High CO but positive LTFT (’98 Camry)

[TS]

Greetings. I’ve asked about this on a make-specific forum but figured people here might have some additional insights. (Sorry about the length.)

I’ve got a 1998 Toyota Camry (4-cylinder, 171,000 miles, California emissions) that won’t pass smog. There are no trouble codes, and power and fuel economy seem normal, but I got these results:

15 mph:
1812 rpm
CO2 14.6%
O2 0%
HC 37 ppm (max 55 ppm, avg 9 ppm)
CO 0.61% (fail; max 0.50%, avg 0.03%)
NO 9 ppm (max 437 ppm, avg 67 ppm)
(lambda = 0.9812, rich, according to a 5-gas smog calculator I found)

25 mph:
1773 rpm
CO2 14.5%
O2 0%
[b]HC 52 ppm (fail; max 39 ppm, avg 6 ppm)
CO 0.72% (fail; max 0.48%, avg 0.03%)[/b]
NO 13 ppm (max 724 ppm, avg 62 ppm)
(lambda = 0.9775, richer still)

Based on the carbon monoxide, it seems like a rich condition. For this test, the spark plugs, wires, PCV valve, and radiator were new, the air filter and thermostat were 6 months old, and the oil had maybe 100 miles on it. The MAP sensor and upstream oxygen sensor had been replaced by a mechanic who got the car to pass 2 years ago. (The last thing he did before it finally passed was some kind of injector cleaning. Immediately afterward, I had a P0171 lean code reappear and hang around for 8 months, but it disappeared randomly on its own.)

After this test I went to see the same mechanic, who offered to hook up his scan tool and give me some free advice. Diagnosis: another dead upstream O2 sensor. I was advised to change it and take the car back for a “tune-up mode” test (no printout for that, unfortunately). Failed again, though I hadn’t driven it half an hour at speed this time around.

(This car does have an A/F or wideband sensor, but on most scan tools it seems these early Toyota setups will show the true voltage divided by 5–stoichiometric is 0.66v instead of 3.3v. I think that might have caused the mechanic to deem the previous sensor lazy. I’ve also read some forum posts saying that the Bosch sensors he used and recommended don’t mesh as well with the Toyota PCM as a Denso would, but it did pass just fine with a Bosch before.)

Said mechanic has been on vacation, so instead I’ve been trying to learn how to use a scan tool. (The one I borrowed is almost exactly like the one in Eric’s videos.) Long-term fuel trim at idle seems to be around +5%, while under load it trends as high as +11%. This may technically be within spec on this car, but it doesn’t seem right given the exhaust failures.

I haven’t found any vacuum or exhaust leaks. I did try (before those test results) cleaning the injectors with Techron in the tank on a few long road trips, but I haven’t tried a more professional cleaning again. Looking at the secondary O2 sensor as shown in the P0420 video seems to suggest the catalysts are okay (steady around 0.5v at 2000 rpm), though (perhaps because of the O2 vs A/F sensor thing) the primary sensor wasn’t switching rapidly over a wide voltage range as shown in the video.

What else might I be forgetting to look at, and what else might I be able to try on my own?
Thanks for reading.

Topic

[Bill]

I strongly think you have a vacuum leak so would triple check for that. Have a look at this maybe it will help.

https://www.ericthecarguy.com/videos/7-videos/27-fuel-and-emissions?start=15&videoid=9CPqbaSgcok

[TS]

That’s the method I used, in fact… perhaps I’ll try a propane torch, especially around the fuel rail and intake gasket. (For what it’s worth, there is a fair amount of black gunk on that part of the intake manifold and fuel rail, too, including on the plastic electrical connectors to the injectors.) But I have also read that a vacuum leak will affect things more at idle than under load–and it’s under load where I’m having more trouble. Can you explain how the vacuum leak would work in this situation?

[Bill]

At 25 mph there isn’t much load on the engine but anything that opens using ported vacuum like the fuel pressure regulator,evaporative controls,EGR components etc.are opening and a vacuum leak in those ares could be a problem.

[exceptionalchris]

I am struggling with CA emissions myself. Is it possible for you to scan a copy of the test results? They have a low income program, the state will help with a max on $500 on repairs. Hear is a copy to the website that has helped me a bit.http://www.smogtips.com/failed-high-carbon-monoxide-CO.cfm

[TS]

Test results are there already. Probably can’t do CAP even if I wanted (limit goes up to $650 on Monday, too, I think). Tried that website initially, but I believe all five things (air filter, O2 sensor, MAP sensor, TPS, and ECTS) are fine on this car.

I’ll check those other possible vacuum leaks… sometime when it’s not 108 degrees outside, anyway. Considering the potential fuel trim issue, a vacuum leak interfering with the fuel pressure regulator would be an interesting idea. However, I don’t see that mentioned on the vacuum hose diagram.

