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Discussion Starter #1
I really need help on this misfire problem.

My 99 Grand Caravan SE 3.3L check engine light went on a while ago, and the scan code show ignition coil error with #6 misfire. The engine ran smoothly at the time. Couple days later, the check engine light went off by it self. Couple months later the check engine light came on, engine starts running ruff (caused by misfire ?). Scan the code again, it shows #3, #6 misfire.

I replaced the Ignition Coil last week, disconnect the battery to clean the code; and the engine ran smoothly with check engine light on. This morning, when I was driving on the local road, the engine start running ruff, and check engine light flashing; after restart the engine, the check engine light stay on, and running ruff. Scan the code again, it show multiple misfire events, #3, #6 again. The ruff running was on and off.

I went home, disconnect the battery to clean the error code; driver around the block, the car ran smooth again. I am not sure what I should do next, I can put a replacement Ignition Coil, is there any other things that I should check / do ?

Thanks,
 

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Cylinders #3 and #6 share a common ignition circuit, so a problem with a spark plug or spark plug wire in #3 or #6 can affect both cylinders.

Did you write down exactly which codes you had both now and months ago?

Check your plugs and plug wires and connections; I suspect you'll find your problem there. It's also possible it could be the coil driver circuit in the PCM, or the primary wiring between the PCM and the coil pack, but that is less likely. You could check the 4-wire connector that plugs into the ignition coil and make sure it is making good contact; sometimes those pin-type connectors just work loose.

- G
 

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Discussion Starter #3
Thanks for the suggestions; I did reseated the wire, and connector to make sure it has good connection. But I did not check the spark plug #3 due to its location; I have yet to find a way to get to the back 3 spark plug without some effort. I did not copied the code, will be sure do it next time.
One thought, If I put back the ignition coil I replaced, and check to see if the "check engine light" comes up, does this means the problem was a bad coil ?

I am new to this, and having some question on how the "check engine light" works -

1, when I start the engine, if the "check engine light" is off, does this mean no fault is detected ? if the light came on, a problem is logged and can be read out via scanning tool ?
2. I disconnect the battery for 5 minutes and it clean the "check engine light", not sure if this is the right way to do it ?

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1) The CE light will light when the PCM detects a fault. That doesn't mean you won't see or feel symptoms before the light comes on. Certain parameters have to be met in order for the PCM to turn on the light. You can read the code with a $20 code reader or a more expensive scanner. On most model years you can also perform what's called the "key dance", meaning you cycle the key from off to on (not start) 3 times and leave it at "on". You then count the flashes of the CE light to determine the code. I say "some model years" because it's my impression from reading forums for many years that 98 and 99s are problematic when reading codes this way. It doesn't work for many.

2) Yes disconnecting the negative battery cable will reset the codes in the PCM. That said, it's not always the best way. It's not probable but there is a chance you could negatively effect other parts of the electrical system due to a voltage surge during the process. Two of the most common problems induced by disconnecting the battery are a spun speedometer needle and flashing HVAC lights. There are others. Again, You could disconnect the battery a hundred times without a problem. The 101st time could get you.

BTW - A flashing CE light indicates the misfires are so serious they could damage the catalytic converter. If you see that happening shut the van off and let it sit for ten minutes or so to let it cool. It's best to get the misfires fixed asap before it happens again. It can cost upwards of $500-$1000 to replace a cat converter.

http://www.obd-codes.com/p0303 (P0306 is identical)
 

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Mine has All Wheel Drive and the only way for me to replace the rear plugs is to remove the big silver plenum. At that point, it's time to replace everything else you can reach with the plenum removed including the pipes to the rear heater, valve cover gaskets, all coolant hoses... I did mine (3 weeks) only to have the radiator burst!

At night and with the engine running, look for arcing in the wires and arcing where the wires connect to the plugs. BE CAREFUL OF MOVING PARTS! Arcing will look like tiny far away lightning flashes.

Replace the wires if they are arcing and replace the plugs if they are arcing at the wires. If they have over 80k on them, just replace them anyway. It's too bad they are hard to get to because replacing them is a cheap diagnostic method and BTW, keep the old ones for spares.

Another Trick: Try swapping the 3 and 6 wires with another pair and see if the misfires follow the wires to the new locations.
 

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Discussion Starter #6
Thanks for the suggestion - I trace the wire harness between coil and PCM, I did not see any erosion. Disconnect the connector on coil side, and PCM side. Measure the resistant to be 0 between two ends, and Infinite to the ground.

Since I did not replace the spark plugs / wires on the van (I am the original owner, and it has 125K), so I decided to the spark plug, wires replaced. With the help from my friend, we went from the side way, lose the alternator to the left, and air intake on the right. He has all the wrenches to work in the tight space, and it took 2 hours. The wires and spark plugs were all in fair conditions.

After 10 minutes drive, stop by the grocer. When I start the engine, the ruff idle came, and CE light back on. The code scan is P0353, Ignition Coil C Primary Circuit (the same as previous time, and before I replaced the Coil).

When the CE light came on, mostly just after a warm start - you experience a ruff idle, then the light went on; if I let it idle in less the a minute, the engine recover (ran smoothly). If I tried to drive the car, the engine stopped. There were couple cases, the ruff running show when I was driving on the highway, again if I let it curies, it will recover in a minute; I did try to accelerate once, it result in CE light flash.. (scan show multiple misfire, Coil C failure - I did not copy the code that time).

