Ugh, got it running, now I've got codes!

[scorpion66]

CEL light came on yesterday, after putting about 20-25 miles on the car since getting it back from the shop.

I did the on-off method for retreiving codes and its showing
12, 51, 55

I know that 12 is because the batter has been disconnected recently, and I know 55 is the end of the message.

All I know about 51 is that it has something to do with the fuel mixture being lean.


I'm not having any trouble accelerating, slowly or full throttle, no rough idle, no excess engine vibrations at any particular speed between 10 and 70.
If any of you remember my 9 month long no start post then you know that I replaced the ECU to solve the problem.

I do not remember any CEL codes before the no start issue, so I'm going out on a limb and saying that this "fuel mixture being too lean", was either caused by the new ECU (unlikely) or something that happened while the car was sitting. I could be way off base, who knows.

Anyway, where should I start looking for a problem? And does that code 51 only speak of fuel problems, or is air also a potential culprit?

If it was the fuel pump, wouldn't I notice performance issues, sluggish acceleration, or rough idles? is it possibly the 9 month old gas in the tank causing issues? (was less than an 1/8th of a tank, and I added 1/4 to it before driving it home from the shop)

I'm at a loss here of where to start, and I can't afford to put the car back in the shop again. I'd however hate to keep running the car with something wrong, and cause more damage. I however may run over to autozone to have them pull the code to see if it can give me any more details. In the meantime, any suggestions, thoughts, or even questions are welcome.

 

[luv2rallye]

Could be a dozen different things but here are the top 4:
Engine coolant temp sensor
Intake air temp sensor
Exhaust manifold cracked
Head gasket failed

 

[scorpion66]

Engine coolant temp sensor
Intake air temp sensor - possible to test these in any way, or am i going to have to take it to the shop to diagnose?


Exhaust manifold cracked - any way of checking for this? If this is it, is it worth fixing? that is, will it do any damage to anything? I plan on making a lot of changes under the hood, including a turbo, so this will probably end up getting replaced anyway. if its not going to do any damage to leave it be for now, why waste my money?


Head gasket failed - don't think this is it, just had a compression test when it was in the shop and everything seemed normal. That wouldn't have been the case if the gasket is/was failing correct?



If I had to guess, I'm going to say that the IAT sensor is the culprit. Before the no-start problem, the fella I bought the car from had one of those cheap aftermarket things installed to override the IAT. Was supposed to increase perfomance by tricking the ECU into thinking the temperature was colder than it really was. I ended up tearing that off and reconnecting the IAT after not being happy with the effect of the aftermarket part, plus it was an eyesore.

 

[luv2rallye]

If compression good head gasket should be ok.

To test N/T engine coolant temp sensor:
Measure the resistance of that 2 wire engine temp sensor near radiator cap. Should be 9k-11k ohms at 77F and 600-800 ohms at 212F. You can also check it's operation by removing it and put in hot/cold water to see if resistance drops/rises.

To test N/T intake air temp sensor:
Disconnect intake air temp sensor connector and measure resistance between the sensor terminals. Should be 9k-11k ohms at 77F and 600-800 ohms at 212F. You can try heating it with a hair dryer to see if resistance changes.