p401 EGR issues

[William839]

Hey guys , I know there’s tons of threads out there related to these issues , and I’ve read about every single one of them trying to figure this out. On my 97 fzj80 I have the p401 that I can’t get rid of. I have tested the egr modulator, tested the egr valve , did the VSV mod using the dorman 911-850 VSV, cleaned and replaced most vacuum lines and nothing is solving the CEL , the modulator was replaced by the last owner and is basically new , everything’s cleaned , the trucks only got 165k , any tips would help, I’m not sure whats left that I can do !!

 

[midfat]

I had the P0401 issue when I bought my ‘97 with 199K miles. I replaced all the vacuum lines, the VSV and EGR modulator, cleaned the intake manifold, put in a new PCV, installed new cats and replaced the O2 sensors. Basically, replaced everything in the entire emissions system. I realize this was a bit of an extreme response, but I spent zero effort chasing dead-ends and pulling out my heat. 50k miles later, everything’s perfect and I’ve breezed through two California smog tests without issue.

Probably not a lot of help answering your initial question, but an optional path to defeat might be something to consider.

 

[William839]

I had the P0401 issue when I bought my ‘97 with 199K miles. I replaced all the vacuum lines, the VSV and EGR modulator, cleaned the intake manifold, put in a new PCV, installed new cats and replaced the O2 sensors. Basically, replaced everything in the entire emissions system. I realize this was a bit of an extreme response, but I spent zero effort chasing dead-ends and pulling out my heat. 50k miles later, everything’s perfect and I’ve breezed through two California smog tests without issue.

Probably not a lot of help answering your initial question, but an optional path to defeat might be something to consider.

Thanks for response, I considered that but considering I’m a broke college kid that could be spending the money on other issues and modifications I’d like to solve this issue without dumping my savings into it! How much did that bill end up being if you don’t mind me asking !

 

[Rusty Marlin]

Did you clean out the pipe that goes through the upper intake manifold. Mine was plugged solid and I had to drill the crap out with a piece of wire.

 

[jpoole]

Was it driven for a while with a bad vacuum modulator? If so I'll share an explanation of what I encountered in case it is helpful. On my truck little chunks of carbon were loose in the vacuum lines associated with the VSV/EGR system and were getting sucked into the wrong places and causing clogs. After I had replaced the modulator, cleaned/tested the EGR valve, replaced all vacuum lines, cleaned the port/tube that passes through the intake, replaced the VSV valve and checked all of the electrical parts of the system I got continued to get PO401s though it would sometimes be weeks in between them. Those tag-along PO401 codes were due to loose carbon chunks blocking one of the two vacuum ports on top of the throttle body a few times and potentially causing intermittent VSV issues as well though that was harder to nail down. I pulled the 90 degree elbows off of the two ports on top of the throttle body and used a tiny pick, carb cleaner and small welding wire to open the holes back up a few times until I figured out what was going on. In the end pulling vacuum lines and blowing/sucking them out in every way I could think of seems to have gotten the remaining small carbon chunks out and it's been many months of regular driving since my last PO401.

In any case, don't forget to confirm good vacuum signals from the two ports on top of the throttle body and of course proper VSV functionality (closing/opening vacuum to the EGR modulator). Once you get through a few rounds of testing/troubleshooting of the entire system it becomes pretty easy to pinpoint what is keeping the EGR valve from opening when it should and it is possible to eliminate these codes long-term so don't give up yet, you are probably very close to success.

 

[William839]

Did you clean out the pipe that goes through the upper intake manifold. Mine was plugged solid and I had to drill the crap out with a piece of wire.

