Plan B Forum

Yesterday I got my car (1994 Sentra) emissions tested. While it passed, hydrocarbon levels while the engine is revving were pretty close to the legal limit (but well within tolerance at idle, as were all other readings). Can anyone suggest if there might be a problem developing that I should get looked at? Exhaust from below the catalytic convertor (resonator, muffler, tailpipe) have all been recently replaced.

Test Results:

Best I could do Scoot:

http://www.aa1car.com/library/tr1196.htm

According to this the 14% dilution means it is running fairly efficiently.

Best to ask a motorhead though...

Thanks Keld.

According to a chart somewhere in the middle, my CO reading is at the high end of the range for a "well-tuned engine", but the HC level is well outside that range, even if still acceptable. And as you point out, dilution is close to the 14.5 that is said to be ideal, which would appear to be contradictory. This:

Hydrocarbon failures mean unburned gasoline is passing through the engine and entering the exhaust. The three most common causes include ignition misfire, lean misfire and low compression (typically a burned exhaust valve). Ignition misfire can be caused by worn or fouled spark plugs, bad plug wires or a weak coil. Lean misfire results where there is too much air and not enough fuel, so check for vacuum leaks, dirty injectors or a fuel delivery problem. In addition to these, hydrocarbon failures can also be caused by oil burning due to worn valve guides, valve guide seals and/or rings.

sounds like a pretty long laundry list of potential causes which might be expensive just to diagnose, let alone repair.

If you have an Autozone nearby they do free diagnostic testing.

Plugs & plug wires are a "tune up" if it's been more than 100K since yer last that might help a bit.

If you don't drive country roads air filters should last awhile but they still need replacing from time to time.

All 3 of those things are actually fairly cheap.

If you are so inclined they could just be cost of parts...

I'd do plugs, wires, and air filter...also, if it has one, how old is the distributor cap & rotor? If not...might have a coil pack going south.

Does it burn oil? If so...there's the culprit.

Doesn't burn oil that I can tell or have been told...I get the oil changed every three months and don't drive it a lot, so not sure if it would be obvious. Air filter was just checked. Plugs and wires were checked as part of my seasonal maintenance last spring. I don't think the distributor cap has ever been replaced...don't know about the other stuff.

Thanks for the suggestions, gives me some idea of what a mechanic should be looking for when I ask him about it...I just didn't want to go in completely ignorant and have him propose a litany of repairs that are unnecessary and wouldn't solve the problem.

I will probably be postiing again to ask y'all's opinion of his diagnosis when I bring it back in, around March/April.

If you aren't driving it that much (but are at least driving it fairly regularly) you don't need to get the oil changed every three months every 5-7000km is plenty and that still leans on the edge of often.

I know it's not a necessity to get it changed that often, but (a) keeping it on a schedule makes it easier for me to remember to get it done, (b) after 16 years the engine still purrs like a kitten, and the only major repair I've had to shell out for was the air conditioning, which went a few years ago, and (c) it's only $30 for the oil change and a 30 point inspection which has, on occasion, caught something before it had the chance to become a major problem, which I think is what accounts for (b).

Not very environmental of you

Having the car period in a city with excellent public transportation isn't particularly environmental, but given that I inherited it already paid for, I have hung on to it.

The HC at speed should be darn near unreadable, at idle is the problem and that is where lean miss will show up.

1994 = 17 years, Id change the O2 sensor (the Denso from rockauto.com is what you have from the factory) and make sure any air injection system is up to snuff.

IIRC that car has a 2 way catalyst (CA rules) so with enuf air the Pt in the cat will decompose any HC radicals and the rhodium will take care of the NOx (which either you dont list or CA dont test)

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Air conditioning in Canada? I don't even understand why you use refrigeration.

Before you do the coil pack make certain the plug wires are not a problem. They may look good but unless you know they are good that is a possible problem and a whole lot less expensive. They will definately come up in conjunction with the coil pack on an anilizer. The coil pack will cost much more that plug wires.

quaddriver wrote:The HC at speed should be darn near unreadable, at idle is the problem and that is where lean miss will show up.

1994 = 17 years, Id change the O2 sensor (the Denso from rockauto.com is what you have from the factory) and make sure any air injection system is up to snuff.

IIRC that car has a 2 way catalyst (CA rules) so with enuf air the Pt in the cat will decompose any HC radicals and the rhodium will take care of the NOx (which either you dont list or CA dont test)

It's listed as NO.

No x...

Crackpot wrote:If you aren't driving it that much (but are at least driving it fairly regularly) you don't need to get the oil changed every three months every 5-7000km is plenty and that still leans on the edge of often.

Distance and speed of driving is key. Driving a car 3 to 5 miles a day (at < 55mph) is considered "harsh driving" as the car does not get up to temperature and does not burn off contaminants in the oil. In that case changing your oil at every 3000 miles or 6 months (whichever comes first) should be the rule. My commute has gone from 50 miles a day to less than 3 miles a day. Oil gets changed every 6 months no matter the odometer reading which is usually only 1k to 2K from the last oil change.

3k miles six months is still too often in all but the harshest of driving. in your case I'd recommend oil changes before and after winter rather than every 6 months.

Crackpot wrote:3k miles six months is still too often in all but the harshest of driving. in your case I'd recommend oil changes before and after winter rather than every 6 months.

My schedule is october and april so that coincides with both your and my schedules. My tractor is on the same routine.