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Black '99 Deville
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75 Posts
Discussion Starter #1
The 99 Deville is up to 95K miles and the engine was beginning to run rough. The check engine lite came on. Pulled the Diagnostic Trouble codes and found one that says "cylinder missing" and another that said "engine stalled once or twice" and a couple of others that meant ignition troubles.
I first thought about having my friendly local garage change the plugs after looking at the engine. The cross wise mounting puts one bank of cylinders right up front and easy to get at, but the other bank of cylinders is up against the firewall and buried under a load of stuff. I finally decided that I'd have to turn in my shade tree mechanic's license if I couldn't even change the plugs on a V8.
So, picked up 8 Autolite plugs and some antiseize compound at the local auto parts place. Asked the counter man about the Autolite for a Caddy, Autolite used to be a Ford brand. Counterman says that's all old fashioned and now GM cars are speced for Autolite as well as Champion and AC.
Got the front plugs swapped out no trouble. They needed a change, the center electrode was damn near burned away and the gap was double that of the new plugs. Then I tackled the ugly rear plugs. It wasn't as bad as I feared. Take off the plastic engine cover, and the cross member running between the spring towers and the rack of ignition coils. Then you can get a 1/2" drive deepwell socket onto the plugs with room for the rachet handle to swing. And they did come out with out busting anything. Whole job took about three hours.
Tools required. 1/2" drive rachet handle and 6" extension. 5/8 inch deep well socket. 1/4 drive rachet handle, 6 inch extension and a 10 mm socket for the bolts on the coil rack. 1/2" socket for the cross bar nuts.
Tricks. Take duct tape, fold it double sticky side out and press it into the deep well socket. This will stick to the plug and pull it up out of those deep deep spark plug tubes in the heads. Beats fishing for the old plug with long nose pliers.
Washed the black off my hands and started her up. Purred like a kitten.

David Starr
 

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Thanks for the report. I will be changing mine soon even though I've only 69k on my 98. I think I'm going with the fancy Bosch with the 4 electrodes. They're costly but you don't change these things very often anyways.
 

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76 Posts
Woah there... Do a little internet research on the Bosch Platinums. I installed a set in one of our Dodge Caravan's. They lasted about 1 year and 10,000 miles before the electrodes were completely worn out... According to my mechanic, Denso, NGK, or AC Delco platinum/iridium plugs are the way to go.
 

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Didn't know that, I'll do a little research first, thanks. I did see that Bosch 4 is available in platinum/iridium now too. The iridium must be needed for longevity??
 

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1997 Eldorado, 2003 Audi RS6
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1,320 Posts
Forget the bosch. Their center electrode insulators are garbage. Go with NGK or Denso if you want an aftermarket plug. The $3 Denso basic platinum plug is better than any of Bosch's stuff. You can also not go wrong with AC Delco Platinums.

And to dstarr: Autolite plugs are worse. This is a Cadillac. Even used, it's hardly the most cost effective car you can buy. Why must people go cheap? I'm all for saving money, which you are by doing the work yourself. Don't buy junk parts. Leave the Autolite to your leaf blower.
 

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White Diamond '03 DHS (with DTS floor shift)
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I seem to recall the Guru once telling about someone using Autolite on a Northstar and the engine blew up. Something about the wrong heat range or something like that. Autolite would not stand behind it. I'd stick with A/C Delco.
 

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I've read a number of threads now for research and I'm now going with 41-950 AC/Delco's. Cost vs performance it appears to be a no brainer! Got all three volumes of FSM coming from ebay, $45 including shipping, so I'm gonna get a whole lot wiser soon.
 

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Black '99 Deville
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75 Posts
Discussion Starter #8
Forget the bosch. Their center electrode insulators are garbage. Go with NGK or Denso if you want an aftermarket plug. The $3 Denso basic platinum plug is better than any of Bosch's stuff. You can also not go wrong with AC Delco Platinums.

And to dstarr: Autolite plugs are worse. This is a Cadillac. Even used, it's hardly the most cost effective car you can buy. Why must people go cheap? I'm all for saving money, which you are by doing the work yourself. Don't buy junk parts. Leave the Autolite to your leaf blower.
On parts, I tend to go with what the counterman at my local auto parts place says will fit. It's his day job, and he probably knows more about it than I do. Parts is parts. I only do this stuff occasionally, he does it every day. Autolite has been around for ever, they used to be owned by/associated with/married to Ford. I've had several Fords over the years and they all took Autolite, they all ran fairly well, and lasted a long time.
Modern electronic ignitions (like Caddy) are so hot that they will fire the cylinder even with badly worn plugs with a gap way too wide. It's the electrical zap from the coil that sets the mixture off, not the metal in the plug electrodes. It also keeps the plugs clean. Even with 95K miles on them, the old plugs were clean as a whistle on all eight cylinders. A little eroded, but clean.
Took the car out for a drive today. The "Check Engine Soon" light went off all by its self, and the Northstar pulled me up three mile hill running smooth as silk. I'll reset the history DTC's tomorrow. Instantaneous indicated gas mileage was up. Should have changed those plugs back about 20K miles ago.
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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:rolleyes: If you're new here, and dig back a couple of years, you'll read horror stories about Bosch plugs and N*'s. AC 41-987 (late N*) platinum electrode is made in Japan, so now we know where Denso went (Denso is the plug named in my '02 STS manual....)
 

