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Misfires on one side of engine

I’ve had a misfire condition on my '01 Dakota for months. It idles a little low, but it’s done that for a decade.
The codes I’m getting are 300, 301, 303 and 305. Multiple misfires, cylinder 1 misfire, cylinder 3 misfire, cylinder 5 misfire.

The most obvious thing to check was that I had mixed up the spark plug wires, but I’ve triple checked that. I’ve replaced the spark plugs, wires, distributor cap and rotor. I’ve tested the new wires for resistance. I haven’t changed the coil, because if the coil were bad, the problem would not discriminate to one side of the engine. As far as I can tell, there is no problem with spark.

I did not replace the fuel injectors, but I moved the injectors from the misfiring cylinders to the opposite side to see if the misfire moved with them. No luck. So it doesn’t seem to be a fuel problem.

I’ve done a compression check on each cylinder. The #1 and #2 cylinders were each slightly lower pressure than #3-6, but within 10%. #3-6 were all within a pound or so of each other.
I haven’t done a back pressure check because I don’t have, or especially want to buy, an expensive gauge to test one thing. There’s no codes coming from the cats, so I doubt there’s a problem with them. There are also no visible leaks to the exhaust at any point.

As a final resort, I bought a refurbished PCM. After installation, the codes went away long enough to get the truck inspected, but about 150 miles later the codes are back.

Dear old dad suggested checking the fuel rail to see if, somehow, that side of the rail is clogged or restricted. I’ll check it, although it seems unlikely that the 1/4" rail would clog before the pinhole sized opening in the fuel injector.

Beyond that, I’m out of ideas. Despite my DIY or die attitude, I would take it to a shop at this point… Except I haven’t been able to find a mechanic who seemed to have even as much knowledge about ignition systems as I do, let alone more. I need a diagnostician, and all I can find are parts changers.

So I’m really hoping that A, my guardian angel reads the cartalk forums, or B, that somebody here has an idea I hadn’t thought of.

Thanks in advance!

Please give us the compression numbers for all cylinders . . . wet and dry

As far as a backpressure gauge . . . which you mentioned earlier . . . they’re not terribly expensive, less than $100, if you shop around. I have seen plenty of cats which were not doing their job properly, causing problems, yet no fault codes, no check engine light, no pending codes. It does happen from time to time.

Some food for thought . . . depending on where your fuel pressure regulator is located, it might be starving one bank of fuel. I’ve seen that happen on certain engines

Check the vacuum hose going to the regulator . . . make sure it doesn’t smell like fuel. It should not. If any fuel drips out when you remove the hose, the regulator is almost certainly done for. You can also use your hand held vacuum pump to quickly check if the regulator’s diaphragm is blown

In spite of what you’ve said, I’d go ahead and check the ignition system again. Use a spark tester on each of the plug wires. It should fire extremely bright blue on all of them. Holding each spark plug to ground is not a definitive test. A spark tester is only a few bucks, and available at any auto parts store. It doesn’t really sound like you’ve got an ignition problem, but it would be good to definitively rule it out, in any case

Your truck’s old enough to possibly have deteriorating wires in the engine bay. Do you have a noid lite kit? If so, you could make sure that each injector is indeed being pulsed correctly

So what are the compression numbers?

I’m not sure what you mean by “wet and dry”. Most of my mechanical knowledge is from YouTube or Google, and I’d never done a compression test before.

I disconnected the fuel injectors and ran the engine until it burned off the fuel before testing the compression, so I assume that’s dry.

Cylinder 1 had 116 psi, 2 had 115, 3 had 121, 4 had 119, 5 had 120 and 6 had 121.

To me, $100 is a lot to spend on a gauge that I don’t expect to use even twice. But then again, I haven’t found a mechanic willing to do any diagnostic work until they’ve tried “putting in the right brand of spark plugs”, so it may be inevitable.

I tested the wires to the injectors using a 12v LED left over from a project. From what I’ve read, a NOID light kit is just a bunch of LEDs with convenient connections. I watched a YouTube video of someone testing an identical truck with a NOID light, and the flashes were the same. I’m pretty sure the problem isn’t there, unless what I’ve read about NOIDs was dead wrong.


What kind of spark plugs are in it vs what mechanics are suggesting???


I second the question.
A “multiple misfire” code needs to be traced back to its root cause. It sounds like you’ve given us a hint that the sparkplugs might be a root cause… and that others have thought the same thing.

I’ve even traced such a code set back to a bad rod bearing. The anti-knock sensor was detecting the pressure waves from the knocking rod as if it were an ignition misfire. The sensor couldn’t tell the difference. That one took a bit to diagnose.

In short, first undo the things you’ve done that have varied from stock. Until you do, you may be chasing “red herrings” forever. There are just too many possible causes, including those that aren’t obvious.

What brand of plugs are in the engine now? My experience with some after market plugs leaves me insisting on discarding them from the start when diagnosing also.

And have you checked the freeze frame from the codes? If the codes always set when idling and hot the problem could be the EGR leaking.

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Compression numbers seem really low.

Yeah, they’re not great

But they seem to be the same, side to side


The compression numbers are too low. Sorry to rain on the parade but the engine is on the way out.

A wet compression test means placing a small squirt of oil into each cylinder and retesting the compression.
Odds are if this is done you will see the numbers go up considerably. That means cylinder bore problems; usually worn piston rings and/or glazed cylinder bores.

Yes, the numbers are low . . . but I don’t see how that would cause misfire on only one bank

I’ve seen plenty of engines start normally and run reasonably well with those compression numbers . . . and on ALL cylinders

I feel the low compression is a red herring . . . in other words, I feel it’s not the reason for misfires on only one bank

But I agree that this is probably a high mileage and tired engine, with its best days in the past

I think your compression gauge is leaking, unlikely that you have 6 failing cylinders.

