Long story short: My car failed the emissions test for HC's many times over the legal limit. My 13MPG also reflects this condition. The car failed with the following parts replaced:

Distributor cap and rotor
Plug Wires
Spark Plugs, appropriately gapped
Oxygen sensor

I have looked over the engine many times and do not see or hear a vacuum leak.
The engine idles fine, and only started throwing a code for running rich when I replaced the O2 sensor.
I suspected, since the car sits for many months out of the year, leaky fuel injectors due to sitting for months in old fuel. So I removed them to have them professionally cleaned. The technician pressurized them and reported no leakage although he said that they were heavily varnished and unbalanced as far as fuel flow. Would this result in a rich condition?

While I have the engine apart, is there anything else I can do to try and track down the problem or are there any other areas to look at. I cleaned the massive amounts of carbon out of the plenum, runners, and throttle body, so that can do nothing but help.

An additional symptom i'm having that i am sure is related to my rich condition is as follows:

In neutral, when slowly and gradually depressing the accelerator, there is a very narrow range of throttle position, slightly below 2000 RPM, where when held there, the engine will sputter and die. I have checked the TPS sensor with a voltmeter and it reads properly through the sensor's entire range and is correctly set, as is the IAC.

My thoughts on the problem, under the assumption that the injectors were not properly operating, was that at that throttle position, the injectors were firing at a certain frequency, where they reached something like a resonating point, causing them to flood the engine with fuel, thus stalling it. Or perhaps that the heavy varnish in the injectors was causing them to operate slowly allowing too much fuel to pass.

Anybody's thoughts on this are greatly appreciated. Thanks for reading.

 

Check the EGR system. EGR valve, egr solenoid, vacuum hoses to solenoid, wires to egr solenoid. Replace the PCV valve and air filter if not done already.

 

Thanks for the reply.

PCV valves and air filter are recent. EGR solenoid was replaced, egr valve operation has been observed during engine operation, and by manual engagment by applying vacuum to the hose. Vacuum was also applied to the Fuel pressure regulator, and it held.

Any chance it could have been the injectors?

This problem is so strange, especially with the dead throttle spot.

I am purchasing an ALDL cable so that I can datalog to get a clear picture of what is happening. Clearly the basics are not working. Although any other suggestions are welcome.

 

I was having the same problem, ended up being a host of problems, but mainly the EGR valve and a bad (although only 7,000 miles) cap/rotor. Do a search on my name if interested.

 

Although not as common: restricted exhaust/clogged cat converter will cause an uncorrectable rich condition.
Easy enough to test: WOT horsepower test between 3500 and 4500 rpm.
It will run like crap if it's clogged/restricted.
(or just kick the pipe loose).

 

I would tend suspect the cats as well, except that the exhaust system is very solid, and sounds fine. Additionally, the engine runs fine at WOT or any other throttle position in the the upper RPM range.

I would say that drivability is barely affected by this rich condition, although it is most noticable in the <2000 range where a slight "chugging/rough running" for lack of better words, is felt. Although I think that this range corresponds with the previously mentioned dead throttle spot.

As far as the EGR valve, I have probably done the most thorough job verifying its proper operation. So I am fairly certain that it is not the problem.

Additional parts that have been replaced are the ECM and memcal chip, in the process of troubleshooting a code 42, which ended up being a knock sensor wire. The MAP sensor was also replaced.

If the problems do not subside with the cleaned fuel injectors, my next troubleshooting angle will be to verify that the EVAP system is not dumping fuel into the engine which I have heard, from TGO, can happen. Although there is no smell of fuel comming from the engine bay, but until I can datalog, I have no further routes to pursue, except those suggested to me.

 

Well, I'm watching this with interest (again). Although the cap/rotor made the motor smoother, its still too rich and has symptoms much like yours (rough below 2k). Took it to work today and got a code 45 (rich exhaust). I can't think of what else to try!!!

https://www.thirdgen.org/forums/tech...ml#post3421564

https://www.thirdgen.org/forums/tpi/...ml#post3427921

Good luck with yours!

 

Here is an update:

I talked again with Accurate Injectors, who are servicing my injectors.

While they don't leak, I was told that they had to sit in a solvent to dissolve all of the rust out of the top. Additionally, they told me that two of the injectors were bad, reading 6 ohms of resistance. The last time I tested them, they all were normal. Although I trust their findings, since I cannot imagine what else the problems could be. As explained to me by accurate, comfirming my theory, the low resistance could cause slow action of the injector, allowing for too much fuel enter the engine.

In any case, they are replacing the 2 bad injectors and flow balencing all 8. He also told me that the injectors were 21 lb injectors. I was under the impression that my car had 19lb injectors. Is this normal?

I'll update this post in a week at the bare minimum after I reinstall everything but will check back for replys.

 

As a followup to my statement about injector size, the majority of the posts on this board lead to the conclusion that all 305's had 19# injectors stock. Except for this post:

https://www.thirdgen.org/forums/tpi/...ize-305-a.html

Which says that the subject car, which is identical to mine (year and drivetrain wise), came with the 22lb injector. I am farily certain my injectors are original, but incorrectly sized injectors would be the root cause of my rich condition since my ECM has not been modified to compensate. So I am slightly confused.

I have heard through various sources that my TPI 305 engine with the Manual transmission came with a higher performance camshaft than the AT cars. Perhaps the higher flowrate injectors were paired with this camshaft. I would like to believe that the injector professional knows what he is talking about. But if anybody could confirm, without a doubt, which injector size is appropriate for my car, I would appreciate it. If I have to buy an AFPR, i'd rather install it while I have everything apart.

I tried looking the injector PN up on GMpartsdirect.com. The search was not thorough enough to come up with the exact number.

 

To conclude. It turned out that the injectors were the problem. The engine runs beautifuly now.