Need help getting friend's 91 ES250 to pass CA smog

[MikeFD3S]

I helped my roomate pick up a pretty nice black 91 ES250 a few weeks ago. Has 117k on the odo, super perfect interior. Was able to haggle and steal it for a clean $1000.

Since he's a poor film student and I'm more car savvy, i've been helping him get the car up to par.

When we bought the car, it had a check engine light.

It originally had an EGR and lean condition CEL (26 and 71 I believe) that was temporarily remedied by reseating some poorly torqued plugs (platinums) and changing the plug wires. Also resealed the valve covers to fix some oil leak problems. The car runs beautifully, with zero vibration and good power. Once the check engine light was off, we took the car to smog, but the HC and CO levels were through the roof.

After about a week or so, the check engine light (CEL 71) is intermittently coming back on.

Usually, EGR would promote high NOx levels, but the car passed those with flying colors, so i'm a bit stumped as what to do next. EGR seems to be working fine, and the low NOx levels proves that.

To remedy the high HC and CO levels, I'm thinking O2 sensor, even though there is no CEL showing for that. If it's not the O2 sensor (there is only one right?) would it be the catalyzer?

I'm hoping it's the O2 sensor, since it's cheap and easy...but if it's the cat, are there any other tricks to pass the sniffer? Slipping a couple bills wont fly since he's broke.

The car is currently using 87 octane.

Any help is appreciated! Thanks guys.

[Doctor J]

The low NOx are due to the compensation from the mixture that is too rich.The code indicates lean condition, so the com. is compensating accordingly.This car is equipped with 4 wire O2 sensor.


Heated Oxygen Sensor
See Figure 1

1. Start the engine and bring it to normal operating temperature, then run the engine above 1200 rpm for two minutes.
2. Turn the ignition OFF disengage the H02S harness connector.
3. Connect an ohmmeter between terminals +B and HT of the sensor. Resistance should be 11-16 ohms at 68 degrees F (20 degrees C). If not, replace the sensor.

Fig. 1: Check the resistance of the heater portion of the sensor

1. Start the engine and bring it to normal operating temperature, then run the engine above 1200 rpm for two minutes.
2. Backprobe with a high impedance averaging voltmeter (set to the DC voltage scale) between the oxygen sensor (02S) signal wire and battery ground.
3. Verify that the 02S voltage fluctuates rapidly between 0.40-0.60 volts.
4. If the 02S voltage is stabilized at the middle of the specified range (approximately 0.45-0.55 volts) or if the 02S voltage fluctuates very slowly between the specified range (02S signal crosses 0.5 volts less than 5 times in ten seconds), the 02S may be faulty.
5. If the 02S voltage stabilizes at either end of the specified range, the ECM is probably not able to compensate for a mechanical problem such as a vacuum leak or a faulty fuel pressure regulator. These types of mechanical problems will cause the 02S to sense a constant lean or constant rich mixture. The mechanical problem will first have to be repaired and then the 02S test repeated.
6. Pull a vacuum hose located after the throttle plate. Voltage should drop to approximately 0.12 volts (while still fluctuating rapidly). This tests the ability of the 02S to detect a lean mixture condition. Reattach the vacuum hose.
7. Richen the mixture using a propane enrichment tool. Voltage should rise to approximately 0.90 volts (while still fluctuating rapidly). This tests the ability of the 02S to detect a rich mixture condition.
8. If the 02S voltage is above or below the specified range, the 02S and/or the O2S wiring may be faulty. Check the wiring for any breaks, repair as necessary and repeat the test.

Prior to blaming the O2 sensor:

Sometimes the crack in exh. manifold near the sensor cause false high O2 content ,therefore the computer makes the mixture rich. Also you may check the the distributor rotor and fuel pressure up system components.


After fixing the rich mixture problem,remove EGR valve from plenum and clean the passage in the plenum from the deposits.

Is your roomate attends CSULB?

[MikeFD3S]

Thanks for the tips Doctor J, he is in fact going to CSULB-- how'd you guess?

I think I can borrow a meter from someone so I can test the voltage ranges and I didn't check the passages to the EGR, so i'll give those a try when I have some time to wrench on his car this weekend.

The manifolds are free from any cracks, and the car isn't shooting a lean condition code anymore.

[Doctor J]

It would be also a very good idea to check the fuel trim voltage aka learned value by attaching digital voltmeter to VF (+ probe) and E1(-) terminals of the diagnostic connector, engine at idle speed. Reading of less then 1 volt indicates rich mixture,
that usually caused by:

• broken vcv on fuel pressure up system (located on the rear valve cover),
• damaged connector on the coolant thermosensor (THW)
• air cleaning element installed upside down
• plugged pcv system (valve)

Is your friend enrolled in ASTR370I class this semester?

[Phatfantom68]

If your friend's having trouble with emissions, tell him to sell me the car for cheap.

[marc780]

I bet you can fix this car, but Kalifornia smog is VERY tough anymore and not getting any easier. I had to get rid of 2 cars, a 1986 Chevy Cavalier and my old 85 Nissan 200sx cause i knew they would not pass smog - but otherwise they ran ok. And my mom had to get rid of her Mercedes 240, in otherwise perfect shape, for the same reason, it needed $2k to fix to pass the smog.

[Ran]

Seafoam.