Spark plugs, idle, and engine temp

OK, many here know I waste too much time trying to fine-tune my USA-spec 1988 V12 HE (Lucas), but I actually enjoy it. (Keeps me out of trouble)

My latest adventure, I still am trying to figure out why one day my idle is smooth as silk, and other days it is near perfect, but I can feel a wee bit shake sitting at idle in Drive. I get no misfires now - over the last year I’ve pretty much found several other issues having to do with fuel injection and plug wiring and fuel mixture and fixed all of them. The car is running very very well.

About a month ago, I removed all of my spark plugs and checked the gap. All were still good at about 0.025". I am running the NGK V that are recommended in Kirby’s book. I decided to index all of them, and put them back in. The car idled SOO smooth, so I thought indexing worked.

But a few days later, the odd rough idle came back now and then.

Some insight - when I reinstalled my spark plugs, I had sprayed them all down with carb cleaner. They all looked good, no fouling nor signs of running too hot. Both electrodes were perfectly clean. But the metal ring at bottom of threads was a bit black, and the insulator tip seemed to have a bit of grey/brown ash dust on it that came off with the carb cleaner.

More insight - 90% of my driving is around town. Usually 10-15 miles over about 20-30 minutes. I checked today, and the RPMs are always between 1000 and 2000, sitting at 1500 most of the time. I sometimes get above 2000rpm, and rarely get above 3000RPM. I haven’t seen 4000 RPM in a week.

I also am running 180F (82C) thermostats. I live in Seattle, a very mild climate. Most of the year, the temp gauges just sit at 180F (I have A and B bank gauges). It may creep up to 185 or 190F (88C), but only if I idle for a very long time. It also takes 10 minutes for the engine to hit 180F, so I"m really only driving at 180F for 10-20 minutes. In the hot Seattle summer months (only July and Aug) the gauges can creep up to 200F. But my aux fan turns on at 200F, so I have never seen temps above 200F.

I’ve read here that these V12s like the high RPMs, they’re designed for the highway not the city, and the Italian tune-up does wonders. I’m wondering if my low RPM driving and 180F temps is not doing enough to self-clean the plugs and the ash/deposits cause erratic sparks at low rpm, as well as my low temperature driving not doing me any favors elsewhere in the engine.

I need to do more highway driving, I know.

But, I know I’ve talked about this before, I’m going to try upping my thermostats to 190F (88C), which will let the car run hotter, between 190F-200F. I hope that will help things. I know my USA HE was designed to run at 190F, not 180F, perhaps it will help for my city driving to keep the engine hotter for those 20 minutes.

The Italian Tune-Up is as much about WOT as it is about RPM. Hold the pedal to the medal until the engine gets past 5000, let it coast back to 2000, repeat. I don’t think the 180F thermostats hurt anything.

My idle RPM is about 550 when hot and if I put it in drive it drops to 500-550 then begins to miss occasionally on both banks.
I put this down to low the RPM and nothing else. She runs very smooth all other times.

well, there’s something to these V12s and not liking city driving.

So today, I took her out on a 20 minute drive, but made the 2nd half freeway. Due to traffic and speed limits, I could only get up to 5000 RPM for a few seconds at a time, did that about 10 times.

Got off the freeway, on the way home, she idled smooth as silk again.

The 190F thermostats can’t hurt, I’ll try them. I know my cooling system can keep temps under 200F. So I’ll just run 190-200 instead of 180-200. Perhaps just getting the oil/internals a bit hotter helps.


If you have a soul of real adventurer and some caps in the pocket - I suggest to get more input data. Have you ever heard of CHT and EGT sensors used in airplanes? You can get 12-channel gauge with appropriate set to fully monitor your engine.

The entire set is approx $600.

If you really do that, pleasepleaseplease document it as much as possible, identifying every difference you possibly can, including performance and fuel economy. And include snapshots of where the needle on the gauge sits.

A few years back I’ve tried both and it seemed to me that the engine was happier with the 190F thermostats.
If you do, indeed report back, I’m also curious to see what differences you will find.

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