[college man]

You could have carbon build up on the back
side of the intake valves acting like a sponge.
Try inducing sea foam in the intake and running
it in the tank. see if the symptoms get better.
You can also use the spray.Have you replaced the cap
and rotor?

http://www.seafoamsales.com/sea-foam-motor-treatment/why-sea-foam-motor-treatment-works/

[TS]

This same 5S-FE engine in the Gen 3 Camry had a distributor, but the Gen 4 (1997-2001) uses a distributorless wasted spark system.

I could try Sea Foam sometime if I can be absolutely certain it won’t harm anything, but aren’t fuel system cleaners like Techron supposed to clean the intake valves as well?

[college man]

Its supposed to be used as maintenance. But if there is a
heavy accumulation it won’t get rid of years of build up.
The seafoam will not hurt the car if done correctly.

[Its me]

All I have to say is I highly recommend to anyone they only use OEM O2 sensors on Toyotas…

That is all.

-Karl

[TS]

Yes, definitely insisting on an OE sensor next time.

Meanwhile, I had a chat with my mechanic from earlier. He thought the fuel trim might suggest a vacuum leak as well, perhaps at the intake gasket. However, that might have to wait for a smog shop’s diagnostic procedures. I can’t spray it accurately from above and don’t have a propane tool handy, but I may try with soapy water (as in some other videos) from under the car if there’s good access.

[hondaguy453]

Wouldn’t O2 be high if it was a vacuum leak?

[TS]

That might be the case for a lean exhaust generally, but I have a rich exhaust. However, O₂ was also zero when I passed two years ago with numbers closer to those averages. (CO₂ was higher at 15.0% on both speeds.)

[TS]

Time for an update! The car just barely passed. A professional diagnosis pointed to at least a faulty downstream O2 sensor, so I put a new Denso in there and re-checked. CO went down to normal levels, and HC came down a bit from this other shop’s first test but was still elevated (at 15 mph, just 1 ppm under the cutoff). There’s about 1.4% O2 in the exhaust now, and my fuel trim numbers seem to have adopted the pattern expected of a vacuum leak. Before this diagnosis and repair, I’d also started to hear an occasional exhaust leak in city driving.

So I’m good with the state for another two years, but perhaps I should try to make sure the catalyst doesn’t get ruined by the elevated HC. Other than checking the spark plugs for evidence of burnt oil, what might I consider? (The car loses some oil between changes, but at least some of that seems attributable to leaks.)

[Its me]

[quote=”trescal” post=67021]Time for an update! The car just barely passed. A professional diagnosis pointed to at least a faulty downstream O2 sensor, so I put a new Denso in there and re-checked. CO went down to normal levels, and HC came down a bit from this other shop’s first test but was still elevated (at 15 mph, just 1 ppm under the cutoff). There’s about 1.4% O2 in the exhaust now, and my fuel trim numbers seem to have adopted the pattern expected of a vacuum leak. Before this diagnosis and repair, I’d also started to hear an occasional exhaust leak in city driving.

So I’m good with the state for another two years, but perhaps I should try to make sure the catalyst doesn’t get ruined by the elevated HC. Other than checking the spark plugs for evidence of burnt oil, what might I consider? (The car loses some oil between changes, but at least some of that seems attributable to leaks.)[/quote]

This is weird… A downstream O2 sensor (which just reports the condition of the cat) had an effect on the HC level…. Hmmmm OK a win is a win, I just don’t see how that helped…. Maybe better to be lucky than good…..

The exhaust leak is more understandable.. Since cars are tuned for catalytic converter performance (not necessarily for best fuel economy or peak engine performance)… A pre cat leak/even a pre downstream O2 sensor leak can cause O2 sensors to read incorrectly.

I would seriously look at what is going on with that exhaust leak…

Cheers.

-Karl

[TS]

Somehow the new sensor massively reduced the CO as well. I’ll put the passing results below, but the last smog check before my diagnosis had CO readings of 1.10% and 0.82%. HC on that test was 97 ppm and 68 ppm, and there was zero O2.

15 mph:
1812 rpm
CO2 13.90%
O2 1.43%
HC 54 ppm (max 55 ppm, avg 9 ppm)
CO 0.05% (max 0.50%, avg 0.03%)
NO 56 ppm (max 437 ppm, avg 67 ppm)
(lambda = 1.065, lean)

25 mph:
1773 rpm
CO2 13.90%
O2 1.44%
HC 22 ppm (max 39 ppm, avg 6 ppm)
CO 0.01% (max 0.48%, avg 0.03%)
NO 15 ppm (max 724 ppm, avg 62 ppm)
(lambda = 1.068, lean)

I was a little suspicious when I got that diagnosis as well (and fearful that I’d immediately receive a P0420), but I think I subsequently read a claim that the downstream O2 sensor can have a small influence on the fuel mixture. Perhaps it’s not so much that it hampers fuel economy or driveability, but it’s just enough to make the computer target a fuel mixture that’s just outside the catalyst’s optimal range. Just curious–does that seem plausible at all?

I think I occasionally hear an exhaust leak, anyway. I don’t have a great ear for these things, but sometime I’ll try to look harder for one.

[Author]

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