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Discussion Starter · #1 ·
On my 2003 V6 Frontier, I've been having engine misfire problems for a while. It's particularly noticeable during heaving engine loading at lower RPMs. I've replaced sparkplugs, wires, distributor cap and rotor but no change. Even with highly noticeable misfiring, there are no engine fault codes or "check engine" lights so diagnosing has been difficult. Any thoughts on possible causes?
 

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An engine misfire is a major fault for which the ECU will register a code. This is true for my generation truck ('06) as well as yours (I believe). After the spark plug fires the engine "looks" for the bump in engine speed. If it doesn't happen, it throws a yellow flag with "misfire" all over it. Per the ECU, you did not suffer engine misfire. This is like the cardiologist saying you did not suffer atrial fibrillation but you felt something bad coming from your heart.

Did you notice any gasoline on the spark plugs? This is clear indication of misfire. A true misfire will ultimately lead to bad cats because of raw gas going into the exhaust.

Where do you go from here? Your truck is old which means many of the systems can partially worn and contribute to the misfire.
 

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The actual distributor - is it original?
 
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Discussion Starter · #6 ·
CT-Nismo - this is what is truly vexing - it's clearly misfiring yet no codes. Spark plugs looked fine when they were changed.
shift_rush - it's the original distributor
 

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How many miles? Might be time for a remanufactured or aftermarket one...as IIRC there are no new OEM ones available.
 

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Discussion Starter · #8 ·
I now believe that the problems may be with the fuel pressure regulator. While checking the fuel system per D13FrontierLE's suggestion, I noted that the vacuum hose was disconnected from the regulator. The problem wasn't solved by reconnecting the hose but who knows how long the hose was disconnected so the vacuum line could be clogged or the regulator could be bad. Now I have a couple of new things to check once the weather clears here.
 

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I now believe that the problems may be with the fuel pressure regulator. While checking the fuel system per D13FrontierLE's suggestion, I noted that the vacuum hose was disconnected from the regulator. The problem wasn't solved by reconnecting the hose but who knows how long the hose was disconnected so the vacuum line could be clogged or the regulator could be bad. Now I have a couple of new things to check once the weather clears here.
Good find, disconnect negative battery cable to reset ECM parameters and see if condition resolves or at least improves now.
 

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Any misfire or repeated vibration in the crankshaft the ECU will detect. You need to check and see if the ECU have capability of detecting misfire by disconnecting one of the spark wire and take it for drive. To see If the ECU trip a code on that cylinder with the disconnected spark plug wire.If all is good, I will have to say injector spray pattern might not becoming atomize for pure energy. Try changing the fuel filter and put a bottle of LUCAS fuel injector cleaner.
 

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... there are no engine fault codes or "check engine" lights so diagnosing has been difficult.
A few full throttle 0-60 runs while misfiring are usually enough to set a code for misfire.
Use a section of garden hose for a stethoscope, and listen to the injectors while idling. There should be a fairly loud consistent click every time they fire. If one is silent it is either stuck open or stuck closed, no good either way.
If all are good, time for a compression test.
 

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Discussion Starter · #12 ·
I wanted to close out this string with a final solution - thanks to all who contributed. Turns out shift_RUSH was correct. It was the distributor. All internal steel parts were severely corroded and the bearings were shot. Not sure how it was running at all. Luckily new distributors are pretty inexpensive. Replaced mine and the engine runs like new.
 

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Cool!
It took 7000 posts, but I finally got one right! 🥳🤘
 
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