Changing Spark plugs and Spark plug wires (Tuneup)

[Qristofer]

I have a 1995 Deville which I've had for about 4 months. Everything has been fine so far except the idle. I don't know how to describe it, but it's a kind of vibration between idle(miss). From my previous car (I had the same prob) I think the problem lies in either the spark plugs or cap n rotor. I have decided to eliminate the problem step by step starting with the spark plugs and wires then the cap and rotor. But therein lies the problem. I went to shop recently where they were offering tuneups for $59 but on getting there I was dismayed to be told that a tuneup on a Deville is around $250 and even higher at a dealership. This is because they had to remove parts to get to the rear spark plugs. My question is how hard is it to change all eight spark plugs(and wires) plus cap n rotor. and can the average Joe with a small tool kit do it.

P/S I know how inflated the cost on these shops are. The Previous time I had to change wires (on a four cylinder car)alone I was quoted a price of $175. Not wanting to spend that much I stopped by autozone, bought $13 wires and installed them myself in 5mins flat.

[Allante North *]

Welcome Qristofer,

I too recently changed my wires, but I choose to go with genuine GM wires thanks to some advice from a GM engineer that worked on the design of my Caddy. He advised that the plugs and wires the come stock on North*'s are the best available. They are not cheap at $100+ for the wires and $8 each for the plugs, but you know your getting the real deal. The example he used was a North* in an experimental aircraft with non AC Delco plugs that fouled and he crash landed. If your plugs or wires fail, you can pull over and get a tow. My advice is to use original equipment and take no chances.

[jadcock]

I would test the resistance of all eight plug wires before replacing the plugs. One of the wires is possibly "leaky" or almost broken. That may be your problem. There is no cap or rotor on a Northstar -- it's a distributor-less ignition system. Everything is controlled by the ignition module, which is mounted to the right side valve cover (at the rear, as the engine sits in the bay). The mechanic is likely talking about this piece that he has to remove to get to the rear plugs. Other than that, they're supposed to be fairly easy to get to. Again, I wouldn't try to change the plugs unless you know one or more is bad. I've never changed the plugs on mine...even at 115k miles, it runs like new, so I'm not going to mess with it.

[Katshot]

Since I don't have my books home here with me, I can't say for sure but in '95 is it possible that you still have a 4.9 engine and NOT the Northstar? Either way, anybody that tries to tell you that it's a big major deal to do the tune-up on your car is just trying to muscle more money out of you.
Unfortunately, you HAVE stumbled on some useful info. Generally speaking, those shops that advertise tune-up specials are only suggesting that they will replace plugs and air filter. Read the fine print where they detail what they ACTUALLY replace. When you start asking for wires, cap/rotor, fuel filter, etc that's when the price goes shooting for the stars.
And one point about "testing" your wires before replacing them, ohm meter testing an ignition wire is all but useless these days. Unless you have access to a scope and know how to use it, forget trying to "test" ignition wires. On Caddy's, I ALWAYS replace wires with plugs, ever since the first HT4100. I've had entirely too many Caddy's come into the shop with a driveability complaint that was then traced to the ignition wire(s), and quite often it's been right after somebody had tuned the car. The wires are very fragile and for that reason I would do plugs and wires as a set.

[Qristofer]

Originally posted by Katshot
Since I don't have my books home here with me, I can't say for sure but in '95 is it possible that you still have a 4.9 engine and NOT the Northstar? Either way, anybody that tries to tell you that it's a big major deal to do the tune-up on your car is just trying to muscle more money out of you.
Unfortunately, you HAVE stumbled on some useful info. Generally speaking, those shops that advertise tune-up specials are only suggesting that they will replace plugs and air filter. Read the fine print where they detail what they ACTUALLY replace. When you start asking for wires, cap/rotor, fuel filter, etc that's when the price goes shooting for the stars.
And one point about "testing" your wires before replacing them, ohm meter testing an ignition wire is all but useless these days. Unless you have access to a scope and know how to use it, forget trying to "test" ignition wires. On Caddy's, I ALWAYS replace wires with plugs, ever since the first HT4100. I've had entirely too many Caddy's come into the shop with a driveability complaint that was then traced to the ignition wire(s), and quite often it's been right after somebody had tuned the car. The wires are very fragile and for that reason I would do plugs and wires as a set.

