Hi I am new here and this is my second post. I have a 1987 240 with an Automatic Transmission and I have always struggled to pass the state's emissions test. It often fails and the guy at the shop always says it needs a new catalytic converter. I checked my log book and found that in 8 years that I put a catalytic converter in every3-1/2 years.

I took it to a local garage for the inspection this year and it failed. The NoX was too high. The limit is 2 and the actual reading was 6, HC and CO were OK. Again the guy told me it needs a new Cat. I took the car home and replaced the Oxygen sensor. It failed again, this time scoring a 7 on NOX, and now CO was over the limit. Again the guy said that I need a new Catalytic converter.

When I took it back home I hooked up my Fuel - Air meter to the oxygen sensor. At idle it seems to be trying to adjust the mixture. I see the meter moving up and down. Once I put the car in drive and give it a little gas, it goes lean. It is lean at any speed and any load, except no load at all while coating.

I changed the fuel filter, checked to be sure the external pump is working, checked the timing. Timing was OK. The pump is operating, I don't have the fittings to check the fuel pressure. And I got the same result after changing the fuel filter.

It seems like the problem is that the engine is running too lean. I can confirm that when under loads going up hills and hard acceleration it will ping. So I belive that my meter is correct.

What else should I be looking at? Are there other things that I should change?

The car is now 2 month over do and I am getting desperate. I am at the point that I am ready to take it to the dealer.

Any help wil be appriciated.

US cars 85-88 have a Chrysler designed ignition system that can cause high NoX because it advances timing to the point of knock, and then backs it off slowly. To get around this plug both ends of the hard white vacuum line that runs from the intake to the ignition ECU on the passenger fender. Afterward you will have a traditional timing advance curve.

Look for and fix vacuum leaks, especially between the air mass meter and the throttle body (large air hose). You may also have an intake manifold gasket leak, causing a lean burn.

Thanks for the quick reply Robert.

I should have mentioned that I took the intake tube out yesterday looking for leaks and it looked good. I cleaned out the flame trap while I was at it, but it was OK anyway. Good idea about the intake gasket or any other intake leaks. I will look at it again.

I have blocked off the vacuum retard in the past and this has enabled me to sqeek by. This time, I forgot to mention that I did this and it still failed. I had learned this trick a number years ago from another online source.

I also listened to the fuel injectors and they all seem to be clicking.

Since I want to state all of the things that I have tried I will also include that I used a wrench to tap on the intake manifold while using a timing light and didn't see any movement in the timing as a result.

Will the ignition timing affect the fuel/air ratio? I guess it doesn't change the ratio but how effeciently it is burned.

Here is another quick fix band-aid: plug both ends of the vacuum line connected to the fuel pressure regulator, this will simulate WOT causing max fuel pressure, which may help your mixture to become more rich.

What has been done, coolant change wise? You might (probably) have super high combustion temps ...

Check accuracy of inlet air temp sensor against std. If it thinks the air is hotter than it really is, it will lean off the mix above idle-be very particular-suggest new only AMM to fix this fault, unless u can get someone to repair it precisely, or use an external cheat resistor network to fool it - precision is necessary.

AMM works by comparing ambient temperatures with the power needed to maintain a sensor at a higher temperature against the wind-chill effect. the greater the difference the more air is going into the motor, and the more fuel needed. (Simplified synopsis.) The sensor(s?) have a self cleaning ability on switch off-might pay to check it works? A dirty sensor is enough to throw it off track and lean it out, but I suspect they're fragile/sensitive to touch-so read up how to test/clean them. The device is expensive new, AND there IS a (factory preset) adjustment for CO under a plug that has to be carefully drilled out, but that is a last resort before replacement, the factory indicates. If done, another plug should be fitted. I guess that they have confidence they've done their part, and if the CO is out, then it's more likely that something other than that adjustment is wrong.

NB Rich mixtures/over-hot exhaust gases destroy CAT's. They should last for years, until the casing corrodes away. Harsh driving isn't good for them. Prolonged loaded driving with the throttle on too far is also bad practice, as it wastes fuel and that's what cooks the CAT. The still-burning excess fuel literally melts the guts in it-leading to blockages &/or non-function. Perhaps that's why the CAT is nearly 1/2 way down the car-to let the excess fuel burn off before it enters the CAT? Brainz, trains.....

