cliffyk

Are you running the stock returnless system?

If so, the returnless fuel system maintains 39 to 40 psi across the injectors, by changing the fuel rail pressure to be 39 psi plus the manufold pressure (negative pressure, vacuum). At idle manifold vacuum will be 18 to 22 inHg or -9 to -11 psi, so the rail pressure will be 29 to 31 psi.

Your 26 to 35 psi figure is a bit low, however gauge accuracy may be an issue.

The scan tool was likely reading the generic fuel pressure PID 010A which will always report the pressure across the injectors--it will be around 39 psi at all times, however you may see higher and lower readings as the PCM makes no attempt to filter values that appear during the feedback processing.

compgt10

yea i'm using the stock returnless system we used an actron scanner.... if that gauge was reading correctly what would you say the problem is?

cliffyk

How many miles on the car? The rail pressure is a bit of a concern, however that the scan tool says 39 psi like it should means it is likely there's enough pressure there to idle the engine. Does the car accelerate well and run smoothly at cruising speeds--if yes it is unlikely the fuel pump is a problem.

Have you disconnected the battery for 5-7 minutes to clear the Keep Alive Memory (KAM)--this will force the PCM to learn new idle air trims.

What happens when you unplug the IAC while the engine is warmed up and running? The idle speed should drop and perhaps even stall--if it stays the same then the IAC is bad.

Has anyone played with the throttle stop screw (sometimes called incorrectly the "idle adjustment screw")? If so it will need to be reset, here's how.

Have you done a compression test, if so were the pressures within 10% or one another?

Have you checked the vacuum with the engine warm and idling? It should be steady at 18 to 22 inHg, if not it could be vacuum leak or sticking valve(s).

Bman2000

hey cliffk the car will run with the maf disconnected wont it? on just some pre determined numbers right? i could be wrong but im thinking ive done that before, just wanted to know

cliffyk

Yes, with a failed MAF the PCM will go into a "limp mode" and use load values pulled from the Load w/Failed MAF table. On the later new-edge models, with the 6-wire MAF plug, you also lose the IAT measurements when the MAF is unplugged.

Normally the load is calculated using the MAF flow, the Standard Air Charge scalar, and a few other inputs and settings--with the MAF being the prime mover. When the MAF fails the Load w/MAF Failed table lets the PCM look up a "guesstimated" load value based on throttle position and RPM--and use that value to calculate injector pulse width, timing, etc.

There are also other settings in the tune that define "limp mode" operational parameters.

Bman2000

is that an effective way to determine a faulty MAF? assuming of course its causing irratic problems that could be solved by at least getting the readings to become more stationary, meaning the fixed limp mode tables. Or is there a better way to test the MAF?

cliffyk

The MAF can be best tested by monitoring it's output voltage, or by monitoring the mode 01 generic PID 10 (PID 0110, normally represented in g/s) with a data logger.--and comparing the observations with what one would expect to be reasonable values.

The math to determine the expected values is not complex, but is quite likely more than anyone cares to read here--I already get chastised for being overly technical so I will let it rest at that.

Basically however, if the MAF is producing outputs that seem to make sense vs, throttle position, rpm, and load, then it's probably OK. It's a simple device with little to go wrong other than contamination that would affect the thermal transfer properties of the hot wire and sensor, or outright mechanical failure.

Bman2000

people who complain about you being overly technical are the same people who ask a doctor whats wrong and go can you say that in plain english please? just means they have no clue what your talking about therefore they choose it better to not hear it. I personally appreciate your extensive knowledge. everyone wants a simple answer, but without someone who knows what they are doing right next to you.....you pretty much have to be told everything so that you know what your looking at.


Its basically similar to an O2 sensor voltage monitering though right? its been forever since i learned this stuff, but when warmed up they are suposed to run around 600ish milli amps? i cant remember lol but similar idea right? it has a range that its suposed to be operating in.

cliffyk

I got to thinking about this, and fact is that in the 20 years or so that MAFs have been around I have never seen a failed MAF that could not be identified readily from visual observation--I.e. physical damage and/or gross contamination that could not be corrected.

I am sure it's happened but I've never seen it...

compgt10

the car has a SLIGHT shudder when i accelerate from a stop... the shudder lasts about half a second after that the car runs smooth at cruising speeds...my teacher explained to me also that due to the power of the motor itself i may not notice any shudder at speeds, we have noticed that the problem increases when we run the motor around 2000 RPM's

we have disconnected the battery for periods of time when i installed new 02 sensors as well as when i installed a new 70mm TB and CAI (the problem occurd prior to that installation)
I have yet to unplug the IAC or do a compression test (i'm going to perform both later on today)

we did a vacuum test and the vacuum is running normal