Rebuilder,

I am now trying to resolve a hign NOX failure that doubled from last year and failed emissions.

Do you recall the number and location of bolts holding down the PCV oil separator?

I do not know but a NOX jump from 1338 to 2765 ppm seems huge.
Have you ever seen reading that high?

'88 244 DL

First I would like to thank all of the people who took the time to respond to my request for the experience of other users to diagnose the high NOX emission problem that I have.

Step 1: I examined the exhaust manifold, its gaskets, the header pipe, gaskets between the pipe and the catalytic converter. No cracks or sources of leaks were found.

Step 2: I examined the inlet air side of the engine downstream of the air mass meter. Results were:

1- the inlet black plastic line had at least six small holes
2- the throttle plate housing was found be to coated with gum and residual motor oil from the PCV system
3- the throttle plate switch was filled with condensed motor oil resulting from a throttle plate seal failure between the throttle plate body and the switch housing. While the switch had no failed yet, the microswitch was on the verge of failing due to hydrocarbons in the switch.
4- All the other inlet lines were tight and no leaks were evident. Check of the idle air valve showed that it was working correctly.

Repair work in progress:

A- ordered new inlet air conduit (Volvo # 136454)
B- removed throttle body from inlet manifold. Cut new throttle body/ inlet manifold gasket.
C- removed throttle plate shaft lever, spring and seal on opposite side of shaft from switch.
D- removed switch case, flange mount and seal (V-Ring SKF 400085) from throttle body.
E- Soak throttle body in carburetor solvent. Reassemble throttle body with new shaft seals.
F- ordered new switch (Bosch 0 280 120 325). Install and adjust switch to throttle body.
G- Check of idle air valve showed not problem with the valve or any of it's hoses.
H-There were no apparent hose leaks in any other inlet air hoses.

Conclusions: It seems clear that unmetered air leaks were coming through the inlet conduit. Also it is clear that the throttle plate shaft was leaking air into the throttle plate body.

Check for vacuum leaks, in particular at the intake manifold gasket and of course the inlet snorkel. Vacuum leaks lean out the mixture, driving up combustion temps which favors the formation of NOx. Ideally the O2 sensor compensates for this, but the leak may be too big for the O2 sensor system's range of compensation. One would think that a vacuum leak big enough to drive the NOx high would be accompanied by poor power and maybe a weak or rough idle, but... stranger things have happened.

Sounds like you've covered all the other bases. One other long shot would be the fuel pressure regulator; if this hasn't ever been changed, it would be a good idea to do so. Pressure regulator springs (in general) weaken and the control pressure schedule drifts with age and millions of cycles.
--
In God We Trust. Everyone else, keep your hands where I can see them.

Check your timing. My '88 240DL with 570,000 miles on the original engine had high NOX numbers a few years back (over 2,500). Correcting the timing cut NOX in half. I then replaced the oxygen sensor and Cat and drove NOX down even lower.

90-240.
In california we have smog check every other year.
In o2 my nox measured 68(Low, low, low) the max in CA. is 791.
In o4 " " 2501! A repair shop corrected to 756.
In 06 " " 749
in 08 " " 766
As you can see, Im right on the margin. I would love to know whats needed to get it back into the 68 range again. But as long as Im passing, Im going along to get along.
ONe suggestion I have not follow is to remove the egr pipe and make sure its open and not all gunked up. I dont know what affect that would have. The system appears to be working. But the egr definitely has a part to play in combustion temp.
I dont know what the repair shop did back in 02 to make it pass, but whatever it was seems to be holding up. I shouldve asked, but at the time I was just to thrilled to be rid of the problem. Now each year Ive got both fingers crossed.

It sounds like you did all the right things to fix the other causes of high nox. So check the simple things first.

The most likely problem that would get past all your fixes is an exhaust leak before the catalytic converter. Any leaks will cause the Nox to rise, and it could be a crack in the exhaust manifold, a leaky gasket, or a leak in a seal between pipes. Just replaced by exhaust manifold gaskets this year and found that the seals under three and four were about start leaking. Those are the two that can have the most movement since they are further away from the down pipe. Ended up having to, grind, file, and polish quite a bit to get a good seal surface. Also check the area where the pipes come together for a small pin hole or crack. Have seen postings about cracks there.

How is your idle? Nice and smooth move on, but if you have a ramble or a slight hitch in your idle take a look from the AMM to the intake manifold. Check all the vacuum lines for cracks or splits, make sure your hose clamps are tight, and look at the under side of the large hose for holes. Good solid connection to the breather box by the hoses. Any air that gets sucked into the system after the AMM can result in a wrong fuel air mix.

Really tossing things at the wall is double the check the gap on your plugs. Also check and make sure you have them torqued in to spec.

Bet you have a slight exhaust leak based on all you have already done.


Good Luck and post back what you find.

Regards,

Paul