davemoscow

Hi All
For all of you that have a SC400, or a Lexus with the 1UZFE engine, and are having misfiring or weird performance after changing your spark plug wires or coil. I think I have figured out what a lot of you are experiencing.
A little history on my car. The coil on the top of the engine went south, but I didn’t know it. I changed the gas filter and ran the car with injector cleaner at 3000 RPMs for quite a while. This didn’t help. I eventually determined the coil was bad. I changed both coils and the plug wires and it ran fine as long as I didn’t punch it. Under certain load conditions it would misfire (car jerk that could be misdiagnosed as a tranny shifting issue). I checked and checked and didn’t find anything. No codes, nothing. I eventually decided that the injector cleaning procedure, when run w with a bad coil, may have damaged my catalytic converters since they got damn hot. I pulled the exhaust and found that the ceramic inserts had been seriously melted by the unburned gas. I pounded the ceramic out, reinstalled the exhaust, and while the missing was less pronounced, it wasn’t completely eliminated. This suggests that the blocked exhaust was artificially affecting the back pressure, and subsequently, the cylinder pressure. I’ll get back to this.
My next thought was that the throttle position sensor had a dead spot since the missing occurred around 3000 RPM. I replaced it without checking the one on the car because it was a $118 part with shipping from Rockauto.com. Well, it didn’t solve the problem, but the car was more responsive when accelerating. Some of you may be thinking electronic control module. Nope. I spoke with a good transmission guy and he said he has swapped out hundreds of ECMs and only a handful solved any of the problems in question.
Here is what I think is happening. First, don’t buy the cheap replacement plug wires. The wires are fine, it is the poorly duplicated rubber boot and cheap connector on the end of the one that goes to the coil body. The male rubber boot doesn’t sit nicely in the seat because it is too big and the flimsy plastic connector can’t keep it properly seated. The higher end wires have better connectors and slightly better male rubber boots. For the coil on the drives side of the engine, a poorly seated coil connector isn’t a problem because it is so far away from ground that an arc from the coil to ground is unlikely. This is not the case for the coil on top of the engine. This one is snug up against the spark plug wiring conduit. This is the problem area. The aftermarket coil plugs don’t seat very well in the coil receptor. If there is a small air gap, at high load (RPM) it jumps the air gap and arcs to ground. Recall, at high loads the impedance (AC resistance) of the ignition can get quite large. The coil will always search out the pathway of lowest impedance. I think my clogged catalytic converters created too much back pressure and at high loads the exhaust isn’t adequately flushed from the cylinders, which creates high pressure in the cylinders and raises the spark impedance.
Here is what I did. I noticed that the clip that holds the coil plug on the coil was cracked. I heated a chisel with a torch and melted/welded the plastic, thereby eliminating the crack and restoring its mechanical integrity. Next, I heated up the male rubber boot on the coil wire and rammed it into the coil in order to get a better seat. Problem solved.
I have an alternative approach that I will try if the connector breaks or the problem returns in time. I will get some silicon rubber and put a coating around the male rubber boot of the coil wire, but making sure I don’t get any on the metal end of the plug. Ram it into the coil and let it set. I think this will eliminate air gaps and keep the coil from arcing to ground. They should have placed the coil somewhere else. The OEM wires, especially the one between the coil and the distributor are top drawer. When replacing the plug wires, if the coil wires look good, keep them. Alternatively, relocate the coil from the top of the engine to a place far away from potential grounds.
I can’t say this will solve everyone’s problems, but I think it will help a lot of you. To summarize:
If you ran your engine with injector cleaner and it turns out you had a bad coil, check your cats. Drop the exhaust and look down the pipe. If damaged, it is pretty obvious. Replace cats if your state has stringent emissions testing. Otherwise, pound them out.
Keep the original coil wires that came on the car when swapping out the sparkplug wires. They are better than any of the replacements.
I think everyone should swap out their throttle position sensor. This is a mechanical wiper resistor that wears out. It is easy to replace. I put mine on and then adjusted its position until the idle was 600 RPM.