Good question. I am using a polypropylene anti-freeze mixed at about 50/50. I haven't checked the specific garvity in a while, it is full though and it is about 2 years old. The engine seems to run fine temperature wise. On long trips the temp needle stays below halfway and when I open the hood after it has been running it doesn't seem to be too hot.

Are these good indicators of combustion chamber tempertures though? Does the radiator have enough capacity to dump this extra hot coolant? Is there some other way to check the combustion tempertures?

Thanks for this tip, Manx. What you are describing sounds like what I have. Just off idle it leans out. I will try to check the mass air meter before ordering. Although expensive, if it works, it is worth it. Groton has it for $128 plus a mandatory air box thermostat ($9).

Is this described in the Bently or Haynes manual? I will check tonight.

I did some checks today. I plugged the vacuum line for the fuel pressure and the car still runs lean.

I checked the resistance of from pins 2 to 3 on the air mass meter and I got 3.1 ohm. The reading should have been between 3.5 and 4.O ohms. Then I checked between pins 2 adn 6 and got 1.007k ohms. This reading should have been betwen 0 and 1000 ohms. I then tried to adjust the potentiometer and found that the screw doesn't change a thing no matter how much I turn.

I will buy a new air mass meter and install it. I will keep you posted.

ANY low ohms reading is difficult, error is inherent. DMM's are NOT particularly accurate at these readings, and the last digit is always a possible + or - 1 error. It may still be ok. best way to test low ohms readings is with a wein bridge, using a restricted current and a wire for a resistor with a moving contact and sensitive analog meter, then compare lengths either side directly with the multiplier-like the science experiment in tech school-its a very good method.

You might also investigate the o-rings on the injectors leaking, allowing air in. I would spray them with WD40 (and the gaskets, etc) and see if your RPM's spike upward.

I recommend changing ur coolant much more often too. 6 mths is in my books. 50% Ethanol Glycol, because of its higher latent heat than water, and anti-corrosion characteristics. Techalloy Gold seems a good brand here, tho we don't freeze. BTW you have a std air filter don't you, and it's working ok?

Thanks for more suggestions, I still need them. I bought a new AMM and installed it this morning. No improvement. It is still running lean. I couldn't find a vacuum leak but I did not check the fuel injectors. I will check them and recheck for vacuum leaks anywhere else.

I haven't tried the WD-40 method. This sounds better than using propane.

The air filter and fuel filter are new. I replaced them last weekend.

I will keep you posted on progress.

It takes ~2400 deg. F of combustion temps to produce NOx. Do everything you can to bring this temp down. You said you checked timing? If neccesary, retard it manually (bring it closer to TDC), to 6-10 degrees. Flush and change your coolant. Spark plugs do ALOT to reduce heat - replace them with NGK copper:
http://www.fcpgroton.com/product-exec/prod...category_id/225

Continue to look for leaks. Measure your O2 sensor voltage while plugged in and running - you should have +12v for the heater on one of the small black/white wires. The big green wire should jump very quickly between .2 and .8 VDC.

Beyond all this there could be problems with the ECU coolant and knock sensor, and the idle position switch - all of this is used to advance/retard timing.

Is the emissions testing idle only or idle + load?

Fuel pressure is 36psi @ common rail-bad/old regulator might be troublesome-screw thread SEEMS slightly BIGGER than 1/4" BSP(?). 'Metric' BSP????? @ 21 yo, it's prob due. mine was. Fuel filter in tank too.

Thanks for more ideas. The test is under load. They have a chassis dyno. The car goes on the dyno and the mechanic maintains a the speed with a load applied to the drive wheels.

I am using a fuel air guage to read the O2 sensor. I have it set up so that I can drive around and I have it sitting on a box on my dashboard. I tried to diagnose the knock sensor. According to the Haynes manual I hook up a timing light and then knock on the intake manual with a wrench. I should see the ignition timing change. I didn't see any change though. What do you think?

I will have to look into what fittings I may need to connect the fuel pressure guage. It is too bad there is no schrader valve. I will also try new spark plugs. Look at the idle position switch and change the coolant. Probably in that order.

I will keep you posted of any news. Thanks for the help. You guys are helping with my sanity. This is frustrating.