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Failed Emissions - 142.0 Carbon Monoxide

talon39

NAXJA Forum User
Location
Phoenix
1990 XJ 4.0L
I Failed emission last week with 85.2 Carbon Monoxide. It needs to be 20.0 to pass.
So we replace the Injectors, Airfilter, Spark Plugs, Distributor(not cap) and oil.
All of the above were due or needed.
After doing the above I noticed the pedal to be a bit more responsive.
I Just got back from the Emission center and Failed again. This time I got 142.0 Carbon Monoxide. How can it be even worse than before?
The Jeep seems to be running good. It doesn't miss. The Idle seems ok. It seems to have good accelleration.
We put it on a MT2500 Scan tool and it has no error codes. Everything looks normal except the O2 sensor reads 5 volts. The O2 sensor is less than a year old and has read 5 volts since it was installed. We compared it to my buddies Jeep that did pass emissions and evething on the MT2500 looks the same including the O2 sensor reading. I'm baffled. I have no Idea what to do next. :dunno:
 
From what I understand after reading these forums a Bad EGR would cause the CO reading to be lower not higher. Is that correct?
142.0 CO is extremely rich, it seems unlikely that a bad Cat alone could cause such a high number. Am I mistaken?

Here are results of the emissions test

Hydrocarbons 4.17 g/m (passing 2.00)
Carbon Monoxide 142.00 g/m (passing 20.00)
Oxides of Nitrogen 0.97 g/m (passing 4.00)
 
Last edited:
I thought EGR has more to do with NOx...maybe I'm wrong too. But, you have failed the hydrocarbons test and CO which to me looks like you're running rich. Getting too much gas. Also how is your crankcase vent system? Are you sucking oil at all in through the air intake. I guess your could even have weak spark or spark timing off but you said the jeep's running fine... How about exhaust or vaccuum leaks? The 02 sensor is the upstream sensor youre talking about right? How warm is the jeep when you take it in? Someone I know that does smog reccomended driving for at least 30 mins, hwy speeds using a lower gear than normal. Try to get your car on the tester right away after pulling in and in the parking lot rev it up to 2000 rpms and hold it for 30 secs before pulling in then let it idle. All this is supposed to get the cat hot, even though protocol for CA smog is not to heat the vehicle being checked (but the cat doesn't work at full efficiency until hot). Furthermore, have you done a decarbonization w/ sea foam which is also supposed to clean out the cat.
 
Check catalytic converter, MAP sensor, IAT sensor, and HEGO sensor. Any of those running out of spec could cause a rich condition, which results in the high CO and HC numbers. Since the NOx is within spec, I'd not suspect the EGR (that, and a failed EGR causes the CO and HC to drop to near zero.)

The MAP and IAT are pretty standard items (GM curve and sensor style,) and you can get away with pretty much any catalytic converter, but the HEGO sensor is a resistance unit (rather than a voltage generator) and can't be substituted.

MAP check - resistance should change smoothly between terminals B and C while vacuum is gradually applied. (disconnected)
(In situ testing) - KOEO. +5VDC between terminals A and C on harness connector (acceptable, 4.5-5.0VDC)
Connect harness connector and backprobe at terminal B - should shot 4.5-5.0VDC relative to ground. (Replace if fail)
KOER - allow idle to stabilise. Should show .5-1.5VDC between terminal B and ground. Replace if fail.

IAT check - disconnect and check resistance vice temperature. Use chart at end of post. You can use a hairdryer to check higher temperatures if you have a fairly accurate temperature gage or a fairly quick thermometer

HEGO check - it's a resistance unit, so you'll first check the heating element (5-7 ohms between terminals A and B on HEGO sensor connector.) You'll need to find a way to make the fuel/air mix "rich" or "lean" on command (haven't sorted this yet - it's on my list) and the HEGO resistance goes up at the mix leans, and down as it enrichens.

Thermistor Test Chart - use these values for the IAT and CTS sensors (not the one on the cylinder head, but the one down on the side of the block..)

212*F 100*C 185 ohms
160*F 70*C 450 ohms
100*F 38*C 1600 ohms
70*F 20*C 3400 ohms
40*F 4*C 7500 ohms
20*F -7*C 13500 ohms
0*F -18*C 25000 ohms
-40*F -40*C 100700 ohms

This can be checked with an accurate ohmmeter and a fairly accurate thermometer (so you know what to expect.) I don't have have the chart in a format I can post at the moment, but there is a predictable result per temperature.

I'm still trying to brain out some workable RENIX diags (since we don't have OBD-compliant ECM units) that can be done with minimal equipment... Gimme time - it will be the next chapter in my book....

5-90
 
Thanks 5-90. I will try your suggestions this week-end

Bajacalal- There does not appear to be any vacuum leaks and the exhaust is new from the cat back.

Why does my O2 sensor show 5 volts on the mt2500 scan tool? Is that normal. That has bothered me since I installed it.
 
If memory serves, the HEGO sensor is fed +5VDC, which is then divided by the variable resistance of the HEGO element. The voltage output from the HEGO sensor (between connector pin C and ground) is converted by the ECM to a digital signal, and is then used for AFR monitoring. Voltages higher than midrange (~+2.5VDC) indicate a rich AFR, and lower means lean. The +5VDC is the supply voltage.

