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Susepecting Coil Rail

3K views 5 replies 3 participants last post by  MechanicJay 
#1 ·
Hello all,

We'll I've had my '06 LJ for about 2 weeks now. I've put about 1000 miles on it so far and am catching up on some neglected maintenance from the PO.

It's currently got 52K miles. This last week I changed the oil, set tire pressures, greased the front-end etc.

I was getting some stumble at idle and a pretty good ping under load at higher RPM (such as pulling out onto the freeway).

I pulled the plugs, and am fairly certain they were original. The gap measured at between .045 and .060. I put some Auto-lite platinum plugs. And dumped a bottle of STP fuel injector cleaner in the tank.

This cured a lot of the idle stumble and most of the heavy load ping, but it's still happening.

In addition, yesterday after heavy traffic and the heat, I was sitting at stop light, and it felt like there was a bad mis on 1 or more cylinders. I revved it and the issue cleared, though it was still a bit shaky. It was then fine the rest of the way home, but I was no longer sitting in one spot, with no airflow through the engine compartment. No check engine light was triggered.

I'm also getting that weird 2000 rpm dead spot, mostly noticeable the 2-3 shift which I've seen other people report on.

Since plugs didn't cure the situation, I feel the next weak spot in the system in the Coil Rail, again going on what I've been reading on here from a number of searches. It seems like spark is breaking down when things get really hot and under high demand.

Anyone have any other suggestions?

Thanks,
Jason
 
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#2 ·
One thing that you may check is that you used the Autolite APP985 plug. I know a lot of discussion has been done on the fact that on the newer 4.0 ignitions if you use any other plug you can have all kinds of issues. In fact I think the Autolite parts guide may even incorrectly ID the wrong plug to use, if I remember another thread correctly.
 
#4 ·
Yes, change those plugs. Use the APP 985's, double platinum. Like Meyers said, Autolites catalog may recommend them,(AP 985) but they cause problems in the distributorless 4.0's (2000 and newer).

Your problem could coil related.
 
#5 ·
Doing some availability searches from work, I can't get the APP985 locally. Have to special order them and see them in a couple days.

I can however, pick-up a set of the stock NGK ZFR5N on my way home from work for 1/2 the price of the Autolites. I'm anxious to resolve this, so this is the route I'll take. I'll let you all know how I make out.
 
#6 ·
I swapped in the ZFR5N plugs last night. What a difference a plug makes! The idle has smoothed out *significantly* and I couldn't detect any high-load/high rpm pinging this morning.

There is still a very occasional stumble at idle, but nothing I'm too concerned about.

Thanks, you guy might have just saved me the $150 on a new Coil Rail. :)
 
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