Blow Off Valve or lack there of?

skates04 · **Posted:** Wed Dec 12, 2012 12:17 am

First let me say I am not trying to "Rice" out my Jeep or looking for a cool sound like the cars in the fast and furious movies.

The other day after taking the CRD for a test drive with out the plastic cover over the engine after some maintenance I obviously could hear much more noise from the engine. One thing that worried me was the sound when I let off the gas. I could definitely hear the turbo as I left off the gas.

This is my first diesel however it is my 5th turbocharged vehicle. Every vehicle has had a blow off valve, I believe the CRD does not. I did a quick search on the forum for "blow off valve" and nothing came back. Is there a diesel specific reason it does not have one?

The purpose of the blow off valve is not to make a cool noise. When you let off the gas and the throttle plate to the intake manifold closes all that pressurized air gets shoved backwards into the turbo which is horrible for the bearings and causes it to fail sooner as it basically causes the turbo to jerk to a stop when it is spinning at thousands and thousands of RPMS, hence then noise I believe I was hearing (A fluttering sound).

Usually turbocharged vehicles have a blow off valve, or relief valve, between the intake manifold and the intercooler. When you let off the gas pedal the valve, which has a vacuum/boost line from the manifold, senses the change to vacuum in the manifold, opens for a split second to relieve the pressure and then closes back up sealing the intake track. Normally on stock vehicles the valve has a tube going to the intake tube and the discharged air is dumped back into the air intake before the turbo. The fast and the furious types vent to the atmosphere to get the "cool" whoosh noise.

I am curious if the blow valve function is built into the EGR Valve some how. It is my understanding the GDE tune disables the EGR valve but still opens for a split second when you let off the gas under boost, right?

I do not have a GDE tune, but I have done the ORM mod, as have many others on this forum which I am assuming could probably be destroying our turbo's by disabling this possible important function of the EGR valve in our CRD's.

Curious to see what others think, or maybe I am completely off!

skates04 · **Posted:** Wed Dec 12, 2012 12:54 am

Did some more research, looks like there is no throttle plate, and most diesels have compressor housing designed to deal with the surge of air. Answered my own question...

ChooChooman74 · **Posted:** Wed Dec 12, 2012 12:55 am

Never heard of a blow off valve. None of my diesels have had anything called that.

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Glend · **Posted:** Wed Dec 12, 2012 4:06 am

My understanding is that the stock ECU software contained an 'overboost protection' feature which provided the equivalent of a pop off valve. The way it works is that the EGR valve is momentarily opened to bleed overboost boost into the exhaust system (think of it as a reverse EGR). This feature is designed to protect the turbo. Now I have heard that GDE retained this feature in its flash tune, again to protect the turbo. So it would appear the function is there already - unless you have blocked off the EGR completely.

flash7210 · **Posted:** Wed Dec 12, 2012 8:37 am

I have never seen a blow off valve on a diesel because diesels do not have a throttle plate.
Most will only have a waste gate.
Our CRD does not have a waste gate because it uses a variable vane turbo.

naturist · **Posted:** Wed Dec 12, 2012 9:07 am

OP got it right eventually. Diesels don't have blow off valves because they have no throttle plate. The purpose of a blow off valve is to relieve pressure behind a suddenly-closed throttle plate, so you will only find them on a gasser. Indeed, there is no practical way to install/activate one on a diesel. What's more, wastegates are pretty much no longer used on new turbo-diesels, having been replaced by the much better variable-vane turbos. So, save for the very high pitched turbo whine, there's nothing there to impress your guy-buds. Fortunately, the girls can hear that whine much better than the guys, so you can still impress them.

Anyway, OP, welcome to the forum, and best of luck with your new-to-you Jeep. Enjoy your ride for what it WILL do.

NotPicky · **Posted:** Thu Dec 13, 2012 12:47 am

They do make them for diesels. Usually for big boost applications to keep from low RPM surge or to keep compressor wheels from exploding when backing off the throttle quickly (drag racing, sled pulling).

http://www.dieselperformance.com/index. ... /index/62P

skates04 · **Posted:** Thu Dec 13, 2012 1:04 pm

Thanks for the replies.

It was'nt so much I wanted a blow off valve it was more that I was concerned the ORM mod maybe disabling an actual import feature of the EGR, I guess since there is no throttle plate there is not much of an issue.

GreenDieselEngineering · **Joined:** Sun Jun 07, 2009 5:10 pm **Posts:** 1111

There is an intake throttle on the 2005 and 2006 Liberty CRD engine. It is used in conjunction with the EGR. As EGR valve opens further, the intake throttle will start to close to increase the pressure drop to help flow more EGR. It also closes when shutting off the engine to reduce shutdown shake. Chrysler calls the intake throttle a FCV (flow control valve). It is located on the inlet to the engine intake elbow where the CAC hose attaches after the intercooler.

Big Montana · **Posted:** Fri Dec 14, 2012 8:01 pm

Do these FCVs trip a CEL if they break Keith? IIRC mine broke years ago and the symptom was a lag in acceleration from a dead stop. I can't remember if there was a CEL though. I think I went in for something else under warranty and they fixed something. That may have been my EGR though.

I'm asking because my Jeep has sputtered and lagged for awhile on accelerations from a dead stop. I'm going to change my fuel filter again soon because I'm not sure this could actually be the cause.

CATCRD · **Posted:** Fri Dec 14, 2012 9:35 pm

When mine died I eventually got P1140. Search that code here for a lot of stories on it. Lag in acceleration was not one of the symptoms though.

Blow Off Valve or lack there of?

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