# Weird miss in the engine.....HELP!!



## Pntr815 (May 29, 2011)

Hello all. First, to the moderator. I found the "new thread" link. Took me long enough. lol So, here's my issue: Two weeks ago I bought a 1998 Pathfinder V6 3.3 with 126K on it. The car is in absolutely pristine condition. No accidents, CarFax is clean as a whistle, all maintenance records etc etc. It was really babied by the previous owners. T-belt and water pump R&Red at 105K but not very well. The gent I bought the car from RE-replaced the T-belt and water pump only to find out the previous shade tree mechanic burned out the transmission. Soooo....that is now replaced. The car literally just hums and I really love it. The seller is a 30 year technician who sells some 20 cars per month. He told me that if my Service Engine light comes on it's the knock sensor. He knows about it but just didn't have the time to mess with it because apparently it's under the manifold...?? He told me the sensor doesn't really do much of anything really and it's a waste of time and money to try to mess with it. Lo and behold....the light came on. Today is the first day I've been able to get the car out on the highway. So, up to about 40-45...smooth sailing. Above 50 (approximately 2200 rpm) and especially on ANY type of incline at any speed there is a very noticeable "miss" in the engine. Sort of feels like someone punching the front of the car as I'm driving along. It's not really audible per se but very noticeable all the same. A hesitation or sputter. In driving along I took the car out of Overdrive. RPMs went up an additional hmmmm....800-1000 (3200-3500 rpm) and the "miss" seemed to dissipate somewhat. I've only put in gas from very busy name brand stations and had pumped in 3/4 tank 87 octane regular just before the road trip. A buddy of mine in Sweden and I were talking back and forth when I stopped to grab a bite to eat and he had me check the vacuum of the gas cap as a first measure. I unscrewed the cap....great vacuum. Could this issue be a knock sensor? O2 sensor? Throttle position sensor? Any clues at all? Please understand I'm a carpenter and definitely NOT a technician so please be gentle in your help. Thanks in advance guys and have a nice holiday weekend.


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## rogoman (Dec 16, 2004)

When the "check engine" light comes on, there could be more then one fault code set. Get someone to use a scan tool to see what codes are set. Post the codes here and then maybe we can tell you what may be wrong.


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## smj999smj (Jan 1, 2006)

For a 30 year technician, it sounds like the seller doesn't have a clue of what he's talking about. First of all, the knock sensor will normally not set of the MIL, even though a code for a knock sensor is generated. A code for a knock sensor sets when a misfire occurs, which you say you are experiencing, so it sounds to me that the knock sensor is doing its job. The job of the knock sensor is to sense engine knock, which triggers the ECM to retard the ignition timing to help prevent engine damage. 

As far as removing the gas cap and having good "vacuum," this is not actually the case. The "whoosh" you hear when you loosen the valve cover is not vacuum, but vapor pressure. Ever have a plastic gas can in the heat and see it swell up until you relieve the pressure? It's essentially the same idea, although the vent in your gas cap will relieve anything more than 0.5 psi. This is normal and should have nothing to do with your misfire.

As far as the misfire, it could be a number of things. Yes, check for stored codes other than the knock sensor. If you have no other codes, this will make diagnosis a little harder. Start by checking the state of tune of the engine. It's best to stick with genuine Nissan or NGK parts when replacing any ignition componants as aftermarket parts have been know to cause Nissan engines to not run properly or not last very long. A professional fuel injection cleaning would be a good idea as these engines have had a history of misfiring due to dirty or sticking fuel injectors. Check your vacuum hoses and make sure they are in good condition. Check your air filter and also check to make sure there is not debris on the MAS screen. Make sure the throttle plate and inside of the throttle chamber is not gummed up with carbon and clean if necessary. Check your ignition timing per the service manual procedure and adjust as necessary.

Not related to your problem, your 98 is currently being recalled for issues with the lower strut tower rusting out and affecting the steering. Contact your local Nissan dealer for info.


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