Misfire on #4 on semi fresh rebuild after track outing


What's Fuel Injection?
Mar 9, 2009
I put together a 4.1 SFI setup that I swapped into an 80 century wagon this winter. Fresh rings and bearings, dingle ball hone on a clean 4.1 .030 block, hyper .030 bottom end, stock LC2 setup (turbo intercooler ect). Maf translater with LT1 Maf, l67 injectors at 4 BAR with adjustable FPR and matching chip, new caspers LED cam sensor, scanmaster. Just want a fun cruiser.

When putting the engine together I had a bad coil. Engine ran good cold but misfired under boost. Ohmed coil open across 2 pairs of posts and like 20k Ohm occross the other pair. I purchased a new to me, used but known good coil and module online, it ohmed at 11.5k Ohm.

Been working out some bugs and took it to the track a few weeks ago. 1st pass was a light low boost roll out shake down and it ran a high 14. 2nd pass it started running crappy half way down the track and got to the pits and was hard misfiring and smelt rich.

The parts I put the LC2 setup together with were parted out in the early 00's and the fuel rail was rusty from sitting for 20 years exposed to air, I vinegar dipped and cleaned it out with a large copper wire when I put the setup together and had plugged 2 injectors previously so I thought I plugged another injector. Limped the car home, swapped injector to another l67 white inejector, still misfired but better. #4 was very fuel smelling and my IR thermometer said it was burning cold. Let the engine dry fuel out, put it back together and still misfired. Basically I determined later on that my coil went dead causing weak #4 spar I think which caused #4 to stop igniting and flooded the cylinder

Checked compression, good 125-135 PSI on all 6. Did a leak down since I assumed I could have hydro locked #4 and pinched a ring or popped a head gasket, check the link for the youtube vid but it was fine. Almost no pressure drop, no air flow, no bubbles out the rad. Also put my dial indicator on the #4 exhaust and intake pushrods to check cam isn't wiped and they were in spec for lift comparing to #2. Cam is an erson 200ish duration mild cam from the mid 90's I got when I bought the 4.1 long block.

After checking a bunch of mechanical stuff since it was still misfiring, but not smelling of fuel I checked the coil and it was high ohms like 17-18k Ohm on all 3 sets of post. Coil and module only had 100 miles on but nothing mechanical could be found so I thought I had a bad coil. I would take it for a drive and it didnt smell of fuel but #4 was burning cold by about 100F and would pop, miss, and run like garbage. Assumed the coil reads bad, so the coil is bad.

Got sick of buying expensive coils that blow up so I bought a dynamic EFI LS coil near plug system and used some heat sink LS truck coils I had on the shelf. Got setup put together and started, ran significantly better. I used the same "new to me" ignition module that was under the bad coil I had 100 miles on. Engine still studdered, and misfired under boost. Could feel a miss at idle in gear when creeping in reverse or drive. Overall better than with the stock type 1 module and coil but not 100%. Maybe 40% fixed.

Thought I might have a bad module so I put the old module on that I had with the bad coil when I put the engine together this winter. This solved the misfire under boost and cleaned it up significantly. However, it still has a light throttle cruise miss and for sure a miss in gear when warm at idle. 80% fixed.

All cylinders burn within 20F of 600F right after a light drive around but #4 is 50-100F colder. Car runs good under boost.

I took a video of unplugging coils. You can hear the engine chug a bit at idle when everything is plugged in. I unplug #4 coil and it gets moderately worse but not much. Unplug any other coil and the engine for sure has a misfire and even died when i unplugged #3. #4 sounds and feels like it has a 50% miss at high vacuum situations.

I have checked plugs and they look good, #1 is top, #3 is bottom right, #4 is bottom left. #4 might have a light brown tint to the ceramic? If the 1/4 coil wiring was bad I should see little to no change when unplugging the #1 coil since its waste spark OR it would also burn cooler, but it doesn't. #1 is burning good.


Anybody have a clue what causes a 50% miss when all mechanical and electronic parts seem to check out? Do I just throw another ignition module at it? Pull the head off and check head gasket?
Check Juan's thread. I posted about a running compression test in there too. Could be a lifter problem just as a wild guess since you've covered a lot already.
I would not pull the heads as you have done both compression and lask down test with good results.

