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Jerry Newton
Jerry Newton, GMC Technician
Category: GM
Satisfied Customers: 4829
Experience:  ASE Master Technician, L1, Master GM Technician. Over 20 years of bumper to bumper GM experience.
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1998 Pont Bonneville SSei with EGR code P1404. Aftermarket

Customer Question

1998 Pont Bonneville SSei with EGR code P1404. Aftermarket CAT 14 months ago - welded on.
Upper intake gaskets replaced last Feb and replaced supercharger coupler and oil, clean plenum between throttle and superchgr and EGR tube and EGR chamber to plenum. Replace all vaccum hoses and T's, check MAF, IAT, EGR.
Submitted: 1 year ago.
Category: GM
Expert:  Jerry Newton replied 1 year ago.

Scan tool should be telling the story here. P1404 tells us that the EGR valve is not closing all the way when the PCM is commanding it closed. A look at the scan tool would tell the technician what the EGR position sensor voltage is, and therefore why it's setting that code. It's quite possible that there is loose carbon in the EGR system that is hanging the EGR open, but careful diagnosis should pin this down.

Delphi and other EGR valves are completely compatible, I don't think this is a problem with the EGR valve itself. That voltage isn't what the PCM expects to see, and the technician needs to find out why that's the case.

Customer: replied 1 year ago.
Two shops had $20000 scan tools and they have yet to find the root-cause to this issue. The EGR flex tube passages are clear/clean from exhaust pipe all the way to throttle body. Throttle body pulled twice. The EGR wiring checks and one shop swapped in a new MAF sensor to see if it had any effect. The EGR re-learn has been performed 10+ times. The new Delphi EGR's pintle can be commanded to move and position readings are normal ... using their scan tool. The PCM/ECM reflash was a suggestion by a local shop and I had it performed at a GM dealer. Pin holes in CAT welds were repaired. Shop claims it passes smoke tests now. Could an aftermarket CAT cause flow issues for PCM and appear as an EGR issue? P1404 appears with old EGR at 1/2 to 3/4 WOT. Hooked up old EGR to multi-meter ohms... response is erratic when I push in the pintle - should be smooth response. Indicates resistance potentiometer is worn out. New EGR pintle movement gets smooth response. I don't have the ohm numbers for both, but they were slightly different - 4000 to 10000. The new EGR's just get the P0401 and no P1404. Engine has 190K miles. Some light soot in intake plenum near EGR passage. IAT and area in intake were recently cleaned, but no effect to this problem. What next? Have GM verify leaks repaired again? Keep replacing other related sensors, or replace the PCM/ECM? Would need to find a GM dealer with a dyno to keep car stationary, simulate driving and hook up scanners to see problem in real time. Is this possible, and how much $$$.
Expert:  Jerry Newton replied 1 year ago.

GM dealers don't use dynos, we/they take the scan tool out and road test the car to replicate the concern. It's a simple process, we just go out and drive the car the same way you would drive it, with a scan tool attached, until the problem occurs (in this case, the MIL comes on). At that point, the technician can either observe the data in real time, or record it and review it later.

I understand your issue, and I understand that many have taken a swing at this and missed. The expensive scan tools not withstanding, things like this can often get past the technician if he is not fully competent in scan tool use, or doesn't have a full understanding of what exactly is causing the light to come on in the first place.

I may have been confused by your original post. At this point, you have a new EGR valve and you are only getting P0401, is that correct? And when does this code set while driving, do you have to be going down the highway or at idle? Any sort of clue you can give on that would be helpful.

There was a calibration update to address P1404, it sounds like you got that done at the dealer, so as a result of that, we have the most current software loaded into your car's PCM. No worries there.

Aftermarket cats don't give issues with EGR, necessarily; however, a restricted cat can absolutely manifest itself as an EGR issue, because the computer isn't programmed to recognize whether exhaust back pressure is excessive. Under hard acceleration, if air isn't moving through the engine and down the exhaust pipe as it should, it will skew the EGR, MAF, MAP, and other readings, and the PCM can set codes for any or all of these things as a result of a faulty catalyst. I'd be wanting to test exhaust back pressure, with a gauge made for the purpose. Allowable back pressure is 1psi, measured at the front oxygen sensor port while driving. Make sure your technician is performing this important test.

Customer: replied 1 year ago.
The P0401 code appears with new EGR after clearing with scantool and driving at low speeds, (i.e., 20 to 35 mph). GM dealer technician comments (before I had pipe weld leaks plugged): C/S CHECK ENGINE LIGHT IS ON, EXHAUST LEAK AND AFTERMARKET [CAT] CONVERTER P0401 INSTALLED LATEST CALIBRATION IN ECM. TEST DROVE AND CODE P0401 SETTING. CHECKED MAP SENSOR AND VACUUM TO MAP SENSOR. CHECKED VACUUM PORT FROM EGR TO INTAKE PASSAGE -CLEAR. CHECKED EXHAUST PORT -CLEAR. CHECKED EXHAUST SYSTEM AND FOUND EXHAUST LEAK IN WELD AND AFTERMARKET [CAT] CONVERTER INSTALLED. AFTERMARKET CONVERTER AND EXHAUST LEAK COULD BE CAUSING LACK OF BACK PRESSURE NEEDED TO PASS INSUFF. FLOW FOR EGR SYSTEM. The last sentence is confusing. INSUFF FLOW through what? the CONV or EGR? Excessive pressure meas at ox sensor means CAT clogged. Ok, so now with leaks plugged and still have P0401...still CAT issue. The converter is a universal/standard walker weld on type and not their EPA model. So could this std converter still be flowing too little? If that ends up being the issue all along. I think the 1st Delphi EGR was leaking betw the base plate and upper body as I noticed soot streaks betw the parts. It seems a lot of pressure would cause that issue too - again bad CONV. I will have GM tech check back pressure while driving it around. I may be regretting not buying GM OEM converter in the first place.
Expert:  Jerry Newton replied 1 year ago.

The technician is referring to insufficient EGR flow, which is what the trouble code is telling us: periodically, the PCM opens the EGR valve during deceleration, for the express purpose of seeing whether there is sufficient gas flow through the system. It determines this by watching the oxygen sensor, when the EGR opens, the oxygen sensor should go lean very quickly. The PCM is not satisfied with the EGR flow, and so is setting this code.

I am not convinced that a converter will fix this, I think the tech is shooting in the dark looking for an answer. If the scan tool is used to open the EGR valve at an idle, does the engine stall?