Okay, so this 98 Ford Windstar 3.8 L is making no sense.

Main issue is a cylinder 3 misfire. (Only current code is P0303) After the engine warms up.

Here what I have done. It started running poorly a few months ago whenever it was hot, ran fine for the first few minutes after cold start. I pulled 3 codes.

P0174

P0506

P0305

Did some research and found out about replacing the isolator bolts, so I did that and cleaned out all the egr ports. Some where dirty but none of them were horrible. I read that these ports could cause a misfire and i checked the fuel injector on cylinder 5 and also spark. The injector clicked and the resistance was in spec. I also had spark. So I figured this isolator bolt job might be the solution.

No codes went away. I know heat increases resistence in electrically systems so I looked at the electrical system more closely, spec'd the resistance in ignition coil, it was good but then I found 2 large cracks on the bottom.

I changed out the coil with one form the junkyard with no cracks and within spec.

All three codes went away and were immediately replaced by P0303 and P0171 as soon as the engine warmed up.

My first thought was well this coil is bad just like the other one except for cylinder 3, the part new is 90.00 so I looked into other options.

Checked my plugs and wires which were pretty old and so I put new ones in, even over priced motorcraft plugs. What happened...

No more codes...but after warming I the engine would idle rough and sound like a misfire while driving. The engine light was working, but no codes came up. I brought this up at autozone and a couple guys said the idle air control valve needed to be replaced. I had already replaced this about a year before and had a warranty on it. So I swapped it out.....

Immediately I had three codes P0303 P0171 and the other code was a vacuum leak or bad IAC valve (Code P1507, I think).

?

I thought maybe the computer had to adjust to the new iac so I drove it for a bit. It drove really rough. I went back and switched the iac the next day for another new one, this one worked. The only code I have now is P0303. If I clear the code before a cold start I can run the car just fine (and it feels great driving it) for about 6-9 minutes, then P0303 comes up and the engine light blinks soon after.

I thought maybe I should spend the money on a new coil pack as this one from the junkyard might be bad too. I did, it ran better for longer off a cold start but I had the same problems 10-15 minutes into driving it. The car runs horrible when hot. I have to stop, turn it off, let it cool some, and then drive again.

Some one brought up that a clogged cat could cause a misfire or lean condition (didn't make much sense to me) I checked my tail pipe and I have pretty good flow that increases with acceleration. So i don't know...

Please any ideas, I'm so close to the end of my rope with this car. It is older and has 219,000 miles on it, but it seems like only one small thing is causing all this trouble...I just can't for the life of me find the cause.

 

I'm delaying ripping the cowl off, cause its a real pain and I've already done it once. I know I should get in there and do compression tests on the cylinders but I just really don't think it is very likely. What would cause it to run so poorly when hot, but just fine when cooler?

Oh and I forgot to mention I changed the fuel injector on cylinder 5 before I replaced the coil pack. probably a stupid idea, but I thought that was probably causing the lean code and misfire. It wasn't.

 

When the engine light blinks, that means the engine is polluting so badly it can ruin the converter(s).
When that condition takes place the engine probably has what we call a dead miss and should be a lot easier to find than an occasional misfire.
There are several possible causes among which could be an electrical connection that is poor to start with and gets worse as it gets warmer, an injector that malfunctions after it warms up, a defective coil assy or even a cracked plug. If your vehicle has 2 pre converters one of them may expand enough to block exhaust passage through it.
A vacuum gauge test will usually indicate a possible plugged converter with vacuum dropping when a steady engine speed (2000 rpm) is run.
MOST of the time Ford computers will provide the correct cyl # with a misfire, MOST!
Switching injectors, coils , plugs with different cyls will help pin down the problem by watching if the problem moves to a different cyl.
If none of those cause any change then it is something relating to that specific cyl and requires some detailed investigating.

One of the tricks of the trade is " use OEM" parts.