: 93 Seville 4.9L sporadic miss at idle. Spark Plugs?



Hairball
12-13-10, 11:41 PM
I just started noticing my Seville has a very slight miss when I'm sitting at a light idling. It's not a regular miss like a bad valve or something but more sporadic like maybe it needs plugs.

BUT, now I'm worrying!!! I'm hoping it's just a matter of new plugs and wires and not something major! If I lightly let off the brakes, I can really feel it.

Anyone have any better luck with a certain brand/type of spark plug for the 4.9L? I've run Autolite in 99% of my cars over the last 25 years or more, but they're inventing new spark plug technology almost daily!

Are those Iridium plugs worth the money? Or should I just replace with OEM spark plugs? Or just ANY plug?

the recluse
12-14-10, 09:58 PM
The Iridium plugs are awesome. I haven't tried them in a 4.9 yet, but run them in my motorcycle and my dads 350 in his Chevy van. I've run them in a variety of other vehicles with some 70% of these vehicles showing much improvement over other plugs. They're no good for a Northstar motor.

Do NOT run Bosch Platinum's (or any Bosch for that matter) in the 4.9. COMPLETE waste of money with massive reduction in power/performance.

I run and would install the appropriate A/C Delco plug for that car/motor. Look it up at your local parts house.

Now, with hesitation that you speak of, it may just be an old fuel filter or condensation in the lines/tank. When was the last time you replaced the fuel filter? It may be time. When/if you replace the fuel filter, collect the fuel that drops in a bucket or bowl. Look for water in the fuel; it will be on the bottom below the fuel. If you find water, go to the store and get some Isopropyl alcohol (70%) for about $.75 and dump half a bottle to a full tank of gas.

See if that fixes the issue. If not, then look into plugs and wires. That there will be the best preventative $15 you ever spent....

Hairball
12-14-10, 11:14 PM
The Iridium plugs are awesome. I haven't tried them in a 4.9 yet, but run them in my motorcycle and my dads 350 in his Chevy van. I've run them in a variety of other vehicles with some 70% of these vehicles showing much improvement over other plugs. They're no good for a Northstar motor.

Do NOT run Bosch Platinum's (or any Bosch for that matter) in the 4.9. COMPLETE waste of money with massive reduction in power/performance.

I run and would install the appropriate A/C Delco plug for that car/motor. Look it up at your local parts house.

Now, with hesitation that you speak of, it may just be an old fuel filter or condensation in the lines/tank. When was the last time you replaced the fuel filter? It may be time. When/if you replace the fuel filter, collect the fuel that drops in a bucket or bowl. Look for water in the fuel; it will be on the bottom below the fuel. If you find water, go to the store and get some Isopropyl alcohol (70%) for about $.75 and dump half a bottle to a full tank of gas.

See if that fixes the issue. If not, then look into plugs and wires. That there will be the best preventative $15 you ever spent....

That's good to know about Bosch. Ya know, this may sound odd but I've run NOTHING but Autolite in EVERY car and motorcycle I've owned in the last 20+ years because they tend to outlast every plug I've ever tried. Not sure how they would do in my 4.9L though.

My car has no hesitation, just a very slight occasional light miss when I'm idling in gear at a light. It's very light and random, but I really feel it if I slightly ease up on the brakes. It really does sound/feel like it just needs plugs. Maybe wires, but I'm guessing plugs. I'll check out the AC plugs as you suggested. My PROM chip came in, so I finally get to go back to the dealer in the morning and have it installed. They said I would probably notice better gas mileage with it but I highly doubt it.

cadillac_al
12-16-10, 08:34 AM
"Do NOT run Bosch Platinum's (or any Bosch for that matter) in the 4.9. COMPLETE waste of money with massive reduction in power/performance.
Read more at http://www.cadillacforums.com/forums/ht4100-4-1-4-5-4/217472-93-seville-4-9l-sporadic-miss.html?ktrack=kcplink"

Poor old Bosch getting a bad rap again. I have used them in ALL of my cars for the last 15 years with zero problems. I am even using them in a waste spark ignition Buick for the last 7 years. I am going to pull them this week to see if the waste spark system has caused them to break down at all. My 4.9 Caddy always starts in below 0 weather and lugs up hills at slow speeds with no missing or chuggling. I think the Bosch plus 2 and plus 4 plugs seem gimmicky and give them a bad rap for overpriced gimmicky plugs but the plain old Platinums are tried and true

I got sold on the Bosch platinums when I put them in my 76 Camaro. The Camaro always had a certain stubborm personality when starting and I always put in new AC plugs in every couple years. It was always like this for 15 years; 1980 until 1993 or so. Then one day I got the idea to try these Bosch platinums and the Camaro started like a normal car with no personality issues. 15 years of stubbornness gone with a simple plug change; I have been sold ever since.

drewsdeville
12-16-10, 08:58 AM
Same. I've used Bosch platinums on all of my 4.9's, and have Bosch coppers in my current '90. All purr nicely. My '90 routinely displays 27mpg on trips up north and performs just fine.

In the end, they all do the same thing. It's a friggin spark plug and aren't the rocket science that advertising would have you believe. They are all just 2 electrodes with a gap between them, with a porcelain insulator. As long as they aren't defective, they are going to work regardless of brand. Spark plug discussion is about as informative as the stupid oil discussions that happen here.

Any spark plug gapped at 0.060" with the correct heat range will work just fine.

the recluse
12-16-10, 05:32 PM
Poor old Bosch getting a bad rap again.


Same. I've used Bosch platinums on all of my 4.9's

I don't know, maybe it was just me, but I put those plugs in and lost 2 to 3 mpg and the motor lagged its ass off :noidea: I put the Delco's back in and it runs great.

Not all plugs do the same job in all motors. Strictly looking at it from a non-performance basis, yes, ANY plug will fire. However, if you plan any performance mods, your choice of plugs is a must study. :yup:

For instance, platinum plugs are not recommended in a car running nitrous, as platinum oxidizes at higher temps already, and so adding an oxidizer (and one that adds heat to boot) the life expectancy of that plug is reduced and the tip can possibly separate. You don't want that at 6000 rpm. :helpless:

With the differing heat ranges, fouling characteristics, electrode wear, etc., one can solve a lot of running issues on both street and strip. He stated it here:


Then one day I got the idea to try these Bosch platinums and the Camaro started like a normal car with no personality issues. 15 years of stubbornness gone with a simple plug change;

Oil talk I can agree with you on, useless;:thumbsup: plug talk, well, not so much. :nono: But then again, I like to modify the crap out of everything :sneaky:

drewsdeville
12-16-10, 08:32 PM
However, if you plan any performance mods, your choice of plugs is a must study. :yup:


Sure, I can agree to this, but most of the spark plug discussion on CF.com, including this one based on the OP's 1993 Seville, isn't about modded or built engines. It's about replacing spark plugs for maintenance reasons on stock engines. In the case of a stock engine, the manufacturer already selected the correct gap and heat range for you, and I'm sure they've done far more homework than any of us.

AC Delco, Champion, NGK, Bosch, Autolite, and any others will most likely all perform exactly the same on a stock 4.X. Usually the only time they don't is when they've been improperly handled/installed and are cracked/broken as a result.

the recluse
12-17-10, 10:11 PM
In the case of a stock engine, the manufacturer already selected the correct gap and heat range for you, and I'm sure they've done far more homework than any of us.

:thumbsup: