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Hey guys, so a little backstory on how this happened, I gave my sisters car a jump and 2 days later my car is harder to start, rough idle, and check engine light came on for a P0300 random cylinder misfire code. This all happened late September. About 2-3 weeks ago it also spit out a P0316 code for a misfire detected in the first 1000 revs. Recently I've changed the spark plugs, spark plug wires, and coil pack on the mazda witch has definitely helped with a big time the cars almost completely fixed but changing those 3 things changed my P0300 code to a P0303 code which narrows the problem down to cylinder #3. I'm pretty positive the only thing left to try is cleaning and/or replacing the fuel injector. I don't feel any misfires and the car is extremely quiet not that I replaced those parts. I'm going to put it in the shop in 2-3 weeks if I can't figure it out and fix it my self. But do you guys think it could be? Autozone said I could use Lucus fuel injector cleaner but I said no because I don't think those even work and I thought about a compression test also but idk what the pressure for that cylinder should be at and I don't have the money for the tool until next time I get paid in 2 weeks. What should I try?

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There is no P0316 in the WSM but I did find it on OBD-Codes.com.  That's kind of odd. :(

Check battery voltage with the car both off and while running.  Were the plugs wet?  Misfire during initial startup is usually due to leaking valve guides or blown head gasket as the oil drips down when the engine is off then next startup the first fire is trying to burn off the oil (poor initial combustion).  The same goes for a leaking fuel injector as too much fuel will cause inefficient combustion as well.  Another thing to consider is the timing being off. 

 

First watch this video on how to manually engage the fuel pump. Yes 98-02 vehicles are included in this one.

 

Now you're ready to do a fuel leak test.  This checks for a leaking fuel injector.  I've since learned that you'd better strap the injectors down or they WILL blow off (and spray fuel everywhere).  I didn't have to do that because I had really low fuel pressure.  I got pretty lucky it could have gone badly but would have been funny to catch on film. Always strap down your injectors.

 

 

Compression test never hurts.  I have a video on that.  All 98-02 I4 FS engines have the same compression specifications.

 

 

That should keep you busy.  Let us know how the tests go.

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Check and see if that spark plug on #3 (and might as well check the rest) are soaked in oil, if so your valve cover gasket(s) need changing. Also check the spark plug wire, might be grounding itself, if you get new wires try to stick with NGK, definitely get NGK V-Power plugs if they need replacing

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Thanks the the info djdevon3, I havt started the tests yet because of work but I hopefully will have time to do it monday. Also how would I be able to tell if the plugs are wet? Do I just unscrew it and see if its dripping oil? And how do I know if the wire is grounding itself? I switched the wires with another cylinder to see if the code would change but it stayed on that cylinder. I might replace the fuel injector first with a new one off eBay for $30 since it'll be quicker and it's probably needing a new set anyways. But I think I'm also going to start with a compression check, just gotta find $40 first so auto zone will lend me the tool =\

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Another Update: today I started my car up to go to school and the check engine light didn't come on. It didn't come on when I drove home either and I havnt don't anything else to the car since the spark plugs, wires, and coil pack a few days ago. I guess the problems fixed and gone now?

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Yeah that's what I was thinking and it's back on now unfortunately. I'm letting the shop do diagnostics tomorrow if not Friday and I'll report back with the results.

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Alright so I got back from the shop and it turns out it's the fuel pressure regulator that's the problem. It's letting some fuel drip from there into cylinder #3 which is causing the rough idle when stopped and the misfire because it's running lean? I think, it was either lean or rich one of the two but now I know what the problem is not to go and fix it.

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If the fuel pressure regulator diaphragm was leaking it would most likely leak into cylinder 4.  Having it set a code for cylinder 3 is odd but a misfire on one cylinder can actually cause the ECU to show a misfire on a different cylinder.  Without going into a ridiculously long explanation of why I'll just leave you with the appropriate videos to test a fuel pressure regulator.

 

If you don't know anything about how a fuel pressure regulator works I'll briefly explain.  Only vacuum should be controlling the diaphragm not fuel.  The diaphragm regulates the flow of fuel using a diaphragm but fuel does not go THROUGH the diaphragm.  On older style fuel pressure regulators like ours it's a common occurrence (especially with the introduction of ethanol) for fuel to rupture the diaphragm and now you have a faulty fuel pressure regulator.  When fuel goes through the diaphragm it eventually winds up in the intake which then gets distributed to the combustion chambers as unmetered fuel which can cause pre-detonation.  If the leak is big enough it will arrive in a combustion chamber as liquid fuel.  If it's small it will probably arrive as mist.

 

 

 

If you decide to do this test please take all necessary precautions.  You will be working with pressurized fuel lines.  I take no responsibility for the legitimacy of the information contained in my video.  Follow at your own risk.

 

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I replaced the fuel pressure regulator with a used one that I got off eBay and the eBay regulator has a big dent in it. I tried it anyways and now the check engine light is blinking on and off. I'm guessing that the one I got from eBay was in fact damaged just by looking at the big dent on it.

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Hey thanks, I'm not home right now so I can't test it yet but I had the codes read and it brought back a random cylinder misfire, cylinder 3 misfire and a running too rich on bank 1 code which wasn't there on the old one so I bought a new one earlier in the morning so I guess I'll try that out first before testing because I thought there's no way it's misfiring randomly because I just fixed if and it has all new ignition parts. But I'm curious if that running too rich on bank 1 was cause by the pressure regulator or me bumping into the mass airflow sensor wires even though I barley touched them

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If the MAF wires are in good condition (not broken off) then it's not going to be a MAF issue.  Bumping the wires is inconsequential unless you damaged them.  If you're running rich you have to figure out why.  It's probably not going to be an injector issue.  Faulty injectors usually cause a lean condition not rich.  A leaking injector might cause a rich condition but that would have to be one hell of a leak.  I would start by checking for air leaks, testing the MAF signal, testing the TPS, testing the fuel pressure regulator, hooking a scan tool up and checking the TPS throttle angle during idle VS. the O2 sensor.  Might be best to take it into a mechanic if you can't perform these tests yourself.

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Forgot to come back to post the fix but like I was saying the fuel pressure regulator was the problem. When I detached the vacuum hose fuel started coming out of the regulator so I put a brand new one on and it fixed the problem for good. Two months of driving later and it hasn't had any problems since. So in short replacing the fuel pressure regulator fixed my misfire.

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Good job and congratulations on fixing your issue.  Thank you for returning to post the fix.  Yeah if fuel comes out of the fuel pressure regulator vacuum hose (with the engine running) that's a 100% sure sign of a failed pressure regulator.   

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Remember reading these forums sometimes is like reading WebMD.  After a while you start thinking you have osteoporosis, aids, alien implants, and your pregnant.  :P  We are all human, we all start life with roughly the same body, but along the way things change and we become unique.  Same goes for cars to an extent.  Just because you have the same symptoms as someone else doesn't necessarily mean the cause will be the same.

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