I have a 1990 Maxima SE with automatic transmission and have replaced the following:

Mass Air Flow Meter, Distributor Cap, Rotor, Plug Wires, Plugs, Fuel Injectors, Fuel Filter, Ignition Coil, Coolant Temperature Sensor, Oxygen Sensor, Timing Belt, Thremostat and Water Pump. I have also repaired all vacuum lines and cleaned the Throttle Position Sensor and Idle Air Control modules. I have also checked the codes on the ECU and it showed no problems.

In spite of all the replaced parts and work I’ve done, my car is still not running correctly. On chilly days it will start and run great until warmed up. Then after the car warms up and kicks into overdrive it will start to stumble. On warm days it just gets to this point quicker. Also, after warming up, it tends to lag and sputter between 1500 and 2000 RPM from a standing start and then will do the stumbling thing when reaching highway speeds and kicking into overdrive. The car idles good whether it is warm or cold and the exhaust gas smells like it is running rich.

I think I am down to replacing 1 of 2 things or both: the Throttle Position Sensor or the Knock Sensor. Please let me know if I'm on the right track or completely off base. Thanks.

 

Did you loose your old thread?


http://forums.maxima.org/showthread.php?t=544968

 

No, just thought I could get more than 1 response by re-wording it a little.

 

If it smells like raw gas from the exhaust,then one of your injectors may be faulty or leaking past the o'ring seals..

 

what do your plugs look like?
they will tell you volumes about how the engine is running.

 

Knock sensor will make your max run like ****. Check into that. That's pretty easy to fix. What do you mean by "stumble?"

 

My car was like that some time before. The nice guy in a repair shop said it was because of exhaust leak.

 

My car sometimes does lag once warmed up and I'm also thinking its the Knock Sensor. I've been using nothing but 93 octane premium gas lately and it hasnt been as bad but my MPG has taken a hit within the past 2-3 months for some reason. I've also replaced all the stuff you mentioned.

 

When stumbling it has a jerky feel like the engine is cutting out in little spurts. It only happens when warmed up with not much of a load on engine.

 

probably a leak in the intake manifold. had the same issue. car idled like crap, i eventually took the entire intake manifold of and gave it a good clean out, then stuck it back on, replaced the seals and its been great ever since. however i still get the overdrive "kick" every now and then, im thinking its an issue with the tranny as it only happens on overdrive.

Note that this is on a maxima 96..

 

higher octane than necessary can lower mpg

 

As a direct result of the mere presence of the higher octane fuel iso of lower?..................... on a VG/VE, I doubt it.

Perhaps if you tweak timing etc when using higher octane fuel and then start using the extra power mpg will decrease.

Can you explain the reason for that or link to a decent page on the net?

 

I've got my timing advanced to 17 degrees but I always gotten my 24city/29 highway mpg EVEN with advance timing and 89/93 octane...its just as of like August-ish that its been laggier.

 

Octane is resistance to burn. the higher the octane the harder it is to burn. since it is harder to burn you use more to get the same burn and in turn power.

 

I seem to have the same problem once and a while but my problem seems like it has to do with the power. I have been constantly loosing power causing me to jump start the car every time. I hooked up a multimeter to the battery while the car is off and the voltage will drop a tenth of a volt every 2 seconds. Could it be a short?

 

So can a bad knock sensor.

 

Give your Max a parasitic drain test. Do you have anything extra using your battery ie: Remote start or aftermarket alarm, stereo system, a freakin' deepfryer, anything? lolz

 

I was so hoping that your answer was not going to be that.

Higher octane fuels have a slower flame propagation characteristic than lower octane fuels, but burns and produce the same amount of power from the same volume of fuel if the base fuel (before the small amount of additive to tweak brand and octane characteristics gets added) for both are the same.

In an internal combustion engine, power is generated by the ignition of the fuel/air mixture, and the pressure created by the burn process exerts a force on the piston top/crank to do useful work. Irrespective of pressure created by the burn process, there exists an optimum point (ito degrees ATDC on the power stroke of the motor) where with mathematics, it can be proven that if you can arrange the maximum pressure to occur, in time, so that it occurs at the time the piston is at around 15 degrees ATDC, then you will exert maximum power on the crankshaft at the right moment in order to create maximum torque on the crankshaft - maximum power used on the crank = maximum efficiency = best mpg.

Static timing on the motor tries to ensure this in theory. In practice, by design, it is/was achieved when the motor was new - ie - there was no carbon deposits in the combustion chamber, there was no oil blow-by etc etc etc. On older and worn motors, where most all of those parameters tend to work together to cause ignition to occur at an unwanted earlier relative time (in my example above lets say shifting ignition by lets say 1 or 2 degrees), you find that the motor starts running in a state close to/of permanent ping/pre-ignition. So the engineers knew about this and devised the KS to (when pre-ignition/knock/ping is detected) retard the timing in small increments to try and get the motor to not run in that condition to prevent mechanical damage totally irrespective of the optimum ignition point for maximum power extraction. If you now assume that your static timing was indeed properly set when the motor was new, then it follows that the maximum power is no longer extracted from the burn process because the ignition has to be made to occur later than the optimum 15 degrees ATDC to prevent ping/damage.

Most VGs/VEs here suffer from this for obvious reasons - mileage/age/wear - ie - the KS is basically permanently and correctly retarding timing to prevent knock and its therefore down on power and thus efficiency (mpg) relative to when the motor was new - even though it may be using the exact same and correct octane fuel as when new.

Higher octane fuel simply allows you a method to move that max power point back to the 15 degree mark in order to once again extract maximum power from the motor IF YOU SO WANT TO DO UNDER THE STATED CONDITIONS

So back to my original comment:
As you can see from the above, the exact results you achieve with using a higher octane fuel iso the manufacturers suggested rating, is going to be huge function of the state of wear and quality of tuning of the motor you do it on - on a nicely worn and carbon infested VG/VE in optimum state of tune (for lower octane fuel use) I would think that, results with higher octane fuel use may actually increase efficiency and thus mpg.

On my own VG - short of tearing down the motor, I have done everything ito quality maintenance and its imo as clean of carbon as I can possibly get it YET .......................

Setting my timing at 17 degrees at 1375m above sea-level, in the AM with air temp around 15C the motor is absolutely wonderful - you can jump on the throttle at any rev range in any gear, and there is not the slightest trace of pre-ignition with 89 octane fuel (what we have here and with the KS bypassed for testing purposes) ...................... now doing the same late PM when the air temp is around 35C, you are guaranteed to hear a faint ping. I can get 93 fuel here too and it does solve this particular issue on my car, but both my driving style with the worn auto (270000KM) and the extra cost of the higher octane fuel, now makes it a non-issue.

 

Man what can't wikipedia do? lolz

 

be 100% accurate.
wikipedia can be edited to say whatever by whomever wants it too.

 

http://www.lubedev.com/smartgas/octane.htm

 

+1 .................. and whenever too.

Just as there is no real substitute for cubic inches in a motor, there is no substitute for relevant experience - using Wiki to demonstrate ones ability to find pages on the net does that and nothing else.

 

Hi,
My '94 does the same, I haven't had it replaced yet, but I'm told it is the EGR valve. I had a '91 that did the same. Never got around to fixing that one either. I also read that you can clean the carbon out of the valve, and kind of milk it for a while before having to replace.