My 2003 has developed an odd miss that occurs different ways. First it was a stumble like the engine was stalling then caught or missed repeated 2 times as I rolled slowly. Engine really rocked for about 1 second. This was a Warm engine while idling at light to get on interstate. Smooth for 150 miles til after fillup and did some kind of miss at 80 mph on freeway--just a 1/2 second uneveness and done.

No miss for 50 miles and then 200 miles home.

Then it missed at 55 mph on freeway about 2 miles after stopping for fast food around town here. This was just an uneveness but I caught the check engine light flashed during the 1 second episode.

Then it missed after stop at cruise-in for 15 minutes later in warm part of day. This time it stumbled as it started up and continued for 15 seconds or so. The check engine light was flashing all the time. Felt like it was really lumbering on 4 cylinders. Engine was really rocking. As I gassed it to pull out of short drive from parking it smoothed. Lasted for about 15 seconds with engine light on and flashing. No problem after parking for 20 minutes at furniture store and 20 minutes drive home. NO CODES when I got home even though the check engine light flashed for 10-15 seconds or more.

Next event was a miss on freeway at 55 after driving cross town to dinner and returning 1 hour or so later. Didn't catch a flash of CEL.

The only common thing is that the engine is fully warm when these occurred.

I've been having a P0440 which I assume is vent valve or rubber tube to it that I've been putting off til I can get a wrench for the fuel filter and replace/check all while I'm under it. But this has been for a couple months. Also have fuel sender not reading right but this has been for a year.

MAF is clean.

Fuel pressure checks at 58 engine off or w/ no vacuum; 48 when running and stays at 48.
Belden plug wires and AC Delco plugs about 40K.

I'm suspecting coil breakdown.

All secondaries read in right range and primaries read .5 ohm or something like that.

What should I be checking.

First check for corrossion at the coils where the plug wires connect- will only take a few seconds. Plugs and wires are 110k miles old. Replace them next. Finally, you may have a failing coil.

My guess is you have a plug wire starting to fail- from a dirty contact either at the coil or plug end.

Your issue is common, the fix is typically easy. Plug wires, plugs or coil. Other than the back plugs, all very easy to replace. Start with inspection of the plug wires, if all look clean then replace the plug wires, if problem still exsists then replace plugs, if that does not solve it get a coil pack and swap it out, one at a time. I bet the above will solve your issue. At 110k on the wires and plugs, you should be looking at replacing them at this point- even without a problem.

Plugs and wires are 40K on them.

I've checked the front wires and they are clean at plugs. Checked all wires at coils when off--metal looks new inside Belden wires. Will check contact end on back plug wires this afternoon.

Any ideas why 15-20 seconds of CEL didn't leave a code set? Does it have to occur longer to set a code?

I have had the exact problem twice. Once was caused by coil corrosion, the other by a dirty plug wire at the plug. The coil corrossion did through a code, the dirty plug wire didn't. I am guessing the misfire is not long enough when the error happens to set a code, maybe if problem worsens it will.

Did those give both a rare high speed burp of a miss and a miss at idle?

I am guessing that the crankshaft position sensor is not a possible cause here.
Fuel injectors and circuits not a likely cause.

I'll check the back plug wires tonight. And I'm thinking about seeing about getting a recycle yard ISC module and 3 coils on it. Or I can take 3 coils from my 98 and substitute, and eventually substitute the ISC, if I can figure out how to get a 14 mm wrench onto the two bolts for the ISC.

icm is what i think you are talking about and it is removed by taking the screws out for the coils and it comes out up

You're right. Thanks. I was making it too hard.

Plug wires on the back look great just like the fronts. Clean. Bright metal. No powder residue.

I put the metal heat shields from the original plugs on the plug ends when I put on the Belden wire. Do those cause any trouble?

What do you think about them? Leave them on? Take them off?

I run the Belden'* also and run them without the heat shields. The heat shields should not be an issue.

You are searching for an intermittent ignition problem that will not leave you stranded and is currently not throwing a code. A course of action you may consider is replacing the wires and plugs. If the problem shows itself after that action, you can replace one coil and rotate the replacement coil. The rear plugs take some time to replace, the rest is super easy.

Nobody likes throwing money at a problem, but in your case I don't think there will be a diagnostic process that will pinpoint the problem in advance. Note that running with a misfire over time may cause premature catalytic converter failure (found out the hard way).

RockAuto had the belden spark plug wires for our Bonneville on wholesaler closeout a few weeks ago. I just checked and they are no longer available. RockAuto does have some decent pricing on other plug wires.

JACKPOT!!!

This morning on a warm restart after getting coffee at my local quick market, it ran rough but didn't feel like the lumpy miss. I let it idle like that instead of speeding it up and the CEL came on. It ran great on the trip up to the store.

It stayed on most of the 1.5 miles home. Got a Misfire #6. I pulled the wire off at the #6 coil and the spark was a yellow white that jumped down the coil to ground. It jumped the distance in air. But should it have been blue?

The idle did not change with that wire off at the coil.

So I'll recheck my #6 wire and pull the plug and switch the coil with my parts car (1998 leSabre) which is on its way to the junkyard or trade.

I'd had that wire off twice at the plug last evening. I also had rechecked the primary on that coil to compare with the middle coil, which was the one I was suspecting.

When I check the primary, my meter would give changing readings. It would show .05, .01, .00, .05, .00, and so on. It'* an autoranging meter, and I wondered if that'* what causes that variable reading? I couldn't get the wire to make any better contact inside the coil. I also used a metal tab inserted that was like the prong on the ECM (electronic control module) and those did the same thing.