Engine running rough after rewiring
12-23-2016, 12:35 PM
#1
Engine running rough after rewiring Hi all,
I will post this over on the 67-72 Chevy trucks forum later, but I figured this is the best place to start.
First, I will give a summary of my issue, as well as everything I have tried to resolve it, and then I have 3 questions at the end
I have a 1971 C-10 with a 2003 LM7 in it. It has been running/driving great and I've put about 4000 miles on it with the "new" motor with little issues. Last weekend, I decided to rewire the motor as when I did it the first time, I only removed unnecessary wires, and left all the original wires at factory length which resulted in a big ugly pcm sitting on the fender.
I ended up mounting the ecu on the firewall, and ran all the wires to the injectors and coils under the intake and to their respective places. In order to do this, I cut the whole wire loom in half, and shortened every wire as necessary. The truck fired up without any issues but immediately revved up to around 3500 rpm and sounded like complete garbage. I initially thought it was a vacuum leak because of the fact that I had wires under the intake, but after I re torqued the bolts, I got the same results, so I sprayed starter fluid all over the intake and got no change in noise or rpm.
After ruling out a vacuum leak, I started looking for issues in my wiring, and I noticed that the passenger side bank manifold became very hot after only 20 seconds of running, where the driver side bank was barely warm to the touch. That made me think that there was some sort of issue with the ignition timing so once again I pulled the intake off and followed every ignition coil signal wire back to the ecu, making sure that they were going to the correct pins. After knowing that the signal wires were correct, I did the same for the injectors. All of those wires were correct.
At this point, I am thinking there is some sort of ground issue, and this is because when I rewired the motor, I used LT1 swap to tell me which pins went where. I noticed that some pins stated they were for "ground" while others were for "pcm ground." I grounded both ignition coil plugs to the "pcm ground pins" and verified that they were getting a ground by using a circuit tester. Also, I verified that every other ground pin was indeed getting a ground. When I was testing these grounds, I noticed that when touching the pcm case with the circuit tester probe, it completed the circuit meaning the case itself was a ground. I wasn't sure if this was a potential issue or not, so I relocated the ecu temporary to the top of the intake manifold, and it still completed the circuit. I am not sure whether or not that has anything to do with my issue or not.
Also, when testing to make sure the injectors were pulsing correctly, I used HP tuners to disable certain injectors. Ie I would disable #1 injector, crank the motor with the ignition on and make sure it was not pulsing, but I noticed that even when all injectors were disabled via hptuners, they were still pulsing as nothing had changed. I did not know whether this was because the motor was not actually running, or if it was because there was once again a potential issue with a ground screwing things up.
And lastly, this morning when I fired it up, It seemed to me like only half of the cylinders were actually firing because when I touched the exhaust manifold, it seemed to me like every other cylinder was hot/warm, where as the others felt like they hadn't warmed up at all.
I have 3 questions that I would really appreciate being answered. 1, Is the PCM case supposed to complete the circuit as I described. 2, what is the difference between a "pcm ground" and a "ground" as described on LT1 swap, and lastly, is hp tuners not supposed to disable injectors when the motor is cranking, or is that another issue with my wiring.
Any help/tips/suggestions would be sincerely appreciated. I am currently a junior in high school and this is my daily driver so getting it back up and running before school starts again is a high priority to me.
Thanks
I will post this over on the 67-72 Chevy trucks forum later, but I figured this is the best place to start.
First, I will give a summary of my issue, as well as everything I have tried to resolve it, and then I have 3 questions at the end
I have a 1971 C-10 with a 2003 LM7 in it. It has been running/driving great and I've put about 4000 miles on it with the "new" motor with little issues. Last weekend, I decided to rewire the motor as when I did it the first time, I only removed unnecessary wires, and left all the original wires at factory length which resulted in a big ugly pcm sitting on the fender.
I ended up mounting the ecu on the firewall, and ran all the wires to the injectors and coils under the intake and to their respective places. In order to do this, I cut the whole wire loom in half, and shortened every wire as necessary. The truck fired up without any issues but immediately revved up to around 3500 rpm and sounded like complete garbage. I initially thought it was a vacuum leak because of the fact that I had wires under the intake, but after I re torqued the bolts, I got the same results, so I sprayed starter fluid all over the intake and got no change in noise or rpm.
After ruling out a vacuum leak, I started looking for issues in my wiring, and I noticed that the passenger side bank manifold became very hot after only 20 seconds of running, where the driver side bank was barely warm to the touch. That made me think that there was some sort of issue with the ignition timing so once again I pulled the intake off and followed every ignition coil signal wire back to the ecu, making sure that they were going to the correct pins. After knowing that the signal wires were correct, I did the same for the injectors. All of those wires were correct.
At this point, I am thinking there is some sort of ground issue, and this is because when I rewired the motor, I used LT1 swap to tell me which pins went where. I noticed that some pins stated they were for "ground" while others were for "pcm ground." I grounded both ignition coil plugs to the "pcm ground pins" and verified that they were getting a ground by using a circuit tester. Also, I verified that every other ground pin was indeed getting a ground. When I was testing these grounds, I noticed that when touching the pcm case with the circuit tester probe, it completed the circuit meaning the case itself was a ground. I wasn't sure if this was a potential issue or not, so I relocated the ecu temporary to the top of the intake manifold, and it still completed the circuit. I am not sure whether or not that has anything to do with my issue or not.
Also, when testing to make sure the injectors were pulsing correctly, I used HP tuners to disable certain injectors. Ie I would disable #1 injector, crank the motor with the ignition on and make sure it was not pulsing, but I noticed that even when all injectors were disabled via hptuners, they were still pulsing as nothing had changed. I did not know whether this was because the motor was not actually running, or if it was because there was once again a potential issue with a ground screwing things up.
And lastly, this morning when I fired it up, It seemed to me like only half of the cylinders were actually firing because when I touched the exhaust manifold, it seemed to me like every other cylinder was hot/warm, where as the others felt like they hadn't warmed up at all.
I have 3 questions that I would really appreciate being answered. 1, Is the PCM case supposed to complete the circuit as I described. 2, what is the difference between a "pcm ground" and a "ground" as described on LT1 swap, and lastly, is hp tuners not supposed to disable injectors when the motor is cranking, or is that another issue with my wiring.
Any help/tips/suggestions would be sincerely appreciated. I am currently a junior in high school and this is my daily driver so getting it back up and running before school starts again is a high priority to me.
Thanks
Last edited by aruss99; 12-23-2016 at 12:43 PM. Reason: Grammer
12-28-2016, 10:15 AM
#2
TECH Regular
The pcm ground circuits, the coil grounds, and any other engine grounds need to be spliced together and then bolted to a cylinder head. Then you'll need a large gauge wire ground strap that runs from the engine to either the battery or frame/body. The frame, engine, body, battery negative all need to be somehow connected by ground straps. This will ensure that there are no ground issues. The pcm case itself is not supposed to be a ground.
You need to make sure you have power to all of the injectors while the key is in the on and crank positions. Also make sure you are not using the resistor wire to the original distributor because it will only supply 9v or so instead of 12v. Is the pcm programmed correctly for this application? My pcm was a 411 pcm from a 4.3 v6 and needed to be flashed with the correct operating system to work a v8.
You need to make sure you have power to all of the injectors while the key is in the on and crank positions. Also make sure you are not using the resistor wire to the original distributor because it will only supply 9v or so instead of 12v. Is the pcm programmed correctly for this application? My pcm was a 411 pcm from a 4.3 v6 and needed to be flashed with the correct operating system to work a v8.