I would look at the plugs them self.

Have you tried a completely new set of spark plugs ?

At what gap are the plugs ?
Did the cylinder balance test. No drop on cylinder 4 when the coil is unplugged, 50-100 rpm drop on all others.

I changed the plugs to brand new NGK UR5 gapped at .035 tonight. No improvement.

Fuel pressure is 43 PSI with 20" vacuum, 52PSI with it disconnected. I have l67 injectors that I am running at 4 bar, so I do need to turn the fuel pressure up to 57 PSI to get the 37lbers acting like 42lbers. It's not perfect but they are acting like 40lbers at the fuel pressure its at so its not that bad enough to cause an issue.

14 volts at the scan master.

Still missing.

And the temp loss on #4 is more than 50F. Its 50 sometimes bit it was 120-170F tonight.



And I was looking at wiring diagrams.

Why does the P pin have resistance wires going from the fuse to the ignition module? If I hooked that up directly to the CCCI module without those resistance wires could that be a cause for me burning coils so quickly?


Also, the P pin on this diagram calls only for type 1, does that mean I need it with the COP LS setup? Might be a question for Bob from Dynamic.

Am I at risk of blowing up the CCCI modules if I have voltage directly from an auxiliary fuse box?
Unplug #4 injector install compression tester. Have helper start the engine. Note the reading. Release the gauge pressure and take another reading while it runs. Then do a " good cylinder.". This test will show you if a lift is over traveling or pumping up excessively.
It helps if your comp gauge has a relief button.
Connect a noid light on the injector plug, to see if the ECM driver is working .
Flat tappet cam used? Could be wiped.
I will try the running compression check, thanks for the instructions. I'll try it next.

I'll try the noid light on the inector plug also.

Yes, it does have a flat tappet cam, but I did dial indicate lift on #4 both intake and exhaust and it was within a couple thousands of #2 which is good.

Other odd thing, this diagram does NOT show added 0.7 ohms of resistance in the harness between the CCCI D4 fuse and the M pin on the module. I will have to check resistance on that wire and see if my setup has resistance across that. I have the stock harness that I cut the GN plug that goes into firewall off, and then soldered what wires did exist to the 80 century fuse block plug since 78-82 and 83-88 G body had a different style. The wires I didnt have power for from the stock 80 fuse panel are on an auxiliary fuse panel near the hood hinge. Possibly that 0.7 ohms does or doesn't exist in real life on a GN? Was it buried in the wiring harness or the fuse block?

Not saying the lack of resistance is causing the issue but it might be a reason I keep killing stock coils?
Also what no disintegrations said is dang good test as it would keep the valve open and not allow it to build full cylinder pressure and it might only be sticking or over pumping after it runs for a little bit
Go old old old school and put a large face vacuum gauge on the car.

See what the needle says.

Ok, so I ended up getting a hold of Bob and figured out the wiring stuff, basically the wiring diagram I found was probably an error from GM so that helped clarify that question. Great service from someone who answers tech problems quick!

I ended up swapping the #4 injector, replaced batteries since all my old ones were questionable and it seems to have fixed the issue. #4 is burning within 50F of all the other cylinders.

I found it really odd that I replaced the injector that came out of running and driving car with a bad one out of a running driving car but there is no other explination.

Wish I would have swapped the injector again long before I went down this path measuring cam lift and stuff but I'm glad I did it. It must have been sticking at very short pulse width or had a slight plugging issue.

Thanks for the suggestions!
Glad to hear that you solved your problem. Years ago I encountered a no start problem and found out the no start chart is not correct in some sections for our cars.. Amazing no one has mentioned it until now. It lead me on a wild goose chase.
Those truck coils suck believe me I ran them and was having all sorts of Miss fires just get some stock Ls coils and watch everything improve 10 fold . I promise you will be much happier When u swap them . Did bob send you a different dwell times for those coils because that was another reason I swapped from those coils was because they need different dwell times ect. To make them actually work as they should . Hopefully you take my advice if not then at least I tried & Keep on spoolin !! Here is a pic of the ones I was running and the ones Im currently running with zero problems