: Engine 'misses' at stoplights



human
08-25-09, 02:42 PM
I just got a '97 Deville Concours from my dad. Pretty sweet car with 80k on the odometer, albeit a bit on the big side. The car runs great going down the road, but sitting at stoplights, it feels like it's misfiring every 3-5 seconds. I can alleviate the symptoms somewhat by putting it in neutral, but its still there.

A couple of possible contributing factors: Dad's had the car for about three years and during that time he burned mostly regular gas in it. I've had the car for two weeks and am on the second tank of premium. I've noticed a slight improvement or else I'm getting used to it. Last fall, the car started running really badly and a mechanic friend of his replaced one of the four ignition coil packs. The coils appear to be a motley collection. Two appear to be original with white numbers on them to indicate which cylinders they fire. The one that was replaced last fall is obvious because it's much cleaner than the others and has no numbers. The remaining one is also without numbers but is about as dirty as the two numbered ones, so it's likely it was replaced some time ago.

Any ideas would be greatly appreciated.

Submariner409
08-25-09, 02:57 PM
Sounds like the ignition system could use a bit of TLC. www.rockauto.com has coils (DELPHI OEM units) for $34 each, wire sets (DELPHI OEM) for $77, and plugs (ACDelco Platinum #41-950) for $6 each in an 8-pack.

So, for about $260 you can do a $700 "tuneup". Clean all the ICM connectors while you're at it.

The plugs go in COLD at 13 ft/lb of torque, no anti-seize, gap .050". You might find a bit of oil in a plug well or two - normal, as the wells are sealed with O-rings and they tend to weep.

Use the diagrams for wire hookup - it's important, as the engine uses a "waste spark" system in which 2 plugs fire at the same time. Google.

If the plugs that come out are branded DENSO, they're the originals - the newer ACDelco Platinums are also DENSO and Made in Japan.

If you're into fuel additives, fill up with Shell/Chevron/Texaco 93 and ONE TIME ONLY dump in a 20 oz. jug of Chevron TECHRON. Google "top tier gasoline".

Ranger
08-25-09, 04:27 PM
Dad's had the car for about three years and during that time he burned mostly regular gas in it.
Not a problem at all. The engine has a very effective spark knock sensor in it and will retard the timing appropriately if a knock is detected long before you can ever hear it. I burned 89 in my '97 for 3 or 4 years before I sold it. Never had a problem or noticed any difference.

Skiller.
09-08-09, 10:44 AM
Sorry about the bump, but I was also having a similar problem with my SLS, but with mine, I had a cylinder 2 misfire because I believe my spark plugs have never been replaced...I just bought the car a month ago with 96k on it. I changed that 1 spark plug to see if that was the issue, and it was...and I also have a little oil on the end of the spark plug, which worried me. I'm going to go through with the tune up suggested above and see how it goes.

Glad I discovered these forums :)

Submariner409
09-08-09, 01:08 PM
Welcome Aboard !!! :welcome:

Post your car and history down in Member Introductions, read the entire Cadillac Technical Archive up in the black bar ^^^ and wade through as much of Seville and Northstar Performance that you can tolerate.

Ranger
09-08-09, 08:59 PM
and I also have a little oil on the end of the spark plug, which worried me.
Don't worry about it. It's seepage from the O ring in the cam cover that seals the spark plug well.

IXSLR8
09-09-09, 04:26 AM
I noticed that you said no anti seize compound on the spark plugs. Why? Does it foul them or cause a misfire?

Submariner409
09-09-09, 12:00 PM
The GM manual is antsy about anti-seize compounds in an aluminum head. Exactly why, I'm not sure. The manual recommends changing the plugs with the engine cool and torque to 13 ft/lb and that's it. Maybe they feel the compound gives a false torque reading. Unless you really get carried away with the stuff it should be OK.

Maybe a few phone calls to Trick Flow, World, Edelbrock, or AFR - all makers of aluminum engine heads - would shed some light on the subject.

EDIT at 1205: Just spoke to a machinist at Dart Heads. A small dab of anti-seize is OK - don't get carried away and don't do it every plug change (that's based on frequent tuning changes, though).

Skiller.
09-09-09, 09:03 PM
Ok great, thanks!

Ranger
09-09-09, 10:15 PM
Anti-Seize is conductive so if you get sloppy, yes it can cause a misfire.

As Sub said, it will affect the torque. That's one reason you don't use it on head bolts, but I never torque plugs anyway.

NHRATA01
09-13-09, 11:35 AM
Anti-Seize is conductive so if you get sloppy, yes it can cause a misfire.

As Sub said, it will affect the torque. That's one reason you don't use it on head bolts, but I never torque plugs anyway.

It will affect the torque but in a way such that it gives a more true reading - you don't want any sort of grime any friction causing material to give a false torque reading as then you don't get the proper stretch out of the bolt. ARP actually includes a thread lube for their head bolts. I thought GM recommended either a lube or a sealer for factory bolts but I can't recall off the top of my head, and GM factory LSx bolts are torque-to-yield while ARP's aren't.

I've always used the anti-sieze on plugs, moreso on an aluminum head because its easier to wreck the threads if the plugs seize over time (especially likely with these 100k recommended intervals!). Strange that the Northstar shop manual shies away from it; the LS1 shop manual actually recommends it.

Ranger
09-13-09, 11:51 AM
Factory head bolts had a sealer applied to them. So do the bolts you get from the dealer. Something similar to Loctite I think. If I remember correctly, I thought that the plugs had a special plating on the threads to eliminate the need for anti-seize and prevent galvanic action and such. When I removed mine at about 80K (on my '97) they came loose with no problems.

Skiller.
09-13-09, 07:32 PM
I think the plugs I removed were way over cranked on my car..I could barely get them off..I put the new plugs on just tight enough so they are snug.

Submariner409
09-13-09, 07:40 PM
I'll create a bit of work for you: Spark plugs should be torqued to the recommended spec. Why? First, the plug has to stay in the cylinder head under some pretty radical heat-cool cycles. Second, and most important, the correct plug torque assures the correct rate of heat transfer from the plug shell to the head and cooling jacket.

The "heat range" of a spark plug is based on the ease, or lack thereof, of heat transfer from the center electrode and outer shell to the cylinder head metal. A plug that is too loose will tend to run hot (poor heat transfer), burn electrodes, and create a possible preignition condition.

The second paragraph explains why we now have tapered plug seats instead of crush washers: easier to control heat transfer on a metal-to-metal contact surface than through a washer.