Another EGR Question! (1 Viewer)

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Apr 4, 2014
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Middle Tennessee
I have gone through so many threads to try to figure this out.

Replaced EGR Valve (new OEM part came with truck) and VSV. Modulator tests fine. Still getting P0401 (Insufficient Exhaust Gas Flow). According to the EGR System Test in the FSM I T'd between the Modulator and the EGR valve, and hooked into a mighty vac. I should get low vacuum with warm engine @ 2500 RPM. I GET ZERO. The FSM doesn't give IF this, then THAT scenarios. What does it mean? Clogged ports or vacuum lines? If so, which ones should I check? BTW, I checked the flow of the "P" and "R" lines going into the throttle body. Not great, but there is some flow. Also, replaced the lines. What other lines, if any, should I check? How can you tell if ports in intake or TB are blocked? Do you have to take them off or just throttle body? I have not messed with the temp sender since many say that is not likely problem. However, could that be the reason why I am not getting any vacuum as described above? How do you test it?

Other Questions:
FSM says to connect port "R" on modulator directly to the intake manifold. Anybody know exactly where on the intake manifold that should be?
Correct me if I'm wrong: The VSV turns vacuum on/off by opening or closing to atmosphere. Modulator controls the amount of vacuum applied to EGR valve which in turn opens up the spigot for the hot exhaust gases. Can anyone give me an explanation of the DIRECTION of flow of the system? I believe the vacuum comes from the engine sucking in air - more RPM, higher vacuum. But, where is the line that provides it, the "P" and "R" lines? Why two of them?

After I deal with this, I have to figure out why the CAT doesn't go into a Ready State.

Help,
help...I'm sinking
 
Joined
Mar 5, 2014
Messages
377
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outside Atlanta
If the new parts are good sounds like you probably need a modulator. It pretty much controls the flow of vacuum to the egr. You should be able to hook your vacuum gauge up to the lines going into the modulator and you should have vacuum on one of those ports- port e -don't think port r will have vacuum- that's why they have you get vacuum from somewhere on the manifold. If you have vacuum on port e at 2500 rpm then you should have it going into the egr. If you don't then plug the other side of the egr going to your vsv. If you still don't have vacuum it is probably venting in the modulator.

Personally I don't like the modulator test in the FSM - what did the filter look like. Probably black from the diaphragm blown out. The modulator uses e for modulating low egr flow. Port r comes into play to override the pressure from the exhaust( going into the bottom of modulator) - giving you full egr flow
 
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Queens, NYC and sometimes Orlando, FL
I've found that the best and easiest way to properly test the modulator is to remove it completely, cover the P,R and Q ports and gently suck/blow on the bottom port :cool:

If it holds vacuum and pressure, it's good :)

If it doesn't, it's bad :(

This method is much more straight forward and better than the FSM's testing procedure IMHO :meh:


Link to my modulator thread:


https://forum.ih8mud.com/threads/it...ive-had-an-mil-401-egr-code-revisited.660812/
 
Joined
Apr 4, 2014
Messages
12
Location
Middle Tennessee
I've found that the best and easiest way to properly test the modulator is to remove it completely, cover the P,R and Q ports and gently suck/blow on the bottom port :cool:

If it holds vacuum and pressure, it's good :)

If it doesn't, it's bad :(

This method is much more straight forward and better than the FSM's testing procedure IMHO :meh:


Link to my modulator thread:


https://forum.ih8mud.com/threads/it...ive-had-an-mil-401-egr-code-revisited.660812/

98 Snake Eater, thanks for the modulator test. I hooked up the Mighty Vac to the bottom port of the modulator and can get vacuum, but without having to cover the P, R and Q ports. It holds vacuum without those ports being blocked (see picture). What does that mean?

Modulator.jpg
 
Joined
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outside Atlanta
That would tell you the diaphragm is good in the bottom of modulator. Maybe you just had the two lines going into the modulator swapped. Check the two lines going into the modulator for vacuum at 2500 rpm. The one that has a vacuum will hook up to "p" on the modulator. Then u should have your vacuum going to the egr (line "q" from modulator)
 
Joined
Apr 4, 2014
Messages
12
Location
Middle Tennessee
That would tell you the diaphragm is good in the bottom of modulator. Maybe you just had the two lines going into the modulator swapped. Check the two lines going into the modulator for vacuum at 2500 rpm. The one that has a vacuum will hook up to "p" on the modulator. Then u should have your vacuum going to the egr (line "q" from modulator)
Thanks, I will check that out.
 
Joined
Apr 4, 2014
Messages
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Middle Tennessee
Okay, I have vacuum going into the modulator. However, I got no vacuum when I T'd between the modulator and the EGR valve. After I plugged the line from the EGR that goes into/through the manifold to the VSV, I get vacuum. The VSV is new though not exact OEM. I am going to do the VSV test per the FSM. Could it be the temp sensor? I know people say those hardly ever go bad, but with my luck, anything is possible. Is there anything else it could be besides the VSV or temp sensor? The lines seem good.
 
Joined
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Messages
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P0401 EGR problem solved (hopefully)! I checked the line going from the EGR valve through the intake manifold to the VSV by blocking it off at the VSV side. It would't hold vacuum. After searching with an inspection mirror, I found the culprit was the vacuum line coming off the underside of the intake. It was split right where it goes onto the nipple. I bypassed the intake by going right from the EGR to the VSV. I got vacuum, EGR ready, no CEL. This was a big PITA. Thanks for those who provided answers, I hope I can do the same for someone else.
 

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