Detroit 60 12.7 cyl3 misfire

My first thoughts were the same, if it’s only an injector, and he has one, why doesn’t he just fix it. But could just be the case. Could just be his guess. You can take the valve cover off and turn it over. See if everything’s in place. Being a 91, you really don’t want to try to build it much. Just get it running, and use it. I’ve changed injectors twice on mine. It’s not that hard. Rockers are 2 pc. Each half covers 3 cylinders. Bar engine over, getting the bad injector at full height, check the height, using the correct height tool, placing the point in the dimpled spot on hold down clamp, and the top of the tool on top of injector spring. It should be already set at spec. Then just remove the one rocker assembly, 2 wires, one bolt holding injector. Remove it, using a small crowbar if needed. You’ll see all the fuel drain from cup into the cylinder, get the fuel out of the cylinder. I used a hand pump for gear oil, adapted down with hoses, to a 1/4” plastic airline, then blow it out with air, make sure nothing (like a washer, or broken o ring) is stuck in the injector cup. Put new injector in, using a new graphite crush washer on the bottom of injector, and a big fat o ring on the top, they sell the whole kits, about $20 or less. Crush ring insures any damage to the cups seat, is sealed, new o rings on the injector, big one on top. Bolt it back in, hook up the 2 wires, then set the height. Put rocker assembly back on, lining everything up, it has to go on the same direction it came off, for oiling purposes, snug it down, evenly, some of the valves will be higher, and under more spring tension, than others. Try to tighten them first, but all bolts evenly, no big deal, just don’t want to bend the rocker arm. Torque to 100lbs. from center out. Just need to know correct height. Set it, by loosening the nut on top of injector, with tool in the hole on hold down bracket, and top of tool at top of injector spring, turn the adjuster on top of injector, till correct heigh is set, tighten the nut, and everything else,(valve adjustments/overhead) stays in place. Buy the right tool. About $35 on eBay. There’s a few different heights .78-.81 roughly. One tool has all the different heights, in one, using different tabs. If he has a correct injector, cost you less than $50 for o rings and tool. The problem is knowing which injectors bad. I knew because I had fuel in pipe between exhaust manifold and turbo. Looks like thin oil, but it’s sooty fuel. Also saw dampness, raw fuel seeping from bad cylinder exhaust port, at the exhaust manifold. There’s your bad injector. The second injector I changed was a no brainer, it was squirting fuel from the selinoid everywhere, while running with valve cover removed. Of course a cut out test by a mechanic would tell for sure. I’ve been told disconnecting them one at a time, and running the engine can do damage. I don’t know myself, how or why. But decided against it. Probably best to change all of them, and run an overhead. But you can just change one out, and leave everything else alone. It sounds like a lot, I tried to cover everything detail from memory. It’s really pretty easy.