(Posting here since this section gets more traffic than the truck section)

Took my Dakota in for its annual smog test, and it failed the Low Speed NOx.

Am I wrong to think that if it was the catalytic convertor more readings would be out of spec?

I'm leaning towards the EGR, but that's just a guess. When I had the intake off (last year), I cleaned the EGR and the port out as much as possible, but, did not test or replace the EGR.

Plugs, cap, coil were all replaced spring of 2015.

Truck other wise runs fine.

Here's the link to the test results: Results



Oh, this is my '93 Dakota with 3.9 and 168K miles...

Suggestions?

Thanks.

Sorry to hear, when we used to have those tests here we would have folks come in with there mini vans etc to work and fill up there cars with our Sunoco 110 octane LEADED racing gas and then pass the test! I am talking about 10 years ago since we had the test here.

I know I took my 340 challenger through one time with a huge racing cam on straight 110 leaded race gas and it passed the first time.

Was your car really hot and blown out on the highway right before the test? They used to say that would help in some cases.

I spent the morning running errands, ran it around town, 20 miles down the freeway and back, and then when I got to the station, they took the truck right in, so there was no cool off period.

And, it was about 80 degrees out when I took it in.....

Got to clean it up at low speed it looks like? Got a fresh air filter in there? Good gas does make a engine run better and cleaner...

Have you noticed any pinging? My dad has a 1994 Dakota with a 3.9 and 180,000 miles. It pinged really bad under part-throttle acceleration and pulling hills. The water pump went bad, so I decided to replace the timing chain at the same time due to the mileage. The chain was stretched, not excessively, but enough to just rub the tip of the drip tab. Just finished last weekend and it runs so much smoother and quieter. The pinging was nearly eliminated. The engine has quite a bit of blowby, so I plan to install some sort of PCV catch can to eliminate the pinging completely. BTW - I did pull the intake a few years ago and installed the Hughes Engines' Intake Plenum Repair Kit to seal up the bottom of the intake. If you do replace the timing chain, be sure to install a tensioner to eliminate the knock and clatter that is characteristic of the timing chain in the 3.9 engine.

check the egr

My wife's '93 Ramcharger had problems just like yours nearly every year. Every time it was the cheap muffler shop cat was shot. The last one we had installed was at a real shop with the correct Walker cat con. It was about twice the size of the cheapy univeral muffler shop junk. Once, several years ago, we thought it was the EGR, replaced it, we were wrong, only last year did it really need replaced.

Robert

If there was no cat, the answer would be to lean out the idle and idle. A little readjustment of the idle mixture screws and maybe the throttle opening (idle speed). But the cat is also a possibility and to a lesser degree the EGR.

IIRC, the early catalytic converters were mainly to reduce the NOx. The other emmissions were controlled by running higher idle temperature (leaner, combined with an idle stop solenoid so it won't diesel on shut down). Your '93 probably came with a 3-way cat which was designed to help reduce all the bad stuff, but again IIRC NOx reduction is the main job of the cat.


So, if you fiddle with the idle-off idle, bring them to the specs on the hood sticker (if that's option on a '93) that could squeak it by.

The EGR valve operation should be testable with a vacuum gage and pump. If the EGR exhaust gas passages are partly plugged up - that's harder to do anything about. You can access the valve orifice but thats about it without a teardown. EGR systems usually have temperature and vacuum switches Off the top of my head there is no EGR until coolant reaches a certain temperatures, no EGR at WOT, etc.


If you want to read more about diagnosing exhaust for relatively modern cars there was a Toyota (ASE?) pdf online in various locations. This may be it here:
http://www.readbag.com/autoshop101-forms-h56

The catalytic convertor that's on there is of unknown origin and age.

When I bought the truck, the exhaust consisted of the original cat, a newer cat back where the muffler should have been, then a stupid little 'race' muffler not 6 inches off the back of the second cat.

When I had a new muffler installed, they took the second cat and replaced the stock one with it (the truck only has one cat now in the stock location).

There was a VERY noticeable difference in how the truck ran after doing this.

As for pinging, it hasn't pinged at all since I did the plenum and intake gasket (and when it did, it was only slight).

I'm going to get after it in a few minutes......

Well, got the EGR off. Still just as clear as when I cleaned it.

Not sure how the electrical connection plays into it, but, lacking a vacuum pump, I gave it a suck with my mouth.

No resistance.

Being no resistance, I carefully blew into it, and felt air coming out from the diaphragm.

That's not good, is it?

Had a '90 Dakota w/ 3.9 w/120,000 miles that failed inspection. Took EGR off, cleaned it, truck passed. The following year, truck failed b/c of EGR. Took it off, cleaned it; truck still failed. Replaced EGR and truck passed. Sometimes the cleaning helps, but only to a certain extent.

Replaced the EGR this morning. It made a noticeable difference with how the truck runs and sounds.

Drove all over this morning running errands, took it even further on the freeway, then took it back for another test.

Too soon?

It failed NOx by a much larger number this time.

Grrrr......

A scan tool would be helpful to look at engine data. Ensure the engine is in closed loop fuel control,O2 sensor is functioning properly,fuel trim data is in range,coolant temp is correct,etc. NOx is created by combustion chamber heat.....anything that could cause combustion chamber temps to go up will increase NOx production. EGR is designed to reduce those temps...just because it's new doesn't mean it's working correctly. Lastly the reduction portion of the 3-way cat is designed to reduce NOx at the tailpipe.

its probably the cat.
i went thru the same exact thing with my old truck...

The Lambda shows in the lean, but acceptable range. The aftermarket Cats are marginal on a good day and garbage otherwise. Before doing that I'd throw an O2 sensor in there, disconnect the battery for awhile to clear the adaptives and give it a shot. In '95 O2 diagnostics were lousy, the sensor can be pretty far off and not throw a code. It should only run $50 or so to replace- I'd recommend the 1) O.E. or 2) NTK sensor, I've had mixed results with the Bosch sensors. O2 switching is very important to keep the A/F in the range(s) that allow the Oxidizing AND Reducing beds to work properly. HTH, Steve

I'm not 100% at this point, but, I think the original O2 sensor is still in there. I ran out of daylight before I could find the plug end.

I'm disappointed about the results of the EGR even more so because of how much better the truck is running. Even the inspection guy made a comment of how it sounded different when I pulled up to his shop.

Are Bosch sensors that bad? It's all I have readily available without ordering.

I can't do anymore until Saturday.

Thanks for the replies!

Oh, and about that scan tool.....I checked with a couple of shops, none had the ability to scan an OBDI system.

Maybe if it were a Chevy or Ford.......

If it's just a matter of a day or 2 I'd order one. I got rid of all my personal OBD1 stuff years ago, no $$ in it, even where I work we have just about gotten rid of all those- I work for a school district,and the 20+ year old stuff is HD or diesel, so no diagnostics. Finding guys that are proficient in those old systems is getting tough, we are all retiring or getting out of it, I've got another decade min, if my body can hold up.

Is this the same Dakota that suddenly started sucking down oil, then fixed itself?

You have lots of room on your HC. If you put in some 94 Premium or whatever you have available, it will lower NoX but it will raise HC.

Make sure your cooling system is doing its job ie running the lower side of normal not the higher side. NoX is caused by high combustion temps so everything you can do to control temp helps. That's why Premium fuel will reduce NoX because it burns slower/cooler. It will raise HC for the same reason.

I used to have a 3.0 V6 Spirit that failed the same way. Lots of room on HC but failed NoX similar to yours.

Went to town and filled it with premium and it passed NoX by about the same margin it failed by and BARELY passed HC.

Kevin