bad cats to blame for not starting
06-18-2012, 01:45 PM
#1
bad cats to blame for not starting
i have simply tried everything a maxima owner can think of...only thing i can think of is my main cats are getting stopped up..on long trips it will heat up a bit but soon as u punch it she cools back down...have to do it a few times while on a short trip...and added a new muffler to the car to put a little bit better back pressure on it and now even harder to start then it was with the glass pack where it has no flow restrictions....ive done knocked out the secoundary cat...just a pain in azz how the cats are on the front lol...gonna try and take it off and cut a square on top of the cats and pull the guts out then weld it back up save me a few 100 bucks..any one try this before
06-18-2012, 02:06 PM
#3
just hard starting takes awhile but after it starts its fine and dandy..gotta have some back pressure if u dont have none u loose hp torque and gas millage...thought it was a sensor going on but ive done replace the iacv both crank sensors the mass air flow sensor knock sensor cam shaft position sensor fuel filter plugs wires a few coil's and the throttle position sensor cleaned my egr valve and pipe fuel pressures great which i knew it was ok...not getting any codes or anything so ii figured has ta be the precats stoping up on her cause it started easier with the glass pack which doesent have any back pressure on it and put a different design muffler on it to quiet it down some and its harder to start..ive had this issue for about 4 months now..
06-18-2012, 02:33 PM
#4
just hard starting takes awhile but after it starts its fine and dandy..gotta have some back pressure if u dont have none u loose hp torque and gas millage...thought it was a sensor going on but ive done replace the iacv both crank sensors the mass air flow sensor knock sensor cam shaft position sensor fuel filter plugs wires a few coil's and the throttle position sensor cleaned my egr valve and pipe fuel pressures great which i knew it was ok...not getting any codes or anything so ii figured has ta be the precats stoping up on her cause it started easier with the glass pack which doesent have any back pressure on it and put a different design muffler on it to quiet it down some and its harder to start..ive had this issue for about 4 months now..
Have you checked the ignition switch, cleaned the mounting surface between the starter and bell housing? Are you getting fuel and spark? What is your fuel pressure cranking?
If it was a sensor you would have codes. Try removing the igntion switch and starting with a screwdriver to eliminate the lock cylinder/switch slop.
I see you spending alot of money on parts, but no diagnosing.
It would still start and idle with plugged exhaust.
06-18-2012, 02:48 PM
#5
when im starting its bewtween 36-38..before it was like 29 but fuel filter was a little dirty...the car turns over with no problem..just getting it to start...its getting fire great and fuel cause it tends every once in awhile to try and flood after about 7 cranks...i took my strering column down checked the connections on the key switch and greased it with white litheium grease and seams to be fine...i did replace the fuel injectors when i first got her because of one was bad so i was like why not...its just weird cause it runs perfect after it starts besides it getting a little warm when u drive at a certain speen for a period of time but soon as u kick it it cools back down just fine....i had a camaro one time and when the cats where clogged it wouldnet start cause it couldnt get rid of the fumes..so i thought could be the pre cats..
06-18-2012, 02:56 PM
#6
Member who somehow became The President of The SE-L Club
iTrader: (19)
Join Date: Jun 2001
Posts: 16,033
It's called exhaust scavenging. Back pressure is never good.
An incorrectly sized system will not allow the exhaust pulses to flow correctly. Hence, bigger is not always better, restrictions (back pressure) is even worse.
06-18-2012, 03:18 PM
#8
06-18-2012, 05:23 PM
#9
in the FAQ section of the forums there's a sticky on it.
edit - I found the thread
http://forums.maxima.org/4th-generat...und-clips.html
and specifically...
http://forums.maxima.org/3583294-post2.html
^^read and enlighten yourself 
edit - I found the thread
http://forums.maxima.org/4th-generat...und-clips.html
Doesn't an engine need backpressure to make power?
Exhaust system backpressure is ALWAYS BAD! Engines do not need backpressure to make good torque. What an engine needs is a system that maintains high exhaust stream velocity at low RPM.
Exhaust system backpressure is ALWAYS BAD! Engines do not need backpressure to make good torque. What an engine needs is a system that maintains high exhaust stream velocity at low RPM.
http://forums.maxima.org/3583294-post2.html
BACKPRESSURE = TORQUE?
