86HOGT

You have to check for vacuum leaks and exhaust leaks. Check the upper plenum gasket, the lower gaskets, check the vacuum tree, EVP solenoids, TAB and TAD solenoid, and all those components' vacuum lines. Another thing could be that you have a broken signal wire from the O2 harness which will make the computer see 0V which is lean.

90Mustangfan

will small vaccum leaks cause it to run lean? and how do I check If the signals are good between the O2 sensors and the comp.

fastmfer

Hey i had the same problem in my 1994 cobra. Took it to autozone let um run the test thing said my o2 sensors were reading lena and my maf was bad. Turn out to be the fuel pump the whole time. it maybe the little filter liek thing on the end of ur pump stopped up or ur fuel filter.
I repleaced my pump and the check engine light hasn't came on since.

lunchbox99

If your CAI tube is leaking the MAF sensor isnt getting a true reading. Could cause it to run lean. If your egr is stuck open, this also could cause it to run lean, setting an egr code. ??? Just an idea

86HOGT

Yes, even small leaks will cause lean codes, I've seen it. Also, if your BAP sensor is reading the wrong altitude, like 135Hz, this will also make it go lean. See this all the time on newer cars where the BAP sensor is built into the MAF sensor.

signal20gsxr

I had a similar problem on my 87 GT I was chasing a code 41 forever. turn out to be the computer.

90Mustangfan

For me its always codes 41 and 91......and 34....heres what I did yesterday, I redid all the vaccum....I mean I replaced all the vaccum hoses....while doing taht I noticed that the vacum hose that goes into the passenger fenderwall wasnt connected to the vaccum storage container......now the heres the new question, my egr still doesnt get vaccum....but when I hook the vaccum hose that has full vaccum directly to the egr valve, it opens the egr valve completely and then the car runs like it should, it has a nice lopey idle the way it should sound with that cam... except the car bucks and studders until 1500-rpm, witch I think the cam was designed to run in rpm range 1500-6500rpm....The car sound healthy like that, it doesnt sound like it running weak, it sound strong and powerful....New question too, when should the egr valve be open(idle, Wot, half throttle, part throttle)??? I think my soloniod is bad, because it never lets vacum to the egr valve...can somone explain when the egr should open and close?

stanglx2002

the EGR opens at idle and part throttle and closes at WOT

90Mustangfan

is it completely open at idle? and or part way....how can I checkif the egr valve is bad....one thing i noticed is that the egr valve is sticking open sometimes( if i manualy put vaccum on the hose to the egr valve?