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The Shop Truck

1971 Chevy C20 with a custom flat bed.
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OK, I had a serious misunderstanding about the flow chart he gave me apparently...
IMG_20250903_154547834.jpg

In the Nitrous PPH column he has an X2 which I thought that was the flow for a single 036 jet and I needed to multiply that by 2... That's why I have twice as much fuel as I need. :doah:

On the bright side, I flow tested the 100 and 150 shot jets and they flow fairly close to what the chart says so I should be able to use those numbers for tuning from here on out.
Now I just have to wait for the injectors to show up and refill my bottles.
 
I did not, I clearly have an issue so I'll send them off to get cleaned and tested. I want to have spares for drag and drive stuff so I ordered another set.
 
Try an infrared temp gun on each of the header tubes. They should be within 20° of each other
 
I had been using my FLIR to check, initially it wasn't running at all and it was obvious the cylinder wasn't firing. I think it's running part of the time at idle, so it makes it hard to tell with a temp gun. We checked numerous times since it magically fixed itself and it always looked like the cylinder was running. It wasn't until I pulled the plug that it was obviously not running all the time.
 
Well, I thought we figured it out...
We changed injectors, then test fired the injectors and coils using the Holley output test. #2 injectors and coil appeared to be firing properly.
We did a compression check, #2 looked good.
I fired up the engine and watched the header tubes with my FLIR. You could see #2 header get hot right away like the other cylinders, then it would shut back off and cool down.
Previous to this, I had swapped #1 and #2 coils, the issue stayed on #2. I had ordered a couple spare Holley Smart Coils, so I went ahead and swapped #2 to one of the new ones.
I fired up the engine and everything appeared to be running properly again. It was running so well that I had to adjust the throttle plate and do a TPS auto set.
We took the truck for a drive, I pulled 3 datalogs.
- The first one is a gentle acceleration from a stop through the gears to about 45mph, then you see me idling at a stop light for a while. This datalog looks good, it's pulling some fuel from the tune, but nothing major, both widebands are tracking close to each other. The 4 lines at the top are target AFR (pink) AFR Right (green) AFR Left (blue) AFR Avg (red) The dark blue line that's hard to see is CL Comp.
1757283782328.png
- The second one was a WOT pull through 3rd getting on to the freeway. Once again, the datalogs look good.
1757283931710.png
- We pulled off at the next freeway exit and got stopped at the light. It seemed like the engine was running rough again and it was noticeably missing at idle. We did one more WOT pull merging on to the freeway and datalogged it.
1757284037981.png
The third datalog shows that we're losing a cylinder on the right side again. It seems like the coil is once again having issues. On restart, the misfire cleared up and all 8 pipes got hot.

We're working on pulling the wiring harness for the coils so we can make sure we don't have any bad connections there (it's still too hot). When we were testing the coil with the output test feature, we were wiggling the wires and couldn't get it to fail. I'm running the Holley Big Wire coil near plug harness but maybe #2 has an intermittent power or ground? I'm not sure and I’m just looking for some ideas.

I ordered the EGT probe kit last week to help troubleshoot, but that won't be here until Tuesday. I'm hoping to get it installed on Wednesday.



Brent
 
That has to being frustrating as hell!

I'm looking forward to you finding and fixing the problem, then showing us the results!
 
If you've got a mechanics stethoscope you can hear the injectors ticking if they're running.

But at this point there's either something in a wire or the injector driver.
 
JoshHefnerX said:
If you've got a mechanics stethoscope you can hear the injectors ticking if they're running.

But at this point there's either something in a wire or the injector driver.
Click to expand...
I don't believe it's an injector issue, it really seems to be a coil problem.

I'll work on it some more tomorrow. It's just too heat soaked to work on.
 
It seems like whatever the issue is, it kills the coil slowly. That's why it's so hard to see. If it's only dropping at WOT initially, I can't test, but the egt probe would show it.
 
I pulled the coil wiring harness out this afternoon and tore it apart. I couldn't see any bad connections anywhere in it, when they take the 4 individual coil power or ground wires and connect them to a single 10 ga wire they used some sort of press to just smash them into a solid copper block, I'm not sure how that could be a bad connection in there.
We contemplated our options at this point and decided to go scorched earth.
IMG_20250908_184313364.jpg

I put factory coil packs on (temporarily partially zip tied) and the factory coil wiring. The LQ4 coil packs wouldn't work at all with the smart coil bracket so I used some other ones I had (D585 coil packs) but I ordered the proper bracket for the LQ4 coil packs.

We had to swap plug wires to the old set I was running before and when I did that we discovered a couple damaged plug wires (damage was inside the fiberglass boot). I'm not sure if that was causing the problem or not, but those plug wires were new not long before it went to tuning. I need to get some ceramic plug wires.

We took it for a drive and it feels so much smoother now, it also feels like it has quite a bit more power. Datalogs all looked good as well. Tomorrow I'll probably start testing with the nitrous, hopefully everything is good for drag racing Saturday.
 
After a few test hits everything is looking good. I think all my problems were caused by fried spark plug wires. Without the ceramic boots they start melting pretty quickly (first WOT drive) and I'm sure that would be even worse with nitrous.
I was running the FireCore plug wires with fiberglass covers which is why I didn't notice they were getting fried. I never had an issue with that setup on the old engine so I'm guessing it has to do with the exhaust overlap causing hotter headers.
I've got some Summit Racing Ceramic boot wires in there now and I've got some Ton's Performance wires on the way.
We've tested a 150hp shot and a 200hp shot with good datalogs. The plan is to go to the Drift and Drag on Saturday with the 250hp jets installed. I'll likely just do an 1/8th mile pull the first run just to make sure the tune is good, then go up from there.
 
Well, I wanted to swap the plugs to the next step colder (8) before Saturday. Apparently I needed to do that before I stepped up to the 200 shot.
A couple of the plugs had some light peppering (from preignition).
IMG_20250912_132158105.jpg

But #8 was considerably worse...
IMG_20250912_132141382.jpg


I'm not sure why #8 was so much hotter than the rest, but I'm going to pull a couple extra degrees of timing out before the first 1/8th mile hit and keep an eye on it. I'll probably move to 9's or 10's before I go to the 300hp jet.
 
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