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Failed CA Smog, High NOX '89 240

philski o'flood

Well-known member
Nov 7, 2005
San Diego
'89 240 with EGR. This car passed fine 2 years ago. I have a Walker cat that is just used for testing, it has maybe 4 hours on it. I found a few small vacuum leaks and fixed them but they were leaking 2 years ago too. Any repair or check suggestions would be appreciated. Thanks


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Keep the cat on and get it hot a few times before testing again. A new, properly functioning cat is almost always the trick for passing. If it was sitting on the shelf, perhaps corrosion is keeping it from doing it's job. Either way, new/good cat is always a good start.
Confirm that the EGR is working good.
Always helps to go for the test during rain- this way they have to dry tires on the dyno which warms up the Cat real good at the same time.
When was the O2 sensor replaced last? With a hose or stethoscope, do you hear any exhaust leaks upstream of the sensor that might throw off the O2 measurement?

With a 3-way cat, emissions reduction requires the O2 mixture to swing up-and-down around 14.7. HC and CO are reduced when there is excess oxygen (lean), and NOx is reduced when there is a lack of oxygen (rich). LH2.4 sweeps the O2 mixture up and down every couple seconds.
Here is the same cat results when on the '90 240 a few weeks ago. I have no idea when the O2 sensor was last replaced. 2 years ago the whole system tested fine. I'm going to test the current O2 now


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Have you recently swapped in different injectors or MAF since you last "pass"?

The CO being almost nothing and the NOX so high makes me think LEAN.

Smoke test the induction system for leaks. Maybe put known clean injectors, a different MAF, and consider a fresh O2 sensor; they can oscillate, but still not be working properly. EGR helps lower NOX but I don't know how much it factors in to a high load test like on the 15 and 25 mph test. BTW, 15mph is twice the load of the 25mph test.

If all else fails, try a fresh cat.
If you apply vacuum to it at idle the engine should stall or be very close to it.
Like ZVOLV said I also thought it was lean but comparing the numbers from your previous test the only thing I see new is the NOx being high. The O2 and CO2 are the same, besides you said you fixed the leaks you found.
Using the calculator I posted above your Lambda ratio is near 1 (ideal) and Cat efficiency is 80%.
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I haven't anything to the '89 since the last test except drive it around, top the oil off.

the second set of test results are in a different 240, my '90 w/ EGR. They were a week before the '89 failed

Maybe I just didn't drive it around enough before the test? I did maybe 10-15 minutes on the freeway over a half hour and stopped to get gas and then to the smog place
Interpreting 5 gas analysis results isn't as straightforward as one may think. After the cat, the three pollutants are all reduced, so low CO doesn't really mean it's running lean.

I just know that over the decades years of owning LH.2.4 cars, a fresh kitty has brought the numbers down to zero unless there is something way off.

With a test being about $75, after a couple/few fails, and the time off work/day off it's gets close to being the price of a $300 cat.
OK, I had to read the thread over to understand.

Nox can be too lean, too hot, overadvanced ignition timing. EGR operates at light throttle cruise, so I don't know how much it factors in for the dyno test. It can't hurt to apply vacuum to the EGR valve at idle and check if it stumbles/stalls. I would be really interested to see EGR enabled VS disabled smog numbers.

I would try different injectors and MAF. I had a 1986 740 that had numbers all over the place over different tests and it was the injectors that fixed it.

Did you put the cat in the correct direction? You know that shops record that cat number...I wouldn't be swapping it between my VINs..
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I feel your pain, not easy getting these old dinosaurs to comply to a test they weren't designed to pass when they were originally built. Been down this road many a time! I have a couple of suggestions, these are what I always checked out before heading down to the smog station. Question : is this the same cat that was on the car when you smogged it 2 years ago? check for any exhaust leaks (head to cat), coolant t/stat ok, check EGR valve operation (disconnect hose, apply vacuum to hose going to valve at idle, listen for idle change, also look down at EGR valve, and see if valve stem is actually moving, if it's moving, but idle doesn't fluctuate, it's plugged up/ not flowing), check for vacuum leaks in intake (spray brake parts wash at different areas, and listen for hissing/ idle fluctuations), areas of concern are IAC hoses (splits usually right where hoses go over the hump of IAC valve), splits in intake accordian hose between MAF and TV ( take it off, stretch it out and look for splits either in the ridges, or valleys, check pre heat t/stat in lower area of front half of air cleaner housing (these are almost always bad, they need to be fully closed, with tension on the flapper, when they're open you're running exhaust heated air into the MAF, could cause some leaning out, and WILL KILL your MAF), check fuel pressure regulator (pull hose off at idle, check for idle change, and there shouldn't be any fuel leakage, check EGR amplifier/ hoses for leaks (split hoses/ loose vacuum elbows), check oxygen sensor for good swing in operation. Assuming that plugs, air filter, clean engine oil etc OK. If everything checks out OK, I think you're probably looking at either MAF or CAT. I'm sure you've probably done most of this already, if not all of it. Hope this might be helpful, and good luck.
PS : definitely take it in after a hot road test, and don't don't shut it off while you're waiting in line