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Old 02-17-2021, 02:07 PM   #1
Keblar
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Default Power loss under load at speed. 0401 code after new EGR.

I'm currently experiencing loss of power and hesitation under load around or above 55 to 60MPH. Idle is fine at ~870-900 RPM. Around town power seems steady, if a little more sluggish than usual.

I am getting a P0401 code since I just replaced the EGR last week with a new Denso from RA. The old one seemed to be working ok, but after many cleaning cycles it had started to consistently leak a carbon dense air (exhaust) from a seam in the base (not the gasket), and idle had crept up over 1100 RPM and had become unsteady. It wasn't throwing any Codes prior to replacing other than P0422, which I assume was gas cap and is unrelated.
I couldn't find the screen gasket I'd gotten so I reused the old one, which had separated into 1 metal and 1 fiber piece, but otherwise seemed ok. A new screen gasket is on the way.

Problem cropped up yesterday heading home. I was cruising around 70mph, when under strain uphill the power just dropped, almost like I'd lost a gear. At first I could barely maintain speed holding a steady high RPM, but adding throttle would make it falter, as if it was running rich and throttle was "flooding" it. Dropping off throttle acted as if the engine was going to die. Over the last 10 miles and hills though top speed dropped down to ~60mph, and I could finally barely maintain 50mph going up a step interstate hill.

After getting home I checked codes and restarted and took Torque snapshots, expecting it to run poorly, but idle seemed fine and local driving was ok. I was low on gas, so I added some seafoam and filled tank in case it was bad fuel or water in the tank (it's been cold and damp here in NY). A Test drive trying to get up to speed in a 65 zone showed it was still happening unfortunately.

The most Obvious culprit is EGR system, since that's what was changed, but I don't know if it's a bad new EGR, or possibly problems with the old gasket. Could changing EGR have loosened enough carbon that now the Catalytic converter is clogged? Without a screen could the new EGR have fouled already in as few as 200 highway miles?

I'm hoping I can get some input diagnosing this. Normally I'd do more legwork before seeking help, but my current project has me working 10 to 12 hour days 6 days a week. So, my hands on diagnostic time is limited to about an hour a day and Sunday. Ironically, this is all to catch up from a 10 day Covid shutdown, which is the only reason I even had time to replace the "bad" egr. I have another vehicle, but I share that with my wife and try to avoid putting the commute miles (120 round trip) on the newer car.

I'll swap the old EGR back in when I get a chance and see what happens, as well as trying the new gasket when it comes in to see if anything changes.

I'll keep scouring old posts for clues as I get the chance today. I would love suggestions troubleshooting in the meantime though, since my time hands on with it will be extremely limited over then next few weeks.

Thanks

Edit: I should add that I just did extensive front suspension work: including new lower co trol arms, new struts and mounts as well as a new driver side axle. I don't expect any of that to be necessarily relevant at all, but I didn't want to leave it out incase it was an obscure clue for someone.
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2002 SL2 335k + Miles

Last edited by Keblar; 02-17-2021 at 02:10 PM. Reason: typo. clarity
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Old 02-17-2021, 03:55 PM   #2
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Default Re: Power loss under load at speed. 0401 code after new EGR.

The first, and easiest, thing to do when there is any "poor power" issue is to take compression readings.
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Old 02-17-2021, 06:25 PM   #3
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Default Re: Power loss under load at speed. 0401 code after new EGR.

Quote:
Originally Posted by billr View Post
The first, and easiest, thing to do when there is any "poor power" issue is to take compression readings.
I should probably inspect the plugs anyway to see their condition, although I'd expect to get a misfire code if it was a fouled plug. I can check compression then, but I have no idea when I'll be able to get to it.
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Old 02-17-2021, 06:52 PM   #4
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Default Re: Power loss under load at speed. 0401 code after new EGR.

You can also eliminate or confirm the EGR as the culprit by using a metal gasket to block flow as a test.
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Old 02-17-2021, 07:20 PM   #5
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Default Re: Power loss under load at speed. 0401 code after new EGR.

Is the vacuum reading off the OBDII in Torque accurate and useful for anything? The screenshot I took yesterday only showed 1.3 in/Hg at 3555 RPM.

I may wind up with a snow day tomorrow. If so I'll be able to troubleshoot then.
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Old 02-23-2021, 03:08 PM   #6
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Default Re: Power loss under load at speed. 0401 code after new EGR.

I was finally able to troubleshoot on Sunday. The problem seems to be alleviated for now.

I took off the new EGR to put on a new screen gasket. While I had it off I sprayed some more throttle body cleaner into the mount orifice and ran the engine for the Richpin style blowout cleaning cycle, again. I also sprayed the new EGR a little to clean off some of the carbon that had already accumulated. I put the new EGR back on and pulled the PCM fuse for at least 5 mins, as suggested in a few P0401 troubleshooting posts.

Pulling the fuse, did not clear the codes (had new codes from running with the EGR off too), so I cleared them from the OBDII on Torque.

A test drive couldn't replicate the problem after that, even under heavy strain accelerating uphill at interstate speed, and I haven't shown any further codes since then.

It could be any of the steps I took, but I suspect it might have been pulling the PCM that corrected the issue. It was suggested that this helps the PCM relearn the EGR position. I'd thought this only applied to older OBDI models and that resetting was all that was necessary on OBDII models, but I suppose a full power drain may be needed for a hard reset.

Incidentally, it was OLDNUC who suggested the PCM fuse fix on several posts I read, so he's still helping me out, even now. RIP Old Nuc.

