help! 1/04/08

[99rangler]

i had a header put on my 99 2.5L and on the way home i started having a miss at 2000 rpm in fourth gear (its automatic) and at idle and i have changed the plug wires, plugs, cap, rotor, and 02 sensor and it still does it, and it doesnt read any codes on a scanner. any ideas?

[RAT]

carbureted or fuel injected?

[TJ Clay]

I thought 99's were 3 speed autos, mine is.

[steelman]

carbureted or fuel injected?

'99 should be F.I.

[Mr. Orange]

Silly question, but are the headers specific to your make and model of engine?
Sounds like a small leak or the gasket isn't sealed completely. Check the bolts on the header and make sure none snapped or are too lose. The slightest bend will let a leak through. Almost sounds like not enough back pressure at low RPM. With little or no resistance in the exhaust, the engine could sound like a miss. If you already tried all the electrical, and it doesn't miss at other speeds/rpms, then it has to do with the exhaust setup. Did you leave the cat. on? If that's off, that would make you loose back pressure also.
Just check the header closely. What you could do, is get one of those 'mechanic's stethoscope'. Like what a doctor uses for hearing your heartbeat, only it has a long tube on the end in stead of the little paddle.
Take that little tube, and carefully probe around the header while the engine is running, and see if you hear any little pops or noises.

[peteyg]

There is a myth that engines need back pressure. This simply is not true. What a header does is allow better scavenging, completely evacuating the cylinder when the exhaust valve is open.

Think about it; if you have back pressure, it would resist all of the combusted gases from exiting the cylinder. Those gases that did not exit would be mixed in with the air-fuel mixture on the next intake cycle, which would then become more inefficient.

Good reading on the subject here:

http://www.uucmotorwerks.com/html_pr...torquemyth.htm


I would suspect an intake leak, and also, check to make sure your 02 sensor is connected. The intake was probably removed to install the header.

[kirkandsylvia]

I had a similar probem with my 96 cherokee, and the fix for mine was the fuel filter. I'd try checking the intake manifold first, and maybe replace the fuel filter

[Mr. Orange]

That's just it, it needs a little pressure or resistance....'Back pressure' is not some insane amount of force, but rather the 'controlled' release of exhaust.

Or why have the header in the first place. Haven't you ever heard an engine with just the headers? Sounds like crap, and doesn't run well, unless you modify other stuff, fuel delivery(amount/pressure),cams, intake....
Theory, you can't put out what you don't put in....

It is very inefficient to think that there is some stuff left in the chamber, but there is...that's also what the cat is for...the 'unburned mixture' is forced through the cat, which makes a reaction to 'burn' the excess.
The factory design is not always the most efficient for the consumer, but rather the factory's mass production and longevity. The exhaust is 'tuned' by the factory, to give the best engine response at 'all RPMs'....The whole setup is designed for the street, and not for max power, or speed. That would deteriorate the engine much faster.

Random thought....maybe you should flash the computer...tell it that you changed something on the motor...can't overlook anything that is controlled by computers....

[97sahara]

you do not want any back pressure, an engine is a vaccum, the faster it gets the air in and out the faster it goes, if it needed back pressure why would people take the cat and muffler off to get more power, wouldnt those add power if it needed back pressure?

[sahara0207]

you do not want any back pressure, an engine is a vaccum, the faster it gets the air in and out the faster it goes, if it needed back pressure why would people take the cat and muffler off to get more power, wouldnt those add power if it needed back pressure?

you need some back preasure in some situations. you can get more low end torque with a bit of back preasure plus engines now days are desighned to work with this preasure.... some times when you remove muffler, cat, or have too big of an exuast diameter you will lose power.

[Kodiak Spirit]

I installed a high flow intake, and then a magnaflow exhaust and cat, and then I was wondering where the power was... and then I upgraded my fuel injectors, and now I got the whole package. I am not talking monster anything, but I do see that making the air flow is not the end all of the engine, you gotta address the fuel as well....

[sahara0207]

ya with out the fuel matching the increase in air you can not get the full potential out of your modifacations. like why have more air flow in if you can not let air out any faster? the back preasure keeps the engine running on time and in sinc. other wise its messy and all over the place

[sahara0207]

but i am a young kid... what do i know lol im no mechanic

[peteyg]

I hate to sound like a broken record, but...

The need for backpressure is a MYTH. Sahara0207, you are so close to being right. Just remove the line that says "some backpressure is needed" and you'd be fully correct. The idea is that, in CARBURATED applications, when you remove exhaust restrictions, you create a lean condition because you can't get enough fuel into the combustion chamber to compensate for the increased air flow. So if you don't address the need for more fuel, you can do damage to the engine, not because you lost torque, but because you created on overly lean condition in the engine.

Modern fuel injected systems have a computer to compensate for this. It's not an issue. However, once you start making modifications, you have to follow through with everything.

Increasing air flow through the engine does not decrease torque. It does, however, tend to move the torque curve higher up the rpm band. This is why there are such things as RV cams, which adjust the valve timing and duration to make maximum power in the lower end of the rpm band. Not good for high rpm stuff, but great for low end grunt.

In some dual exhaust applications, it seems that putting on a true dual system can decrease power. This is true. The reason is, a true dual system can increase backpressure. That is why on most dual exhaust systems, you see an "X" pipe that links the two sides together. The reasone for this is for the one side to create a vacuum on the other, which dramatically improves exhaust scavenging.

I know that people want to believe in backpressure, but it just ain't so. Go take a look at a Nextel Cup car or an NHRA dragster. Show me where the have restriction in their exhausts...

[RAT]

X2 on peteyg's post-
I built race engines for drag and road racing back in the early eighties. The ONLY time we wanted backpressure was when we built a turbocharged road racing engine, and even then, the only place the backpressure was desireable was between the exhaust port of the head and the inlet side of the turbo. And even then, it was only momentary (upon acceleration). You want backpressure? Stick a potato up the tail pipe. Oh, and in a normally aspirated engine, backpressure, if excessive enough to cause a miss, will usually result in a backfire.