I'll apologize now for the overly long post but I believe the details are important.
A couple weeks ago, my wife's 01 MPV had the check engine light come on and it started to misfire. We put in some gas treatment thinking it was condensation but that didn't fix the issue. It was between paychecks so we didn't get a chance to fix it right away. The misfiring got worse so we had the code pulled and it returned a P0300, multiple random misfire. I bought new spark plugs and replaced them. Unfortunately, Autozone gave me the wrong wire set or I would've replaced those too. Overall, the old plugs weren't too awful looking but the one that came out of the #4 cylinder was completely black and wet and smelled of gas. I gapped the new plugs and put them in and unfortunately had to put the old wires back on. Misfiring didn't really change any, leading me to believe coil pack and/or wires. Around this time, the idle started being off as well, to the point that the car stalled a few times while driving it. It had a rapid foghorn like sound coming from the airbox for a couple years but didn't seem to affect drivability so we ignored it mostly. I found out from another forum that this was likely the idle air controller, so I pulled it and cleaned it but it hasn't helped much.
Being our only car at the moment, we've unfortunately had to keep driving it, even with all the issues. The CEL blinks constantly now and idles roughly but once up above 20mph or so, it runs pretty decently with only a slight misfire but the CEL continues to blink. I was forced to drive it 20 miles (one way) to work yesterday and today. It was fine, more or less. Except on the way home, I pulled out of the parking lot at work and had no power. I nearly mashed the gas to the floor before it finally managed to pick up and get moved again, though reluctantly. I got to a stop light and it did it again, except this time it wouldn't do anything. I pressed and pressed and the engine choked and sputtered but I was getting no power at all. I turned the car off, put it in park, started it up again and tried again, same thing. The car was completely not getting power. I heard some low clunks in the engine while I was trying to get it going again. I managed to get the car through the intersection and pulled over and put the car in park for a few minutes. I turned it off, turned it back on and it finally was putting power to the wheels again. It was a very tense drive home. About 10 miles of my trip is highway. I pulled onto the highway and as I'm driving, I notice that I started losing speed going up a slight hill. I pressed on the gas harder but it didn't seem to help. The second hill was the same way, still a slight incline but enough that it slowed me down to about 50 from 65. About this time I noticed a clattering/sputtering noise coming from the engine. This stopped if I left off the gas but even the slightest touch on the gas, and it started again. It got louder the closer to home I got. I was about a mile from home when I went to stop at a light and pressed the brakes but the brakes didn't grab. I had to press as hard as I could to get the brakes to grab and stop. It did this a second time about a 1/8th of a mile down the road and then the brakes were fine when turning into my neighborhood but then lost response when pulling into the driveway.
I'm taking the day off work tomorrow to look into it and replace the coils, hopefully the wires and a cracked fresh air intake hose between the airbox and throttle body. Is there anything else I should look into? I've also heard PCV could need replacing. I wasn't overly concerned with the car until the clattering/sputtering noise and the brakes losing power but now I'm worried because this is my families only vehicle at the moment.
I went out last night and was looking around and happened to notice that the coolant reservoir was pretty much bone dry. There was about a 1/4" in the bottom of the tank. I had some coolant so I filled it and started it back up. After starting and being able to hear the clattering noise, it's not a clattering noise. It sounds more like short, loud pops, sort of like if you were to quickly jab at a valve stem on a tire, coming from the engine and they sound like they're coming directly from the cylinders but I'm not sure. I've never heard a car sound like this so I'm at a loss.

 

You forgot your mileage! 4 cyl or 6cyl? Automatic or standard?
Do have a coolant leak? check your driveway.
If you don't then you are burning it: This is expensive as it may also be mixing with the oil causing connecting rod bearing failure (giving you a knockig sound), check your oil. Coolant is not a good substitute for oil when it comes to connecting rod bearings.
Your car is old enough to change coils, wires.
Driving around with a misfire could clog up your catalytic, especially if coolant is burning.
I hope your catalytic is OK. Don't forget about it. Read this article: How can I tell if my catalytic converter is... — Yahoo! Autos
and this article: eBay Guides - Reasons for catalytic converter failure.
Check out this video:
Check out this video:

 

Chris:

Noggin's advise is good and valid. You really let it go to far with deferred maintenance. On time maintenance saves money.
The engine bogging down may be caused by a clogged converter or even worse, a failing head gasket.
Maintenance items for the engine include oil and filter changes, coolant changes, plugs, wires and coil pack replacements, air filter replacements, PCV check. At this age also include vacuum leak check and MAF sensor cleaning. Engines with timing belts need them replaced.
Automatic transmission: Preferably have a complete fluid replacement done every 30000 miles. If there is a filter it must be replaced as well.

