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Code p.o 402

EGR flow excessive is that bad.

If it really is excessive, yes, that’s a bad thing. Excess exhaust gasses routed back to the intake dilute the air and gas mixture, and can result in various drivability problems, poor performance, loss of engine power, and even stalling. The EGR is only supposed to flow during certain situations, like during the time the engine is already warmed up and you are accelerating up to speed on the freeway on ramp. There’s a valve that opens and closes to allow the correct amount of flow. I’ve had that valve stick open before on my truck, and it definitely isn’t something you want to ever happen. The idle becomes really rough, almost to the point of being un-drivable.

Interesting. If my interpretation of what CS posted above is correct, the engine computer may be disabling all EGR flow to prevent rough idling. If that’s the case, the rough idling problem may be being masked, but then there’s another problem to worry about. No EGR flow can cause the internal engine parts, valves, valve seats etc, to overheat and major engine damage can result. This would be especially worrying if the truck is driven hard, or towing a boat or trailer uphill for long distances, etc.

thanks george it funny the check engine light came on.i read code p.o.402 but truck run great no bad performance problems.

Perhaps, but I’d still recommend addressing it.
The EGR system is diluting the incoming air with a bit of inert exhaust gas in order to prevent the cylinder temps from becoming too hot, which can cause preignition conditions that can damage the parts. If George is correct and the computer is shutting the EGR system down by keeping the solenoid-operated valve closed, the problem could be causing internal damage.

If the EGR system is allowing too much flow, you could have other issues. The exhaust gasses carry with them carbon. Carbon buildup on intake valves and/or parts inside the cylinder can also cause operating problems. Carbon retains heat and can interfere with the sealing of the valves and even buildup enough to upset compression values designed at their highest margins, causing preignition in that manner. Excessive carbon can also contaminate the catalytic converter.

In short, even though the engine is running great, I still recommend that you follow up with correcting the EGR fault.

For the record, the EGR system is also there to prevent the creation of excessive NOx emissions, banned by the EPA, which skyrocket when the cylinders get too hot, but that in itself won’t do engine damage, so for the purposes of preventing engine damage I consider it secondary. Others may disagree, but that’s okay.

I’m wondering if I’m missing something.

When the EGR is disconnected, what evidence is there for engine damage risk?

The EGR was introduced purely as an emissions device (to dilute the incoming charge with pre-burned gasses which leads to lower combustion temperatures so NOx would not form). Years ago it was standard practice with a number of us mechanics at the dealer I worked at to disconnect them during new car prep.

I agree there will be higher combustion temperatures, which means slightly more power due to higher gas pressures to push on the pistons.

What would cause the risk of denotation now to be any worse than it was before the EGR was used?

EGR systems were introduced as a response to the side effects of the emissions-reducing benefits of leaner operation and higher cylinder temps (which were exacerbated by leaner metering) to prevent the generation of NOx, but they also prevent preignition and subsequent damage due to the higher temps. Prior to the leaning of fuel mixes and the intentional raising of cylinder temps in order to reduce emissions, preignition was only a factor for high compression engines and could be controlled effectively for street engines simply by using a higher octane fuel.

Um… really? With your dealership’s blessing, I assume? It’s good that your dealership never got caught. Disabling emissions systems on new cars was and is a huge no-no.

Agree. That is the way I remember it. And I do agree that leaner mixtures which showed up in that timeframe aggravated preignition.

What I’m still wrestling with is that EGR use was driven by emissions requirements. Yes it does have a secondary effect of helping to reduce preignition. But I’ve never seen any engine damage from unplugged EGRs.

I don’t remember discussion other than with fellow mechanics. It was based on pride in work. When we invested time to “prep” a new car, we wanted the vehicle to run well, which often included tweaking emission controls with the various tricks we knew. Recall the 70s and early 80s were years when there were often driveability issues related to emission controls.

If we “prep’d” a new car without tweaking those controls and the customer returned complaining about driveability, we did what we needed to do in order to make the customer happy.

With a disconnected EGR system I’d be be concerned about damage due to uncontrolled unwanted ignition. Pinging and possibly knocking.
With an “open” EGR line, as like when an EGR valve is stuck open, I’d be concerned about carbon buildup.

I’m unsure which of these you’re thinking of as being an “unplugged” EGR, but these would be my concerns.

I understand that what you were doing was of concern for the customer, and I think I understand that you were considering the EGR system as not being an emissions system but rather a system to control the secondary effects of the emissions systems. Correct me if I’m wrong about that. But I think the feds consider the EGR system to be an emissions system, since you cannot pass a car with an EGR fault.

It is perhaps truly a gray area as to whether it’s an emissions system.

Can you elaborate, as to why you feel it may be a gray area?

I would consider EGR part of the vehicle’s emissions system. My state’s bureau of automotive repair does

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Your assessment of my mindset back then is correct.

Thinking back on it more, those were the days when emissions controls (especially the California Emissions Package which was required for any new vehicle sold in counties over 4000’) made a big negative impact on driveability. As mechanics, we were often regarded as “heros” if we could make a new car have the driveability that customers expected.

I don’t remember anyone ever discussing the illegality of modifying emission controls then. That thinking hadn’t caught on yet. It’s definitely been ingrained in recent decades.

I have to admit, disconnecting the EGR added little benefit. Other mods made the most impact.

My thinking is that the EGR system could be looked at as not itself reducing emissions but rather compensating for the consequences of the leaner operation and higher cylinder temperatures that were made for emissions reductions. I recognize that the EGR system controls the proliferation of NOx, however elevated NOx levels wouldn’t be present without the aforementioned changes.

As I already stated (see above) I believe the feds consider it to be an emissions system. But I accept that it could be considered a gray area. The world is not always black & white.

“An EGR valve can stick open when it should not, causing a rough idle or hesitation. It can also stick closed, causing valve clatter and a check engine light.”

I would add to that “carbon deposits” for a stuck open valve, but I absolutely agree with it all.

Interestingly, a year or so ago I might have said that engines run clean enough now that carbon deposits are no longer the concern they once were, but having read this past year about the carbon deposition issues with directly injected engines, I recognize that carbon is still a byproduct of the combustion process sufficient to be considered.