An old hot-rodder's tall tale: Engines need some backpressure to work properly and make torque. That is not true. What engines need is low backpressure, but high exhaust stream velocity. A fast-moving but free-flowing gas column in the exhaust helps create a rarefaction or a negative pressure wave behind the exhaust valve as it opens. This vacuum helps scavenge the cylinder of exhaust gas faster and more thoroughly with less pumping losses. An exhaust pipe that is too big in diameter has low backpressure but lower velocity. The low velocity reduces the effectiveness of this scavenging effect, which has the greatest impact on low-end torque.
Low backpressure and high exhaust stream velocity can be achieved by running straight-through free-flowing mufflers and small pipe diameters. The only two exceptions to this are turbocharged engines and engines optimized for large amounts of nitrous oxide. Both of these devices vastly increase the exhaust gas volume and simply need larger pipes to get rid of it all.
Some stock mufflers and exhaust systems have up to 18psi of choking, power-robbing backpressure. In direct contrast, a well-designed, high-performance street exhaust system typically has about 2 to 6 psi of backpressure. For an interesting comparison, an un-muffled straight pipe on a real racecar usually has 1 to 3 psi of backpressure.
An old hot-rodder's tall tale: Engines need some backpressure to work properly and make torque. That is not true. What engines need is low backpressure, but high exhaust stream velocity. A fast-moving but free-flowing gas column in the exhaust helps create a rarefaction or a negative pressure wave behind the exhaust valve as it opens. This vacuum helps scavenge the cylinder of exhaust gas faster and more thoroughly with less pumping losses. An exhaust pipe that is too big in diameter has low backpressure but lower velocity. The low velocity reduces the effectiveness of this scavenging effect, which has the greatest impact on low-end torque.
Low backpressure and high exhaust stream velocity can be achieved by running straight-through free-flowing mufflers and small pipe diameters. The only two exceptions to this are turbocharged engines and engines optimized for large amounts of nitrous oxide. Both of these devices vastly increase the exhaust gas volume and simply need larger pipes to get rid of it all.
Some stock mufflers and exhaust systems have up to 18psi of choking, power-robbing backpressure. In direct contrast, a well-designed, high-performance street exhaust system typically has about 2 to 6 psi of backpressure. For an interesting comparison, an un-muffled straight pipe on a real racecar usually has 1 to 3 psi of backpressure.
Last edited by Amerikaner83; 06-18-2012 at 05:30 PM.
06-18-2012, 05:55 PM
#10
Member who somehow became The President of The SE-L Club
iTrader: (19)
Join Date: Jun 2001
Posts: 16,033
Thank you Amerikaner for bringing those facts to life in this thread.
I find it amazing that some guys think back pressure is good for what basically is nothing more then an air pump. It's a lack of education of the basics of a combustion engine. Just bolting on engine mods and increasing the size of your exhaust system is not necessarally the way to increase "performance".
Note: The word performance is key here.
06-18-2012, 07:18 PM
#11
i agree some people say o imma put 3 inch exhaust on my car and gain 30 hp..wrong lol i wont go nothing bigger then 2 1/2 but read this aswell Destroying a myth.
Some say that "an engine needs backpressure to work correctly." Is this true?
No. It would be more correct to say, "a perfectly stock engine that cannot adjust its fuel delivery needs backpressure to work correctly." This idea is a myth. As with all myths, however, there is a hint of fact with this one. Particularly, some people equate backpressure with torque, and others fear that too little backpressure will lead to valve burning.
The first reason why people say "backpressure is good" is because they believe that increased backpressure by itself will increase torque, particularly with a stock exhaust manifold. Granted, some stock manifolds act somewhat like performance headers at low RPM, but these manifolds will exhibit poor performance at higher RPM. This, however does not automatically lead to the conclusion that backpressure produces more torque. The increase in torque is not due to backpressure, but to the effects of changes in fuel/air mixture, which will be described in more detail below.
The other reason why people say "backpressure is good" is because they hear that cars (or motorcycles) that have had performance exhaust work done to them would then go on to burn exhaust valves. Now, it is true that such valve burning has occurred as a result of the exhaust mods, but it isn't due merely to a lack of backpressure.
The internal combustion engine is a complex, dynamic collection of different systems working together to convert the stored power in gasoline into mechanical energy to push a car down the road. Anytime one of these systems are modified, that mod will also indirectly affect the other systems, as well.
Now, valve burning occurs as a result of a very lean-burning engine. In order to achieve a theoretical optimal combustion, an engine needs 14.7 parts of oxygen by mass to 1 part of gasoline (again, by mass). This is referred to as a stochiometric (chemically correct) mixture, and is commonly referred to as a 14.7:1 mix. If an engine burns with less oxygen present (13:1, 12:1, etc...), it is said to run rich. Conversely, if the engine runs with more oxygen present (16:1, 17:1, etc...), it is said to run lean. Today's engines are designed to run at 14.7:1 for normally cruising, with rich mixtures on acceleration or warm-up, and lean mixtures while decelerating.
