jns

I just moved to Vegas and was introduced to the smog program here with a failed emissions test.Hydrocarbons sky-high at 1351.This 91 4R 3.0 has 159K miles and new plugs,wires and cap/rotor.The hydrocarbons passed at 2500rpms and everything else passed.Why are just the hydrocarbons high at idle?Please help.

jns

Any smog techs out there?I'll try changing the plugs back to stock NDs since the Boschs looked like the engine was running rich.I put vacuum on the EGR with the engine running and it died so I think the EGR is OK.Anything else?

SLC Punk

It could be an O2 sensor.
What is your idle set at?

jns

Idle's between 750-850.It won't sit at 800 all the time.I'm changing the plugs back to NGK,changed the PCV and cleaned the throttle body.I just came in from working on it since it got dark and will change the fuel filter tomorrow.I checked the Airflow meter and the plastic looking probe looked clean(am I looking at the right thing?).How do I check the O2 sensor?I don't have a check engine light on so I thought it might be OK.Thanks for any info.

Mystickal

Just because the check engine light isn't indicating that the O2 sensor is malfunctioning doesn't mean that the O2 sensor is working properly. The only real way to determine if the O2 sensor is functioning properly is to measure the voltage coming from the O2 sensor going into the ECU while also monitoring the Air-Fuel ratio. Kind of a pain in the rear, but the basic procedure is this:

With engine cold, O2 sensor voltage should be below .2 volts (lean condition) as the sensor has not yet reached 'operating temperature'

After about 20 minutes or so, you should note voltage varying between .1 and .9 volts (sensor is warmed up and properly sensing the fuel-air mixture).

Rev up to 2000-2500 RPM, then artificially enrich the fuel mixture by directing propane into the intake until the engine speed drops by 200 rpm. Or, if you can remove and plug the vacuum line to the fuel pressure regulator (so the EFI pumps more fuel into the engine). NOTE - WHEN YOU DO THIS (ESPECIALLY WITH PROPANE IN AIR INTAKE) WEAR SAFETY GOGGLES AND MAKE SURE NOTHING COMBUSTIBLE IS NEAR THE EXHAUST! ! ! You have the potential to shoot fireballs from your tailpipe (or anywhere else there may be an exhaust leak).

O2 sensor should read .9 volts (rich mixture)

Undo the propane or plug in the vacuum line, then induce a small vacuum leak

O2 sensor should read .2 volts (lean mixture).

Secure the leak. The O2 sensor should then read about .5 volts (stoichiometric ratio of 14.7 parts of air to 1 part of gasoline).

All of these changes should occur very rapidly, not sluggishly at all. Generally it should respond within a second to these changes.

Like I said, a pain in the rear.

~Bill

Mossback74

Or just replace it

jns

Great info,thanks.How many O2 sensors are on here?I was under there knocking on the cat to see if it was hollow or rattling and saw one sensor there.I'm considering just changing the sensor under the "will need one soon anyway since it's at 159K" clause.Would the O2 sensor be responsible for a high HC reading at idle but low HC at 2500rpms?

Mystickal

Not sure on a '91, though I can't imagine it having more than 1 O2 sensor - check a shop or haynes manual to be certain (or even call a dealership), but I'd feel safe betting that you only have one sensor.

And it's very possible that the O2 sensor would be repsonsible for that condition. As different amounts of air flow into your cylinders, the O2 sensor is responsible for ensuring that the proper amount of fuel is being sent into the combustion chamber. If the O2 sensor is malfunctioning, it may tell the ECU to send too much fuel at a higher RPM, but the proper amount at a lower RPM.

Of course, it could be something else too. Another possibility is that your timing is off. But I'd check the O2 sensor first, especially since your vehicle is a 91 - if it has the original O2 sensor then it is almost certainly an O2 sensor problem.

~Bill

jns

I just checked with the dealer and found that there are 2 sensors at the bargain price of $139. a piece.I better go play the casinos so I can win enough dough to fix my ride. Would you recommend using some Auto Zone-sourced Bosch sensors(their plugs suck,but maybe they got the sensors right)?I might need one for my Ford F150 and wanted to go with the Ford OEM but saw Ford/Bosch printed on the stock sensor.Who knows?

Yoda

How long had you driven before your test? We used to send customers to a station a half hour away (telling them it was easier to pass there). Most ususlly did pass, but it was because the engine was thoroughly warmed up. A 5 minute drive is not enough.
Don't sit and idle in line (death for testing, come back when there is no wait). Change your motor oil if it is old. Add a couple bottles of gas dry to your gas tank, the alcohol promotes a cleaner burn (areas use "oxygenated" gas to lower smog...just has 10% or so alcohol).

Do they do the high speed test first? If so, don't just dump the throttle to idle, slowly release over 3-4 seconds.

jns

Yep,it was less than 10 minutes to drive to the smog shop and a lot of stop and go traffic.Since my failed test,I have changed the plugs to the NGKs recommended in the owner's manual and gapped them.Already had new wires and dist cap/rotor.I applied vacuum to the EGR while running and the engine died(must be working OK?).Changed the PCV.Air filter was already clean before test.Timing was found 2 degrees advanced,now it's stock.Put in some "gas kicker"?(in a 2.5 oz. pink bottle).I sprayed the throttle body clean(and split the dry-cracked intake hose in the process which cost $70 ).I'm thinking of getting another smog test done after a long drive on Monday.I have a coupon for $12.95 to get a test done and if it fails again I'll change both oxygen sensors.

My last test was:
Limit Reading
HC
2500 220 29
idle 220 1351

CO
2500 1.20 .03
idle 1.20 .47

CO2
2500 15.10
idle 14.60

O2
2500 0.1
idle 0.9

The HC at idle(1351)was the only thing that failed.Any more suggestions?Thanks for all the help.

jns

That last reply didn't come out the way I typed it.The limits are on the left and the readings are on the right.There are no listed limits for the CO2 and O2,just the readings it got.Sorry for the confusion.

jns

Update!I just got it resmogged and it went from a 1351 HC level at idle to an 8.I had raised the idle to 1100rpms,cleaned the throttle body,changed the pcv,took out the new Bosch platinums and put in stock NGKs,set the timing to stock,ran some gas additive (in a 2.5oz pink bottle) through it,drove it hard out of overdrive to a shop 20 miles away to warm it up good and revved it to 3000K rpms for 30 seconds just before they pulled it in to test.Thanks to all for your help.