Smog Question.

[SeanPlunk]

What would cause a 4cyl Jeep to pass HC, and CO with flying colors, but fail NO at 15mph but not at 25mph. Any ideas? It has a brand new cat (smog legal) that is clearly working for the most part. Help :bang:

[jedhead]

I would check the EGR system. Faulty EGR can cause the NOX to be high. Also if you have too much timing advance can cause elevated NOX.

Bob

[SeanPlunk]

I would check the EGR system. Faulty EGR can cause the NOX to be high. Also if you have too much timing advance can cause elevated NOX.

Bob

Stock tune. I'm thinking EGR.

[Vettezuki]

What would cause a 4cyl Jeep to pass HC, and CO with flying colors, but fail NO at 15mph but not at 25mph. Any ideas? It has a brand new cat (smog legal) that is clearly working for the most part. Help :bang:

Assuming the engine is otherwise workign to spec, the system is correctly metering emissions, that is clearly a failure of the reduction catalyst stage. I don't think there's any real break in period for a cat and I can't imagine it was a failure in manufacturing. Was the cat hot? It's always best to get a system good and warm before testing for emissions.

NO is (to my knowledge) only formed from high combustion tempratures. This can happen for a variety of reasons. The top two that come to mind are a failing EGR (which lowers charge temps) or if there's a ton of carbon buildup in the cylinders that can cause some high residiual heat.

[SeanPlunk]

The smog tech thought it might be EGR too. Besides the EGR valve, what else is the system should be checked/replaced?

[jedhead]

You may want to pull the plugs and see if the engine is running too lean. Too lean condition will also elevate NOX because the combustion temps will be higher.

Bob

[Vettezuki]

You may want to pull the plugs and see if the engine is running too lean. Too lean condition will also elevate NOX because the combustion temps will be higher.

Bob

:iagree: Lean = higher temps = NO

[kdracer73]

What year Jeep?

[CorruptCulture]

Hey everyone,

My Jeep is the one in question here. I appreciate all the feedback. It's a 1998 4cyl 2.5L with manual transmission (AX-5).

The two popular opinions in here are that it is either running lean or the EGR is failing, both of which were mentioned as possibilities by the smog tech. He indicated that CO2 and O2 levels gave further encouragement that it was running lean.

I'll post up a scan of the results in a little bit.

[CorruptCulture]

http://imgur.com/Wh3lR.gif

Here are the results. If someone could provide some guidance as to how correct the air/fuel mixture (if that is indeed the problem), that'd be great.

I'll look on the Jeep forums and try to find a guide to replace the EGR.

Also, another relevant factor, I'm running 31's. Would running smaller tires help pass smog as the engine wouldn't be working as hard?

Thanks again.

[enkeivette]

You guys pretty much have the causes: high cylinder temp, bad 3 way cat (obd2), stuck closed egr, or a lean running condition.

So buy a new EGR, don't screw around with cleaning out your old one. Make sure to clean off the gasket mating surface really well, scotch brite and a razor blade and carb cleaner. If it leaks at the intake, the vacuum leak will make your motor run leaner. Make sure the vacuum lines going to it are sealed.

Yes, put on smaller tires. If you're failing at a lower mph, and you run cleaner at 25mph, with smaller tires your engine will be spinning faster at 15mph with smaller tires, hopefully running cleaner. Also with less load. I think Eric has his stock ranger tires he would prob let you borrow. I think his bolt pattern is 5x4.5

Run the car with the heater on full blast on the way there, that pulls heat off the cylinder heads, drive the car there fast, WOT a few times to get the cat hot (hot cats work better). Then keep the heater on during the test! Full blast.

I don't know much about FI, but if you have a fuel pressure regulator you should be able to back it out maybe 1/2 a turn to increase fuel pressure, making it run more rich. That's how we tuned Mike's turbo Avenger. Do you have a fuel pressure regulator somewhere obvious?

I would also pull out the fuel injectors and clean the tips off with some carb cleaner.


Might as well try some of that guaranteed to pass stuff while you're at it, I've heard first hand that it works, even with faux cats.

[kdracer73]

What model ? I have been searching wire diagrams/diagnostic info for a Wrangler 2.5 and it does not show EGR

[CorruptCulture]

What model ? I have been searching wire diagrams/diagnostic info for a Wrangler 2.5 and it does not show EGR

That's the thing, I don't think mine has an EGR. There is no mention of it in the Factory Service Manual.

