peterls

NOTE: THE ISSUE WAS RESOLVED. READ THE LAST FEW POSTS TO FIND OUT HOW.
I've been too busy to post here or even work on my car... (except for oil changes and break jobs)
Anyway, to my shock, after I changed spark plugs, wires, oil, filter, air filter, and oxygen sensors, my 1990 LS which is running like a champ, failed a smog test due to high hydro-carbons!
I don't have the numbers on me, but if normal HC at 15 mph is around 15, and max is around 70, mine is like 180!
At 25mph it gets slightly better, was around 120.
In any case - way too high.
Now, NOx is almost zero.
Oxygen - zero.
CO - very low, very normal.
So, cats seem to be working great, engine is doing great, but how is it that HC is so high and only HC, not even CO or CO2 to point to too rich a mixture?
My mechanic checked the numbers and entered them on his computer, and it calculated that the mixture was a bit too rich. What is the ideal ratio, is it 1:14.7? Mine was 14.65 or thereabout. Didn't seem like such a big difference to me, but he assured me it was too rich. How come the CO and CO2 is not too high?
My only suspects are:
- Engine coolant sensor - the one at the front of the engine, just below wires. But could that thing really make the fuel mixture THAT rich???
- Maybe the injectors themselves, although doubtful. Why would they go so bad, so suddenly. Two years ago, I passed the smog with flying colors, I mean, the engine was giving the values like it was new.. and every time before that too.
- Charcoal canister with that valve? Could that be the culprit and inject that much fuel into the mix???

Any good advice is appreciated.

Yamae

I have experienced several high HC, low CO and very low NOx problems exactly like you are experiencing now. Those were caused by the slight/intermittent miss fires. There were variety of root causes. Some were caused by the vacuum leak and others ignition problems such as leaking high voltage line or a weak ignitor.

dlcoffin

another cause can be worn or misaligned oil rings. oil gets sucked up and burnt, causing high hc. leaky valveguides can be another unwanted entry of oil into the combustion chamber. i imagine you keep tabs on oil consumption, smoking from the tailpipes, suchlike?

YODAONE

ECU capacitors...Replace them.
Check this forum for instructions

If original,. replace coolant sensor.with OEM part

OEM distributor caps, rotors and wires?

Original engine vacuum hoses are probably tired..

Have ignition coils checked.

Use NGK Ruthenium spark plugs.

peterls

Thank you Yamae for your reply. I was wondering if I may be having a vacuum leak somewhere, as I noticed that my power steering seems to be a bit too heavy, but the change was so gradual that I couldn't tell for sure (besides, I am sure that my power steering module needs new caps anyway)
Would I be able to hear the misfires at all? Because I really haven't noticed any.

peterls

In all these years, I've never had to add oil (and I never put in more than I should). I do my own oil changes, every 3-4k miles and I've never seen smoke from my tailpipe either, so that should not be the culprit.

peterls

Most of what you mention was already done, except for ECU (Have another new one ready and waiting, so I could swap them easily), and coolant sensor.
I am leaning toward either the coolant sensor which was replaced last time about 6 years ago (I think it was OEM as I try to use OEM always) or vacuum leak somewhere.

peterls

The more I think about this, the more I am leaning toward a vacuum leak.Any pointers on how to check for that, besides the obvious - "Go over all the black, dirty hoses in a dark engine bay and see if they are cracked or if you hear any hissing"?
Peter

Stroock639

does your car still need to pass the emissions part of inspection despite being OBD 1? here that's not the case but i know CA has their own emissions standards

peterls

Yeah, in CA, it's smog for everyone, unless the car was made before 1975.

peterls

One more thing I noticed the other day: standing at a red light, uphill, after driving for about 30 min, the car misfired twice in few seconds. Quite noticeable, almost felt like a big hiccup or something. Does that seem related, and what would cause such a misfire while the car is idling? And, that was the only time too!

spuds

We have basic and enhanced smog.Enhanced is the dyno smog test.I passed that down the hill where purchased with only about 8% away from hitting max.

In the mtns Im in a low smog area and only needed basic smog, idle and 2500 rpm testing,I flew thru that, both hitting only 13% or so of whats allowed,Im EXTREMELY lucky living where I do.

Sin1UZFE

And is among if not the most strict

Yamae

Your symptom tells me exactly what I have experienced. The miss fires were only occurred intermittently when the engine was fully warmed up. It was caused by one of the spark plug cables which was like this below. You can see the evidence of the tracking inside. There happened a surface leak and the rubber is damaged by the heat and making a tracking line.

Originally the local shop owner was having a hard time finding the root cause and he asked me for a help. I used an oscilloscope and checked the current waveforms of each cylinders and one of them was intermittently showing unusual spikes and found this below.



Another item you need to consider is the vacuum leak. The vacuum level is strong while idling and the air is relatively more absorbed to cylinders when some minor opening is there at some intake area. This tends to cause miss fires only while idling.

peterls

Thank you Yamae and others for replies. So far, this is what was done:
- A couple of months ago, car ran on four cylinders only. It happened about a mile from my home, so I limped back slowly, without forcing it, and then changed the ignition coil. Changed the passenger one first, because that's the one that tends to go out first. But, it made no difference. Then I changed the one in the center - no difference either.
- Didn't have the time (busy at work) so I towed it to a mechanic. He diagnosed the distributor rotor and changed it (one only - when he should've change them both really). He put in a denso distributor/rotor and at the same time changed the spark plugs (I had them in my trunk, as I was planning to do that anyway), and wires. ALL OEM (ngk/denso etc)
- Then failed the smog test with HC ~ 180
- Changed the o2 sensors (put in Denso) on advice of my mechanic, and failed the test again. Same numbers all over.
- Changed both igniters (5 min job anyway). Failed the test yet again with same high HC number.
At this point, the only explanation would be either some kind of a problem with wires (but they are new!) as Yamae says, or maybe the ECU (I changed that also, just now, after I failed the test yet again) or a vacuum leak somewhere.
Anyone wanna bet (joking), as to which of these three it is?
I have spent some time inspecting vacuum tubes, and heard no hissing, found no problems, but that was only looking at what I could see when I opened the hood. Are there any hidden vacuum lines that I should look at?