6.5 diesel egr flow insufficient codes

[gardentractorworld]

I'm new here to the forum and I have a question about my 98 YMCA sierra 2500 2wd with a 6.5 diesel. Every time I drive the truck I get a P0400 and P0501 code for Egr flow and Egr flow insufficient. The truck is all stock other than I cut the muffler and converter off and put flex pipe on instead. I have done a few tests and I have vacuum to the first two black solenoids on the drivers side while the truck is ideling 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. Does anyone 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.

[JohnC]

Cutting the converter off probably reduced the back pressure and hence flow through the EGR system, enough to cause the code. The code might not affect performance, so, unless you have an inspection problem, it could be ignored. (Check the service manual to confirm this.)

[gardentractorworld]

Is there a back pressure sensor? If so is there a way to trick the computer into thinking it's fine. There is a noticeable loss in performance when the codes come on.

[Warren96]

Have these codes been there before the modification, or have they always come up? The solenoids are not very expensive and the EGR valve is easy to clean. I have had to replace a couple of them that held vacuum. I had the muffler and catylast off mine for a short time, and the computer didn't care. The reason I put them back was because there was no increase in mileage or performance, and I preferd a quieter engine sound.

[JohnC]

There's a mass airflow sensor on the intake. I'm not up on the particulars of the ODB II but I suspect if the airflow is not within certain parameters in assumes the EGR flow is responsible.

[gardentractorworld]

The codes have been coming up before muffler and cat removal but they seem to come on sooner since removal. Does anyone know if I should have vacuum coming out of the middle solenoid while the truck is ideling. I have plenty of vacuum going into the middle solenoid or at the right fitting but no vacuum coming out or at the left fitting

[Yukon6.2]

Hi
Seeing you are already "testing" without the cat and muffler...
I would "test" with the EGR system removed...

My.05 cents,i'm from Canada and we don't have penny's

[gardentractorworld]

So you would suggest taking off all the vacuum lines and electrical connectors and sensors? Wouldn't this throw even more codes

[DmaxMaverick]

So you would suggest taking off all the vacuum lines and electrical connectors and sensors? Wouldn't this throw even more codes

Yep. Just find another ECM of the same year/model as your truck, only from a 2500HD (8600 GVWR, F code) or 3500, or purchase a reprogram. The computer is already complaining. Save yourself a lot of headache and eliminate the problem.*

*This advice is for off-highway and diagnostic testing purpose only. It is unlawful to modify the emission system of any vehicle.

[Warren96]

Have you had the egr valve off for cleaning? They fill up with a huge amount of carbon on mine. Besides, who is to say that the Canadian govt might change the law and now all your smog parts are missing.

[More Power]

I'm new here to the forum and I have a question about my 98 YMCA sierra 2500 2wd with a 6.5 diesel. Every time I drive the truck I get a P0400 and P0501 code for Egr flow and Egr flow insufficient. The truck is all stock other than I cut the muffler and converter off and put flex pipe on instead. I have done a few tests and I have vacuum to the first two black solenoids on the drivers side while the truck is ideling 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. Does anyone 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.

P0400 - EGR Flow Malfunction
P0401 - EGR Flow Insufficient
P0501 - Vehicle Speed Sensor (VSS) Circuit Malfunction

The only real component in the system that can monitor EGR flow is the MAF sensor. There is no differential EGR flow detector like those used with gas engines.

So, the question is, what can cause EGR flow DTC codes?

A bad MAF sensor.

A bad vacuum pump or loss in vacuum (leaking lines/hoses), which means the EGR valve won't open.

A bad EGR solenoid, which is a vacuum switch that controls the EGR valve.

A bad turbocharger wastegate solenoid, which can affect incoming airflow over the MAF sensor.

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 these tests can be performed using a Tech II scan tool from the driver's seat. Might be worth the 1/2-1 hour of shop time at a dealership if the tech knows these diesels.

Otherwise:

1- Measure the vacuum.
2- If vacuum pump and lines are OK, replace EGR solenoid.
3- If that didn't help, replace wastegate solenoid.
4- If that didn't help, measure exhaust system pressure differential across the catalytic converter to determine whether the cat is plugged. Replace cat if above about 3-psi at WFO (a hard run). Test this by drilling a 1/8" hole in the exhaust system right before/after the cat, install pressure fittings, do pressure test, then weld holes closed if pressure is OK. Most cats slowly plug over time. A few years of driving is enough to completely plug a cat when the vehicle is lightly driven in cooler climates.

Jim

[Kennedy]

When the EGR valve opens you are recycling so the ECM expects a reduction in MAF readings. With the exhaust opened up there is slightly less back pressure, greater gross flow and less EGR flow.

This can all be taken care of in the ECM programming.

[gardentractorworld]

I haven't cleaned the egr valve yet it didn't look that bad. I was assuming it had something to do with taking off the cat and muffler.