I think you will find out how much money you may have saved. In addition there is a loss of value. You can avoid making that loss real by keeping your vehicle going and going.....That's why we keep our vehicles very long and maintain them appropriately. We don't baby them.

I am not posting this to pontificate. I understand that it can be hard to come up with the cash for the maintenance. Going forward, plan ahead and budget it in.

 

Odd it didn't throw p0304. I'd suggest swap plug wire from 4 to whatever is close. Drive see if the problem follows. If not, I'd suggest coil pack.

 

Thanks for the support.
I want to add, due to coolant loss, a flush and fill of coolant is needed. This should be done every 3 years ish. You will also need a new coolant thermostat and coolant sensor.
You should also do a compression test as your # 4 plug was wet.

Also almost forgot about this video on diagnosing a misfire. Good Luck!
RANDOM MISFIRES discussed from 5 minutes in video.

 

Thanks for everyone's replies and advise. My apologies for not including mileage and engine size. Mileage is 135000 and it's the 2.5L V6.

I just finished up replacing the wires and the coil pack. It started up, bogged down and died almost immediately. I started it again and it started up like it was brand new.
The only thing I hear wrong with the engine now is that it has a definite "puttputtputt" to it that it didn't have before. The engine idles right where it should, just below 1k. I noticed that at first, there was a good deal of black liquid coming from the exhaust, which I imagine is from driving it so long with it misfiring.
The most concerning part of this now is the fact that the brakes are not grabbing how they should. I had to press the pedal almost to the floor to get the brakes to engage. It was not doing this before the issues yesterday. I haven't had a chance to look up anything about the brakes but I thought I would mention.

 

Have you driven it since you did these little repairs?

I would disconnect the negative terminal on the battery followed by an application to the brake pedal. Wait 10-15 minutes. Re-hook up battery. re-start it and let idle for 10 minutes before driving. Re-learning of the computer has begun, as well as clearing any codes.

Brakes: Sounds like you have a vaccuum leak to your brake booster. This happened to me. Mine was difficult to diagnose as we had to pinch the hose from intake plenum to brake booster and watch the changes on a monitor to the engine fuel trims. I also had to press really hard on brake pedal. I found one in junkyard as they are expensive.

I am still leary on your coolant loss and catalytic, especially with that mileage, and that you're trying to do a quick fix on a major problem.

 

I gave in and took the car into the shop to have it looked at. Turns out it the reason for the "puttputtputt" sound was I had bad intake manifold gaskets, which was also causing poor vacuum at low speeds and causing the brake booster not to have enough air pressure to function properly. Gaskets have now been replaced and the brakes work again. The puttering noise from the engine is also gone. However, the shop ran a compression test and found that I have zero compression in the #3 cylinder. Shop tried a fuel system flush but it didn't help. The mechanic started it up and pulled the #3 spark plug wire and the idle didn't change at all but pulling the #4 had a clear impact on the idle. They recommended engine replacement with a low mileage used engine but that's not something I have the means to do at the moment. Mechanic is suspecting a cracked or broken exhaust valve but said he wouldn't be able to tell without pulling the head off, and I didn't have the money for them to do that right now. It's able to be driven but has a bit of a miss and you can hear the valve clattering during acceleration. Nothing more can be done for it at the moment, unfortunately.

 

Sorry to hear about your bad news.
Thanks for following up, we appreciate that alot.

 

Indeed, thanks for letting us know. I hope you can find the funds to replace the engine. You certainly will be more vigilant about maintenance from now on.