P0401 Code
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Cross-Country
Joined: Jul 2013
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From: Hanover, MD
P0401 Code
Have a p0401 code which I need to diagnose and solve. However, I don't believe the symptoms are indicative of the EGR system, so I am asking for help. Its a 1999 4.6L auto approximately 200k miles. Truck runs fine until it warms up, and then occasionally it begins to act like its missing under load. steady state on the freeway it may have an occasional miss, but put a load on the motor and the miss becomes real bad. This doesn't seem to me to be symptomatic of a P0401 code, but I don't really understand the EGR system enough to be sure. Searching on the forums for other p0401 codes doesn't seem to have the same symptoms. Thoughts?
Lead Driver
Joined: Sep 2006
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As an education, you do need to know what the 401 code tells you.
It is a 'test failure' of the EGR system.
The failure is a flow test that failed due to 'insufficient' flow when the computer tested the system.
Cause is generally a blocked passage behind the EGR valve but could be before it or even a faulty DPFE sensor that is used to report test results.
.
There may be a faulty EGR or vacuum regulator as a second fault causing the misfires.
Solve for the 401 code then see how it runs otherwise.
Good luck.
It is a 'test failure' of the EGR system.
The failure is a flow test that failed due to 'insufficient' flow when the computer tested the system.
Cause is generally a blocked passage behind the EGR valve but could be before it or even a faulty DPFE sensor that is used to report test results.
.
There may be a faulty EGR or vacuum regulator as a second fault causing the misfires.
Solve for the 401 code then see how it runs otherwise.
Good luck.
Lead Driver
Joined: Sep 2006
Posts: 7,907
Likes: 143
The EGR is more complicated that you might think.
You cannot delete it in program.
You lost fuel mileage from disabling it in any way.
It normally has no effect on engine power so there is no gain in deleting it's operation.
EGR only operates during third gear or overdrive at light throttle, not when you call for power. It closes up at that time so has no effect on power.
But...but during it's operation the system leans the fuel and advances the ignition timing to gain some extra fuel mileage by design, so you lost that gain from disabling the EGR.
Once any EGR code is set you lose the fuel gain because the program does not use it again until a change has been detected by either a repair or a code clear where the computer either find it good or still faulty on the next self diagnostics run on the system..
When the EGR system is disabled you won't see any change in apparent performance except for fuel mileage.
Good luck.
You cannot delete it in program.
You lost fuel mileage from disabling it in any way.
It normally has no effect on engine power so there is no gain in deleting it's operation.
EGR only operates during third gear or overdrive at light throttle, not when you call for power. It closes up at that time so has no effect on power.
But...but during it's operation the system leans the fuel and advances the ignition timing to gain some extra fuel mileage by design, so you lost that gain from disabling the EGR.
Once any EGR code is set you lose the fuel gain because the program does not use it again until a change has been detected by either a repair or a code clear where the computer either find it good or still faulty on the next self diagnostics run on the system..
When the EGR system is disabled you won't see any change in apparent performance except for fuel mileage.
Good luck.
Go Bills !
Joined: Jan 2008
Posts: 2,778
Likes: 65
From: Buffalo, NY
I had a 401 code all of last year Untill I built up the energy to pull the throttle body off and clean out the plugged passageways . They were plugged solid at 175k . My 401 issues gave me an occasional snotty idle while the motor was warming up. Like sitting at a stop light it was flooding out like the choke was on. Once it was warm it ran fine . After I cleaned the throttle body it all cleared up . Not sure if the 401 caused my idle issue, but cleaning the throttle body, aic and egr system, all was good .
I had an occasional misfire when she warmed up and it took a few days for it to tell me what cop was failing. It was agravating because I knew what it was but it wouldn't tell me which # cop it was and I didn't want to change all 8. It eventually spit the misfire code and what cylinder it was after a few days and I then changed out the culprit cop. . Why it took a few days , I do not know
I had an occasional misfire when she warmed up and it took a few days for it to tell me what cop was failing. It was agravating because I knew what it was but it wouldn't tell me which # cop it was and I didn't want to change all 8. It eventually spit the misfire code and what cylinder it was after a few days and I then changed out the culprit cop. . Why it took a few days , I do not know
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Cross-Country
Joined: Jul 2013
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From: Hanover, MD
Well I pulled things apart and cleaned the passageways. Some Carbon for sure, but it certainly wasn't as bad as some of the pictures I have seen on here. Everything is back together and I drove it into work this morning. Stopped at a light the truck stumbled maybe every 2-4 seconds, otherwise ran fine. No check engine light. I will check to see if there is anything pending at lunch today.
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Cross-Country
Joined: Jul 2013
Posts: 83
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From: Hanover, MD
Interesting thought on the COP - I had wondered whether one could be causing an issue occasionally once it warmed up. Perhaps some more driving will cause it to throw a new code.
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Go Bills !
Joined: Jan 2008
Posts: 2,778
Likes: 65
From: Buffalo, NY
I know that stumble.. I had same thing. I can sit at a light and watch it on the rpm gauge. I actually had a video of it but i accidently deleted it.
Cleaning the egr, throttle body and AIC cured that for me.. Maybe your aic is causing the stumble ?
Your 401 is gone?
Cleaning the egr, throttle body and AIC cured that for me.. Maybe your aic is causing the stumble ?
Your 401 is gone?
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Go Bills !
Joined: Jan 2008
Posts: 2,778
Likes: 65
From: Buffalo, NY
Yup remove it . Spray it out with the motor upright and clean it out real good with choke cleaner . And reinstall. Most time the gasket will stay in place and won't need to be replaced. But sometimes. You need to make a gasket or buy one . It is an easy process. Maybe 10 minutes . With a shot. I don't think your 401 had anything to do with the idle stumble either. But your 401 is gone now
Lead Driver
Joined: Sep 2006
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EGR system and 401 code.
The EGR never opens at anytime other than during light throttle in third and overdrive between 45 and 60 mph when the PCM see the TPS in a relatively stable position as the signal to open the EGR, as long as the other parameters of speed and throttle are present.
When the EGR is called to open under these conditions, the fuel is cut back and ignition timing advanced.
These conditions call on the ignition coils to supply their max voltage. If any coil has low output, misfires /stumbling will be felt. If the throttle is opened causing the EGR to close and or downshifting occurs the RPM goes up and fuel, and ignition return back to normal power levels such that the misfire goes away until the next time these conditions are entered.
.
The 401 code is the results of a (test failure only) and has nothing to do with idle.
.
Idle is a function of the IAC control by the PCM.
If there is a fault such as sticking from gum or carbon it cannot control the idle properly.
If there are air leaks large enough, they tend to add un controlled air and interfere with idle control.
Hope this helps with understanding of these operations.
Good luck.
The EGR never opens at anytime other than during light throttle in third and overdrive between 45 and 60 mph when the PCM see the TPS in a relatively stable position as the signal to open the EGR, as long as the other parameters of speed and throttle are present.
When the EGR is called to open under these conditions, the fuel is cut back and ignition timing advanced.
These conditions call on the ignition coils to supply their max voltage. If any coil has low output, misfires /stumbling will be felt. If the throttle is opened causing the EGR to close and or downshifting occurs the RPM goes up and fuel, and ignition return back to normal power levels such that the misfire goes away until the next time these conditions are entered.
.
The 401 code is the results of a (test failure only) and has nothing to do with idle.
.
Idle is a function of the IAC control by the PCM.
If there is a fault such as sticking from gum or carbon it cannot control the idle properly.
If there are air leaks large enough, they tend to add un controlled air and interfere with idle control.
Hope this helps with understanding of these operations.
Good luck.
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