Hey Folks,
(bit of history)
Recently just did a spark plug and wire swap on my 98' F150, this truck was my dad's and I got it around 27k miles, currently has 72k on it.
Dad said it was suggested the spark plugs were bad when I got it from him, now I think he was right. The original plugs were pretty much burnt the platinum tips were gone
all the old plugs gap measured around 0.80 - not sure how it was running with that large of a gap.

Dad never ran nothing but 87 oct fuel in it and upon me getting the truck I had to up the octane, added fuel injector cleaners and such and have currently been running 93 oct that way since just so the truck wont sound like
a garbage disposal with a fork pinging around in it.

My thoughts were 25-30k is too early for spark plug swap, and I prob should have done it anyway around 60k - so I'm a bit late. But it's done, just installed new Motorcraft wires and spark plugs about 2 weeks ago.
Truck currently still has some 93oct in it, but it starts quickly now, accelerates with barely touching the gas pedal, much better than before.
new plugs I went with 0.54gap.
I also had to replace the fuel vapor hose that runs from the firewall driver side to a pipe connector on top/middle of the engine, original hose was deteriorated and mice chewed.
The issue:
Since doing this work I've noticed a fairly loud new noise ONLY on engine shutdown.... almost sounds like a large amount of air escaping in a rush or like a HVAC unit makes the sound doing it's cycle.

I wondered if I somehow cross threaded a spark plug, but I can't hear a leak while engine is running and after you shut off the engine it's a slight pause, the 'whoosh' like pressure being let off.
Sounds like it's on the passenger side of engine best I can tell.
When I removed the old plugs, I did one at a time, wire brushed the threads, oiled them with some marvel mystery oil, re-inserted the old plugs by hand to run the threads good before I
put the new plugs in, I did have to use the spark plug socket and extension to get them started -but did by hand, once after 4-5 turns and they started to get tight I used the ratchet to tighten.

Truck runs too good and i'd assume i'd have a constant noise with engine running but with the fan and all going it's not sticking out as a leaking spark plug, just one big whoosh after engine shut down.

I know this is fun to describe but anyone ever noticed this before?

My only other thought to test is to pull all the wires off the coil and crank (but not start) the engine to remove the idea of a cross threaded plug, I think this should help prove that but have not tried it yet.
Thinking the turning over of the engine would produce the pressure and if I got a leak i should hear the same noise.
If I don't, then it could be vacuum related somehow, really not sure, kinda stumped on this one.

This isn't my first rodeo on installing spark plugs, done it many times on different vehicles over the years, but this is the first time I've ever experienced this, and not saying I'm perfect but this is a odd one.

Hey Thanks for any help/tips/suggestions.
T

 

Not much of an update but I ran this by a few mechanic friends and seems what i may be hearing could be a vacuum leak - NO idea where/why this would have popped up but i'll have to get a helper and see if we can pinpoint
where the 'whoosh' noise is coming from.

IF anyone has experienced this before I appreciate any help on this one.
Thanks!
T

 

Could be backpressure in the exhaust system being expelled.

 

hey Alloro, thanks for the reply, and yeah I've read up on this as a possible issue - weird thing is this all happened after the plugs and wires were replaced so it's highly likely i've cross threaded something, got a loose vac hose or a bad plug (at the moment).

I'll not rule it out (exhaust back pressure) as it is all stock and looks it, probably not far away from a muffler replacement it's kinda crusty.
A friend that has a 03' f150 recommended a possible leaking spark plug - as in leaking past the ceramic, not the threads - apparently, he had that happened to him once. I've never seen that before.

Well either way I'm going to have to find someone to help me try to diagnose this, too difficult to do alone at the moment, I can't put my hands on my 'remote starter button' yet, if I could find that I could undo all the sparkplugs and turn the engine over (just not start it) and see if i can find
any escaping air pressure. (it's one of those tools I've used like once since I bought it)

Thank you for the input I'm working a 1/2 day today and will try to work on this later this afternoon and see if I can at least diagnose the source of the "whoosh" noise.
I'll report back what I find.
Kinda thinking plan b: I could just get 3 more spark plugs and replace the passenger side (where it sounds like it's coming from) or even install the originals 1 at a time on that side and see if i can get the noise to go away, if it's the "plug itself" and not a cross thread
that could work too.

I hate it when stuff just doesn't work right the first time! (although I should have learned to expect it by now! LOL).

Thanks for the suggestion I appreciate the help!

T

 

Ok after some more investigation I think I've found the source of the "whoosh" noise on engine cut off.
I found my bump starter switch so I cranked the engine a few times and found nothing.
Had my wife start the truck - let it run a few sec and turn off a couple times and it appears its this part (bottom pic) but I took a few pic's to help locate where it is on the motor.
I don't have a repair manual yet for this truck but just wanted to show the group.

Now, here's the weird part... went to the gas station later that day, heard it clearly at the pumps when engine cut off.
Back home barely heard it, so I don't know if it's getting quieter or was a trick of sound play.

But doesn't' appear to be the spark plugs or any vac hoses that I can find.

Anyone know what that thing is in bottom pic?
Should i be worried here or am I freaking over nothing?

Also as part of my test, I got about 5-10 gal of 89 octane vs 93 premium, wanted to see if the new spark plugs change the way the engine runs/requiring 93 cause without 93 (on old plugs) would require a double start to get engine running
and any lower octane just pinged the engine like crazy, especially in winter, 89oct would barely start the truck. 93oct worked every time.

So far (minus this whoosh noise the truck seems to be running fine now with new plugs/wires).

Well, Thanks for any input yall...
T


1998 4.2l v6

1998 4.2l v6 - unknown part

1998 4.2l v6 - unknown part close up

 

That's the purge valve. Try this, next time before you turn the engine off, remove the gas cap, then turn the engine off and see if it still makes the whoosh noise.

 

hey Alloro, ok thanks for identifying that part.
So I tried that test.

1st try: still got the whoosh noise, put gas cap back on and truck wouldn't start. (took like 3 tries and it finally did).
2nd try: still got the noise but it started the first try.

Not sure if this means anything but that's what testing resulted in.
I wondered if the reason i never noticed that noise was due to the fuel vapor hose I replaced had to be leaking, now with a new hose it's changed the rule somehow? (total guess) but besides plugs and wires that hose also got replaced.

If you look at the post with the 3 pics, the top pic....just above the Ford Oval in the plastic engine cover that says 4.2l you see a white plastic plug thing on the firewall,
the hose i replaced is right above it, black hose with a grey connector, it went to a metal pipe on top of engine just under the intake.

Well thanks again for any input here, so far truck seems to be running fine...Just becoming a rolling wasp nest hotel, geez, had one in the gas tank area and one under the driver rear tire!

T