2.6L emission dilemma

[KManiac]

I have a 1980 Dodge D-50 Sport (Mitsubishi) pick-up with a normally aspirated, 2.6L inline 4, with jet-valve, swirl-port head and 5-speed manual. This is a California-built engine with a dual control EGR valve.

My dilemma: When I took it in for California's famous bi-annual smog test, it failed because of high NOx. All the other numbers were acceptable.

I tested the EGR valve. It is functioning correctly. Both the primary and secondary valves operate when vacuum is applied and recirculate exhaust. The vacuum signals are correct, so the thermo valve is working. And vacuum exists in the hoses when and where it needs to be. The Sub EGR valve on the carburetor is functioning correctly, as well.

What could I be missing here? Is it possible my exhaust ports are partially plugged, limiting the amount of exhaust recirculation? Could the jet-valves need adjustment or replacement or do they even have an effect on NOx?

Any emissions "experts" out there are welcome to respond.

[denisond3]

If your Mistubishi engine has that Mikuni carb on it - I wish you all kinds of good luck. We have two minivans with the same 2.6L motors. Under the carb and cast into the intake manifold, are small passages for the EGR gases. When we got each minivan, both of their intake manifolds had those EGR passages plugged solid with crud. (No air at all could get through!) As part of rebuilding the engines, I spent quite a while cleaning them out, with different shapped pieces of stiff wire, as the passages curve around. I also soaked them in gunk degreaser overnight, then would remove what I could with the fine stream from the garden hose. So with that, and blasts of air from my air compressor and my assortment of twisted bits of stiff wire - I got them cleaned. Both vehicles passed the emissions test here in Northern Virgina, and both of them now have over 200,000 miles on them.
Sometimes an engine will have lower NOx readings if you run the ignition 2 or 3 degrees later than the factory settings. Its one of the few advantages of having distributors instead of computer controlled spark.
P.S. On both minivans we have had to replace the module that is inside the distributor - and if I were driving into remote country, I would prefer to have an extra module with me. They arent cheap though!

[KManiac]

Thank you for the update. The plugged passage scenario was one that crossed my mind. But I think that they are clear as the engine boggs down when the EGR valve is operated.

One important thing I did this weekend, though, was to adjust the valves. As I recall, it has been over 15 years since I attempted the last valve adjustment. This adjustment has to be done with the engine hot, so now you know the reason for my delay. Anyway, I did find, on initial inspection, that the intake valves were loose, what you would expect for over 70,000 miles without an adjustment. The exhaust valves, however, were too tight. I guess I let the engine cool off too much before I set the exhaust valves last time. This engine is also equipped with the air injection "jet valves", which require adjustment as well, for emission control purposes. Now that I adjusted the valves, the engine runs different, a bit smoother and just a little better. It could be that I wasn't getting enough injected air through the jet valves. I will take it back to the smog center for a retest and see how it goes and keep you posted on the results.

[KManiac]

Well, I got my truck retested after the valve adjustment. It failed again for high NOx. The numbers were similar as the first test, 2371 ppm at 15 mph and 1565 ppm for 25 mph. Maximum allowable is 1523 at 15 mph and 1383 at 25 mph. Since I have a two-stage EGR valve, with two diaphrams and one small port for low speed and one large port for high speed, I got to thinking that maybe the low speed port was partially plugged or something like that.

Anyway, I removed the EGR valve to inspect it and clean it out, if needed. I found a little bit of carbon build up inside the valve, but nothing serious enough to plug it up. I cleaned out what I could. It put vacuum on both diaphrams and both appeared to move. The primary diaphram was inside a cover held on by three screws, so I decided to remove this cover to make sure everything was good inside. As I started loosening the screws, engine oil started to leak out from the diaphram housing. Enough oil came out that I figure at least half of the vacuum chamber was full of oil. I guess this oil somehow passed through the vacuum hoses and into the chamber. With a large volume of oil in the chamber, I suspected that the valve would hydrolock half open, restricting the flow of exhaust gases. So I cleaned out all the oil and put it back together.

The next day, I took it to a different smog check place and it passed.

So, if you have a similar engine that keeps failing smog tests on high NOx, this might be your issue.

[KManiac]

I decided to give the engine a valve adjustment, since I last did this 15 years ago. I found that the intake and jet valves were loose and the exhaust valves were tight. I guess I didn't do it quite right the first time.

Well, I got my truck retested after the valve adjustment. It failed again for high NOx. The numbers were similar as the first test, 2371 ppm at 15 mph and 1565 ppm for 25 mph. Maximum allowable is 1523 at 15 mph and 1383 at 25 mph. Since I have a two-stage EGR valve, with two diaphrams and one small port for low speed and one large port for high speed, I got to thinking that maybe the low speed port was partially plugged or something like that.

Anyway, I removed the EGR valve to inspect it and clean it out, if needed. I found a little bit of carbon build up inside the valve, but nothing serious enough to plug it up. I cleaned out what I could. It put vacuum on both diaphrams and both appeared to move. The primary diaphram was inside a cover held on by three screws, so I decided to remove this cover to make sure everything was good inside. As I started loosening the screws, engine oil started to leak out from the diaphram housing. Enough oil came out that I figure at least half of the vacuum chamber was full of oil. I guess this oil somehow passed through the vacuum hoses and into the chamber. With a large volume of oil in the chamber, I suspected that the valve would hydrolock half open, restricting the flow of exhaust gases. So I cleaned out all the oil and put it back together.

The next day, I took it to a different smog check place and it passed.

So, if you have a similar engine that keeps failing smog tests on high NOx, this might be your issue.

['97ventureowner]

KManiac - Welcome to AF. There is no need to post the same thread in 3 different forums. It is against the Community Guidelines and is considered spam and is a bannable offense. Refer to the guidelines ( http://www.automotiveforums.com/vbul...uidelines.html ) for proper posting techniques. Here is the excerpt from the guidelines that pertains to this:
"BE COURTEOUS: AVOID DUPLICATE POSTS
Do not post the same discussion more than once on a discussion forum or on many forums. Duplicate discussions can be frustrating for other members, especially for those whose time and energy is limited. Weeding through the same discussion in multiple locations can be not only frustrating, but down right annoying. Duplicate discussions will be deleted. Reposting the same message repeatedly can be interpreted as SPAM and could result in the loss of your membership. Also be conscious of your fellow members and avoid making meaningless posts for the sole purpose of increasing your post count."

[KManiac]

My apologies to all. I was not trying to increase my posts or create meaningless chatter. Given that the engine in my truck was used in numerous applications, I just thought I would get a quicker response and more knowledgible eyes seeing my problem sooner using my approach. I didn't realize it was against the rules. And I know that ignorance of the law is no excuse. It won't happen again.