1990 Coupe. Engine runs perfect once normal operating temperatures are reached. However, about a minute after start-up from a cold engine (typically in the morning on the way to work after the car has sat overnight) I get a Code 44 and an extremely rough idle, to the point of almost stalling. I also get a decent amount of black smoke out of the exhaust. This all happens until temperatures reach about 210-220 and then, almost as if the car trouble god snaps his fingers, the issue dissapears and everything is fine.

I've asked around and from what the local mechanics have told me, the car senses, for whatever reason, that there is a lean mixture (hence the Code 44) and overly richens the fuel mixture causing the rough idle and black smoke. I've heard everything from O2 sensors to valve seals.

I just got the car back from a week long stay at a local auto shop which claimed to have solved the issue. Not the case. At the shop they replaced spark plugs, EGR valve, Induction cleaning, Throttle body cleaning and did some work in the ECM. I plan to take the car back, but would like to try to narrow this issue down so I can give them a place to start looking.

 

Bump?

 

Just went through this. My IAC (Idle Air Control) motor was stuck. $22.00 part from auto zone.

 

But if the IAC was stuck, wouldnt there be a rough idle at all times, rather than simply dissapearing once the engine warms up?

 

My advice would be to find a different shop that knows Corvettes. I would like to know what ECM work they might have done and how they did Induction, and Throttle body cleaning. Code #44: Lean Exhaust indication could be caused by a number of things. If you have the OEM injectors the first thing I would do is a resistance check with a Ohm meter.

 

I agree, find a different shop that has the Proper Equipment and Experience to properly diagnose an engine.

 

have a similar problem on my 91 and I'm almost sure it's the O2 sensor.

My understanding is if the sensor is bad the ECM puts the engine into "closed loop" mode until reaching operating temperature.

I have my new sensor it was cheap enough (I hear not to use Bosch) just haven't gotten around to crawling underneath the car and replacing it yet...

 

Bump/update

Issue seems to be getting progressively worse.

Replaced the O2 sensor about 2 weeks ago with a Bosch. Almost no change at all, if anything it made it worse. After replacing the sensor, if I went WOT while the car was in "stupid mode", as I call it, the engine would work its way up (slowly) to a 4,000 RPM rev limiter where it would then make a loud bogging/growling sound and emit a thick cloud of deep black smoke out of the exhaust. I've seen turbo diesels with less coal train ability than this thing.

The car was also deciding to jump back into stupid mode at random times after engine temperatures had reached normal operating level, which previously was smooth sailing. Typically this happened after going from Highway speeds to a stop light where engine temps were sub 200 degrees.

Today, after reading that the Bosch sensors are somewhat unreliable, I switched to a Denso. I drove the car around for a bit to let the exhaust expand and to let the car jump out of stupid mode and then ramped the car and switched out the sensor. This took no more than 10 minutes. While I was underneath I noticed that someone had bunched the O2 wire up next to the oil filter and zip tied it off very tightly and that it is now covered in a light coat of oil that appears to be coming from the rear main seal. YAY! A few cat inflicted third degree burns later, I started her up and she was once again in stupid mode (I'm guessing from the cold sensor) and now instead of the Bosch 4k RPM limit, I simply steam locomotive all the way to the factory rev limiter, stumbling and growling the whole way. Yet, just like before, once the sensor warmed up everything was fine.

So far I've replaced spark plugs, EGR, Ran Seafoam, and had the throttle body cleaned. The ECM is the only thing I can think of that's left. IAC was mentioned as well. Is there an easy way to test it? It seems to hold the stupid mode idle of about 550-600 rpm well enough even if I cycle the A/C on and off. I'm kind of worried about the bunched up, oil covered sensor wire and it possibly being shorted out or something of the sort. I didnt want to tug it out of its hole as I wasn't sure how brittle it was.

I don't have the cash to simply throw parts at the car as my wallet and Corvette have become mortal enemies and a clutch replacement is coming up very quickly. I do have access to an ohmmeter and just about any tool you can think of. Is there anything I can test myself to try to narrow this down? Will Ohm the injectors tomorrow. Should I be worried about this turning into an engine rebuild if I don't get it fixed soon?

 

I would also check the coolant temp sensor,good luck

 

It sounds like a heat (or lack of heat) related like the coolant temp sensor. Since the car runs great after it is warmed up at would suspect that ohming the injectors is a waste of time because the do work correctly given the warmed up temp. It could possibly be a throttle position sensor that reacts correctly when it is warm but it is a long shot. IAC could be suspect as previously stated.
I'm sure it will be something simple and obvious once you figure it out. Think of the sensors that effect the computer related mixture and it should get your problem solved.
Barry

 

I'll give the coolant temp sensor a try since it's relatively cheap. Where exactly is it located? I'm guessing I'll need to drain atleast some of the coolant before pulling it out.

 

you removed and cleaned the IAC?

I think the coolant temp sensor (on a '91 anyway) is on the upper front of the block...