The CE light (ruff engine running) never came on when starting cold; and once it recover, it will not show up again shortly (couple hours of driving time plus / minus), the interval between occurrence is getting shorter. Since it show up intermittently, I do not know what is the best way to check other then replace the "possible" parts. Should I replace the PCM next ?

Any suggestion are greatly appreciated.
 

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PCM replacement is expensive, and I wouldn't at all suggest replacing the PCM as "diagnosis by part replacement". Based on the diagnostics you've done so far, though, PCM replacement is the next step, according to the FSM code chart.

Do you still have your original ignition coil pack? Have you checked the primary/secondary resistances on it to see if it was indeed defective?

One more wiring thing to check -- disconnect the coil pack connector, and try to snug up those four pin sockets just a smidgen, if you can. BTW it is often difficult to find intermittent wire/connector problems just checking using an ohmmeter.

And - double check the primary resistance for the ignition coil. I normally just measure all 3 coils, and if one is way different from the others, there's a problem. The FSM says below 2 ohms for the primary. Be sure you identify the common on the coil connector properly (otherwise two out of three "coils" will appear to have double the resistance).

If you are handy with electronics, and you do determine the PCM is at fault, you could try replacing a blown driver transistor or re-flowing a broken solder joint, whichever the case may be. FYI the PCM circuit board is encased in a vibration-dampening (or, tamper-discouraging...) gel material, which complicates DIY repair.

Be aware that PCM replacement involves some reprogramming (by a dealer or by a shop with the right diagnostic equipment), and can be complicated by the VTSS (anti-theft) feature (if your vehicle does not have VTSS, and you install a used PCM from a vehicle that did have VTSS, you could end up disabling both the new PCM and your van's existing BCM).

- G
 

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Discussion Starter #8
I checked the resistance of the ignition coil, it is 0.7 ohms for the new one, and 0.8 to 0.9 for the old one.

As an engineer by training, I would need to be able to reproduce the problem to fix it, to deal with a intermittent problem like this, so frustrated.
At this point, I am ready to bite the bullet if I can confirm the PCM is faulty, I'll get one from ebay ($145). I'll just keep on driving as it is, and see what happen. I plan to have a road trip next month, that is the major reason I would like to fix it sooner.
 

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Discussion Starter #9
One of the sale in Advanced Auto suggest to check the fuel injectors; I put a 16oz can of Seafoam engine treatment before my short road trip to PA. Since then, 600 miles driving, the P0353 (ruff running) has not show up yet.

I'll update in couple weeks to see how it goes; thanks everyone for your suggestions.
 

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Here's a stupid thought, bear with...

I had a 1996 Dodge 15-passenger van with the 5.9 litre, and one day in 2000 (4 years later), at just under 59,000 miles, the engine started running rough and I got the Check Engine Light, which translated "Misfire - multiple cylinders". The tank was at 1/4 capacity, and when I filled it up, the problem vanished...until I got to 1/4 tank again. After a mess-up and fight with one Chrysler dealership (in which I single-handedly had their 5-star rating pulled by Corporation), I went to a far better Chrysler dealership further south...and they said it was the fuel pump, and they had one on the shelf for $300. I needed the van for work, so I bought the pump, installed it and never had the issue again. Seems that in that instance, the pumps only make it 50K miles and then start frying out. I got 9,000 extra out of it.

You may want to see what your tank is sitting at, then refuel it and see if it goes away...and returns as you drink down. Not the perfect solution, but a great diagnostic.
 

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Being an engineer will certainly help you solve it and not jump to conclusions like many do. For sure don't jump into replacing the PCM unless you enjoy frustration. I have had the common mis-fire codes in both my 2002 (V-6) and 1996 (4 cyl) minivans. First, don't put much weight in the cyl# shown. The computer looks for variations in crankshaft speed to try to infer the cylinder. My 1996 always shows P0303 (cyl 3), regardless, so I suspect it is the first one checked in the algorithm. The one time the cyl# was correct was when my 2002 had a failed coil pack (cyl 2 & 5 coil). In that case, I clamped a timing light (for old cars) on each plug wire and #2 & #5 didn't flash the light, plus I could hear clicks inside the coil pack. Of course, fuel problems can also cause mis-fire.

Causes of mis-fire codes I recall solving in my vans:
V-6 3.8L engine:
Degraded fuel injector harness above exhaust cross-over (common on early 2001+ engines)
Old spark plug wires - hard on rear side where hotter (a maybe fix)
Erratic main engine harness wiring, fixed by unwrapping, but not sure exactly how.
Cracked & rotated flex-plate
Bad coil pack (above)

4 cyl 2.4L engine:
Too much spark gap (was at factory setting), missed at WOT getting on freeway. Read TSB for turbo engines (not mine).
Bad cam sensor - tentative conclusion, was leaking oil anyway.
Bad battery - strangely seemed to get code right when cranking in the morning for ~4 months, finally my battery brain started tripping, Autozone confirmed the 6 mo old battery was bad. No code since I changed it.

Might be a few more I forgot. Main conclusion: mis-fire code = "could be anything in engine controls". Good luck.
 
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