5-90
 
talon39 said:
Thanks 5-90. I will try your suggestions this week-end

Bajacalal- There does not appear to be any vacuum leaks and the exhaust is new from the cat back.

Why does my O2 sensor show 5 volts on the mt2500 scan tool? Is that normal. That has bothered me since I installed it.

One thing, if on your smog report there should also be %02 and %c02 which aren't pollutants but tell you how the cat is working. If you are rich and high 02 (more than say 2%) you probably aren't burning the gas completely or the cat's not working.

The exhaust leaks in question would be the ones before the oxygen sensor, aparticularly the one were all familiar with in the exhausts manifold or joint between the pipe before the cat and exhaust manifold. Also the gasket joining the header to the head but I think its unlikely. If there is an exhaust leak, it will seem like only exhaust escapes but also between each cyl fire, air sucks back in making the computer richen up the mix. Do you have 2 O2 sensors? If you have one connected to the cat, it just monitors if the cat is working or not ant seems to do very little anyway. The upstream ones monitors exhaust 02. Also I checked my service manual and it says for the RENIX models the reading should be between .1 AND 1.0 VOLTS so around 1/2 volt is normal and it should respond quickly if propane is introduced to richen the mixture. The 02 sensor produces full voltage at lean (atmospheric levels of oxygen, lean mix or have air in exhaust) and low voltage if gas rich. So you are 1.) set the multimeter wrong, 2.) have a short from the 02 sensor heater circuit, 3.)my non factory manual is worng 4.)have installed the wrong part or maybe a downstream sensor 5.) dropped the sensory, contaminated it w/ grease, or for some reason it is getting air. But if its really reading so high continuously the comp would think the mix is lean but really squirt too much gas. Why hasn't the computer thrown an 02 sensor out of range code?
 
bajacalal said:
Why hasn't the computer thrown an 02 sensor out of range code?

A very simple reason - the RENIX ECM doesn't store or generate hard codes in the OBD sense. The RENIX system is "pre-OBD," and lacks a MIL. The use of a DRB-II/III or MT2500 allows the tech to check relatime driveability data, and most diagnostics are done with a good meter.

That's one thing I like about RENIX - it's a system that demands a little skill in repairs. I came up on points/condensers and no electronics, and you had to know what you were about to work on the things effectively. I'm not overly fond of self-diagnosing systems...

5-90

Oh - I'll have to check those numbers I tossed out on the HEGO voltages - they were from memory, and I've got a few things going on up here (tapping forehead...)

5-90
 
talon39 said:
1990 XJ 4.0L
I Failed emission last week with 85.2 Carbon Monoxide. It needs to be 20.0 to pass.
So we replace the Injectors, Airfilter, Spark Plugs, Distributor(not cap) and oil.
All of the above were due or needed.
After doing the above I noticed the pedal to be a bit more responsive.
I Just got back from the Emission center and Failed again. This time I got 142.0 Carbon Monoxide. How can it be even worse than before?
The Jeep seems to be running good. It doesn't miss. The Idle seems ok. It seems to have good accelleration.
We put it on a MT2500 Scan tool and it has no error codes. Everything looks normal except the O2 sensor reads 5 volts. The O2 sensor is less than a year old and has read 5 volts since it was installed. We compared it to my buddies Jeep that did pass emissions and evething on the MT2500 looks the same including the O2 sensor reading. I'm baffled. I have no Idea what to do next. :dunno:

The bottom line is that your engine's running too rich. The commonest culprit is the O2 sensor but since this reads 5.0v, it's therefore working normally and sensing the richness. The MAP sensor would be the next most likely candidate but if this sensor was bad, the engine would be hard to start and run poorly so that one's pretty unlikely. I'd check the following:

1. Vacuum lines from intake manifold for leaks, especially the one to the MAP sensor.
2. CTS and IAT sensors. If the resistance across either of these is excessively high i.e. over 20kohm or infinity, the ECU will think that either the coolant is cold or the intake air is cold and increase the injector duty cycle to add more fuel.
3. Plugged cat.
 
Well, I can't help you much compared to the hi-tech advice you got from those with the know-how. My little help would be to suggest using HEET in the tank plus a fresh tank of gas, next time you go in for smog check. Getting the truck very hot is also an excellent suggestion. Also, do you use OEM O2 sensor? I hear that does make some difference, but I don't have first hand knowledge.

My 2 .. Maz
 
Dr. Dyno said:
The bottom line is that your engine's running too rich. The commonest culprit is the O2 sensor but since this reads 5.0v, it's therefore working normally and sensing the richness.


After doing a little reading it seems that my O2 sensor works the opposite of later models. On later models the sensor has a voltage range of 0 to 1 volts. Higher voltage indicates a rich condition. However the O2 sensor on my 90 has a range of 1-5 volts, higher voltage indicates a lean condition. If that is correct then my computer is incorrectly sensing an extremely lean condition. So my O2 sensor must be bad. I replaced it yesterday. I havent had a chance to put it on the scan tool to see if it still reads 5 volts yet. I will check it this week and let you guys know if that was it.
 
Yup That was it. I passed emissions no problem. The new O2 sensor made a huge difference. Kinda makes me mad that the last time I replaced it with a bad one.
Thanks for your help.
 
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