Getting back to the discussion, the reason that exhaust valves burn is because the engine is burning lean. Normal engines will tolerate lean burning for a little bit, but not for sustained periods of time. The reason why the engine is burning lean to begin with is that the reduction in backpressure is causing more air to be drawn into the combustion chamber than before. Earlier cars (and motorcycles) with carburetion often could not adjust because of the way that backpressure caused air to flow backwards through the carburetor after the air already got loaded down with fuel, and caused the air to receive a second load of fuel. While a bad design, it was nonetheless used in a lot of vehicles. Once these vehicles received performance mods that reduced backpressure, they no longer had that double-loading effect, and then tended to burn valves because of the resulting over-lean condition. This, incidentally, also provides a basis for the "torque increase" seen if backpressure is maintained. As the fuel/air mixture becomes leaner, the resultant combustion will produce progressively less and less of the force needed to produce torque.
Some say that "an engine needs backpressure to work correctly." Is this true?
No. It would be more correct to say, "a perfectly stock engine that cannot adjust its fuel delivery needs backpressure to work correctly." This idea is a myth. As with all myths, however, there is a hint of fact with this one. Particularly, some people equate backpressure with torque, and others fear that too little backpressure will lead to valve burning.
The first reason why people say "backpressure is good" is because they believe that increased backpressure by itself will increase torque, particularly with a stock exhaust manifold. Granted, some stock manifolds act somewhat like performance headers at low RPM, but these manifolds will exhibit poor performance at higher RPM. This, however does not automatically lead to the conclusion that backpressure produces more torque. The increase in torque is not due to backpressure, but to the effects of changes in fuel/air mixture, which will be described in more detail below.
The other reason why people say "backpressure is good" is because they hear that cars (or motorcycles) that have had performance exhaust work done to them would then go on to burn exhaust valves. Now, it is true that such valve burning has occurred as a result of the exhaust mods, but it isn't due merely to a lack of backpressure.
The internal combustion engine is a complex, dynamic collection of different systems working together to convert the stored power in gasoline into mechanical energy to push a car down the road. Anytime one of these systems are modified, that mod will also indirectly affect the other systems, as well.
Now, valve burning occurs as a result of a very lean-burning engine. In order to achieve a theoretical optimal combustion, an engine needs 14.7 parts of oxygen by mass to 1 part of gasoline (again, by mass). This is referred to as a stochiometric (chemically correct) mixture, and is commonly referred to as a 14.7:1 mix. If an engine burns with less oxygen present (13:1, 12:1, etc...), it is said to run rich. Conversely, if the engine runs with more oxygen present (16:1, 17:1, etc...), it is said to run lean. Today's engines are designed to run at 14.7:1 for normally cruising, with rich mixtures on acceleration or warm-up, and lean mixtures while decelerating.
Getting back to the discussion, the reason that exhaust valves burn is because the engine is burning lean. Normal engines will tolerate lean burning for a little bit, but not for sustained periods of time. The reason why the engine is burning lean to begin with is that the reduction in backpressure is causing more air to be drawn into the combustion chamber than before. Earlier cars (and motorcycles) with carburetion often could not adjust because of the way that backpressure caused air to flow backwards through the carburetor after the air already got loaded down with fuel, and caused the air to receive a second load of fuel. While a bad design, it was nonetheless used in a lot of vehicles. Once these vehicles received performance mods that reduced backpressure, they no longer had that double-loading effect, and then tended to burn valves because of the resulting over-lean condition. This, incidentally, also provides a basis for the "torque increase" seen if backpressure is maintained. As the fuel/air mixture becomes leaner, the resultant combustion will produce progressively less and less of the force needed to produce torque.
06-18-2012, 07:29 PM
#12
but anyways i wasent tending to be talking about back pressure was asking ta see if anyone has tried that before and the results..taking the cats off...cutting squares in them taking all the guts out and then welding the square back and then bolting it back up and going on with it...i think imma give it a shot this weekend couldnt really hurt anything
Thread
Thread Starter
Forum
Replies
Last Post
kjlouis
4th Generation Maxima (1995-1999)
11
11-24-2018 06:09 AM
FlaMark
5th Generation Maxima (2000-2003)
25
08-28-2015 10:15 AM