YAP0401 Thread (1 Viewer)

[jatree]

Hi all - I got a P0401 driving back home through the pass last week. When the CEL came on, I plugged in my OBD and had my passenger read it while I kept driving. Once I saw it was a P0401, I decided to just keep going since we were only 15-20 miles from home.

I have driven it around the neighborhood once since then to confirm the code and everything felt and sounded fine. I'm in the middle of a remodel and need the truck over the next two weeks so figured I'd wait to look into fixes.

Well today I popped the hood before going out and saw a melted/blown EGR Vacuum Modulator. As you'll see in the picture, it even messed up the hood insulation. Come to think of it, I am wondering if my premature paint oxidation in that area of the hood is related to all this.

19-0921_VacuumMod1.JPG

• jatree
• Sep 21, 2019

19-0921_VacuumMod2.JPG

• jatree
• Sep 21, 2019

Most recent work I did in June was:

Replaced both ventilation hoses and clips

Replaced PCV valve

Replaced distributor cap, rotor, and packing (dust-ring)

Replaced spark plug and ignition wires

Replaced PCV valve grommet

Cleaned out tuna can below air filter

I know from my last shop trip visit in April that there was a slight exhaust leak (I think between the two cats).

One week before the code, I took a picture near the modulator because I noticed some yellow-ish powder/dust. Just a little but it caught my eye.

19-0907_yellowpowder.JPG

• jatree
• Sep 21, 2019

Noticed 1 week before P0401 code while highway driving

So, how deep into this rabbit hole do I need to go? I'd like two options: A) Quick fix and get back to light driving for 1-2 weeks and B) Diagnose entire system and correct for long-term enjoyment and peace of mind.

My plan for next spring was to work on the exhaust system, replacing whatever parts needed to be replaced and removing the noisy system the previous owner put in. But I'm new at this and don't fully understand how it all fits together and don't want to go overboard.

I'm reading the FSM procedures now but would appreciate any help from the experts. Due to my time crunch, I'd even consider taking it to the shop if necessary.

 

[jatree]

Bought a new EGR Vacuum Modulator today (25870-66011, $100) but before connecting the vac hose between the EGR valve and Port Q, I instead connected it to my vacuum pump. I started the truck and while it was idling, pumped but was not able to draw vacuum. Does that mean the EGR valve is either stuck or bad?

 

[jatree]

I did a lot of diagnostics today based on this board, FSM, and youtube. Thought I'd post up what I did if it helps anyone and as a way of figuring out next steps if anyone has done similar. Or whether I should just try to drive and clear the P0401. It's my only vehicle and I need it to take me 5-10 miles at no more than 35-40mph for two weeks before I have time to go deeper into removing and cleaning the TB and Intake Manifold. My concern is having the same failure again - melted Vacuum Modulator, or messing up more.

So I did a mixture of the FSM P0401 diagnostics (non-Toyota OBDII reader) and some of the great tips on here (testing VSV).

0. Check function of EGR Valve via direct vacuum. I applied vacuum to the passenger side of the EGR "T" and could get it to stumble if I pinched the driver side at the same time. Not otherwise. Calling it good for now.​

1. FSM - Check resistance of EGR Gas Temp Sensor, should be 600 kilo-Ohms or less. Mine read 210 mega-Ohms...seems I need to replace it, this is getting expensive (opens egr-delete tab again).​

2. FSM - Check voltage of ECM or Harness (what the temp sensor plugs into). Slightly worried I pulled too hard on the harness side of the connection while trying to unclip. Should be b/w 4.5-5.5V, mine read 62mV.​

Of course now that I'm writing this I can see that the ignition should have been switched to ON before measuring. FSM confirms this for the ECM Voltage check but doesn't say for Gas Temp Sensor. Both connections looked good, clean. Unfortunately inconclusive until I can do again.

3. Check VSV Connector - way more straightforward. Thread says 30-35 Ohms, I read 60 Ohms. Not sure what that means.​

​

4. FSM - Test Modulator outside by blowing, useless test. Probably why this thread exists. The bottom could hold vacuum so I called it good. There's another test where you connect it up and blow through Port Q with the engine off (should be easy) and again with it on (should be hard). I felt like it was hard from the get go but it's brand new, the filter is clean, should be fine.​

​

5. Feel whether two small vacuum hoses (P/R) from TB are providing vacuum to the Vacuum Modulator while revving the engine. I felt strong vacuum on hose-P and very weak on hose-R. So I know there is likely a clog here.​

​

6. Reconnected all small vacuum lines and added a T-fitting between Port Q and EGR Valve to read vacuum pressure. You should see some vacuum at 2500rpm but I saw absolutely 0 whether at idle or ~2500rpm.​

So, my best guess is that the TB port (at minimum) is plugged and little to no vacuum is being provided to Port R on the new modulator. I know this was very long so thanks to anyone that read it.

7. Bonus - As I was putting everything back together, noticed a deformity next to the right EGR stud. Could it possibly be a pinhole leak? Vacuum test seemed to suggest valve is working ok...​

 

[jatree]

Unsure if it is related but I got a few months after replacing the EGR Vacuum Modulator, I got a P0402 on a regular drive. I never got around to further diagnosing the TB ports or whether all components are working. Looks like I'll be digging back into this to make sure the vacuum system is working as intended since it seems to control so much including emissions and when vacuum is applied to the charcoal canister.

 

[jatree]

This keeps getting more interesting (at least to me). I've been gathering parts for various vacuum and fuel hoses and putting off looking into the P0402 while reading many threads on how to diagnose and that it's probably the VSV. Well after replacing the original Charcoal/Vapor Canister with the VC120, the CEL (P0402) went off...I'm not sure why but I will post up whatever I find as I plan on going through various EGR/VSV tests.

edit: I wrote P0402 up there but after checking my log, what I really had at some point was a P0420 which is related to catalyst flow. Either way it went off and hasn't come back.