cylinders. I might not be right, so don't quote me but i'd be afraid that overtime it could be a problem if that happened. Hope I could help a little bit, and i'll give you the resistance that the coils should be testing at, i have it on a document at home.

Ted

[awesomeame]

fyi--how to test & coil tolorances can be found in a $15 haynes manual ;) it has lots of other info as well [img]smile.gif[/img]

--matt

[Guardsman]

Quick and easy basic check of the coil packs (got this from a Chevy Service shop). You'll need a second person.

Remove both wires from the coil in question (ONLY DO ONE COIL AT A TIME!!!!!).

Have your friend/wife/gf/whatever (somebody you trust) turn the engine over a few times (only a few seconds).

While the engine is turning over, you should see a nice blue arc between the coil towers.

No arc, bad coil :D

[bu-bye]

Well SOB, its not the coils. I first tested all the coils by using the arc test. All the coils looked good. I then went to the parts storeand bought one coil. Using my ohmmeter on the good coil i got a reading of 8.22. the coils in my car came in a 6.12, 6.22, 6.23. Why the old coils came up with less ohm's gets me. I expected to see it the other way around. So anyway I bought two more coils for a total of 3 and installed then. Went for a drive and the problem was still there. I pulled the coils and took them back to Autozone. I told them I never ended up using them ;) and they gave me back my $90. I went home and got back under the car. I found the number 6 wire was touching the header and was black. I went to pull it off and the wire snapped in 2 because the plastic was so hard. I thought that might be the key to my problems so I went back to autozone and bought a 8mm set of wires. Installed them and my SES light went off but I still have the problems that I had before. The only thing left is the sparkplugs.

Now, can a bad sparkplug work fine when cold but turn bad when hot? All the signs I have point to a bad coil but I know thats not it. What else could it be? It only misfires when it gets hot. The first five min of driving are great. All the power is there and it feels like a new car, 6 min later it feels like crap. Help

[ December 09, 2003: Message edited by: bu-bye ]</p>

[RAR]

I had the same problem with my 3.4, but it ended up being the coils. I just bought one and put it in, drove the car for a while then moved it to the next location and repeat...

I am not sure, but it could be the coil driver (I am not sure about the 3.8, but the 3.4 has a driver that the coils plug into). If I remember correctly the Hanes manual did have a way to test it, but basicly if I remember correctly it basicly said "if nothing else solves the problem replace this part" - just something to look into

[bu-bye]

<blockquote>quote:</font><hr>Originally posted by RAR:
I had the same problem with my 3.4, but it ended up being the coils. I just bought one and put it in, drove the car for a while then moved it to the next location and repeat...

I am not sure, but it could be the coil driver (I am not sure about the 3.8, but the 3.4 has a driver that the coils plug into). If I remember correctly the Hanes manual did have a way to test it, but basicly if I remember correctly it basicly said "if nothing else solves the problem replace this part" - just something to look into<hr></blockquote>

Would that be the plate that the coils plug right into or do I have to follow the cables to another device? My Chilton book says nothing about that...Stupid POS Chilton. Could you maybe post what they say for the 3.8's. That would be such a great help. I'll be replacing the plugs this weekend but if that does not work I'll try the coil driver.

[Guardsman]

He's talking about the ignition control module, and yes, that's what the coil packs are mounted on. The ICM on our cars handles all of the ignition control except for deciding what the advance is. the PCM controls the ignition through the ICM, so that could very well be the problem.

Also, check your sparkplugs, you have one that's broken. I was reading an article on engine testing they were doing (GM High Performance or whatever), and one of the motors had a broken insulator on a plug. It ran fine initially, but then started causing problems at high rpm, etc.

[bu-bye]

Well, the problem is solved. I bought new Plat plugs and was installing them when I noticed the boot on the #3 wire was cracked and the top of the boot was exposed to the header. I had inspected the the wires a few days before but looked at them from under the car so I did not see the exposed part. I inspeced the plug from #3 and it did look like is was running rich. I replaced all the plugs and wires anyway. I also cleaned all the connections on the ICM with a fine sandpaper just incase. I have had bad wires before but it always gave me a misfire the second I started the car not just when it got hot. I guess car tech is not an exact process. Thanks for the help guys.

[ December 17, 2003: Message edited by: bu-bye ]</p>