My cars been having a misfire at idle for awhile now but it hasn't been until recently that it threw a po300 code.The misfire is pretty bad but only happens at idle and power has been lacking lately. Im not sure where to even start with this. The spark plugs are new and were changed less than 1k miles ago so that can be ruled out. I was thinking about installing a 300zx fuel filter to see if that would solve the problem but I dont want to start throwing money here and there to only get me back to sqaure one. What do you guys think?

 

Just to clarify, the plugs were ngk's. Not sure which ones exactly but I remember them being around $12 each so I didn't cheap out on them.

 

Misfires that are worse at idle than higher rpms are usually vacuum related. Check for vacuum leaks. What work have you done on it lately? A 300zx fuel filter shouldnt help unless your fuel filter now is really bad. Stock filter is just fine anways, a 300zx filter just lets you go longer without having to change it again.

 

$12 a piece for ngks would mean you got the laser platinum, which are fine. Did you gap them? If so what was your method?

 

The spark plugs were not gapped. I just installed them fresh out of the box. About a month in a half ago I took off the upper and lower intake manifold so I could get to the knock sensor which I ended up replacing. I can't honestly remember if it got worse after the knock sensor was installed. At about 1500ish rpm, when I rev it a little I can still hear a slight misfire. But like I said at idle it is pretty bad. If it is a vacuum leak, do you think its coming from the intake manfold gaskets? Maybe its time to upgrade to nwp intake manifold spacers.

On a side note, I also forgot to mention that previos to the code coming up, the same misfire was still present and I had a forum member who is familiar with the 3.5 swaps check the car out. He was able to figure out that the rear 3 cylinders (firewall side) were firing intermittenly. Do you guys think I have a timing issue on my hands?

 

Wait, 3.5 swapped? When you removed intake did you get new gaskets?

 

No, I just bolted it at all back together using the same gaskets which appeared to be fine. And yes, its a 3.5 swap.

 

yea.. i prolly wd have replaced the gaskets when u took them off or at least used a lil rtv. Now, what this means is that a vaccum leak cant be ruled out.

misfires is air, fuel, or spark. U need to check all three. Since its po300 which is a general misfire, I would test air first.

Then u have to check the plugs. Everytiime i have ever got NGK plugs they were NOT gapped properly. Dont know why u didnt at least check to make sure they were gapped correctly. Either way, check the plugs too.

Also, u do know there is a proven org method of removin the KS that does NOT involve removing the intake manifolds right???

Prime example why ppl need to SEARCH before attempting repairs.

Kudos on the 3.5 swap

 

Yes, I know there is a how to on the forum on the knock sensor removal with all the extensions and what not. But my hands are way to big to fit in the tight space. Removing the upper and lower manifolds wasn't a big deal at all and was a matter of 30 minutes or so. It makes the job a million times easier IMO. If the problem seems vacuum related or due to a leak at the manifold gaskets, I wouldnt mind upgrading to nwp spacers. But I want to be sure first so Ill do a leak down test soon.

The spark plug gap looked fine to me. But if you think I should go ahead and take them back out to gap them, ill go ahead and do that. How big of a gap would you recommend?

 

Idk if the org method of replace KS wd have worked with the 3.5 swap.

I dont have a 3.5 engine so idk what the gap should be. Im sure a quick search will yield the info u lookin for.

 

Leakdown test doesnt find vacuum leaks. Try the intake cleaner method of spraying around possible sources. People say platinum plugs arent supposed to be gapped, but most of the time when you get them their a little off. If you gap them right it wont hurt them. Be sure to only pry on the ground electrode and not the platinum tip. If nothing else take it to a shop and have them hook up a smoke machine to find an intake leak, if thats what it is.