Author Topic: Help with HT 4100 pre-detination for model year 83-84-85 requarding spark plugs  (Read 385 times)

Offline collector

  • Posts: 22
  • Name: john rutenberg
I have a 6K mile all original Eldorado Biarritz and have a slight pre-detonation issue I need to resolve.

In 83 the car used an R43NTS6 plug
In 84 they used an R45NTS6 plug
in 85 they used a R42CLTS6 plug

Now all the valves open and close at the same rate.

What were they trying to do? Improve fuel economy or get rid of pre-detonation or both?

I currently have the originals plugs in the car and they look like brand new.

I wonder if I need to go to a cooler plug to help with the issue or do I go with the 85 plug and that would solve the issue all together? They also have the 1985 version in a platinum plug as well. What should I do?

Offline Scot Minesinger

  • Posts: 4182
    • CLC Member
      CLC Member #20543
It may not be that the spark plug replacement will resolve this problem.  6k miles on a 30 plus year old car might just mean it needs a good highway run (3 hours minimum, at speed - 60mph or so).  Very low mileage cars suffer mechanical issues that can easily be resolved by primarily replacing rubber parts and use.  Do a search and you probably will find a list of what to do if you have not already. 

Enjoy your Cadillac!
Fairfax Station, VA  22039 (Washington DC Sub)
1970 Cadillac DeVille Convertible
1970 Cadillac Sedan DeVille
1970 four door Convertible w/Cadillac Warranty

Offline TJ Hopland

  • Posts: 6590
R=Resistor

4=14mm

3,5,2= heat range lowest is cooler so they were apparently playing with the heat range

N = 3/4" reach 3/8" thread length.  Looks like this is just an older designation back when there was only copper.
CL =  copper long reach.   Looks like a newer designation that came out when platinum was introduced.

TS6 = tapper seat extended tip, 0.060 gap


I'm with Scott in thinking this likely has some carbon or other build up in the engine that is likely causing the issue.

It may also be worth checking to make sure the base timing is set correctly using the specific process and specs in the shop manual.  A lot of people didn't do this correctly on these early computer controlled cars that still had distributors.
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
75 Eldo rusty but trusty
80 Eldo Diesel
And other assorted stuff I keep buying for some reason

Offline Dave Shepherd

  • Posts: 1897
Also if your egr system or valve is inoperative, you will get pinging under certain conditions.

Offline Scot Minesinger

  • Posts: 4182
    • CLC Member
      CLC Member #20543
Yes faulty old vacuum lines also cause knocking and rough idle issues too, all originals should probably be replaced.
Fairfax Station, VA  22039 (Washington DC Sub)
1970 Cadillac DeVille Convertible
1970 Cadillac Sedan DeVille
1970 four door Convertible w/Cadillac Warranty

Offline smokuspollutus

  • Posts: 54
  • Name: A. Molinaro
Assuming EGR works (believe me, you will know if it doesn't, my 84 car had a hellacious ping on everything but 93 octane when the thermal vacuum switch failed causing a no-EGR condition, sounded horrible) I don't think the plugs are the problem. As I daily drive my car distances, I use platinum plugs from a 4.5, same heat range but lasts 100k miles. After I fixed the EGR, I set base timing to 12 instead of 10, it will chirp the tires and run 87 octane without any pinging. It is exempt from emissions inspections at its age where I live. I doubt anyone has screwed around with the distributor given the low mileage but I guess it couldn't hurt to verify.

In searching for the cause of the ping on my car (at the time had approx 47k miles) I did replace all engine vacuum hoses. The originals were in good shape and probably didn't need replacement, but I was really scratching my head. The vacuum system on one of these is very simple fortunately. Might be worth it to verify the vacuum pump is operating properly. Again with the low mileage I doubt there's a problem, but it doesn't take too long to rule it out.
1984 Eldorado Biarritz Coupe

Offline collector

  • Posts: 22
  • Name: john rutenberg
I fixed the issue. I put on a Ca Egr and a Ca ECM. No more pinging. I also put in a set of NOS AC Delco plugs from 1984, as they were a cooler plug. Its perfect now. Thanks guys

Offline smokuspollutus

  • Posts: 54
  • Name: A. Molinaro
Interesting. My 1984 is CA emissions as well. Anyone know generally how the tune of the CA cars was different to a 49-state?
1984 Eldorado Biarritz Coupe

Offline collector

  • Posts: 22
  • Name: john rutenberg
They use a different EGR, the larger kidney shaped cat vs the smaller type and the calibration of CCN for 83 and DMN for 84. Nothing else is different. Hope this helps.

Offline TJ Hopland

  • Posts: 6590
So far so good?   Your car is still happy?
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
75 Eldo rusty but trusty
80 Eldo Diesel
And other assorted stuff I keep buying for some reason

Offline smokuspollutus

  • Posts: 54
  • Name: A. Molinaro
Yes, the pellet converter stuck around in California for a few more years after it was phased out in the federal cars. Someone mentioned to me that the Cali tune is slightly richer.
1984 Eldorado Biarritz Coupe