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Hello guys,

I decided to make a thread on this issue and not just post bits and pieces on other threads. I know this topic has been covered ad nauseam and beat to death, but I've searched through so many threads my eyes are glazed over :zdunno: Bear with my wall of text.

I have a rough idle while in park, and an even worse idle when in drive or reverse. If I give it a bit of gas while in park, the engine starts to vibrate even more, but it drives fine when I'm accelerating and on the highway. My symptoms only appear at idle and at a stoplight or parked. I noticed that it gets really bad when I turn the wheel, run the AC, and even if I roll my windows up or down at the same time.

I started by looking at long term fuel trim on my scanner, and I see that the fuel trim is +-1 of zero at idle, but when I start giving it some gas it starts becoming increasingly negative to about -8. I don't think it helps much considering it may be normal.

It's not the misfire you get when a cylinder goes dead and the engine runs really poor and chugs, it's more of a strong vibration into the cabin and steering wheel.
I have not begun throwing parts at the problem and am hoping to avoid doing so.

Here is a list of what I have done so far:

1. Disabled one cylinder at a time to see if any one cylinder was making trouble and found none (each one I disabled made the car run rougher)

2. Checked fuel pressure. I get about 10 psi key on engine off, 30 psi engine idling. When I disable the fuel pressure regulator I get about a 10 psi increase which means its good. No fuel inside the vacuum hose.

3. Replaced all vacuum lines.

4. Smoke tested system, found no vacuum leaks.

5. Hooked up vacuum gauge and got a needle moving from 19 to 21 in. Hg. Within spec (no vacuum leaks), but Haynes and other tech guides say possible ignition issue if the needle vibrates +- 2 in. Hg. Some forums claim it's normal.

6. Tested EGR with vacuum pump, good. Not stuck open. Only about 2 years old anyways.

7. IAC valve replaced with brand new OEM one about a year ago due to foghorn noise and stalling.

7. Plugs were replaced about 10k miles ago with autolite coppers. Wires were replaced with NAPA blue belden cables about a year ago. I inspected the driver side spark plugs, none were fouled and had very little wear and the correct gap. I didn't check the other bank as I felt it was not beneficial. Ignition coils all gave spark when hooked up to tester. I figured if it's ignition related wouldn't I see misfiring at speed too?

8. Alternator replaced a few months ago. Battery voltage good engine off and idling with load.

What I plan to check again:

- Motor mounts. Normally, I would replace these just because they are likely worn and it wouldn't hurt, but they don't look like the easiest thing in the world to install so I want to be reasonably sure they are bad. I did a power brake test but I did not observe much movement. I'll try to repeat it just to make sure.

- O2 sensors for lazy response

- Unplug MAF sensor to see if there's a difference

I'm at my wit's end with this thing. I could take it to shop, but I always feel crummy when I just can't get to the bottom of it. I'm trying real hard to improve my diagnostic abilities.

Any help or things I'm forgetting, would be greatly appreciated.
 

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7. Plugs were replaced about 10k miles ago with autolite coppers. Wires were replaced with NAPA blue belden cables about a year ago. I inspected the driver side spark plugs, none were fouled and had very little wear and the correct gap. I didn't check the other bank as I felt it was not beneficial. Ignition coils all gave spark when hooked up to tester. I figured if it's ignition related wouldn't I see misfiring at speed too?



Any help or things I'm forgetting, would be greatly appreciated.
Bingo!

Most coppers last about 10k before they give trouble and need to be dressed.

The side you looked at (Driver's is easiest, and wears normal) was 0.054 stock, and now is probably at ~060 or more at 10k miles, and the passenger side, that wears the post, will all be bad, likely burned down inside the center. :)

Our cars wear plugs strange unless you get platinum on both spark surfaces, thus "double platinum." It's called "Waste Spark".

Be sure to blow any crap out of the holes bofore you pull the plugs, unless you want it in your engine.
Air hose +brake parts cleaner. (Pull the boots first...)

Pull them all, labelling the position. Take pix and post them; I might actually be wrong, it happens. :)

Do it cold, not having run the engine that day.

Be prepared to replace the plugs as you pull them, 4 will be bad.

Don't crossthread or overtighten; There are THREE Threads. :surprise:

Put anti sieze on the plug threads before they go back in. that lessens the "Don't remove Hot commandment".
Leaving the threads in the plugs is a distinct possibility. You were Warned!!!




Unless you are running a cam or blower, the best plugs for your car are the stock double platinum ones.
 

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Thanks for the reply Grog, I didn't know these cars used the waste spark system. Learn something new everyday lol. I just removed one of the passenger side, but my camera can't get a good close up pic. It's definitely more worn then the other side. I'll try to get a good pic up, but I'm just gonna go ahead and replace them all with the OEM platinum ones today.

I also came to the conclusion that the car is 22 years old at this point with 103k and the mounts are likely on their way out anyways. I figure that the power brake test only really works if the mount is completely blown, in which case I'd probably loose my teeth from the vibrations. We have a 95 cutlass supreme that had a cracked leaking mount for over a year and mild vibrations, until it completely collapsed in which case the engine felt like it was ready to leave the solar system. I'll put in the plugs first, recheck vacuum, then I'll put in some new mounts (maybe make a write-up considering they look like a pain to install) and report back.
 

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Also check wire routing and make sure that the wires weren’t crossed when changed.

BTW, I’ll be in Maryland from mid-February to mid-March maybe we could get together.
 

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The problem with the non-race coppers is they have a relatively small center; so as it wears, it undercuts the ceramic, and the ceramic cracks, and then the plug shorts and misfires.

I had this exact problem in 2004-5 ish; I could search it out, lol.
(goes and looks)

No, my prob was the splitfire plugs; the centers got destroyed on 4 plugs; it's in my log for Lazarus. At 36k miles, lol. (I guess it was Virginus, almost, at that point.)

That was probably my first stupid mod, my latest being "It must have 4 valves!".

Jeez, all the Fucked up bullshit Ive done to that poor, poor, car. :grin2:
 

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But wasn't the SplitFire triple platinum: https://www.summitracing.com/parts/spl-tp416d ?

I wonder what would cause the center electrode to erode quickly just because there's that V ground. I know the hype was better consistency and more pronounced spark (in the ad graphics at least), but was the center electrode really seeing that much extra electrical energy (or for a longer period) vs. a "regular" double-plat plug? Was the "fine-wire" electrode that much smaller than the OEM plats for our cars? Was it just a cheap ass plating job that led to premature wear?

The company is pretty much belly-up, but apparently electrode early death wasn't the reason: https://en.wikipedia.org/wiki/SplitFire_spark_plug

Another amusing scheme: https://e3sparkplugs.com/automotive-sparkplugs/

Has anyone bothered with iridium plugs in the stock 4.6?
 

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It wears the same amount of material off both sides, that big curved electrode has more area to lose from, and is not going to fail quickly.

Most 'special' plugs have smaller tip electrodes; if it's not platinum or indium, our cars melt them in a short time. :) I've seen them on other people's cars that were melted down 1/4" in the middle, with the ceramic all gone. :) Mustangs... :rolleyes:

There's a reason they went away from our ignition; COP setups don't melt plugs and eat wires.

Copper plugs are Great for racing, where they get pulled every day.

Not so great on a stock car you don't want to wrench on every 10k miles. :)

Platinum wears so slowly that the tiny little puck in there will last 100k; double platinum means two pucks of platinum.

Eventually, the puck melts off, and then it wears out fast; that's where they can claim "over 100k miles".
 
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