jeremymilo

1986.5 Porsche 928 5-speed with 150k miles

Hi, I just failed my CA smog test with high NOx levels, which is typically caused by lean combustion mix and/or high combustion temps.

Seems like the place to start is cleaning the EGR system. Can someone point me to a "how to" on cleaning the EGR?

And if you have other ideas for things I should try first, I'm all ears.

Thanks!
Jeremy

RedPanda

Simple cheap things first:
-Use Chevron TECHRON injection cleaner into the tank following its directions w/ big name stations prem gas (Porshe approved only the chevron one so far, so use it).. depending on how bad maybe 2 times dunno that is up to you (dont use two bottles at once though)
(side note chevron gas has a little in it but that option will take a few months, also letting it sit longer also helps penetrate too but it is gunk in your engine)
-Oil change after the clean, or maybe a gas tank afterwards for flushing out remaining stuff (after the Techron your oil is going to be dirty, like spitting black gunk out of your exhaust)
-Drive the car for a good 30minutes and keep it running before its your turn for the smog check. (that and go to a smog place that specializes in older cars)

More $$$ measures:
Get a new O2 Sensor
Get new seals/tubes/hoses
Get another Cat
An actual fuel injection cleaning
Having the shop check the a tuning/tune up (includes chip, idle, and do engine flush of every fluid type etc)


Some people like to run with regular gas in a smog check due to the lower energy content in it..... I don't do it though, our cars are fickle enough.

there are other things, but I only know these.

jeremymilo

Thanks RedPanda. Would you say that cleaning the EGR system is still a good idea (reasonably affordable/easy and likely to help), or best to try your other suggestions first?

Bmw635

I don't see an EGR system in my 88 S4 so don't know if yours is the same.

Having failed NOXes in my BMWs , most fixes were lower temp thermostat, high octane, good spark plugs and ignitions, test right away with hot engine/ don't let cool down.

The 928 failed Nox when I first bought it was due to bad MAF with lean condition- replacement MAF passed smog.

safulop

Did you drive it before testing? I made the mistake of smogging my old 944 by just driving it 3 blocks. I failed with high NOx. I drove it 20 miles down the freeway and smogged it again, and it passed.

Lizard928

No EGR on yours.

Install a new factory fit O2 sensor, and adjust the base CO level as per the manual.

jeremymilo

Ok, new theory: high NOx is simply being caused by mild engine overheating. (And things like the O2 sensor, MAF sensor, catalytic converter, fuel injectors are working fine.) Does this theory sound likely based on the evidence below?

Evidence:
--My smog HC levels were very low. Measured at 39 ppm with 128 ppm allowable at 15 mph; Measured at 24 ppm with 103 ppm allowable at 25 mph.
--My smog CO levels were also very low. Measured at 0.01% with 0.80% allowable at 15 mph; Measured at 0.00% with 0.60% allowable at 25 mph.
--Slow coolant leak evident, and coolant level below the expansion tank mark.
--Cooling fan has been running more often than normal.
--Dash temp gauge reads in the normal zone, but maybe the gauge is not showing high temps properly?

dr bob

Warmer is a symptom of youre lean combustion I suspect, and the tailpipe readings for CO and HC made after the cats don't tell you how the mixture is at the engine.

All it takes is one or two 'weak' injectors to cause a NOx 'fail'. The oxy sensor takes the mean of all the cylinders' exhaust and makes a correction reading that assumes all injectors are flowing the same. At the same time, -any- oil in the intake will cause high combustion temps that will cause NOx regardless of how the mixture is set.

So...

Replace the plugs with new Bosch copper plugs, not platinum or any of the gimmick plugs or multi-electrode options.

Clean or replace the injectors. You might get by with an in-car cleaning, but if they have been in the car since new they really need to come out for a thorough cleaning or replacement. There are several replacement options with newer-tech injectors.

Oil accumulates in the belly of the intake, deposited there by the crankcase ventilation system. There are some interesting flush options like BG, Snap-on, etc that push a solvent through the running engine from the throttle body to clean this stuff out and send it through the exhaust. Most of those services can chem-flush the injectors in the car as part of the process. Results can be good.


My best NOx results, starting with low numbers when I bought the car (22k) and getting worse over time, came after a full intake clean and refresh including new injectors at around 100k. NOx numbers dropped back to as-new levels. Although I didn't think the car idled less than perfect before that little effort, it ran a -lot- better and idled smoother with new injectors.

There are sevices like Witchhunter that will clean and verify the flow on your injectors for you. Some of our vendors offer aftermarket packs of new injectors with the right hose and o-rings for your car that may be a better option than cleaning.

Ninjaxz750

You could move out of Cali.....

The Forgotten On

[/LIST]Some of us like to drive our cars year round without the fear of rust

Replace the plugs, fuel injector gaskets, O2 sensor, and check your MAF to see if it's within spec.

Those should help lower NOx a lot, just like others have been saying. Otherwise you could have a vacuum leak and need a top end refresh, but lets start with the easy stuff for now.

GregBBRD

Short term....good solution.

However, it's only a matter of a few years when all states are going to have emission standards like those in California. With all of this global warming and world pollution standards being such a huge issue, it's inevitable.

As all of the people in other states that have de-federalized their vehicles scramble to find all the missing cats and other pieces they have removed, I believe we will laugh last.

GregBBRD

^^^^^^^^^^^^^^This is 100% correct.

Your 928 has excessive NOX because it is too lean (not enough fuel).

O2 sensors are designed to fail lean. MAF can fail lean....beyond the ability for the O2 sensor to correct. Weak/dirty injectors can make it run lean. Any manifold air leak, after the MAF, can make it run very lean. Temperature senders can have the wrong resistance and cause a lean running condition.

The list goes on, but gets more obscure.

You've got plenty of the common things to check, from the posts, but here's where you should start:

Hook up a volt meter to the O2 sensor lead, when it is connected. We have special jumpers which allow us access to the O2 sensor wire, but you can strip back a small piece of insulation to be able to clip onto the O2 sensor wire.

Get the engine warmed up and monitor the voltage.

The voltage should be right at .5 volts as the O2 sensor (and the engine) warm up.

Ideally, once warm, you should have a voltage output fluctuating fairly rapidly from .2 to .8 volts.

Report back what reading you actually are getting and we can probably help you, from there.

jeremymilo

Closing the loop on this thread, I passed the CA smog test today -- just barely! Thanks for all the suggestions and advice. Here's what got the job done:

--Ran a tank of Chevron gas + a bottle of Techron fuel additive
--Installed a new O2 sensor (the PO had removed it for some reason)
--Adjusted the fuel mix while the car was being monitored on the dyno

Thanks to Kevin's Auto Repair in Mountain View, CA for their very knowledgeable and friendly assistance. Highly recommended if you're in the area.

Now, onto the next set of projects! (Installing a muffler, replacing all the shifter bearings, replacing the fuel pump, and then some minor body work.)

dr bob

Follow-up --

You were able to get the injectors barely clean enough to pass. Still needs the injectors cleaned or replaced to run correctly. New O2 sensor is a good idea every 50k miles or so, good catch. To the best of my knowledge, there's no adjustment available for mixture on our cars. Interested to know what and how they adjusted mixture; I think they may have been trying to bullwink you on that part.

jeremymilo

Bob, there's an adjustment at the MAF that controls the fuel mix. I was at the shop when they had the car on the dyno making adjustments, so no BS. I'll look into cleaning the injectors -- thanks!