89 Lincoln Towncar Sputtering / No Guts

I have a 89 Lincoln Towncar 5.0L with 148k miles. It has idle through mid range roughness. The Bosch platnium spark plugs have maybe 10K on them and I recently changed the EGR valve and the EVP sensor in response to scan
codes. That cleared up the symptoms for about five days but the symptoms slowly returned. The engine also seems to idle pretty slow and the tranny seems to clunk when shifting. It is showing no codes at all and passes KOEO and KOER test and cylinder balance tests as well. It has the original O2 and MAP sensors as far as I know. At 150k miles are either of these past their useful lives and could they be bad without throwing codes?
Any help would be most appreciated.
Thanks, Bob
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Robert L. Wells wrote:

try opening the hood and looking at the engine in the dark.. see if you see any arching from the spark plug wires... might need new wires or better wires if you already changed them....
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Check PCV Valve.

scan
KOEO
and
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Definately be suspicious of the O2 sensors but as far as the slow Idle in addition to all the other stuff that's been mentioned....don't overlook the timing chain....had a 1978 Caddy come into a shop I was working at...second time for car first time I looked at it...said he'd been back and another mechanic adjusted timing after recent tune up....12 degrees retarded the invoice said from factory.....sheesh....checked timing and found it correct...did vacuum gauge check.....14inches at idle....pulled valve cover...tdc'd #1 checked and deduced timing chain had jumped exactly 1 tooth...vary rare to just jump one tooth....customer verified diagnosis after taking to full repair shop, mechanic called me to verify diagnosis...just a thought.....

scan
KOEO
and
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Thank for all the inputs. Definetely will check out the PCV and plug wires. As far as the timing chain goes...I wonder if it has a chain or belt? I'd imagine whichever it is it probably needs replacing so I'll check it out. I just got done replacing the timing set on my 77 Suburban that has similar mileage and it had 11 degrees of slop. What a difference the new set made! Restored the low to midrange torque completely.
Bob

scan
KOEO
and
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not really that familiar with the 4.6 but isn't it overhead cam...if it is I guess on a belt...maybe a chain but I'd guess belt...

through
them
tranny
their
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sorry it's 5.0 first post says so.....forgot with such a long thread......ignore stupidness in previous post...

scan
KOEO
and
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I found out it's a chain. I guess I will be checking it out in the next few days. Argh... not really looking forward to replacing it but with ~150k miles its probably pretty sloppy.

through
them
tranny
their
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