Late model EGR system questions

[izak]

How does EGR work in our trucks? Do the heads have exhaust ports for both the intake and exhaust manifold? I've read posts that say the MAF is just around to control EGR. It must use that and the MAP sensor to evaluate whats going on. I cant for the life of me figure out why the baro(?) sensor on the firewall is plugged into the EGR valve vacuum control line. And why do you need 2 solenoids? So you dont have to design another IC? Its like a Rube Goldberg machine.

I'm asking because my truck started smoking like a train right after getting the IP replaced at the dealer. I'm thinking what is happening is the EGR valve is opening when there's boost and that sends a lot of air out the exhaust. The PCM thinks there's a lot of air, supplies the wrong amount of fuel and all the unburnt fuel comes out the tailpipe as smoke. I'm trying to find an L65 PCM to plug in for a fix, but in the meantime I'd like to get the SES light off before I have to change the bulb.

[Turbine Doc]

High level ruff explanation

MAF is a separate input to PCM used only on L56 tells how much air is being flowed, PCM looks at MAF, RPM, Boost, ECT, fuel, Baro to determine how much EGR is needed for given power demand vs programmed set of curves in the PCM.

Firewall baro looked at only during start, rest of time it is used to compare EGR vac supply via EGR solenoid to EGR vent delta of 2 signals tells PCM how much valve is open. This is why you can blank off EGR valve with shim and not throw a code; as long as firewall baro, EGR sol, and supplied vac are in range you should not throw a code. (Not that I have had any personal experience in running that way, I just heard some folks that exclusive off road have done it so they would not be in violation of numerous laws.

If supplied vac is the problem you will have WG problems as well do Vac TS , if MAF is bad you will overfuel even at idle (low vac will tend to cause problems while driving) lots of black idle smoke bad MAF for experiment unplug MAF no change you have MAF problems.

Poor man L56 upgrade, get late model junkyard L65 PCM do theft learn, blank off EGR junk & 3" PVC in place of MAF, keep with you and keep all devices plugged in to not set up EMI open ground loop to PCM.

All aforementioned is for off road use only, you will want to stay legal, especially if you break down on the hiway in an emissions check locale, takes 15 min or less with practice to be back 100% L56 legal, that is unless your cat fell off when you were off roading.

How to skin the cat is another tail or tale, put your thinking cap on or do some searches you'll figger it out. Add a boost fooler and open up the airway some with bigger pipe better air filter you'd be suprised how lively the stock L56 can behave.

Spend some coin with one of the vendors you see about the house on a buuble here and there, how's Emril go? BAM kick it up a notch or 2 or 3.

[More Power]

In the 94-95 models (OBD I), disconnecting and plugging the vacuum line that connects to the EGR valve won't set a code. This will set a code in the 1996+ (OBD II) L56 (light-duty 6.5TD) equipped vehicles.

The two EGR related solenoids are: 1- EGR solenoid and 2- EGR Vent solenoid. The PCM operates both solenoids based on an electronic control strategy that uses a number of inputs, including CKP (crankshaft position), APP (Accelerator Pedal Position), EGR Control/BARO pressure sensor, ECT (Engine Coolant Temperature) and MAF (Mass AirFlow).

The 6.5TD requires full EGR flow at idle and less EGR as speed increases. EGR is incorporated in the LD 6.5 to reduce Oxides of Nitrogen emitted by the engine. Lowering combustion temperatures reduces NOX. EGR flow, or the recycling of exhaust, reduces the amount of oxygen available during combustion, which results in a cooler combustion temperature and lower NOX levels.

Hope this helps,

MP

[izak]

Thanks for the good info. Do you think I'm right - that any boost while the EGR valve is open bleeds into the exhaust?

I plugged my vacuum gauge into the EGR valve and used the windshield wiper to hold the gauge while I drove. One time it seemed to supply vacuum even while under load and one time it seemed to cut the vacuum. I'll have to test it more. The vacuum gauge seems to swing hard between 0 or 20 though, there's no in-between. Could it be that way because the EGR valve is not hooked up and the PCM is trying to make it open, or is it that the solenoids are bad and they need to meter vacuum better? Or are the solenoids part of a 2-stage full-vacuum-or-none decision process?

I'll try hooking the EGR back up and unhooking the MAF and see what effect that has.

[More Power]

As a very general rule of thumb, boost pressure somewhat follows exhaust backpressure in psi. So, boost pressure won't necessarily be dumped out the exhaust.

Turbo efficiency experts consider boost pressure and exhaust backpressure graphs when sizing turbos for a particular application. The factory 6.5 turbos are pretty efficient up to about 12 psi before the exhaust backpressure begins exceeding boost pressure - in a somewhat logarithmic fashion above 12 psi.

MP