You wouldn’t be able to drive this truck on 3 cylinders, this problem must be intermittent. You should look at the freeze frame data stored in the PCM and identify when the misfires occur.

Those number are all pretty low. A compression test MUST be done with the throttle held wide open, or you’ll get low numbers like this.

Was the throttle held open for this test?

As kind of a hokey, back yard test you could remove all of the spark plugs, add a tablespoon or so of motor oil to each cylinder, reinstall the plugs, and start it up.

Yes, it will smoke for a bit. However, if the engine runs well, at least temporarily, you know the low compression is the problem.

He was suggesting Champion copper core (stock) plugs, which were the plugs I had just removed and replaced with NGK copper core, which I only bought because A. They were in stock, and B. Experience has lead me to believe they are equivalent or better. The truck runs the same, as far as I can tell, with NGKs or champions, gets worse gas mileage with autolights, and runs more poorly if I try anything but copper core.
The Champion plugs I removed looked good, but I’d already spent all of the money and half of the time to replace them, and I wanted to be sure they weren’t the problem. I didn’t keep them. The new NGKs have not changed anything. Truck feels like it’s running exactly the same, no new codes, and the same codes I had with Champions came back. Still misfires ONLY on cylinders 1, 3, and 5.

I can buy some new Champion plugs to replace the new NGKs, but that really feels like throwing away money. A trusted mechanic back home (two thousand miles is the only reason he’s not doing the diag work) who also drives a 3.9l Dakota a year older than mine, says the NGKs are fine. I’m obviously no expert, and I’m not about to turn down advice from multiple experts because one disagrees… but I’d need to have a better understanding of WHY before I replace a brand new part that I trust, and which didn’t seem to change anything.

NGK copper cores, but I’ve used them multiple times in this engine in the past with no issues. I’ve been unable to find any difference between them and the stock champion plugs, whereas any other brand or core material has made a negative difference that I could feel in performance or measure in fuel economy.

I’m not sure how to check the freeze frame codes. I’m guessing that requires a fancier scan tool than my $20 Autozone special? I’d appreciate it if you could recommend a good one that will do the job without requiring a second mortgage. :sunglasses:

I have noticed that the code comes after, but not immediately after, the o2 and heated o2 sensors show “ready” and before the evap sensor shows "ready. I’ve also noticed that, although the mileage where the code comes back varies significantly, it has been triggered most often on a long stretch of road across a mountain, either during the uphill portion or very shortly after.

Or it might be that the youtube video I watched wasn’t completely correct, or it could be that my cheap-o Harbor Freight gauge is only worth what I paid for it; almost nothing. I’ll do some more research on proper compression tests and see if I did something wrong, and then I’ll see about a gauge with better reviews.
Somebody else suggested checking the freeze frame data, and I’ve already asked them how to do it and what scanner they recommend, but I’d welcome your input as well.

ok4450, I hope you’re wrong, but I won’t cry if you’re right. I bought the truck for $2000 and an Army LT’s insurance paid me $1950 in damages when she hit it in a parking lot and tried to flee. Aside from the normal expenses that go with a 15 year old truck, I’ve got $50 in it. I’ll drive it until it falls apart, and then I’ll be more upset by losing the stories than the cost to replace it.
I’ll try the wet compression test though.

db4690, That’s what I was thinking. If I can fix the misfire so it passes inspections until the old (apparently dying) engine finally quits, I’ll be happy.

JayWB, I had a screwdriver holding it open. Is that adequate?

Thanks everybody for the suggestions, and even the rain on my parade. I live in a desert, rain is good.

db4690 brought a good point that compression is likely to be a red herring, it is “not exactly good”, but “not fatal” either, as long as it is enough to keep engine going

misfire is likely to be connected to:

  1. Spark plugs, but these were replaced, so UNLIKELY

  2. Ignition: if it is a “coil on plug” design, then it is UNLIKELY too as it misfires on way too many cylinders (still, swapping coils could find if problem follows)

  3. Ignition again: if it is distributor-type design, I would closelly look under the cap for erosions and bridging, replace wires just as a precaution

  4. Crankshaft position sensor: this is the one telling ECU when to fire, so if it is giving erratic readings, that would explain it

  5. ECU itself: expensive to replace, but cheap Repair&Replace service is available on eBay, just search for it, worst case they will return your ECU back with “it was good” diagnosis and make a partial refund, so you gain a lot, loose not much

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a little bit addition on “compression” topic

I had few cars I bought with 100K+ mileage in USA, with twice as much back in Europe, and I was using one “snake oil” type of product to make it going much better when compression is bad: XADO

I 100% attest it worked on my “old clunkers”, usually gaining 15-30psi and more or less leveling out cylinders, and it is working for way longer than your oil: consider it “until car dies”

I’m not doing any money on this advertisement :slight_smile:

so, if you decide to give it a try once you fix your misfire, you have to also consider WHY your plugs were getting oil on them, as it is likely that your oil-scrubbing piston rings might be “caked” to some extend, so it might be not a bad idea to use some mild “engine cleaner” product like “Marvel Mystery Oil” added to oil for 100-200 miles before oil change - once again, this is from my experience of “reviving” old engines on “old clunker” cars I worked on, then follow up with fresh oil and XADO treatment

oh… misfires are in #1, #3, #5, so it is “ONE BANK”

check if each of banks has its own sensor controlling ignition timing, as it would explain it

another idea would be a bad “knock sensor”: these are individual, per bank