[Q] interesting spark plug issue

iJimaniac · Apr 16, 2015

Another long time stalker surfaces!

So, I've recently acquired a 94 4.0 ranger with "supposedly" 145xxx, but seems to me it might be 245xxx. Regardless, it runs okay, but kinda shakes on the highway and idles a little rough. I'm not sure if the shaking is the engine is misfiring or just wacky alignment/etc.

Anyway, pulling the plugs to check em out, and came across something. 5 of the plugs look just like the plug on the bottom of the photo, which looks normal to me, but there's the one on top that looks a bit more ominous. It just so happens that the plug that looks kinda bad also has a split boot that the previous owner just taped up. I'm a little green (only 18 here), and was wondering what you seasoned people think about it? Could the cracked boot have caused the spark plug to look like this, or am I facing an ignition problem in that cylinder? Could a split boot cause misfiring? I don't think it's misfiring (I don't here any "knock"), but again I'm green, so I suppose the shaking could be misfiring and I don't recognize it.
Thanks for your time, I look forward to spending some more time on these forums, and maybe even contributing! :yahoo:

The image won't embed for some reason, so here is the picture; https://docs.google.com/file/d/0B9pFRXtlF8PQU1NIOGhTQ2xNQUk/edit?usp=docslist_api

Earl43P · Apr 17, 2015

Welcome to The Ranger Station!

Spark plug wires aren't very expensive.
It is the obviously first part to replace in this case, even if you have to throw a good used plug wire in there.
I would clean and gap that ugly plug and swap its position then recheck them after some miles. Plugs are typically ~$3 each, just about everywhere.

Most of us, with $ available, would replace that set of plugs and wires, with new (Autolite or Motorcraft plugs).
Your wallet may vary.

kimcrwbr1 · Apr 17, 2015

Check the compression three times engine cold with a good battery. The numbers should be within 10% between the high and low numbers. You can do a complete tunup wires, plugs, PCV, oil. Get the OEM motorcraft plugs and a can of seafoam and do a good decarb.
Then you can diagnose further if necessary.

iJimaniac · Apr 17, 2015

Alright, thanks to both of you for your input! I was thinking I want to do a compression test, but then again I'm tight on cash so maybe ignorance is bliss in this case [emoji14]. But I probably will anyways, and just hope I don't find anything bad.

I'll definitely be replacing the wires, maybe the plugs if I find the cash.

RonD · Apr 17, 2015

And just as a heads up, Ford uses the Waste Spark system which means spark plugs are hooked up to a coil in pairs, so 2 spark plugs fire at the same time, one spark is "wasted" on the exhaust stroke, hence the name.
To do this they had to hook the spark plugs up in series wiring, so one spark plug sparks from center to tip, and the other from tip to center.
Because of this the original spark plugs in the engine would use 3 spark plugs with platinum tips and 3 spark plugs with platinum centers, so two different spark plug numbers

The heads up is to use double platinum spark plugs, or 3 spark plugs will wear out faster than the other 3

JimRummy · Apr 17, 2015

Interesting, which cylinders need to double platinum? Last time I picked up I only got regular platinums, which need the double?

RonD · Apr 17, 2015

JimRummy said:
Interesting, which cylinders need to double platinum? Last time I picked up I only got regular platinums, which need the double?

Well that's the thing.........they all do, lol, unless you use two different plugs, platinum tip on one bank and platinum center on the other bank.
So did you get "regular platinum" centers or tips?
Double has both

Cylinders 1, 2 and 3 get worn tips
4, 5, and 6 get worn centers

I posted it here: http://www.therangerstation.com/forums/showthread.php?t=160371

First line has a link to pictures that show what happens in waste spark systems

JimRummy · Apr 17, 2015

Good write up. I'm not sure exactly what I have in there.. whatever stupid AutoZone said it took. All I know is they are all the same.
AutoZone telling me I should purchase a set of plugs that will wear out prematurely and cause me to buy more? Impossible! lol

RonD · Apr 17, 2015

Yes, go figure, lol.

