01 Regal S/C p0401 won't leave me alone!

Etes01

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2001 Buick Regal GS
HI everyone! I just joined the forum and I'm in need of help...
I purchased this regal a few weeks ago with a p0401 issue. The previous owner had replaced the egr valve and Map sensor with no change. First i checked the exhaust and inlet ports of the egr... they were clear. Just to make sure i manually powered the egr with the engine running and it stalled. I then checked the wiring to the egr, all seemed good until I found the 12v wire from the pcm unable to light a test light. I disconnected the pcm ran power through the connector to the 5 egr wires and all were ok. I deemed the problem as the pcm. I replaced the pcm, no change check engine light on as soon as I drove it down a hill.

I then started looking at data from my cheapy scan tool. I found the long term fuel trims were sitting at 17%. I looked for a vacuum leak and discovered an incorrect maf sensor was installed. Replaced with correct one and got the fuel trim down to 4% but still the check engine light returned. The engine runs fine by the way... When I test drove the car before buying it did a mean peel to 40mph so I'm convinced the catalytic converter is ok.

I then borrowed an expensive scan tool and manually controlled the egr valve. I was able to control it to the point of almost stalling the engine. The Map sensor appeared to respond.

I decided to check for vacuum leaks again being the Map sensor depends on good vacuum. I found a couple lines to the supercharger boost valve very dryrotted and a couple rubber T`s cracking. I ended up replacing all the vacuum lines from the map sensor to the bypass solenoid and valve. I also found (using a vacuum gauge) a slow leak from the boost valve and when spraying brake clean through the underside of it the short term fuel trims would go negative.

I ordered a boost valve and I'm praying it will solve my issue as I'm clueless what else could be an issue. Have I checked everything at this point? Is anything else known to cause this issue?

Thanks! And thanks for reading!
 

Etes01

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2001 Buick Regal GS
Just to update I bypassed the bypass valve... fuel trims went lower to just a hair below zero. Drove the car and p0401 came back on down the first hill with no change seen from the map sensor readings. I`m officially clueless.
 

Steveyo

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2019 Buick Encore
Could it be a problem with the upstream or downstream O2 sensors? Also, I believe a car can run just fine but still have a catalytic converter problem where the emissions levels are not what they should be and thus produce the P0401 code. Good luck.
 
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Etes01

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2001 Buick Regal GS
Could it be a problem with the upstream or downstream O2 sensors? Also, I believe a car can run just fine but still have a catalytic converter problem where the emissions levels are not what they should be and thus produce the P0401 code. Good luck.
Doesn’t this car use the map sensor to determine egr flow? I would think the only time the exhaust system would be an issue is if the converter is clogged or there is a crack on the exhaust manifold.
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Steveyo

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Doesn’t this car use the map sensor to determine egr flow? I would think the only time the exhaust system would be an issue is if the converter is clogged or there is a crack on the exhaust manifold.
Oops--my apologies, for my memory was in error in that I confused your code P0401 with code P0420 and an issue I had several years ago with a catalytic converter and O2 sensors producing the P0420 code.

In any case, I hope the new boost valve solves the problem, and perhaps someone familiar with your problem will come along and be of help.
 

Etes01

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2001 Buick Regal GS
Oops--my apologies, for my memory was in error in that I confused your code P0401 with code P0420 and an issue I had several years ago with a catalytic converter and O2 sensors producing the P0420 code.

In any case, I hope the new boost valve solves the problem, and perhaps someone familiar with your problem will come along and be of help.
Thank you, and unfortunately the boost valve did not fix it. My next goal is to check the exhaust backpressure and the intake and exhaust ports again. Just to make sure.
 

Etes01

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2001 Buick Regal GS
The test depends on the MAP sensor. You said it appears to respond, maybe not correctly. I'd try replacing it.
Map sensor has been replaced and all the vacuum lines running to it. At idle it it sits at roughly 10.5 inhg.
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Etes01

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2001 Buick Regal GS
Have you put an actual vacuum gauge on the engine? My SC3800 makes a steady 20" of vacuum even in gear sometimes. The actual sensor reading may be different, not sure.
No I have not, maybe I should try that. If the vacuum is low what could cause that? The engine runs at perfect fuel trims I dont think there are any vacuum leaks left.
 

LARRY70GS

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98 Riviera, 70 GS455 Stg1
Yeah, I'm not sure, the sensor might read differently on a scan tool. I have an actual vacuum/boost gauge permanently installed on my Riv. It almost always reads 20" at idle. The PCM does the test on Deceleration. My 98 FSM mentions the MAP and clogged exhaust as possible causes. An easy test with a vacuum gauge is to look at your vacuum at idle, then hold the engine steady at 1500 RPM or so and watch the vacuum. If it drops off, you have an exhaust restriction.
 

Etes01

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2001 Buick Regal GS
Yeah, I'm not sure, the sensor might read differently on a scan tool. I have an actual vacuum/boost gauge permanently installed on my Riv. It almost always reads 20" at idle. The PCM does the test on Deceleration. My 98 FSM mentions the MAP and clogged exhaust as possible causes. An easy test with a vacuum gauge is to look at your vacuum at idle, then hold the engine steady at 1500 RPM or so and watch the vacuum. If it drops off, you have an exhaust restriction.
I checked and there is a conversion. BARO-MAP=VAC. So 29.9inhg minus 10.5inhg equals 19.4in. So I would say I'm good. Also I did the exhaust restriction test at 2500 rpms for about 10 seconds and the number stayed right around the idle pressure.
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Etes01

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2001 Buick Regal GS
I don't know if it will help, but this is the page out of my 98 Riv FSM. Couldn't really get a complete shot as the manual won't sit flat on the scanner.

View attachment 14374View attachment 14375
Thanks, I already have a copy of that for my car, and everything exhaust wise appears Ok (minus the exhaust leak after the cat). I think I'll have to borrow my friends expensive scan tool again and road test to see if the egr valve is being commanded open by the pcm.
 

Etes01

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2001 Buick Regal GS
I obtained a scan tool and analyzed the egr opening and closing, pintle valve position, map sensor pressure etc... what I noticed was when the valve was deactivated after running the tests the map pressure on decel stayed steady. When the egr valve was testing, the pressure would kind of bounce around, but would seem to rise between 1-3 kpa. The valve was responding to pcm commands properly according to the scan tool.
 
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Etes01

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2001 Buick Regal GS
I've got good news!!!! It's fixed! And the whole problem was the "new" egr valve from the previous owner. Managed to get a used one from a coworker and now after 30miles of driving no ck engine light.
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Steveyo

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2019 Buick Encore
I've got good news!!!! It's fixed! And the whole problem was the "new" egr valve from the previous owner. Managed to get a used one from a coworker and now after 30miles of driving no ck engine light.

Good for you, the "new" owner. Well, you've checked everything out, and have new parts installed, so happy driving for many miles for you.
 

Etes01

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2001 Buick Regal GS
Good for you, the "new" owner. Well, you've checked everything out, and have new parts installed, so happy driving for many miles for you.
I hope it will be! So far I'm enjoying the car a lot, despite its issues.
I'm also hoping this forum will be a great help with any further repairs this car will need as I'm sure there will be many. Thanks for talking and listening!
 

Etes01

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2001 Buick Regal GS
Don't you hate it when "new" parts are defective right out of the box?
Yeah and the sad part was there was no way to tell. Doing manual testing and using a scan tool.... both methods did not alude to any apparant issues with the valve. Oh well, this was a learning experience I guess. Thanks for your input and for listening!
 
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