messing with the EGR question

[DaveBr]

The 98 is an S code engine. The upper intake is factory cast about 90% blocked where it bolts onto the lower intake. I think this is because of the EGR system. My Question is how much of the cast blockage can I grind out before it affects EGR and throws a code? Will the extra air flow cause the MAF to throw a code? Anybody out there ever tried this? Thanks in advance for the input.

[Robyn]

If it were me, I would not molest the factory "S" maifold and top hat but instead scrounge up a manifold and hat from an F engine and install them as is.

Plug off the hose going to the EGR pod and give it a whirl.

You will in the worst case scenario have to put the old stuff back on if the puter gets rowdy with ya and tosses up a bunch of complaints.

Not sure what exactly the MAF sensor needs to see to be happy.

I really dont think it will complain BUTTTTTTTT if ya trash the old stuff then your SOL.

The MAF sensor is in the system to allow the computer (ECM) to more accurately control the fuel delivery at any given throttle demand.

In a situation where the airflow decreases due to a plugged filter or air density related to altitude etc, etc, etc.

This device is installed on the S engines to keep the emissions within a tighter tollerance than the F engine is required.

Other than the fact that this is (IMHO) just another electronic device that is going to fail, the concept is great and can be used to increase mileage too.

The ECM on an F engine sees the "throttle demand" signal from the APP plus it sees boost pressure and air temperatuire. Its does not know how much volume is flowing though.

If I were to guess here, (sometimes a bad plan) I would say that the MAF will likely not notice or care about the difference in airflow within reason.

The system can't tell if the EGR is actually doing something or not. There is no feed back loop to the ECM that signals if exhaust gas is actually flowing into the engine.

Dumping all that soot into the intake is real tough on things.
It causes a huge buildup of a thick carbon like substance that over time can break off in chunks and be sucked into the engine.

There is always small amounts of oil vapor being drawn into the turbo from the CDR system, this vapor will leave a little residue behind and this in combination with the dry soot from the EGR Bleed combines to make one HELLUVA MESS.


I have seen carbon built up on the backsides of the intake valves 1/4" thick in a nice little cone all over the head and up the stem part way.

Let this engine sit this way for a month or two with changes in temp and humidity and this stuff can and does break off.

The results can be as little as a Knock while the engine beats the stuff up and blows it out, or if just right, it can lock an engine tight.

The 6.5 has only about .035" clearance between the head of the piston and the cylinder head at TDC.

A large chunk of this crap can be major thick and its hard too.


Hope this helps answer your ??????????//


Missy

[JohnC]

The MAF sensor is primarily to police the operation of the EGR. On the ODB II engines defeating the EGR requires programming changes.

If you keep the EGR, be careful how much you do change the upper manifold. I think it's the way it is to promote mixing of the EGR gasses with the intake air.

[DaveBr]

What if I was to make an orfice plate to go under the EGR valve. That way when it opens there will be a small regulated flow thru the EGR which should satisfy the ECM and allow me to open up the upper intake. The reason I'm asking is I was hoping to avoid buying an ECM & intake from an F code engine.

[JohnC]

If the EGR flow is not within expected values the ODB II software will code. Not sure what else it will do, but if you have emissions testing, likely the presence of the code will cause you to fail. Basically, MAF + EGR = MAP

[DaveBr]

We don't have emissions testing in Alberta. I just want to know if I am on the right track. My guess is that GM undersized the 2 small holes they left in the upper intake just like they did with the exhaust. If I enlarge the 2 holes and proceed with the orfice plate I think it will work.

[JohnC]

Enlarging the holes may affect the mixing of the exhaust gasses with the air and adversely affect combustion in some of the cylinders.

Restricting the flow of exhaust gasses by means of an orifice plate (I assume that is your intent) will cause the PCM to set an EGR code. I do not know if there will be any affect on performance.

On ODB I trucks you could block off the EGR and the PCM was none the wiser, but not so on ODB II.

If you want to get rid of the EGR I'd get a L65 PCM or a performance PCM and be done with it.