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1997 Eldorado, 2003 Audi RS6
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1,320 Posts
On parts, I tend to go with what the counterman at my local auto parts place says will fit. It's his day job, and he probably knows more about it than I do. Parts is parts. I only do this stuff occasionally, he does it every day. Autolite has been around for ever, they used to be owned by/associated with/married to Ford.
...
No. Parts are not parts. Champion was/is Ford. The parts counter guy is not a mechanical expert. He is a monkey who works for an auto parts store. He knows what will fit, probably not much beyond. I love when some old school grease monkey tries to tell me things I can do to make my car run better, prevent a problem, or fix a problem. I tell him "This is a Northstar V8, not a 1965 Chevrolet 350. My engine's more complicated than a lawn tractor engine." Believe it or not, my Cub Cadet's engine is about as mechanically complicated as the afore mentioned 1965 Chevrolet. It has a carbuerator, a fuel pump, all the filters, a vacuum actuated AND a manual choke, an overspeed governor, overhead valves actuated by rockers connected by pushrods to a chain driven cam in the center of the engine parallel to the crank. It even has a pressurized oil system with a full flow filter.
 

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Cadillac Technician
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11,051 Posts
Do not use Bosch platinum plugs in any GM car. They might seem to work great for a while but it won't take long to develop some drivability issues. Heat range is pretty darn important on the plugs, at least in a GM car. If it is too far off the car will never run right.
No real reason not to use Delco plugs that were designed for the application.

Also you can buy "spark plug sockets" that have a rubber sleeve built into them that are designed to hold onto the plug when you pull it out and slide off the plug once it is tight. They are not too expensive.
I use a "locking" extension that holds onto the socket so it all comes right back out.
 

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White Diamond 2001 STS
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1,317 Posts
I pulled the original plugs out of my '97 SLS at 145k miles (or thereabouts), just to say I changed them. The car didn't run much different after the change...and it ran fine beforehand. There was a little gap widening, but not bad...especially for plugs with well over 100k miles on them. I replaced them with ACDelcos.

And by the way, I used to work at Advance Auto. To the person who said that the counter guy at the retail store is just a monkey working at a parts store...YOU ARE CORRECT. There is NO mechanical knowledge required to work at those shops. Most of us did it because we liked cars, but would absolutely sell you a Champion plug for your Northstar because it worked good in a buddy's friend's cousin's little baby brother's Mustang. Nobody knows anything about the Northstar engine at those places (I sure didn't when I worked there). Parts isn't parts!
 

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95 Fleetwood Brougham / 01 DTS / 11 CTS Lux / 11 DTS Platinum
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From what I understand Bosch plugs suck in the N*. AC Delco OEM plugs provide the best performance.

Don't waste your money.
 

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1997 ETC (GAVE TO STEPSON 2011), 2000 DTS (RIP)
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1,702 Posts
I Have Had Good Luck With The Oem Ac Delcos - Don't Really See A Need To Try Something Else - Imho
 

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Cadillac seville sts 1999
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9 Posts
Hey I also have a 99 seville with the same problem....gonna start swapping plugs tomorrow fingers crossed thats whats wrong ...quick question for you about resetting / getting engine error codes off the car....how????
 

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2000 STS RHD 194,150 miles. what's a speed limiter???
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219 Posts
the FSM says that the motor can go for 100k miles before the plugs need changing, thats by using the recommended OE plugs.
the fact that GM would say their plugs can last 100k miles is good enough reason for using them.
would bosch stand behind such a claim?
 

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'96 Eldorado
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60 Posts
My AC platinums died with only 70K or so (they came in a used engine). I put some basic everyday Champions in and they work fine.

Using a magnetic spark plug socket makes the whole job a whole lot easier. I paid excess bucks for one off a famous-brand tool truck, but you can get one for <$10 from one of the mail-order joints if you don't mind the wait.
 

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2015 Mazda3 S GT Hatchback 2013 Kia Optima SXL
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2,817 Posts
I paid $5 for my spark plug socket, and it works great for the N*. I got it from www.harborfreight.com. It is an extended socket that is about 5 or 6 inches long. To retrieve it from the spark plug hole, just use a pair of needle nose pliers. The socket and the holes are the same depth.

Don
 
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