1999 Mazda 626 V-6 Missfire, codes:P0400, P1131, P0174

My 1999 626 V-6 runs very rough. It's has a missfire on #2 and #6. I've had my car hooked up to 4 different Diagnostics machines. The mecahnic yesterday gave me the codes P0400,P1131,P0302,P0306, and P0174. He reccomended a coil, Plugs, Wires, valve cover gaskets(b/c oil is leaking on the plugs) fuel filter, injector cleaner, and a Mass Air Flow Sensor. He told me it needed all of these things but to first replace the coil to see if it solved the missfire. I replaced the coil and it made no difference.

Will a MAF sensor make the engine miss on just two cyliders? He said it read it was lean on bank 2. Should I check the MAF, or the EGR? I spoke with him and told him that the coil didn't fix the problem. He said to replace plugs and wires but before I do this to spray brake cleaner in the plug holes and blow it out with air hose.

As a side note. This missfire happened overnight. It didn't go from bad to worse. That's what makes me think it could be just one part instead of plugs gradually getting a little dirty. Another mechanic acted like it could be the crank positioning sensor. Any help would be greatly appreciated.

Address the egr first! by the way I think your mechanic is clueless when it comes to cause and effect of drivability, A MAF won't make it miss on a single or 2 cyls, A vaccum leak would, an egr flow problem can, but not a MAF! Same with another mechanic telling you it's a crank sensor problem! If it can identify which cylinders are misfiring then it's not likely a crank sensor problem. Have you been to 4 different mechanics each with a different scanner and still haven't settled on a solution. Yeah if the valve cover gaskets are leaking oil on the ign cables then you will have to change the plugs and wires regardless of whether or not they are the only cause or not. As for the coil highly unlikely. Vaccum leaks from the intake gaskets and egr problems are very common on Mazda's any mechanic that doesn't know that shouldn't be pumping gas let alone in business doing drivability and diagnostics. Some days I don't know where you people find these jokers. How could you think there be 4 different causes for your codes! Have any of these "mechanincs" been at a Mazda dealer because I'll tell you when you can't seem to get a straight answer you'll sometimes have the best luck with the techs that work on that brand the most. If a mechanic removed an ignition wire to check a plug, wrote the estimate for you and now it's misfiring as you drive to yet another shop then you'll most likely need an ignition wire, no he's not ripping you off but if they're original then I myself wouldn't remove them until I knew they were being replaced regardless! Pull your car over, find the mecahnic you trust the most and go see them! 4 opinions and 4 estimates don't amount to anything except more wasted paper!

Jeff, thanks for your fast reply. Like I stated above, I know it's not the coil since I've already replaced it and it didn't help.

So I need to check the EGR valve. Also I need to check the intake gasket. The plugs and wires probably have about 15,000 miles on them.

What's the best way to check the EGR valve? And I've been told to use carb cleaner around the intake gasket to see if it's leaking?

Anymore input on this?

Darkrooms? any advice?

check the egr with a vaccum pump, apply vaccum, the engine should stall or stumble!

I definitely wouldn't relpace that laundry list that he gave you before I did some more investigating. I know this conflicts with what COMP said but first I think I'd determine the cause of cylinder 2 and 6 miss. If bank 2 is running lean(according to P0174) cylinders 2 and 6 both reside on that bank. It may be that there is some debris clogging the injectors causing thelean condition or a damaged wiring harness. Check the injector harness for any visible damage. If nothing jumps out at you, then I'd do a fuel injector drop test. Find out if the injectors are in fact plugged or bad. If the injectors are plugged or not actuating, that would definitely cause a lean condition and a misfire.
And, no, a bad MAF won't cause a miss on 2 cylinders only. However it's not impossible that an EGR stuck open would. Still, I'd go with the above tests first and then move to EGR. Don't hate me COMP.

We're not going to agree on everything My approach is based on starting with something that's cheapest probably and can affect all cyls then recheck what's remaining, It's highly unlikely 90% of people on here have access to or the understandings of how to go about injector troubleshooting so I'd rather see them tackle the more basic re and re type stuff if they must before potentially create more problems changing perfectly fine injectors. It may very well be a vaccum leak but it still amzes me how many so called diy'ers don't understand that concept so I forget to remind all of them! I myself have seen Mazdas come in with both egr codes and intake gasket leaks so it is possible has 2 or more issues but Im unsure after to going to 4 different techs how competent he is as a diy-er

Yes, I guess I tend to approach from "if the car was in front of me" rather than the more practical "do what is easiest/cheapest for the diyer". Oh, well, that's why there's different techs, because each guy gives different advice and the poster can understand from all available angles

Thanks for both of your replies. BTW, Jeff you might be right. Maybe I'm not a competant diy-er. I just figured since I replaced my wife's air compresor, dryer, and orifice tube on her 02 camry thursday night, and have replaced everything imaginable, including the EGR valve and the intake manifold on my 96 chevy truck, that I could handle getting rid of a miss in a car. I have replaced quite a few things on this care, radiator, alternator, coil pak. I tell you this not to brag, but simply to tell you that have a little patience with me and I think I can handle this. However, maybe you proved your point since I spent an hour under the hood and a while with the car on my rack looking for the EGR valve and couldn't find it. I did spray carb cleaner around the intake with it running and the engine never changed it's tune. So if anyone would like to instruct me as to where the EGR valve is I sure would appreciate it.

Darkrooms, I will check the injector harness and injectors first chance I get.
Also this problem happened all at once. That's the only reason I didn't pursue the injectors because I didn't think that the injectors for two cylinders would go out both at the same time.

I can tell you this. You guys earn your $ working on these cars. That v-6 is an extremely tight fit.

Please instruct me to where the EGR is located. Thanks for your help.

do you have a vaccum diagram or schematic under the hood that may point to where? Seems to me I remember them being under the intake runners on some Mazda's. The dealer parts dept could tell you where for sure. Otherwise a manual or an alldatadiy subscription would.

Thanks Jeff. No diagram under the hood. I'll bet you may right about it being under the intake.

Anybody else know the location?

Found it!! That thing is in a tricky spot. I pulled loose the top vaccum line and it poured out a good bit anti-freeze. Is this common?