can having a 4 foot vacuum line to your MAP sensor cause inaccurate readings? I am asking this because i was going to mount my MAP sensor inside the car since i have swapped to SD from MAF and it was just easier to run one vacuum line out to the engine and 3 short wires for the MAP sensor. Only thing is, my MAP sensor is reading 66 KPA at idle, but when i give it light throttle it will drop down to like 40KPA where it would appear it should be. Im going to check for vacuum leaks tomorrow, and see how much vacuum im pulling at idle, but i really dont think i have a leak. the cam in the car is comp XR276 cam
thanks alot

 

Something is awry. You have more manifold pressure at idle than you do at part throttle? That doesn't add up.

Did you have to mess with the MAP sensor wiring? Perhaps you got the voltage and ground wires mixed up? There is a 5V reference voltage that the ECM sends out from terminal A4 (gray wire). That reference voltage is then changed by the sensor to a lower voltage depending on the manifold pressure. It sends that new voltage back to the ECM at terminal F15 (Light green). The black wire is obviously sensor ground (specifically sensor ground, NOT general chassis ground) and goes to terminal B5.

Based on this, go back and inspect the wiring to make sure you don't have anything backwards.

 

Something is awry. You have more manifold pressure at idle than you do at part throttle? That doesn't add up.

Did you have to mess with the MAP sensor wiring? Perhaps you got the voltage and ground wires mixed up? There is a 5V reference voltage that the ECM sends out from terminal A4 (gray wire). That reference voltage is then changed by the sensor to a lower voltage depending on the manifold pressure. It sends that new voltage back to the ECM at terminal F15 (Light green). The black wire is obviously sensor ground (specifically sensor ground, NOT general chassis ground) and goes to terminal B5.

Basically, with words, here is the MAP sensor hook up.

ECM A4 to MAP sensor C

ECM B5 to MAP sensor A

ECM F15 to MAP sensor B

Based on this, go back and inspect the wiring to make sure you don't have anything backwards.

 

are you 100% positive about that...i have mine in B6 and im not throwing any kind of trouble codes...i hope your right and im wrong

 

Positive. I have the schematic right in front of me.

Actually, B6 is sensor ground as well. Which is why the computer probably doesn't notice it. B6 is sensor ground for the throttle position sensor and the coolant temp sensor. B5 shares its ground with the MAP sensor and the MAT sensor. All you're doing is running a little more current through the circuit than was intended. Not sure if that's a problem. These sensors require very very low current to function.

 

about the MAP readings at idle, i hooked a vacuum gauge up to the car today and found out that i have 13 inches of vacuum at idle but when i rev it up a little to like 1500 rpm or so, it will cimb to about 20 and really smooth itself out....im gonna check for vacuum leaks tomorrow, but is there anything else that could cause this?
thanks again

 

I think I see what you're saying now. You're right that the vacuum will hold steady at a higher RPM when in neutral and a constant RPM. I thought you were talking about under part throttle load while driving.

What is the duration @.050 and lobe separation angle on your cam? 13 inHg may be normal for it.

 

@.050 its 224/230 and the lobe seperation angle is 112*

the comp part # is XR 276 HR-12

i would have just figured that i would have a little more vacuum at idle than i do. if not, i guess i just have a lot of tuning to do
VE master here i come

 

by the way, thanks for replying so fast to all of these posts i have been having lately...you've really been helping me out

 

Not at all. Always happy to help.

With that cam, I'm not surprised that you're only getting 13 inHg at idle speeds. It's a fairly stout cam for the street.

You definitely have some tuning in your future!

Curious though, what other mods do you have? You picked a somewhat radical cam for a TPI engine (I'm assuming it's TPI). If it's otherwise stock, you may have too big a cam.

 

AFR 195's, stealth ram, 30# injectors, hooker long tubes, mufflex y pipe(sucks), built tranny with 2800 stall, 3.73 rear.....

i just swapped from MAF to SD and im just learning to tune...i really wanna get this thing running ASAP, but i know this isnt something that i can do in one day. its just that the weather might (if it ever stops raining here) get nice and i would want to drive my car

 

Ahhhhhhhh, very nice. No we're talking!


i just swapped from MAF to SD and im just learning to tune...i really wanna get this thing running ASAP, but i know this isnt something that i can do in one day. its just that the weather might (if it ever stops raining here) get nice and i would want to drive my car [/QUOTE]

Sounds like you simply need to start with the idle. If you haven't already, go into the constants and change your injector flow rate to 30#. This alone should help out a little. If you're reduced your fuel pressure to compensate for the bigger injectors, put it back to stock once you've altered the value in the calibration.

If you don't already have one, fo yourself a get a good scanner. AutoXray's has the ability to record data and download it to a PC for further analysis. It'll be immensely helpful in tuning the car. Also, make small changes at a time that way you always no what exactly has changed with each iteration.

 

Ahhhhhhhh, very nice. Now we're talking!

Sounds like you simply need to start with the idle. If you haven't already, go into the constants and change your injector flow rate to 30#. This alone should help out a little. If you're reduced your fuel pressure to compensate for the bigger injectors, put it back to stock once you've altered the value in the calibration.

If you don't already have one, do yourself a favor and get a good scanner. AutoXray's has the ability to record data and download it to a PC for further analysis. It'll be immensely helpful in tuning the car. Also, make small changes at a time that way you always no what exactly has changed with each iteration.

Take a look at the DIY PROM section. Lots of good info and people to ask questions of.