I have a 1994 Corvette with just over 170,000 miles. I have been doing my best to keep up with maintenance and replacing parts as needed for the past two years, but I have had a recurring problem of Lean Oxygen Sensor Codes. Twice on the driver and Once on the passenger side (each time replaced oxygen sensor and had no problems for a long time).

However, I replaced the Driver's side O2 sensor just 3000 miles ago and the code is back again.. The last time I replaced it, I also replaced the fuel pump, fuel injectors, EGR valve, PCV valve, cleaned the MAF sensor and throttle body, and the car has been running better than ever since. I was having symptoms of fuel injector leak, so I suspected that to be the cause of the rapid fouling of the sensor. I also replaced a leaking oil pressure switch sensor that I thought may have fouled the sensor from the outside.

I just recently had the transmission rebuilt and have replaced the fuel filter and spark plugs within the last two years (<10,000 miles). The car has been running great for the past few months until a couple days ago, when I knew the oxygen sensor would fail again. Idle got rough. Hesitation on WOT. And rich smelling exhaust started again. It drives okay until it gets up to temperature, then the code comes on (4. H44)

Today, I pulled out the MAF to clean it again just in case, and I will clean the throttle body too. I need to check the fuel pressure again, but it has not been a problem since I replaced the fuel pump not too long ago. The only thing I can think that is left is a vacuum, Exhaust recirculation leak, or exhaust manifold leak that is evading me. My plan is to get it tested at an exhaust shop tomorrow.

Is there anything else I am missing or that I should try first? I am trying to do as much as I can myself to learn about the car and to save money. Thanks for any help. Your advice in other posts have helped me out so much already,

Evan

Latest Update: I have replaced both oxygen sensors, which were reading lean due to being covered with carbon. I have been trying to find the source of the problem causing that to happen. I had a failing evaporator purge valve solenoid that did not hold vacuum. So I replaced that and eliminated that vacuum leak. I have no more lean condition engine codes and the car is running better, but I am not confident that I do not have other leaks. I am trying to learn about EGR to make sure that is not another source of a leak. I have received mixed messages about how to test the EGR valve and solenoid. I was told today by someone that the valve will NOT hold vacuum when the car is off or at idle, which is contrary to what I have read elsewhere. Testing the solenoid that I have versus a new one I bought, both leak slowly without power and are wide open (holds no vacuum) with 12 volts. So I figured mine was functioning normally (it clicks on), and returned the new one.

I would be very grateful to anyone who can teach me to test/diagnose the EGR system on this car. I will update when I find anything new!

 

What kind of o2 are you useing I put bosch in my 94 so far so good.

 

Ah, yeah I have used Bosch each time as well. Despite having only a few thousand miles on the previous one, it was covered in black soot when I removed it. I'm not sure if that is normal or a sign of running rich, but it was that way on both driver and passenger sensors. By the way, I only use 93 octane from Costco or Shell unless I'm on a long trip and there just isn't one nearby.

 

My 94 is having the same issue, will follow this one closely!

 

I would check the fuel pressure and see if it leaks down. Harbor frieght has a gauge for $20 that has a long enough hose to tape to the windshield.

 

I'll check it and report back soon. Thanks

 

Give all your spark plug wires a good look and push to make sure they are seated.

 

The following link may help:
http://www.corvetteactioncenter.com/...ning-lean.html

Replacing a faulty evaporator purge solenoid, and leaking vacuum hoses solved a lean condition (post #5).

 

Alright I just checked it, and the fuel pressure looks good. The pressure holds at just over 40 at ignition and drops down to 35-37 running and holds that pressure under load.

UPDATE: I went to an exhaust shop and they were unable to hear a leak coming from the exhaust manifold. I was told to check it tomorrow morning when it is dead cold, and I will do that. I was also told to look into the Air Circulation system to see if maybe it is the check valve. I am not sure how to test that though.

They also pointed out to me this potential vacuum leak that could be causing a lean condition (see picture).

 

Is there a way I can test it before replacing it?

 

How long does the pressure hold once you shut it off?

 

I found this as a way to test it. I can try that tonight. http://www.ehow.com/how_7235739_test-egr-solenoid.html

 

I waited about 10-15 minutes after just turning the key and it only dropped down by about 5.

 

If I remember right mine lasted alot longer might have a leaky fpr or injector. Do it again and wait a hour post this and see what others say. Give your oil a sniff.

 

Should I do it when the car is cold or warm? Does it matter? I just replaced the injectors because I thought those were leaking, so it shouldn't be that. The fuel pump is new too. I have clamps so I could test the FPR if it does drop down too fast. If that's good that should rule out the fuel system, I think.

 

Does not matter seems to me mine held over night but my mind is not what it used to be.

 

Does anyone know what this white vacuum thing is? Is it supposed to have a vacuum line connected to it? This on the driver's side air injection hose near the firewall.

 

No but mine has nothing on it either

 

Thats too low under load. Key on engine off pressure should be the same as under load/vacuum hose off regulator.

 

Let me make sure I am testing this right. So I need to pull the vacuum hose off the fuel pressure regulator then open the throttle and watch the gauge?