View attachment 1795061

I did clean it , in fact I punctured a hole in the vacuum line in the bottom and replaced it , I’ll clean the rest and see what happens , thanks

 

[William839]

Was it driven for a while with a bad vacuum modulator? If so I'll share an explanation of what I encountered in case it is helpful. On my truck little chunks of carbon were loose in the vacuum lines associated with the VSV/EGR system and were getting sucked into the wrong places and causing clogs. After I had replaced the modulator, cleaned/tested the EGR valve, replaced all vacuum lines, cleaned the port/tube that passes through the intake, replaced the VSV valve and checked all of the electrical parts of the system I got continued to get PO401s though it would sometimes be weeks in between them. Those tag-along PO401 codes were due to loose carbon chunks blocking one of the two vacuum ports on top of the throttle body a few times and potentially causing intermittent VSV issues as well though that was harder to nail down. I pulled the 90 degree elbows off of the two ports on top of the throttle body and used a tiny pick, carb cleaner and small welding wire to open the holes back up a few times until I figured out what was going on. In the end pulling vacuum lines and blowing/sucking them out in every way I could think of seems to have gotten the remaining small carbon chunks out and it's been many months of regular driving since my last PO401.

In any case, don't forget to confirm good vacuum signals from the two ports on top of the throttle body and of course proper VSV functionality (closing/opening vacuum to the EGR modulator). Once you get through a few rounds of testing/troubleshooting of the entire system it becomes pretty easy to pinpoint what is keeping the EGR valve from opening when it should and it is possible to eliminate these codes long-term so don't give up yet, you are probably very close to success.

I’m going to spend some more time cleaning it , i don’t have any vacuum on the upper port that goes to the T on the left side ( towards the radiator) though it’s clean I’m not sure if the VSV should have vacuum on both sides or not . But I will clean it more today and see what happens ,thank you

 

[jpoole]

I found it very difficult to clean those two ports on the TB without pulling the short 90 degree copper tubes. The holes in the TB are very small and short of welding wire I wasn't able to clear them well with anything else including soaking in cleaner, compressed air, etc. Since these are downstream they tend to suck the little chunks out of the system and then clog on them.

Gentle pull/twisting of the copper 90 with pliers and they should come out pretty easily (they are press fit in the TB). After cleaning the port(s) and getting vacuum the 90s push back in and can be aligned for clean vacuum hose runs to the modulator. As I recall one of the vacuum ports is upstream, the other downstream of the TB butterfly so you should see vacuum pressure variance on one side based on an open or closed TB.

 

[William839]

I found it very difficult to clean those two ports on the TB without pulling the short 90 degree copper tubes. The holes in the TB are very small and short of welding wire I wasn't able to clear them well with anything else including soaking in cleaner, compressed air, etc. Since these are downstream they tend to suck the little chunks out of the system and then clog on them.

Gentle pull/twisting of the copper 90 with pliers and they should come out pretty easily (they are press fit in the TB). After cleaning the port(s) and getting vacuum the 90s push back in and can be aligned for clean vacuum hose runs to the modulator. As I recall one of the vacuum ports is upstream, the other downstream of the TB butterfly so you should see vacuum pressure variance on one side based on an open or closed TB.

Thank you, I cleaned the bottom port that goes to the VSV and replaced the heat temp sensor , even though the stock one tested good , I’m going to drive it and see what comes of it. Do you know what the Vacuum control valve does I’m still not sure what it’s purpose is (though it’s in the name)

 

[vegasfj40]

I had 2 ports on the tbody blocked up. Also had to bypass the intake port with vacuum hose because I couldn't open that up. I removed the tbody and using a sewing needle was able to clean the ports. Also had to replace the modulator and VSV. Took me several attempts and trial and errors but I haven't had a CEL for a year now.

 

[vegasfj40]

Also, if you can get a FSM that will help explain everything. Theres a way to test the VSV without removing it via a connector mounted to the front of the intake. Theres a thread on MUD that explains which terminals to use.

 

[jpoole]

@William839

The VSV is an on/off device in terms of electrical signal and open/close in terms of the vacuum circuit that drives the EGR valve. What that translates into is that if the VSV valve is not powered up the EGR valve should never open no matter how much vacuum is sent it's way because the VSV valve basically creates a leak in the EGR vacuum circuit. If the VSV valve is powered up then it closes off and if vacuum is introduced to the EGR valve it will open the EGR valve since the VSV is closed and has sealed the vacuum circuit for the EGR valve.

