Trying to diagnose a slight rough idle/ missing out

Well as of late I've noticed my truck idles slightly rougher than normal and also a "Thwarting" sound that is heard best in the cab and when the truck is fully warmed up. Best way to describe the sound is to repeat "Th Th Th Th Th Th Th" over and over. The "sound" kinda sounds like it could be an exhaust leak but all that checks out fine. I've never changed the wires or ignition coils so was thinking maybe that could be it?

There are no engine Codes being thrown so that's no help. This has been going on for 2-3 months now and I figured if it was a bad coil pack or bad wires then it should have thrown a code by now. The plugs are NGK dual electrodes and are maybe 3 months old.
I'm just taking guesses at what it could be and I can't afford to waste money on new ignition coils and wires at the moment if that's not going to solve the issue.

Could a faulty spark plug wire cause a slight rough idle and make the engine miss out. The truck starts up fine it's just after it warms up the seemingly thwarting/ knocking sounds is more noticeable as well as the rougher idle. It's not bad to where the truck wants to die out but it's not smooth like it was a couple months back.

Any help is greatly appreciated!

 

Side note... The spark plugs were never gapped if that makes a difference, I just took them out the box and installed.

Also, I changed from single electrode NGK plugs for the reason of the issues I'm having in my post above ^
My truck ran fine with the stock denso plugs that had never been changed for 164k miles. So when I bought the truck I changed the plugs 3 months later to the single electrode NGK's. Then after 4-5 months I noticed the rougher idle and thwarting sound, so that's when I changed to NGK dual electrodes which didn't cure the rough idle and here it is 3 months later and the issue still present, hasn't changed.

 

Did the previous owner give you paper work showing when the last tune up was done ?
New wires , Denso plugs . Clean the throttle body , mass air flow sensor and the idle air control sensor , air filter etc etc .

 

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This & pretty sure theres codes being recorded your just not able to see them unless you have a decent code reader, if you do then re test, with key off plug into service port inside cab, turn key all the way to ignition & see if anything pops up on scan, if not turn off key unplug & re plug into port, this time start it up.. if nothing still doesn't show up as error codes, something else is amiss, what you describe should show up as a possible misfire, & most all other codes are going to be emissions related & those ALWAYS show up from what I've seen in past, alot of error codes are just noted & stored..

 

No there were no previous records from the PO but he has been upfront with me about everything and I still call him from time to time to ask about certain things. I've had the truck a little over a year and the misfire started to happen about 3 months ago. So for the first 9 months of ownership the truck was running fine. About 6 months into ownership I decided to give it a tuneup my self and change the plugs, oil, air filter and seafoamed through the intake. Then 3 months later is when I started to notice the misfire/ slightly rough idle. That brings things to the present time where I've already tried new plugs again (dual electrode NGK) which are about 3 months old now. I'm gonna purchase some cheap coil packs and see if those make a difference. If so I'll save up and buy some DENSO packs.

 

So you're just going to throw some $$$ at it & see what sticks ?? If you go to AAMCO or Vato zone or O'reillys they'll scan it free still , & if a misfire code shows up I'll tell you now that its going to be a plugwire about 99% of the time

 

That's the thing... I have a ultra guage that shows nothing. So I thought maybe a scanner would show something so I went to auto zone and still no codes. Can't figure it out. The coil packs are only $50 which I don't have to spend but I thought if it works then awesome! If not then I'm glad I didn't spend $200

 

NKG plug wires, Advance auto, apply one of their 20% or 30% coupons (they got them all the time). order online, pick up at store, about $20 or so...

It solved all of my 'slight misfire and rough idle' problems, much cheaper than coil packs....runs smooth like new engine (252,000 miles)

 

Got some NGK's on the way from rockauto and DENSO dual electrodes just to replace again.

 

Huh.. they couldn't pull anything up ? You've got a problem with the service port sounds like, I have yet to see a misfire code not show up when engine is running displaying the symptoms you describe... I'd try new plug wires first before buying coil packs, there IS the possibility that its fuel/emissions related, there again those types of codes are almost always thrown, if not displayed, they're stored...

 

I did have the P0420 code being thrown for the last year. So 3 weeks ago I replaced the cats with magnaflow California grade cat converters and I replaced the O2 sensors with DENSO's from rock auto. I knew it would take care of the P0420 code but was also hoping it would fix the "thwarting" sound and smooth out the idle. But no dice! I haven't had any P0420 codes being thrown, which is awesome but now want to get to the bottom of the slightly rough idle and thwarting sound. Also worried that maybe the misfire might ruin the new cats. I've had the P0420 code being thrown for the past year but the rough idle started 3 months ago

 

I guess I'm looking at it as it's time to replace anyway since they've been good for 175k miles. The wires are the OEM ones so I should have changed those when I first bought it along with the plugs. I don't know how often coil packs go bad/ fail but if I do need new ones then I'm glad they lasted 175k miles

 

For anyone who is experiencing rough idles here is some info I found out today about mine...

Took it to the professionals today and had it hooked up to a couple different machines and found out NOTHING! He said all signs are good and where they should be. Compression tests are good too. One thing he did question was me dumping the end of the exhaust just after the axle on the turndown. When I cut the exhaust a couple months back made a small cut about 5" from where I actually cut the exhaust. He says it's a possibility that the small incision at the end of the exhaust is making the sound of an exhaust leak which would explain the "th th th th th th th" sound. As for the slightly rough idle he says to try some BG through the intake and then only use premium grade fuel. I've read enough stories to believe him about both BG and good gas, just not so sure that is gonna solve the idle, but won't hurt to try.

 

UPDATE:

Rotated the thermostat so the Jiggle Valve is in the downward 6:00 position. After doing this my temps went down from 208 to 190. Truck has been running so much better and seems to have smoothed out the rough idle and Thwarting sound I was hearing.

I had the timing belt done about 10 months ago and when they replaced the thermostat they put the jiggle valve up (like most vehicles) but for our engines it has to be down. Well I do remember starting to feel the rough idle about a month after the T-belt job so seems like the engine running warmer than its used to could have been the culprit for me. I'm praying it stays running strong because it feels good to know your truck is running as it should.

for anyone experiencing a rough idle and maybe what sound like an exhaust manifold leak or knock but no codes being thrown, check the temps with an Ultra Guage or similar. If they are in the mid - high 200's then check the thermostat jiggle valve. Im now running in the low 190's!

 

Goin' to try this! Hope it solves my problem, also.

 

Have you had the thermostat changed?

 

I just recently did a engine rebuild due to a spun rod bearing. And I'm positive I put the thermostat hole in the 12 o'clock position. Even if that is not the culprit, I'll be sure to check my engine temp to see if it's below 200.

 

Yeah definently check it. Be careful not to strip those 3 nuts that secure the housing. I think it's the heat/ moisture that softens the metal just a little and when I went to try and remove them a couple months ago I ended up stripping the one closest to the water pump. Well when I took my truck to get diagnosed the other day I asked if they could remove that nut and put a new one on. That's why I was able to remove it and rotate the thermostat just recently, otherwise I would have done it along time ago.

If the Manual says to put the jiggle valve in the down position then it's gotta be for a reason. I was skeptical before doing it but the results have been awesome.

 

Ok. I'll bite. What the hell is a thermostat jiggle valve? Is that similar to muffler bearings?

 

It's next to the bi-metal reed valve sequencer sensor.