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Former 2003 Seville STS, 2016 ATS4 Coupe Premium 3.6
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Discussion Starter · #1 · (Edited)
Little context: I got my car cheap because the reluctant seller had basically dumped too much money into it trying to chase down P0300. That is to say, an honest to goodness multiple random misfire rather than just a generic P0300 code. This was about a year and 10000 miles ago. He replaced pretty much everything. One of the coil packs (the front silver one, 1104076). The PCM. The spark plugs. The wiring harness. Crankshaft position sensors replaced in response to service bulletin (and they used the part numbers that I see are the DENSO ones. 12575481 and 12575482). He had the fuel injectors all cleaned too. The spark plugs were replaced with the proper ACDelco Platinums (the service writeup specifically says that they found "aftermarket" plugs that were hand tightened, so it seems safe to think they replaced them properly. Part number they listed was 12571535, and that seems to be right). Didn't want to sell it, but couldn't justify dumping any more into it. Local mechanic I use hooked it up to a Tech II after I bought it, and watched the misfire count rise to truly silly numbers. Turns out the serpentine belt was bad, and it was causing the engine to throw codes everywhere. He replaced it, cleaned the fuel injectors again; then replaced the air filter and fuel filter too. I lucked out. Suffice to say, the original owner was... not terribly amused.

Flash forward a year. 94,000 miles. Starts up again about a month ago. SES light trips. P0300. No other codes except those silly "U1000" ones that pop up from time to time. Flashing SES when cruising around at ~1900 RPM. Nearly every day coming home from work (roughly a 7 mile drive), starts flashing either around 55MPH @ 1900 RPM in 4th or (much more commonly) >30MPH at ~2000 in 2nd. Flashes for a few seconds, then goes away again. Doesn't do it on WOT. Doesn't do it at idle. Only when cruising at around 2000RPM. Have reset it a couple times since then (but not in the past two weeks), but always comes back.
Checked the onboard DIC. P0300 confirmed. Checked it with my dad's (admittedly cheapo) scanner. P0300. Took it to AutoZone and had them check it. P0300. Bought one of those nice (admittedly a Chinese clone, but highly rated on Amazon) wireless Bluetooth ELM375 adapters and bought Torque Pro. P0300. Check misfire history. Thousands of misfires on every cylinder, mostly around 4000-5000 but a couple around 2000; all I'm assuming since the last time I reset the code. Ran the real time scanning. Car is fine cruising at 55, then the SES flashes at random (meaning with no apparent cause, like going up a hill or whatever). I glance at the real time readout. Anywhere from 15-30 spikes on all cylinders at once. Then it just stops and levels off. You feel nothing. No change in performance. No stuttering or stumbling or any other sort of roughness. No hesitation from the engine if you decide to goose it (in fact, when testing out the new scanner, I put it in park and revved it to the limiter and it cleared the SES, though it still flashed later without turning it back on). Fuel economy stays at ~25 MPG all day long. Don't smell anything to indicate the car is running rich. Wouldn't I have to feel a change in how the car is running with that much of a spike in misfires across the board?

So I'm truly stumped. Searched all over the forum, and all I could find for cars with across the board misfires were examples of cars that actually had serious performance problems, or suggestions of things that were already replaced (like the serpentine belt). What else can even cause a detected misfire that doesn't seem to actually be a misfire, especially with all of those parts having been replaced less than 10000 miles ago? The only (scary) thing I can think of is coolant leaks into the cylinder, but while I haven't checked the coolant level yet the car has never overheated once and dutifully stays at the 12 O'Clock position under all circumstances (plus, both head gaskets at once?). The belt can't have gone bad again, can it? 3 or 4 months ago, I had a code trip for one the crankshaft position sensors, but I cleared it (since I knew they were replaced and thought it might be a buggy reading due to the cold) and it never came back (and isn't back now); so I don't think that has anything to do with it. Gonna have my dad ride along while running the scanner to see if he feels anything, but he doesn't think what the engine computer is saying sounds right either.

· Master of the Dark Art of Diagnostics
2003 DHS - two-2002 DHS, 2003 SLS, 1995 Sedan DeVille, 1989 Coupe DeVille
22,646 Posts
For what it's worth, it was the serpentine belt again. This time they replaced it, replaced the tensioner and replaced the idler pulley.
For what it's worth, it was the serpentine belt again.
that's probably not it -

I remember something from several years ago with similar symptoms -
I think the solution was a faulty knock sensor -

it was picking up/reacting to the sound of either the belt tensioner - or idler pulley -
replacing the serp belt "solved" the problem for a short time -
but the problem returned -
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