i was out a few weeks ago and shredded both my belts... ran it as far as i could on just the battery juice... got towed home... put new belts on... started it up, crappy idle, just crappy everything! turns out the alternator went bad at the same time. got that replaced and pullin plenty of volts now. but still just all around crappy. too afraid to even take it out of the driveway. before this outing, it was running great!!

i hooked up the laptop, and its telling me in pig rich!! with my motor (350H.O.) im running a 350 tbi at 15 psi, there's just no way that it could be. i have changed nothing in the tune. the only thing that's changed is a new alt.

im trying to figure out what i need to be checking out that could of gone bad....
can a new alt cause any of this?

 

Well 15 PSI is higher than stock 13 psi so it could be rich... but theres a lot to know about the motor and whatnot to be sure... are there any error codes stored? Check engine light on? If the charging system is charging 17 volts than yes it would cause all sorts of havoc to a EFI system.

 

thanks for the reply! its pullin 14.2 volts right now.

from my calcs with the formulas on this site i should have to run higher than 15 psi with my size injectors to feed my motor to full potential, but im not worried about full power right now.

no error codes. it'll pop up with a high map, code 33 i believe it was. but only once in a while.

 

A failing MAP sensor (or other intake leak) will result in the computer adding unnecessary fuel. The computer thinks the engine is under load & adds fuel.

 

Yup, high MAP = low vacuum. Low Vacuum means a load = more fuel...

What are the cam specs on this?

 

cam specs: .435 in. intake lift and .460 in exhaust lift with duration at .050 in. for intake is 212 and 222. a vacuum leak is not monitored air, so the o2 sensors seein extra air and tryin to drop the injector time on... correct? how about a header gasket leak? i've dropped my pressure all the way down to 11.5 psi just to see if it would stop reading so rich, and it helped, but its still trying to pull out fuel... my motor would be starving normally for fuel with that little of pressure. thanks guys for helping me out with this!

 

No, the vacuum leak would cause low vacuum on the MAP but the air would be burned before O2 sensor.

That cam should be fine but need a chip tuned for it especally at idle. What is the vacuum reading at idle?

Header leak is definatly not good for an O2 sensor...

Do you have a vacuum pump? If so hook it to the MAP and pump it down to 19 inches of vacuum and see if that fixes the idle rich condition.

 

i have stuff for tuning chips. no vacuum pump tho. it will usually idle in the 40-50 area in the blm chart. but now its all over the place

 

"a vacuum leak is not monitored air"
For a MAF yes directly, SD is a bit different.

Lack of vacuum will cause the computer to add unneeded fuel due to being in an incorrect position of the fuel map (richer), the system fights itself with a rich o2 report.

Quite different.

Extra o2 in the exhaust will report to the computer a lean condition, it will try to add unneeded fuel.

These are completely different conditions with a similar result,
runs like crap & stinks of fuel.

Add them together & your engine will throw a fit lol.

 

im just running a map system so it is unmonitored, but that makes sense what you said in that if the o2 sees more air, its going to add fuel to even itself out. not decrease it which mine is trying to take fuel out. sorry, i get confused trying to follow all these routines sometimes.

so you mentioned about the ecm compensating by adding fuel in those to examples, but what about taking it out??

 

The system fights itself,..the rich o2 driving BLM/INT down, while low vac (high KPA) adds too much fuel because it's in the wrong part of the fuel map (LV8 to some).

 

ok i can follow that. so are we starting to lean toward the map sensor? maybe go rent a vacuum pump and see what happens?

just coincidence that it could of gone bad at the same time as the belts and alt going...

would driving with low volts have screwed stuff up?

 

I think age & excess heat may have led to MAP sensor failure.

Low volts are NEVER good for any IC but the map sensor is not the most fragile or vulnerable component of the system, more-so rather isolated and durable in that respect.

 

hmmm, cuz my map sensor is only about 2 years old... i think ill just rent a pump and see how she wants to run with that hooked up.

thanks a bunch guys so far for all your thoughts! by all means if you think of somethin post it up for me to look at