6.5L DIESEL EGR CODES

I have a question about my 1998 GMC Sierra 2500 equipped with a 6.5L diesel. Every time I drive the truck I get a P0400 and P0401 code, which indicates a problem with EGR flow. The truck is all stock other than I cut the muffler and converter off and put flex pipe on instead. The codes had been coming up before muffler and cat removal but they seem to come on sooner since removal. I have done a few tests. I have vacuum to the first two black solenoids on the driver’s side while the truck is idling, but I’m not getting any vacuum coming out of the middle solenoid. I checked all the vacuum lines and none of them are collapsed and they all look good. Might you know why I keep getting these codes? They come on after 20 or 30 minutes of driving.

And is it possible the codes are on because I removed the converter and muffler? Any help would be greatly appreciated.

Anonymous Via Email P0400—EGR Flow Malfunction P0401—EGR Flow Insufficient The only real component in the system that can sort of monitor EGR flow is the MAF sensor. There is no differential flow detector in the EGR system like those used with gas engines, which can produce an accurate EGR flow value. So, the question is what can cause EGR flowrelated DTC codes? 1. A bad/contaminated MAF sensor. 2. A bad vacuum pump or loss in vacuum (leaking lines/hoses), which means the EGR valve won't open. 3. A bad EGR solenoid, which is a vacuum switch that controls the EGR valve. 4. A bad turbocharger wastegate solenoid, which can affect incoming airflow over the MAF sensor. 5. A plugged catalytic converter will restrict exhaust flow resulting in excessive backpressure in the exhaust system, which reduces the intake airflow read by the MAF sensor. Removing the cat/muffler could still produce EGR flow codes because the EGR system was calibrated to work as a complete system. A total loss of restriction would alter the flow dynamics.

Always begin a troubleshooting process by carefully examining the related wiring and electrical connectors. Some of the following tests can be performed using a Tech II scan tool from the driver's seat. It might be worth the 1/21 hour of shop time at a dealership if the tech knows these diesels. Otherwise: 1—Measure the vacuum at the turbo wastegate. You should see about 20” of vacuum.

2—If vacuum pump and all lines on top of the

engine are OK, replace EGR solenoid. 3—If that didn't help, replace wastegate solenoid. 4—Use a spray MAF sensor cleaner to clean the MAF sensor. 5—Normally we recommend measuring exhaust system pressure differential across the catalytic converter to determine whether the cat is plugged. Replace the cat if above about 3 psi pressure differential during a hard run. Test this by drilling a 1/8” hole in the exhaust system right before/after the cat, install pressure fittings, do the pressure test, then weld holes closed if pressure is OK. Most catalytic converters slowly plug with soot over time. A few years of driving is usually enough to completely plug a cat when the vehicle is lightly driven in cooler climates. Let us know how it goes.