You are right! I used to wonder why my motor din't have that silver cover (plastic or whatever) That says Northstar V8 (even though the manual talks about Northstar). I now know why!!

Right now I only have 82,800 miles on my deville everything is okay as far as I can tell. I guess I will wait till I hit 100,000 to do the manufacturer recommended tuneup. Time to start saving $$$

[jadcock]

Ah-ha, yes! I must keep reminding myself that '96 was the first year for Northstars in all Devilles, not '95. Maybe we should move this thread to the Deville or Pushrod section -- there may be more 4.9-specific experience over there?

[Katshot]

Yeah in '95 only the Concours had the Northstar.

[Ralph]

BTW, does anybody know if I would have platinum plugs in my 1991 4.9????

[jadcock]

Hard to say Ralph. I think they did from the factory, but the car is 12 years hold now...do you know if they've ever been replaced? If you bought the car new and never replaced them, then I'd say they would be platinum.

[Katshot]

If the maint. interval in the manual is 100K, they are Platinum. To be honest, I don't think they were offered back then though.

[Bruce Nunnally]

My 92 STS 4.9L had/required double platinum plugs, so I would not be surprised if the 91 did as well.

The plugs are not impossible to replace on the 4.9L, but you will want to do it with the engine cold, since you have to lie across the engine and do the back bank by feel. A spark plug wrench with an articulated head helps. Be careful, as there is a sensor between the two plugs on the back bank -- on the driver's side -- so don't take it off which wrenching on of those plugs.

I have heard that at the dealer what they do is disconnect the front engine mounts and roll the 4.9L engine forward to access the rear plugs more easily.

Try gmotors.com, gmpartsdirect.com, or parts.com for discount new AC/Delco wires and plugs.

[Ralph]

Thanks for the tips guys. No the plugs were never done (low mileage) but I will eventually. I think you are right Kat, I don't think they are platinum because #1 the change interval recommended is 50,000kms and #2 there is no "P" prefix in the part number for the plugs. (a friend mentioned this to me and maybe you guys know if it is true). As for the plugs along the firewall bank, yeah, I used to have to sit on my Pontiac air cleaner and do them by feel, not as bad though 'cause there were only three there. Bloody knuckles afterwards.

[Ralph]

Originally posted by Bruce Nunnally
My 92 STS 4.9L had/required double platinum plugs, so I would not be surprised if the 91 did as well.

The plugs are not impossible to replace on the 4.9L, but you will want to do it with the engine cold, since you have to lie across the engine and do the back bank by feel. A spark plug wrench with an articulated head helps. Be careful, as there is a sensor between the two plugs on the back bank -- on the driver's side -- so don't take it off which wrenching on of those plugs.

I have heard that at the dealer what they do is disconnect the front engine mounts and roll the 4.9L engine forward to access the rear plugs more easily.

Try gmotors.com, gmpartsdirect.com, or parts.com for discount new AC/Delco wires and plugs.

Yeah the dealer disconnects the engine mounts and rolls it probably just to get more labour out of suckers like me!

[Katshot]

The plugs don't have a "P" prefix. I believe the AC plugs use a "41" series (prefix) which designates them as Platinum.

[Ralph]

Originally posted by Katshot
The plugs don't have a "P" prefix. I believe the AC plugs use a "41" series (prefix) which designates them as Platinum.

Damn, I don't have the "41" series prefix. Man, looking at the far bank, I have no idea how I will change the plugs. One can hardly get a slip of paper through there let alone my hand! I guess some things are best left to the experts (mechanic). Newer cars recommend 166,000 miles before a tune-up, but I would never let it go that long for my own personal vehicles. I think that number is a bit on the high side for people that like to neglect their cars, etc. After my pcv valve froze up on my Merc. and cost me 1200 in new oil seals, I like to stay on top of things, but I cannot find the pcv on the 4.9!!