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Discussion Starter · #1 ·
Hello All:

I know most of my posts don't require immediate attention (suede this, suede that); but this time, I may have a real problem. Help is appreciated.

After standing in prolonged rain, as in all day and all night, I've now had two occasions where after initial startup, the engine made rumbly vibrations. Rumbly meaning enough to be felt through the entire car, but not severe enough to shake the car.

The first time, I had to move the car, which felt somewhat disconcerting.

The second time, I simply let it idle for a good five minutes when the vibration disappeared.
While it was idling, I filmed the video below. You'll have to listen carefully. Hear past the engine running, and you notice occasional (but fairly regular) ticks. They sound to me like flashes from the coil pack; that's why I focus there. However, no flashes were visible (in hindsight I feel I should have looked at the wire boots on the spark plugs; wires and plugs are new though).

Once past those five or so minutes, the car runs normally. Also, between those two occurrences, there was a dry day where the car started and operated fine from the start.

Thanks!

 

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Discussion Starter · #2 ·
If it matters, I should add that RPMs do not fluctuate at all during the vibration, at least not enough to register on the instrument cluster. RPMs also remained at a normal idle (approx. 800 RPM) throughout the entire time.

When I changed plugs and wires, I did apply dielectric grease around all boots.
 

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1997 Thunderbird LX Sport
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Since you recently changed the wires I would just wiggle and tighten the wire there on the coil pack and see if that makes any difference. I forget... did you change the coil pack when you did the wires and plugs?
 

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I had something similar a while ago that would result in a miss fire bad enough to through a code occasionally. I never did find the exact root cause, but ended up replacing the wires and plugs. Since you have done that recently I would agree with Green T's approach above.
You can also mist some water on the wires at night with the engine running to look for sparks. I was unable to see any in my situation, but at some point in my testing I did hear electrical crackling just never saw it.
 

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The best way to find a leaky spark plug wire is to have your most expendable buddy to run his hand up the wire and see if it shocks him. Worst case you need 8 friends... :)
...Running a ground wire to a long screwdriver, and holding it by the insulated handle is safer and almost as spectacular. If the wire is bad you'll get big sparks to the grounded screwdriver.
Word to the wise: if that happens, do not drop it in your lap!! :ROFLMAO:
 

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Discussion Starter · #11 ·
Dry day today, no issues at startup; however, now I'm getting bogging whenever I drive anything but the most conservatively.

The bogging happens from a stop and at any speed, especially during acceleration but also at cruise. It happens occasionally at a standstill. There are no RPM changes that register on the instrument cluster.

I do not believe this is transmission related since the fluid is all new, shifts feel normal, and engagement between D and R happens without delay.

Help please...
 

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97 Thunderbird 4.6, 98 Mark VIII LSC
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Do you get any smell from the exhaust while it's running rough? Rich or lean?
 

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Discussion Starter · #13 ·
Do you get any smell from the exhaust while it's running rough? Rich or lean?
I have no idea. No opportunity to check. And forgive my ignorance, but how would it smell? I can say that there is no smoke of any color.

It only seems to happen in gear, and most often under load. Revving the engine in Neutral feels normal. There aren't any noises to speak of.
 

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97 Thunderbird 4.6, 98 Mark VIII LSC
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Stumbling is a low-grade misfire. The problem is just about anything can cause it. It's going to be a process of elimination.

A misfire under load is often a spark issue - higher cylinder pressure (most easily encountered at low RPM) requires more voltage for the spark to arc the plug gap. If there are weak links in the ignition system - coil, wires, plugs, or the connections, the arcing will occur where the resistance is lowest. E.g. if the plug boot isn't fully seated on the plug or in the coil.

There's also a chance the MAF sensor is providing erroneous readings to the PCM - its data is used to calculate spark as well as fuel delivery. If it's reading low (often seen if it's dirty) it delivers too much spark and not enough fuel. Under normal conditions the fueling issues are immediately corrected by O2 sensor feedback (assuming the O2s are working properly), but the excessive spark remains. If your condition is mild enough/intermittent enough, the PCM may not be throwing a code for it.
 

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Discussion Starter · #15 ·
Would throwing a code pruduce a check engine light? Because there is none.

I did clean the MAF sensor in March immediately after I purchased the car. I never looked at the O2 sensors given that they're a bit hard to get to.

Would any of this be caused by severe rain. It had been raining almost nonstop for three days.
 

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97 Thunderbird 4.6, 98 Mark VIII LSC
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If moisture gets into the engine bay it could cause arcing issues in the spark plug wells if they get wet. Best fix for that is a big blob of dielectric grease around the bottom of the boot to help keep things dry. There's also the wires at the back of the engine bay that lead to the O2s - heat from the exhaust and time causes the insulation to crack - so if they get wet they can cause bogus O2 readings throwing things off too.
 

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Discussion Starter · #18 ·
@theterminator93,
I took a few days to monitor. To reiterate, we had a few days of nonstop pouring (during which I didn't drive).

After that:
Day 1 was dry but cool. That's when it was really bad.
Day 2 was dry and very warm. I didn't drive.
Day 3 was dry and very warm. I drove about 20 roundtrip. It was still bogging/stumbling at the beginning of the drive, but much less so, and it successively disappeared entirely.
Day 4 (today) is dry and cool. It's not happening anymore with the exception of an initial rumbly idle, but no worth than any cold engine.

So...
I'm quite certain that it was rain related. Wherever the humidity accumulates, it seems to remain for 1-2 days, even in warm weather; that's pretty remarkable.

I did smell the exhaust, but honestly I don't know what I'm smelling for (?). I would say there's some sweetness in the smell.
 

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97 Thunderbird 4.6, 98 Mark VIII LSC
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If this was me, I'd do simple things first just to rule them out.

1) Pull the wires and give them the dielectric grease treatment
2) Unplug the MAF sensor the next time it starts up and runs rough - if there's an improvement it could be a bad sensor
3) Pull the plugs and read them (best done if you shut down after it's run rough for a few moments immediately prior) to determine if it's one particular cylinder missing
4) You could also pull the injector plugs off each cylinder, one at a time, to determine which cylinder is missing (if it's a single culprit)
 

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Discussion Starter · #20 ·
Just an update:

Two more rainy days, first start up, back to crackling sound. I absolutely do not see any sparks anywhere; all there is is the sound. It sounds like a whip hitting. I'm 99% sure it comes from inside the coil pack (if that's possible?).
 
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