Van will not go above 40 mph without shuddering- help!

After I got that great UOA I posted a couple of days ago, of COURSE I have another problem!! My wife called me at work today after barely making it home. Here's what happened:

She drove 10 miles to her destination without any problems- the van ran normally. Incidentally, it was raining heavily today- warmest day in months (in mid-40s with rain). The van sat for 45 min. (still raining), started fine, then shuddered on acceleration. It was front to back kind of shuddering, like she was hitting stops; not like a rumble strip like in other descriptions of shuddering on this forum. It was not like a chugging. She noted that she had to accelerate slowly. If she accelerated too quickly, the shuddering occurred. Shuddering was occurring when driving over 40 mph, bad enough to make her back off while driving. She drove home with the hazards on and drove under 40 mph. Once in the housing development, where the limit is 20 mph, the CEL was on solid. When driving faster, the CEL was actually blinking. Trans shifted normally- 1st to 2nd, and 2nd to 3rd was fine. No jerking, bucking or anything while shifting.

I'm thinking this is a bad coil pack. With the trans shifting normally, hopefully it's not the trans or torque converter. It sounds electrical based to me the since CEL came on and speed seemed to be a factor (over 40 mph shuddering). I found it strange how the symptoms came on suddenly like that, but I've read some posts where a bad coil pack presented itself suddenly like that, though maybe not identically.

Given that when I got home, it was freezing rain and flash freezing, I ain't goin' anywhere. The high temps today melted a lot of the arctic tundra snow up here in NNY, and the streets were all soaked. Now they're all a sheet of ice. My little Mazda with it's Blizzaks was having some trouble staying on point, so the van's staying parked for tonight. I'll get it out tomorrow to see if it's still doing this.

Our other theory is that the vehicle is now acting up because we just, LAST NIGHT, sent the bank the balance of what is due on the thing!! Tell me THAT's not ironic! :rolleyes:

So I sure could use some of y'all's expertise here! Thanks in advance!!

FLASHING CEL is almost always a misfire.
You could get the code(s) read....get the actual numeric code.

If it is P0303 and/or P0304.....it could be clogged EGR ports.
To test to see if it is the EGR system......remove and plug the vaccum line that goes to the top of the EGR valve...
Go for a drive.....if the misfire does not happen.....then most likely it is the EGR system that is causing the problem.....and the first step would be to clean the EGR ports in the lower intake manifold.....removal of the upper intake manifold is require..... it is easy to use a small pick and shop vac to clean the ports.

If the misfire STILL happens....then it is not caused by clogged EGR ports.

In either case.....remember to reconnect the vaccum line to the EGR valve.

This test (disconnecting the vaccum line to the EGR valve) will cause a additional code and CEL for lack of EGR flow....as you have disabled the EGR valve from opening.

If it is not the EGR system......spark plug wires, coil, spark plugs and dirty fuel injectors are common causes of misfire.

Wet weather is a good stress on weak electical systems.....like spark plug wires and coil.

Vaccum leaks can also cause misfire.....so a check for any broken or disconnected vaccum lines might be in order....although I don't expect it will be that easy....given the wet weather factor.
A bit of water in the gasoline is another possible cause.......and is the fuel filter in good order?

Thanks for your quick reply! Plugs are new as of last year (Autolite 2x platinum plugs), wires were replaced by the stealership two years ago when I didn't realize how to get to the back ones. So they're OEM wires, but they cost as much as two car payments when all was said and done... Filter was replaced a year ago, but it hasn't acted like this until just this episode. I would think fuel fouling issues would present themselves incrementally, kind of like they're sneaking up on you like a cat would.

I did the iso bolt fix last year as well (about 11 months ago) and cleaned up the EGR ports just before deploying, so I had peace of mind and my wife didn't have to deal with it. Y'all were so helpful to her when the stupid van went kablooey last May- thanks again, guys! So the EGR ports should still be clean. I'm pretty nutty about making sure that there's some Drygas or Heet in the gas tank since it's so God-forsaken cold up here, but there could still be water in the fuel. With the weather being what it was, it being so wet outside, and warm (which I'd think would make it more humid), I'm thinking that moisture got somewhere it doesn't belong. Just took apart the intake hose and associated harnesses to clean the connections last week, so at least that part is solid. PCV, air intake temp sensor and MAF were checked and cleaned then too. So that eliminates a lot. Wires are unlikely but still possible, so I'll check that in the dark. At 112K miles, my money's on the coil, which is original.

Replaced coil and all seems to be fine and dandy. I pulled it, and used my multimeter to read the primary voltages (across the harness connection) and secondaries (across each pair of spark towers). The secondaries came out to be around 10.5 k-ohms (spec is 6.5-11 k), but the primaries (spec'd for 0.3-1.0 ohms) were reading like 2.2-2.5 ohms. I found a crack, and decided to replace it. Pic of crack in epoxy here: http://good-times.webshots.com/photo/2959665570101767595KXtIKe I bought the new one, thinking that even if it didn't fix the issue, I'd have to change it sometime soon anyway (112k mi), and since the cowl was already off, I'd do it. I bench-tested the NEW one, and got slightly lower resistance in the secondaries (9.5 k-ohm), but the primaries were EXACTLY the same. I was sort of miffed, but installing the new coil fixed the problem. Now for the NEXT major failure!! Thanks, Ford Motor Company!! :wink:

Intermittent misfires due to a bad coil is rarely going to show up in a DVM test. If the misfire was continous it MIGHT show up if caused by an internal short between windings or a broken winding.

My bet is that it was caused by the crack, given the humid weather conditions.

Intermittent misfires due to a bad coil is rarely going to show up in a DVM test. If the misfire was continous it MIGHT show up if caused by an internal short between windings or a broken winding.

My bet is that it was caused by the crack, given the humid weather conditions.

Yeah, I was wondering about that test. I figured the crack could only be bad, and it was already all disassembled and it'd have to be replaced soon anyway, so I did it. Glad I did!! The thing runs so so much better now. Forgot how much fun they could be to drive!