1998 Lumina 3.8 EGR Problem

[Market525]

Hopefully there are some GM guys out there who can give me some tips on this one -- my son's car, a 1998 Chevy Lumina LTZ, w/ 168,000 miles. Overall it's been a pretty trouble free car and the original motor still runs pretty well. This recent problem has me puzzled.

Check Engine light is on. Using a scanner, the code is PO401, which is EGR Flow Insufficient. This has been going on for awhile now. I replaced the EGR valve, and the problem went away for a few weeks, then returned. If I clear the code, it returns within one day. EGR valve itself tests good.

Can this be a carbon issue? I've ran a wire through the the EGR tube that I can get to, but other than that, I am puzzled. Does this EGR valve key off another piece of engine managment hardware that I'm missing? Any tips are appreciated.
Steve

 

[Matt_S]

The service manual suggests removing the EGR inlet and outlet tubes from the manifolds and inspecting for blockages. You should also make sure that all the gaskets are sealing properly. I get the feeling that your wire cleanout method may not have been sufficient to clear the blockage.

The only other sensor involved is the MAP sensor, which is used to run the EGR flow test. During a gradual deceleration (i.e. closed throttle, no brakes, speed above 25 mph), the computer monitors the MAP sensor and looks for an increase in pressure to correspond with a measured opening of the EGR valve. It will run that test 9 - 12 times before throwing a code.

 

[JTK]

Try to blast a can of intake cleaner into the port in the EGR valve base that goes into the intake. I dunno how the EGR exit is jetted on this particular engine, but where the EGR finally exits is where it usually cloggs up. I've got the same (GM) feedback EGR system on my 3.2L isuzu rodeo (valve and sensor(s) all in one). Some really stubborn ones have been sucessfully cleaned by spraying intake cleaner into the vac port on the base while running the engine. You have to plug the exhaust port with a rag, rubber plug, etc and be extrememly careful of flame up's

.
G/luck
Joel

 

[mechtech]

Vacuum leaks -either hoses or fittings, should be closely checked - hard to see sometimes.
The ECU tests for EGR operation by artificially turning it on, and notes an RPM change. If none occurs, a code is thrown.
Plugged passages will do this.

 

[brianl703]

Usually the EGR valve is connected to a vacuum regulator or solenoid (controlled by the PCM) which in turn is connected to intake manifold vacuum. You might want to trace the vacuum hose from the EGR valve and see where it connects.

As far as the EGR valve itself, applying vacuum to it with the engine at idle should stall the engine or at least make it run very roughly.

 

[Matt_S]

quote:

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Originally posted by mechtech:
The ECU tests for EGR operation by artificially turning it on, and notes an RPM change. If none occurs, a code is thrown.
Plugged passages will do this.

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I don't mean to be snippy, but this isn't true in this case, although it may be for other cars. The information in my post above came directly from the '98 Lumina factory service manual and should be accurate. Also, the manual makes no mention of vacuum lines in relation to EGR. Visualizing my own engine (a 2000 3800 Series II) I'm almost positive there are no vacuum lines connected to this EGR valve.

 

[brianl703]

If there's no vacuum line connected to that EGR valve, then how does the computer actuate it?

Is there a solenoid built directly into it?

 

[T-Keith]

GM has used digital EGR valves on the 3800 since 88, there are no vacuum lines.

 

[Matt_S]

quote:

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Originally posted by brianl703:
Is there a solenoid built directly into it?

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Yes.

 

[The_Jeff]

Does this car have any aftermarket exhaust like a high-flow cat? EGR needs/expects a bit of backpressure for calibrated flow.

Less pressure= less flow= not effective enough for the PCM to like.

 

[Market525]

Thanks for the replies. To clear up some questions: no aftermarket exhaust - oem system still intact. EGR - no vaccum hoses, electrical connection only, as stated in an earler post.

Matt_S - thanks for the MAP sensor tip; I hadn't considered that. I think my plan of attack will be to (somehow) get the tubes off the intake & clean them, as well as checking the MAP sensor to ensure it is working properly.

Thank you to all who replied. I just can't bring myself to pay $75-80/hr dealership labor if it's a problem I can tackle myself. Just a cheapskate I guess.

Steve

 

[Matt_S]

quote:

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Originally posted by Market525:
Just a cheapskate I guess.

Steve

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No, just being smart.

Good luck with the fix. I sorta' doubt you have a MAP sensor problem. That would show up as bad mileage, low power, hesitation and the like. That should also throw different codes. Anything's possible, though.

 

[BlazerLT]

The EGR passages are probably coated with carbon and the passage diameter is being choked off.

Remove the EGR valve spray the EGR passages liberally with TB cleaner and wih the EGR valve out, start the engine and give it a few good redline revs to blow any carbon and crap out.