Intermittent engine misfires

My 2007 Dodge Dakota with 4.7 L V8 misfires when hot weather and running A/C. Have taken to 2 different repair shops and they say the only information on their analyzers is engine misfires. Does not tell them which cylinders or why. Misfires are so bad sometimes that the check engine light comes on. Engine runs fine during cold weather months but does this every time it gets real hot and run A/C. So far they replaced coils and spark plugs in 2 cylinders that they thought looked bad. That seemed to fix for a few days then misfires started again. Then they tried fuel system cleaner, thinking there is a valve sticking. Could it be an electrical problem?

I don’t get it, the shop replaced 2 of the 8 coils, but didn’t know which one is misfiring? What if they replaced a perfectly good coil? Wouldn’t that mean the bad one is still there?

Get yourself 1 more new coil. Then start replacing one of the old coils with the new one. If the misfire continues put the old coil back in and replace the next old coil. Continue doing this until you find a bad coil.

Misfires can also be caused by other problems, but making sure the coils are good is a cheap start and it is a DIY job.

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Thanks for the suggestion @kurtwm2010

I don’t get it either.
Coils are mass produced in very well controlled, highly automated processes. Coils produced in the same production run have very close lifespans. If a 10 year old daily driver’s coils begin to become heat sensitive, it’s negligence to replace only two. Even if those two are the only bad ones under testing, the others are due to fail in the very, very near future.

The other things I don’t get are

  • they couldn’t tell with an engine analyzer which cylinders were misfiring?
  • they replaced only two spark plugs on a 10 year old car with misfires?
  • they didn’t know how to test for sticky valves? Have they ever heard of a vacuum gage?
  • they added fuel system cleaner because they suspected a sticky valve? Huh?

My friend, you need a new shop.

My guess to the problem is that

  • you need to start by replacing all the sparkplugs and all the coils. At 10 years old, it’s worth changing the plugs anyway, and it’ll give you a chance to “read” them for signs of pathologies. Mark on each one the cylinder it came from.
  • Since changing a few of the coils seems to have helped, and since coils are known to become heat sensitive, I’d change the remaining ones just as a matter of principle.

If after that the problem still exists, post back.

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Presumably the codes said only two cylinders are involved, so they replaced those two spark plugs and the coil or oils associated with those two cylinders. OP should double-check on this theory with the shop, but that’s my guess. hmmm … you know, if I had this problem and I had no other clues at hand, I’d replace the crank position sensor on a flier. That part is well known to be heat sensitive.

As posted above, if the codes don’t specify which cylinder are affected, general misfires in other words, only changing out 2 plugs at random seems a very odd way to address the problem. Suggest to ask the shop techs why they chose those two, and they may very well have a good reason, for example those two cylinders are know to develop this problem on that make/model/year. But if the shop techs are just guessing, might be time to start looking for another shop. I’m not saying it is easy to diagnose intermittent misfires, but if the decision is try replacing parts, which might well make sense, replacing all the parts that are part of the routine engine maintenance schedule should be first. All the spark plugs in other words, engine air filter, fuel filter. Ignition analyzer, compression, and fuel pressure tests should be considered also. The heat and A/C involvement would make me want to check the ignition timing is correct, both at idle and that the advance vs rpm is to spec.

You say it misfires when hot and the AC is running. My first guess is the AC is causing the engine temp to rise above operating norm and the heat in both the engine and engine compartment could be causing partial fuel vapor lock. A lean fuel mixture will cause misfire.
If the engine starts quickly, runs great when under load, then the ignition and timing are probably OK.

Thanks @kurtwm2010 and @GeorgeSanJose I went back and looked at my invoice. They did replace all 8 spark plugs and actually replaced 3 coils. I assume they saw some indication those cylinders may have been where the misfires were occurring. They didn’t replace all the coils because of the expense, but I think you’re right, at 10 years old it is better to eliminate that possibility before chasing other leads. If those don’t fix it, I will find a shop that knows how to use a vacuum gauge.
Thanks Again.

Thanks @patgurr That sounds like a good possibility because it doesn’t do that in cool weather. I’m going to replace all the coils anyway, but if the problem persists, this is definitely something to consider. How do you fix vapor lock/ lean fuel mixture caused by excess heat?

Will your “Engine analyzer” show which cylinders were misfiring last week?

A vacuum gauge has no memory, intermittent valve problems on the 4.7 L engine can be elusive. Freeze frame data in the computer can be a great help when the operator of the vehicle has little information to offer.

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To prevent fuel supply vapor lock you can re-route the fuel line further from the engine and/or insulate the fuel line against the heat.

Okay, but I don’t understand why that was never a problem until the last 6 months or so. Seems if it were a design problem it would have happened in the first 9 years too??

Two shops have put analyzers on, even right after check engine light comes on and they say the data does not tell them what cylinders or reason. Is the computer from 2007 just not sophisticated enough to do that or is problem just affecting different cylinders at different times totally random?

Nevada, I take it you feel they did a good job?
How would you have diagnosed it?

I’m not mechanically inclined, but I feel they have always been pretty straight with me and tried to be as economical as possible. Do I need to find a shop with a better analyzer or are they pretty standard in their ability to read the codes?

You can get an OBD II reader and record data yourself to an android, tablet, etc.
Starting for under $20.

There is a wide range in pricing. Are the $70 ones more capable than the $20 or just have more bells and whistles?

If the stored fault is P0300, Multi cylinder misfire this does not indicate which cylinders were misfiring, the technician will have to drive the vehicle and observe the misfires. What were the stored faults? How frequent does this occur? How long will he need to drive the vehicle?

Can I infer from your response that you think they did a fine job?
Would you still feel that way if it were your car?

The original post states that the vehicle still misfires, has not been repaired. Why do you ask if I feel they did a fine job?

I expect all shops know how to use a vacuum gauge. It would be difficult to stay in business otherwise. Vacuum problems are one of the most common causes of poor idle.

If you’ll allow an educated guess, I’m thinking the problem is the crank position sensor. It is known to be a heat sensitive part, and can fail intermittently causing misfires, and the conditions you describe is when the engine compartment will be at its hottest, when the ambient temperature is hot, and the AC is on. The AC system doesn’t actually do any cooling, it transfers the heat in the passenger compartment to the engine compartment, which further heats the engine compartment. If not the crank position sensor, the other things in the engine compartment that tend to be heat sensitive by the comments we get here are the coils, the ignition module, and camshaft position sensors.