I also found this post on a Jeep Forum:

"The EGR valve was last used on the 4.2L and 2.5L in 1990."

From this thread:

http://www.jeepforum.com/forum/f12/no-egr-valve-84615/

[CorruptCulture]

Yes, put on smaller tires. If you're failing at a lower mph, and you run cleaner at 25mph, with smaller tires your engine will be spinning faster at 15mph with smaller tires, hopefully running cleaner. Also with less load. I think Eric has his stock ranger tires he would prob let you borrow. I think his bolt pattern is 5x4.5

I'm running 31x10.5's and I have my old 30x9.5's at home. I'll give those a shot.

Then keep the heater on during the test! Full blast.

Do smog guys let you do this?

I don't know much about FI, but if you have a fuel pressure regulator you should be able to back it out maybe 1/2 a turn to increase fuel pressure, making it run more rich. That's how we tuned Mike's turbo Avenger. Do you have a fuel pressure regulator somewhere obvious?

Nope, the fuel pressure regulator isn't mounted on the fuel rail. It's part of the fuel pump unit which is in the gas tank; I'd have to drop the gas tank skid plate and tank to get to it. Ain't that a bitch?

Might as well try some of that guaranteed to pass stuff while you're at it, I've heard first hand that it works, even with faux cats.

I'll give that a shot. It can't hurt.

[kdracer73]

Heater on full blast?
Do smog guys let you do this?
.

All accessories should be off, according to testing guidelines.

[CorruptCulture]

I also just thought of something else. Vacuum leaks cause it to run lean, right? Well, I've had problems with my heater/vent switching positions from floor to defrost to face etc. This is a vacuum problem on the Jeep.

Maybe if I track that down it'll solve the smog problem? Supposedly vacuum problems are notoriously annoying to find. Anyone wanna help?

[Vettezuki]

You could have poorly performing injectors and/or O2 sensors. Injectors can be cleaned by various methods, or (in most cases) rebuilt, or replaced obviously. O2 sensors just have to be replaced.

You can try a Sea Foam treatment of the engine (cleans out carbon buildup) and a can in the tank to flush through the fuel system (cleaning injectors). When I've done this on higher mileage older cars, including my 91 Wrangler, the amount of crap in the form of nasty smelling white smoke that came out was biblical. More like a Battle Ship throwing a smoke screen than a car. (Neighbors loved that.) My Vette is clean as a whistle, doesn't put out anything. If the higher temps are from lean burning from clogged injectors and/or huge carbon buildup in the combustion chambers, this *could* be a $20 solution.

[Vettezuki]

I also just thought of something else. Vacuum leaks cause it to run lean, right? Well, I've had problems with my heater/vent switching positions from floor to defrost to face etc. This is a vacuum problem on the Jeep.

Maybe if I track that down it'll solve the smog problem? Supposedly vacuum problems are notoriously annoying to find. Anyone wanna help?

Paul can correct me if I'm wrong, but this would really be mostly for any leaks more around the intake manifold or EGR, where a lot of un-metered air is coming in. :huh:

[kdracer73]

A vacuum leak will have more of a effect at idle. The proportion of air going through at 1700 rpm is not going to be as big a deal. It would have to be sooo big it would not idle well. Exhaust leaks before cat could effect the ability of the cat to reduce nox.

O2 sensor performance could effect it also. If the sensor has a false rich condition, the ecm will lean out the fuel trim. A lazy-slow o2 sensor can do it also.

[CorruptCulture]

You could have poorly performing injectors and/or O2 sensors. Injectors can be cleaned by various methods, or (in most cases) rebuilt, or replaced obviously. O2 sensors just have to be replaced.

You can try a Sea Foam treatment of the engine (cleans out carbon buildup) and a can in the tank to flush through the fuel system (cleaning injectors). When I've done this on higher mileage older cars, including my 91 Wrangler, the amount of crap in the form of nasty smelling white smoke that came out was biblical. More like a Battle Ship throwing a smoke screen than a car. (Neighbors loved that.) My Vette is clean as a whistle, doesn't put out anything. If the higher temps are from lean burning from clogged injectors and/or huge carbon buildup in the combustion chambers, this *could* be a $20 solution.

I've Sea Foam'd in the last 3 months and I changed my upstream O2 sensor about 3-4 months ago as well.