Here is the setup: TPI 350 with a 727 ECM using the P4 board installed in a original carbureted vehicle. This is the fourth swap I have done, going back 15 years or so to when we were burning chips and using the black light(?) to wipe them clean. So I am not new to this and my professional life involved lots of wiring. I bring this up because I want people who are new to this process to know how easy it is to get off track when problems arise, and they will. While I don't follow this forum as closely as most and I am not into the 'perfect' tune, I surely want my vehicle to start, drive with decent performance, and have O.K. fuel mileage. I run all my tunes open loop with a wide band O2 only. I use MSD ignition and auto transmissions. O.K. here is the meat of the story. Latest swap was done about 2 years ago, rebuilt engine, used OEM electronics, wiring, ECM, and P4 board. So all the wiring, relays, grounds, and routing of wires is custom, does not follow any OEM configuration what so ever other than I use all OEM sensors. I use TunerPro software for calibration mods.
The problem I had was code 42 popping up after about 1 year of running, very intermittent and easily cleared, at least for the first month or so. Engine ran pretty good except for slight rough idle. Code 42 started to show up more frequently and I needed to check it out further. Followed all the OEM troubleshooting, ignition modules checked good using the 12v circuit tester I could get it to switch as described. I use the small cap HEI with 2 connector module, 2 pins on 1 connector, 4 pins on the other. Swapped in a different module just for the feel good fix, used GM module, no help. Problem with code 42 getting worse, to the point of not just going to the no advance mode like when you set the timing and disconnect the wire (EST?) it was now killing the engine at very random times.
If I told you all the things I tried you would laugh and call me names I can't print. It got to the point I wanted to send my P4 board back to BobR and have him test it. Then, BAM, early one morning I was smacked upside the head with EMI. I pulled a spark plug out, #8 to be exact, fouled with oil. I checked the rest, all fairly clean not like #8. All my wiring is on the passenger side, including the wiring for the ignition module! When a plug fouls, it requires extra energy to fire, my MSD delivered it. If you read the OEM T/S carefully you will note only 2 wires can cause this fault, and it has to do with the ECM seeing voltage on one of them when there should not be. Or a bad ECM. My spark plug wires were about 2" from the module wires. Changed plugs, moved plug wires further away, problem solved!
Looking back, one thing stands out and was screaming at me, but I did not listen. While setting the timing, with the EST wire disconnected, the timing mark would jump 5-10 degrees every 5 seconds or so, not a steady mark. Also, there was that slight rough idle, but no code 42 at the time. Pay attention to the little things and follow the K.I.S.S. rule, keep it simple stupid! I am going to wrap my ignition module wires with scotch 24 and ground one end, this will eliminate any future problems with EMI influencing my timing, even with fouled plugs, at least that is my intention. Time will tell.
I also can not state how much we owe to BobR at Dynamic EFI maker of the hardware we have used for years and Mark Mansure(?) for his TunerPro software. These guys have spent more time on developing the tools we use to tune our junk then we could ever imagine. They have stuck with us through the years guiding us in the right direction with all the knowledge we lack. I truly appreciate it, thank you!
Bottom line, follow the K.I.S.S. rule and problems will work themselves out. Hope this helps at least one other member.

 

I recently thought about resistor spark plugs. I researched them and determined they are used by the OEM's on engines with any sort of computer controls to knock down E.M.I. issues. Also to remove interference with the radio? Like the one I described in the prior post. So if I was running non-resistor plugs and I oil fouled the plug my thoughts were the resistance would go up and cause my MSD box to increase voltage to fire the fouled plug. If that was in fact the case, the increased resistance should have lessened my E.M.I. problem, not increased it? Maybe the oil deposits actually decreased the resistance? Or the fouled plug had nothing to do with it. Currently I run a non-resistor plug and E.M.I. wrap any wires having to do with the computer controls anywhere near my spark plug wires. So far have not had any issues.
So as I understand it, resistor plugs limit the voltage a spark plug uses to fire, a weaker spark. Personally I always went for the strongest spark, is that not why we use MSD boxes and coils? So if you follow the O.E. M.'s and use the resistor plug and reduce E.M.I. issues, is the weaker spark giving up power? Also, if you use the non-resistor plug, what steps do you take to reduce the E.M.I. interference?

 

A resistor plug affects the rise time of the voltage at the spark plug gap. With the slower rise time there is less EMI to affect the computer & radio systems. With a fast rise time of the voltage at the gap, there is a ton of harmonics that creates a broad spectrum of noise.

Which is why piston aircraft and older Corvettes run shielded secondary ignition systems.

My preference is to run a decent spiral core wire and resistor plugs.

RBob.

 

Thanks for the response RBob. What gap do you use on the resistor plugs?

 

Usually .035" on a standard distributor ignition system. The one car using the LS CnP the plugs are gapped to .040". And that engine is also boosted.

RBob.