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Boost leak testing the LML

Practical Diesel Tuning

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Hey Guys-

Is there is any special procedure to boost leak test the LML with emissions on? Tried to do this the other day and it appears to be leaking air out of the EGR into the exhaust like the valve is not closed. Is there a command to do this with EFI live?

Truck has no codes and runs fine just wanted to check health before tuning?




With the key in the run position, what does the EGR valve position report?


Hi Nick-

Sorry for the delayed reply, work got crazy then finally got a chance to look at the truck today. With the key in the on position (engine off) the EGR reports back 0%. I could push on the accelerator and see the % value increase so it appears to be a live readout.

I loosened the EGR and installed 2 blocker plates to test and I can get the pressure up to 30 PSI now. Looks like there is a very slight leak @30-20psi where the rubber boot connects to the plastic before the elbow going into the heater grid. Below 20 PSI I get very little leakdown. Could probably go at least a few minutes without losing pressure.

I was checking a few other forums and someone had a similar problem. As I recall they mentioned there are two valves in the EGR and they don't seal at the same time do you know if this is typical?

What do you think the best course of action is a this time? Remove the EGR and inspect?

Attached Files

It's a pretty typical situation, a little leak past the EGR on the boost test. Remember during operation the hot side of the EGR will always have higher pressure than the boost side so any unwanted leak will be exhaust polluting the intake stream. My hope was that it might resolve itself with the key in the on position and the stepper motor holding the valve shut.

I usually look for a leakdown of less than 1 psi per second. If it drops quicker than that on the gauge then the boost leak is big enough to cause performance and driveability issues.


should the Key be turned on while boost testing the LBZ and LMM as well?

If your truck is equipped with EGR you'll want to make sure the EGR valve is closed during the test. On LBZ/LMM I've not found a need to test with the key on, but you experience may differ. It's not going to hurt anything, and you'll be able to verify your MAP sensor function if you test key on with a scan tool hooked up. If the truck is losing boost through the EGR system you'll want to verify the valve is shutting completely by physically removing and inspecting it. Odds are it's due on these 10-15 year old trucks anyway.


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