'97 Ram 1500 5.2, Misfire on No. 5
My first post. I've Googled several sites and used the search feature here to learn about my problem, and have most likely just confused myself further. I am a writer, not a mechanic.
Before I go into my sick truck's specifics, I want to note that I did read the sticky thread on plenum problems, and I'm hoping that doesn't apply to my Ram. For starters, I've owned it about three years (running it from 110K, where I bought it, up to 180K miles) and it's never used much oil. No more than a quart between changes, usually more like half a quart. (Though some of the other plenum symptoms perhaps do apply to this truck, to some degree.)
Also, while the catalytic converter certainly is old enough to have failed, one of the principal and "rather quick" symptoms of a clogged CAT as described in that plenum sticky is fouling at the No. 8 plug; my No. 8 looks fine; it's plug 5 that's bad.
So here are my truck's specifics and the problem:
-- 1997 Dodge Ram 1500 4x2, 5.2 V8 with automatic.
-- 179,571, two-owner miles.
-- Underwent a full tune-up with new Champion plugs, new plug wires, distributor cap and rotor, filters, belts, etc., at an ASE award-winning shop in another state exactly two years ago. (I promptly took it on a 1,400-mile trip and got nearly 20 mpg highway.)
-- (Seemingly) ran like a champ until about a week ago.
Suddenly, or so it seemed, the truck developed rough-running conditions, the "check engine" light came on, and it experiences a periodic flashing CEL under load. The truck seems to run OK at idle and not too bad once up to speed -- say, 60 to 70 on relatively even terrain. Temperature is fine and oil pressure is fine.
But the engine misfires badly (enough to shake the whole truck) in a certain rpm range and gear -- that is, about 45 to 50 mph in high gear. I'll be accelerating pretty normally, the transmission up-shifts at 45 mph and RPMs drop back down to 1,500, and suddenly it's "chug and lug." Once you gingerly work it through that chugging, things smooth out considerably as noted prior, around 55-65 mph. However, if you really put the engine under load, such as climbing a hill or merging on the interstate, the check engine light begins flashing. The flashing stops when the engine is slowed back to idle (like if I pull over to let it stop flashing) and doesn't come back on again until under heavy load.
I took the Ram to Auto Zone for the free CEL check and according to the kid behind the counter, it returned codes for occasional random misfires, repeated misfire on No. 5, and a vacuum leak. ... Sorry, I lost the receipt and don't have the actual codes.
I bought Bosch platinum plugs, but declined to buy a new gas cap. (Salesman said "95 percent of the time, that vacuum leak code is a gas cap that isn't sealing anymore.")
I changed the plugs, made sure the wires were routed as they should be (from tips I've read) and definitely took note of glossy (that is wet?) carbon-fouling only on No. 5. All the rest looked pretty good for roughly 40k-miles plugs. The chugging and CEL-flashing situations were not remedied. ... I did notice in some threads that the flashing CEL is often attributed to a lean condition, and I do wonder whether the No. 5 injector is bad. No idea, since I'm not much of a mechanic, whether that would lead to the glossy fouling of the plug on that cylinder.
I do also hear (or can convince myself that I sometimes hear) a repetitive "gasping" sound as the engine slows down to a stop, like at an intersection. But that sound, if it's there at all, goes away when the truck just sits and idles.
Of course I'm fearing "the unspeakable" (as I've seen it described) -- things like a cracked head, a burned valve, etc. (I've seen about a dozen things suggested as possible causes of such a misfire.) Also could be a blown head gasket from what I've read, and many other causes.
But it strikes me as odd that the misfire occurs by far most dramatically at 1,500 rpm in high gear.
With two-year-old plug wires routed correctly, two-year-old distributor cap and rotor, new plugs, no considerable oil consumption, and the fouling of spark plugs not on No. 8 (a quick sign of CAT failure) -- what would you consider next? Taking it to a real mechanic?