You can test the VSV pretty easily by pulling the vacuum line in between the EGR valve and the intake manifold and then confirming first that you can suck air through the hose from the intake freely. You then power up the VSV and should not be able to suck air through the vacuum line that passes up through the intake manifold. To that end, a clogged VSV would never cause a PO401 because a 401 is "insufficient flow" meaning the EGR valve isn't opening or similar. A failed vacuum modulator could lead to the VSV getting clogged with carbon and essentially being closed all of the time which would lead instead to a P0402 or excessive EGR flow. Conversely, if the VSV fails electrically or isn't getting a signal it could remain open all of the time causing the EGR valve to never have enough vacuum to open.

Here is a thread about testing the VSV.

Checking VSV for EGR easy way P0401

If you are troubleshooting a PO401 you can isolate the VSV by pinching the vacuum line between the EGR valve and the intake manifold. This simulates a functional VSV that is powered up and that has closed off the vacuum circuit for the EGR valve. If all else is well revving the engine should lead to the EGR valve actuating. You can gently put a probe into the bottom of the EGR valve and actually feel / see the diaghram move up as the EGR valve opens. Another test on the other side could be to first pinch the EGR to intake manifold/vsv vacuum line and then introduce a vacuum on the vacuum modulator side of the EGR valve. This should cause the EGR valve to open and at idle the truck should stumble/run rougher. Etc. etc.

Here is a quick vid of an EGR test in action:



Hope that helps,

 

[William839]

I had 2 ports on the tbody blocked up. Also had to bypass the intake port with vacuum hose because I couldn't open that up. I removed the tbody and using a sewing needle was able to clean the ports. Also had to replace the modulator and VSV. Took me several attempts and trial and errors but I haven't had a CEL for a year now.

So very single port should have vacuum at idle or some are only going to create vacuum at higher RPM’s?

 

[William839]

@William839

The VSV is an on/off device in terms of electrical signal and open/close in terms of the vacuum circuit that drives the EGR valve. What that translates into is that if the VSV valve is not powered up the EGR valve should never open no matter how much vacuum is sent it's way because the VSV valve basically creates a leak in the EGR vacuum circuit. If the VSV valve is powered up then it closes off and if vacuum is introduced to the EGR valve it will open the EGR valve since the VSV is closed and has sealed the vacuum circuit for the EGR valve.

You can test the VSV pretty easily by pulling the vacuum line in between the EGR valve and the intake manifold and then confirming first that you can suck air through the hose from the intake freely. You then power up the VSV and should not be able to suck air through the vacuum line that passes up through the intake manifold. To that end, a clogged VSV would never cause a PO401 because a 401 is "insufficient flow" meaning the EGR valve isn't opening or similar. A failed vacuum modulator could lead to the VSV getting clogged with carbon and essentially being closed all of the time which would lead instead to a P0402 or excessive EGR flow. Conversely, if the VSV fails electrically or isn't getting a signal it could remain open all of the time causing the EGR valve to never have enough vacuum to open.

Here is a thread about testing the VSV.

Checking VSV for EGR easy way P0401

If you are troubleshooting a PO401 you can isolate the VSV by pinching the vacuum line between the EGR valve and the intake manifold. This simulates a functional VSV that is powered up and that has closed off the vacuum circuit for the EGR valve. If all else is well revving the engine should lead to the EGR valve actuating. You can gently put a probe into the bottom of the EGR valve and actually feel / see the diaghram move up as the EGR valve opens. Another test on the other side could be to first pinch the EGR to intake manifold/vsv vacuum line and then introduce a vacuum on the vacuum modulator side of the EGR valve. This should cause the EGR valve to open and at idle the truck should stumble/run rougher. Etc. etc.

Here is a quick vid of an EGR test in action:



Hope that helps,

Thanks for the response , and I did those exact tests in the video and everything seemed to pass , I even replaced the VSV using the Dorman part and relocated it . Even doing these tests is there a way the egr modulator or valve can both be bad?

 

[Tennessee80]

Can you do the EGR resistor mod and pass in CA? My old 80 had that code and I did the resistor plug mod and the code went away and I passed TN emissions...

