Hey all,

Forgive me if this has been addressed already, but I haven't seen anything on it yet. I have a 95 3.0 with 143xxx miles that I am trying to get smogged and registered; smog has been coming back with elevated CO and HC. I have tried a few times with various fixes and I'll list them as follows, mind you all the papers are at the shop with the rig:

Smog round 1: Came back as a gross polluter (oh ˟˟˟˟.) CAT was passing though. so I took it to the shop. They found that it was an upstream O2 sensor; after replacing that, my rpms gained a lot of consistency and it ran much more smooth all around.

Smog round 2: It just barely failed, with slightly elevated CO. The smog team recommended that I put another 40 miles on it and go for it again.

Smog round 3: Back to being a gross polluter with CO and HC. Took it back to the shop and they flipped out the MAF with no improvement. It's still in the shop, in the hands of who I am sure are terrific mechanics, but I am very into tinkering and trying to DIY. Through my research I have seen pretty consistently that it could be an ignition coil or a fuel injector, but seeing as I do not have the vehicle on hand today, I can't be sure.

Does anybody have any experience with this issue, and/or am I screwed?

Some more pertinent info:
CAT passed smog.
New denso O2 sensor
There are new valve covers.
Spark plugs and wires are clean/new.
New airfilter
I have run Chevron Techron through the tank.
Timing is at 13 (smog guys said it is a little high, but acceptable here in Cali)

 

Have your mechanics determined whether the fuel regime goes into closed loop??

 

I assume it is not throwing any check engine codes? Does the CEL light work?

What do you mean by "the CAT passed smog"? Unless they insert an upstream and downstream probe in the exhaust system, the only thing the smog facility can do is ascertain the physical presence of the CAT.

If the CEL system is working, and there are no codes, the likely causes are:
- not going into closed loop, probably because the engine temperature sensor isn't working properly. This is a different sensor than what runs your temp gauge, so the fact that your temp gauge is reading normally means nothing.
** You can check for closed loop operation by warming up the engine, and then idling at 2500 rpm. Put a high impedance voltmeter on the Ox1 port in the diag connector. It should be switching between 0 and 1 volt around once per second.
- the CAT is faulty. The onboard diagnostics have no way of measuring the effectiveness of the cat.

Oh, wait. Is this a California truck? In that case there are both upstream and downstream O2 sensors, and a bad cat should throw a code.

A faulty injector or coil should also throw a code if it interferes with engine performance enough to cause gross pollution.

From your symptoms, I would guess it's not going into closed loop operation.

 

Misfire is another cause of excess CO and HC. You should be able to hear that at test speed (2500 rpm). Misfire doesn't directly throw a code (modern vehicles use a crank velocity measurement to detect that ). It's hard for the O2 sensor to "catch up" with the excess HC by playing with mixture, because the misfire is intermittent. Therefore, the tailpipe gases could have excess HC and CO, but your ECM thinks it's keeping up with fuel trim (so no code).