I read somewhere that at the Ford factory they installed two different models of spark plugs on the same engine, read something else than confirmed it, that eventually lead me to that post with the pictures...............which are hard to contradict as to the "why use 2 different models of spark plugs".
It may also explain why some "other" brands don't seem to last as long, especially in my old 4.0l

enjr44 · Apr 18, 2015

IF you are really short of cash, you can get a single plug wire from any parts store. Just take to old one with you or measure its length with a string. Or go to a wreaking yard and pull one. They probably won't ever charge you for it if you cry some. LOL

iJimaniac · Apr 19, 2015

enjr44 said:
IF you are really short of cash, you can get a single plug wire from any parts store. Just take to old one with you or measure its length with a string. Or go to a wreaking yard and pull one. They probably won't ever charge you for it if you cry some. LOL

Lolol, I'm not that bad off.

So, I replaced the wires and did a compression test on all cylinders. Here are the results in PSI;

Back of engine
135 140

115 135

115 135

So, do those look acceptable for a 21 year old truck? The two side by side 115's scare me a little. From what I understand, that indicates a bad head gasket. Are those values still within an acceptable range, or should I be looking at doing some work on it?

Also, after I installed new wires, it seems to run smoother at high speeds, but I'll have to wait for my Monday commute to be sure.

As a side note, it gets great gas mileage (at least I'm assuming 20-21 is great for an old 4.0), regardless of low compression.

iJimaniac · Apr 22, 2015

Any opinions on these compression results? Am I driving a ticking time bomb? XD

RonD · Apr 22, 2015

If you are getting 20mpg with a 4.0l then....................your math or odometer is off, lol.
16 around town, maybe 18 if whole tank was used at 55mph..........with a tail wind

Yes #1, and #2 are very low, and yes most likely a gasket leak between the two cylinders, assuming you are not overheating or losing coolant.
But above 100psi means cylinder should fire, just doesn't provide as much power as the others.
Compression is the "rubber band" the tighter you wind it, higher it is, the more power you get when you release it, cylinder fires.
High compression engines have more power because of this, they also can't run regular(87 octane) gas because of this.

4.0l OHV should be about 165psi, when testing with all spark plugs removed and a good battery to spin the crank at full speed, crank speed is the key to get good compression numbers.
My '94 4.0l still gets 155-165 on all 6, 300k on the rings, heads have been off and rebuilt.

But your test is fine, main reason for this test is not high numbers, the numbers are secondary to the DIFFERENCE in the numbers, and as you surmised, #1 and #2 seem to have a problem.
You can retest and put a teaspoon of oil in the spark plug hole before the test.
If 115 jumps up close to 135 then the rings are the problem, if it stays below 125 then it's a valve or gasket issue.

Is it a time bomb, no I wouldn't think so, but I would investigate it more

You can do a Leak Down test on #1 and then see if you hear air escaping in #2.
Leak down test is when you put a known pressure into spark plug hole when cylinder is at TDC, both valves closed.
And read how much pressure is lost, new engine leaks about 5%.
So if you put 100psi pressure into cylinder and pressure gauge reads 95psi then you have 5% leak down, 5psi is leaking out the rings and valves, metal on metal doesn't seal well, but it isn't suppose to, lol.
In your case you would be more interested in listening at the #2 spark plug hole for escaping air, that would confirm a head gasket failure.

JimRummy · Apr 22, 2015

I actually get really good mileage on my 93 4.0 . I get about 18city 24hwy. Last trip I got over 250 miles from a half tank, and that was averaging about 75mph, and with a decent amount up hill. (course this is after just putting in New plugs, wires, water pump fan and fan clutch, and the truck only has 116k miles on her.)

iJimaniac · Apr 25, 2015

I assure you, those are my actual mileage results. I've been presently surprised as well.

it is 2wd, which I'm sure makes a difference.

Thanks so much for your input, I've got some stuff to work on! (Including getting this Tapatalk app to notify me when someone replies to my thread)

[Q] interesting spark plug issue

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