 

[jpoole]

@William839

Both the modulator and/or EGR valve can be bad at the same time but you wouldn't be passing the EGR tests in the video etc. It is pretty easy to confirm that the EGR and the Vacuum modulator work following the prescribed tests online and/or in the FSM without removing them or getting in too deeply. If all of the system is testing out good though it's possible that you have a clogged port at the throttle body preventing the vacuum needed from being able to get to and open the EGR valve.

At idle vacuum is minimal on both of the ports on top of the throttle body, that lead to the EGR vacuum modulator. My gauge isn't great and when I tested the two ports at idle a few minutes ago neither registered until above idle RPM and then the one on the downstream/engine side showed more vacuum response I believe. The test that you should run on those two ports in my opinion is to squirt a touch of spray cleaner into each one to see if it impacts idle rpm. Don't use much and try to use something O2 sensor safe so that you don't end up causing other issues (I don't know if this is really possible I just don't want to give advise that causes more issues). On my truck when I just tested a touch of cleaner caused an immediate and very noticeable rpm dip when squirted into either port while the truck was idling. In my experience these ports are so small that they are likely either clogged or clear so you should get a good sense of these ports being good to go or needing to be cleared with this easy test. A few squirts of carb. cleaner could help clear a bit of carbon too so it shouldn't hurt if you don't go overboard. You can also hook up a vacuum gauge and rev the engine to confirm vacuum on both.

 

[William839]

Can you do the EGR resistor mod and pass in CA? My old 80 had that code and I did the resistor plug mod and the code went away and I passed TN emissions...

How did it effect your engine? Loss of power or mpg? I’m concerned about reliability and power , im pretty obsessive when it comes to stuff like that , do you have a link that can guide me to doing that? Might consider that if I can’t figure this out , and I’m in NM so I don’t think it’s half as strict as CA but I do believe they are somewhat strict with emissions

 

[William839]

@William839

Both the modulator and/or EGR valve can be bad at the same time but you wouldn't be passing the EGR tests in the video etc. It is pretty easy to confirm that the EGR and the Vacuum modulator work following the prescribed tests online and/or in the FSM without removing them or getting in too deeply. If all of the system is testing out good though it's possible that you have a clogged port at the throttle body preventing the vacuum needed from being able to get to and open the EGR valve.

At idle vacuum is minimal on both of the ports on top of the throttle body, that lead to the EGR vacuum modulator. My gauge isn't great and when I tested the two ports at idle a few minutes ago neither registered until above idle RPM and then the one on the downstream/engine side showed more vacuum response I believe. The test that you should run on those two ports in my opinion is to squirt a touch of spray cleaner into each one to see if it impacts idle rpm. Don't use much and try to use something O2 sensor safe so that you don't end up causing other issues (I don't know if this is really possible I just don't want to give advise that causes more issues). On my truck when I just tested a touch of cleaner caused an immediate and very noticeable rpm dip when squirted into either port while the truck was idling. In my experience these ports are so small that they are likely either clogged or clear so you should get a good sense of these ports being good to go or needing to be cleared with this easy test. A few squirts of carb. cleaner could help clear a bit of carbon too so it shouldn't hurt if you don't go overboard. You can also hook up a vacuum gauge and rev the engine to confirm vacuum on both.

 

[William839]

Thank you , I cleaned the egr modulator again yesterday and got somewhat confused I believe it was port P that runs straight through the modulator but when you pop off the cap and look at it it’s the center pin size hole it looks clogged or even broken in my opinion , I just don’t want to stick a needle in it and break something and spend more money , but the filter looks new and the port to the left with the larger size hole (r) is clear I’m thinking of picking a new one up at my dealer and trying it , but it’s the blue topped one and looks new and holds vacuum . So I don’t think that’s the culprit . Gotta keep diggin in thanks for the advice

 

[vegasfj40]

When I put a vacuum pump on the port that supplies vacuum to the VSV at the throttle body it was plugged and would hold as much vacuum as I put on it. It should be clear and not hold any vacuum. I knew then I had to remove the tbody to clean the port.