I am chasing an EGR fault which is driving me crazy

Symptoms: 0401 Code and Engine detonating under load

Removed Hoses and Inlet Manifold Cleaned everything , but nothing serious found no real carbon to clean
Replaced EGR, Valve, DPRE (both hoses) and Solenoid.
Pressure checked vacuum hoses, all seem good.

Idles just fine, drives OK. I can get 30 miles before triggering the fault, so only on a hot engine and only when under load will trigger the 0401.

Any ideas much appreciated !

 

Welcome to the forum

Should post the codes definition, no one knows them all by heart, lol, and you probably did look it up already

P0401 Exhaust Gas Recirculation Flow Insufficient Detected

The engine pinging/detonations means the EGR flow is actually low, so not a sensor fault

Adding exhaust gases to the intake "cools" the cylinders preventing a spike in NOX gases which are toxic, AND as a bonus it also prevents pinging/knocking

So under load the computer opens EGR valve a bit to keep NOX down and under heavier loads it opens EGR valve more

On the EGR tube there two hoses for the DPFE sensor, in between the hoses inside the tube is a restrictor plate: diagram here: http://cyoum84abk-flywheel.netdna-ss...1-1024x574.jpg

The hose closest to the EGR valve will have a bigger drop in pressure than the one farther away on the other side of the restrictor plate, as the EGR valve is opened by the computer
That's the data the computer gets from the DPFE(differential pressure feedback) sensor

P0401 means the pressure difference is not enough, computer "sees" that as low flow of exhaust gases into the intake manifold
If this was a DPFE sensor issue then there would be no pinging or detonation, because flow is OK, sensor is wrong
If there is pinging then sensor is right, flow is too low

 

RonD,

Thanks great explanation.

Therefore I must have a blockage further down in the actual exhaust system as everything else is clear.

or

The EGR Valve is not being lifted enough at higher workloads by a low vacuum pressure which is my suspicion ?

Should have hours of fun chasing all those vacuum pipes back to source ! Where does the EGR system source its vacuum to start with...hmmm.

 

With engine idling put longer vacuum hose on EGR valve, and suck on it, you should be able to stall the engine if EGR can open far enough and you have enough exhaust flow from the exhaust manifold.

Look at the vacuum hose diagram on the rad support top plate, that should tell you where it hooks up

EGR solenoid(EVR) should usually be hooked to Vacuum Reservoir hose after Check Valve, so it always has vacuum regardless of intake's vacuum level
But it can also be hooked directly to intake manifolds "vacuum manifold" which should have several ports with hoses on them

Thing about vacuum in the intake is that it drops close to 0 when you open throttle wide quickly, i.e. accelerate, and thats also when computer wants to add more exhaust gases, lol

Thats were the Vacuum reservoir comes in, its like the power brake booster, it has a check valve and storage tank, so it will hold vacuum at 18" or more as long as it doesn't have too many leaks, lol
Vacuum is not "used up", unless it leaks out
The vacuum reservoir is for the Vents in the cab, defrost, panel or floor selection is controlled by vacuum valves, defrost is default, no vacuum = defrost vent only
For example, if you have Panel vent selected and then accelerate and notice the panel vents are losing air flow but Defrost is getting more air flow then reservoir has a leak, can't hold vent in Panel position
It can't hold the 18" of vacuum, vacuum level is fluctuating with the intake manifolds vacuum

If EGR is hooked to vacuum reservoir, and reservoir is OK, then computer and EVR solenoid will have vacuum at the same level all the time regardless of intake manifold vacuum level
So when you accelerate, computer can add as much exhaust as needed to prevent NOX spike(or pinging)

 

RonD,

Fantastic now I understand where my leaks could be, and how to find them, much appreciated !

A virtual Beer or two on me !