Hopefully that was the fix, but I'll report here if the 0401 comes back.
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Old 02-24-2021, 05:00 PM   #7
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Default Re: Power loss under load at speed. 0401 code after new EGR.

That was nice while it lasted: The 0401code is back. However, my wife says she can't replicate the same sudden power loss issue that I experienced. I'll have to wait to get to it this weekend. I'll try the EGR delete plate and/or swap out the old EGR.
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Old 02-25-2021, 09:27 AM   #8
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Default Re: Power loss under load at speed. 0401 code after new EGR.

Original catalytic converter (possibly disintegrating internally)?
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Old 02-25-2021, 03:35 PM   #9
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Default Re: Power loss under load at speed. 0401 code after new EGR.

Quote:
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Original catalytic converter (possibly disintegrating internally)?
I'd have to go through the original maintenance records that I have from the previous driver, but I'd say almost certainly yes.

If I can get the problem to recur, or it does it on its own again, I'll try the trick of pulling out the O2 sensor.

It crossed my mind that something like that could be the problem: EGR starts working "properly" and suddenly there's a surplus of pent up carbon headed toward the exhaust. I don't know if it works that way though.

As far as the 0401 - Will a blocked cat throw the EGR code?
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Old 02-25-2021, 04:01 PM   #10
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Default Re: Power loss under load at speed. 0401 code after new EGR.

I think an exhaust problem (cat or other) could affect the PCM's EGR flow test.

There is no sensor to measure EGR flow directly, flow is inferred from how opening of the EGR valve affects MAP; but if there are any other problems, like exhaust back pressure, then the MAP will already be different than normal and response of MAP to EGR flow may also not be normal.

My understanding is the PCM does not look at O2 readings in relation to EGR opening; but if it did, the same reasoning would apply.
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Old 02-25-2021, 04:14 PM   #11
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Default Re: Power loss under load at speed. 0401 code after new EGR.

EGR function is measured by the upstream O2 sensor. When exhaust gas recirculation is metered during normal acceleration, the O2 sensor can detect exhaust gases relative to combustion gases since the pcm controls egr valve operation and expects a change from normal combustion (idle rpm shuts off egr flow) to acceleration with egr flow.

When a damaged catcon occurs, many things are affected. At the least, primary and secondary issues can occur without a definitive correlation. Careful diagnosing and troubleshooting can narrow problems. Patience works as problems are sorted out.
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Old 02-25-2021, 06:11 PM   #12
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Default Re: Power loss under load at speed. 0401 code after new EGR.

Bill is correct. It is the MAP that is monitored.
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Old 02-25-2021, 06:24 PM   #13
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Default Re: Power loss under load at speed. 0401 code after new EGR.

Quote:
Originally Posted by Keblar View Post
That was nice while it lasted: The 0401code is back. However, my wife says she can't replicate the same sudden power loss issue that I experienced. I'll have to wait to get to it this weekend. I'll try the EGR delete plate and/or swap out the old EGR.
There is no need to block EGR flow with the metal gasket if you're not currently having driveability issues.

What code did you have when you thought it was the gas cap? Confirm the code and post it.
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Old 02-25-2021, 06:58 PM   #14
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Default Re: Power loss under load at speed. 0401 code after new EGR.

I imagine it was P0442. Also, there is no Denso EGR listed.
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Old 02-26-2021, 05:46 AM   #15
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Default Re: Power loss under load at speed. 0401 code after new EGR.

Quote:
Originally Posted by Cheyne View Post
I imagine it was P0442. Also, there is no Denso EGR listed.
Correct, it was a 0442 code. That has not returned. I suspect that was from the compromised seal on the old original EGR, since the ACDELCO "GM Original Equipment" replacement gas cap never fixed it.

The reason you don't se a Denso listed is because that was bad memory pull on my part. The new EGR is actually the Delphi EG10176
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Old 02-26-2021, 11:20 AM   #16
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Default Re: Power loss under load at speed. 0401 code after new EGR.

The cat could be an issue as mentioned above. The best way to check this at home with it being intermittent is perhaps with a good vacuum gauge if you don't have the back pressure tester.

Since there have been new symptoms and new parts ... At the beginning if I understand correctly, you had a high idle and P0442. Where does the temp gauge read and have you checked it with live data? Did you check the evap lines at the solenoid and throttle body? Your idle is still high at ~900.

The P0401 and sluggishness did not appear until the Delphi EGR replacement with the faulty gasket? Then, after cleaning, starting without the EGR and replacing the gasket, the driveability symptoms have improved with only the P0401 remaining? The aftermarket EGR valves have been known to cause issues.

Perhaps billr and fdryer have other ideas.
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Old 02-26-2021, 11:55 AM   #17
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Default Re: Power loss under load at speed. 0401 code after new EGR.

My bet is on the converter being plugged or there could be a chunk of the platinum broke inside of it bouncing around and intermittently getting stuck causing excessive back pressure. With 300k miles, it's certainly possible.
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Old 02-26-2021, 12:11 PM   #18
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Default Re: Power loss under load at speed. 0401 code after new EGR.

Was compression ever checked?
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Old 02-28-2021, 03:32 PM   #19
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Default Re: Power loss under load at speed. 0401 code after new EGR.

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Was compression ever checked?
It was not. Once the power loss problem was resolved, I didn't bother checking compression, given the limited time I had/have. I decided it would either come back eventually and I could keep digging, or it would stay resolved. So, compression check got moved down on the list.

I do have compression loss, and yes that may even be down from the last time I checked compression, but I would think any loss of power that sudden and drastic would only be related to compression if it were something fairly catastrophic, and I wouldn't imagine something like that that coming and going.
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