Having experience the problem a couple times myself (car seems to be acting sluggish recently - again), I've been keeping an eye out for cars with said problem on the dyno.

Recently, a S2Ki'er from out of state brought his car in for a VAFC tune. Making the baseline run, the car was performing poorly. The peak power wasn't too far off, nor was the AFR, but the car was missing, hesitating and generally running poorly. The torque curve was off by 15 lbs-ft down low, 10 lbs-ft through the midrange and another 5-10 lbs-ft after 8300 rpm.

At the time I thought it might be a VAFC install problem. I started watching my scantool closely and then I saw it - at full throttle the map sensor was reading nearly 1 psi above atmospheric pressure! Now I remembered where I'd seen a dyno curve like this customer's - it was on a car where someone had tried to add fuel with a VAFC. This is a bad idea as the ECU perceives the attempt as seeing positive manifold pressure, and proceeds to go into a limp mode, costing lots of power.

So I pulled 5% fuel out across the board (a normal starting point for a VAFC on an otherwise stock car). Voila - the map readings were now within a normal range and the hp curve now resembled a stock car (perhaps a little better). The rest of the tuning went without incident, although the customer may need to reduce the VAFC settings a bit when the MAP is replaced (if that is confirmed to be the issue).

Now, this is not conclusive data yet, but it would seem that the MAP issue may be caused by the sensor getting stuck in a position that mimics positive pressure (a little vibration, gummed with oil, who knows?), and the ECU behaves accordingly.

Not that it solves anyone's MAP problems, but it may be useful to understand the pathology behind the problem

UL

 

Sounds like a problem I had or am still having. I replaced my Map sensor and my check engine light still came on and the car runs sluggish only when the humidity is near 95%. Other than that the car runs ok when its cooler. When I say cooler, thats everyday weather in California...

Ultimate Lurker do you think its possible when I get back to California I can have you take a look at my car and VAFC2 settings? I am just running the usual bolt ons, I/E/H/ VAFC. Thanks again for the information..

 

Seeing slight positive pressure on the MAP sensor could be due to a faulty MAP sensor, but it could also be due to resonance effects in the intake or exhaust tracts.

I've done a lot of logging with my AEM ECU, using several different intake and exhaust designs. I log for half an hour of high speed highway driving, varying both load (mainly throttle position) and RPM. Then, using AEMLog, I plot the XY graph of Engine Load (MAP reading) vs. Engine Speed (RPM). The perimeter of the graphed points gives the extremes of manifold pressure. I've done this several times on each arrangement.

With one design of intake piping between a large cone filter and the throttle body, I found manifold pressure between 5500 and 6000 RPM as high as 0.6 psi positive. (The manifold pressure at 8500, however, was about 0.6 psi negative.) I'm now running an intake which shows a bit over 0.1 psi positive in midrange and only 0.1 psi negative at red line. I'm not really sure which is better, though they do, of course, need different fuelling.

I also found that changing my exhaust system had an effect on the maximum pressure seen by the MAP sensor -- especially when on the high-speed cam. That's really not surprising; it's what intake/cam/exhaust tuning is all about.

This does suggest that changing out either the intake or exhaust while using the factory ECU can only be guaranteed safe if the new intake and exhaust are no improvement over the stock versions.

 

UL,

since the car went into limp mode was there a CEL indicated and what code. I believe I have experienced this too, but the CEL I received was p1259, which was a vtec solenoid control problem. The obdII tool stated "manufacture fuel air sensor", but looking at the readings on the map in mmHg reading it was approaching a positive value. Just curious...thanks.

 

No limp mode indication outside of the poor running, and I've never seen a MAP problem trigger a check engine light. Its almost as though the ECU just treats it as an unusual occurence.

In the case of the car in question, it had a completely stock intake, which is pretty much devoid of resonances. On CAI's you will see half a psi pretty consistently, but 1 psi of positive pressure! Zoiks!

UL

 

UL,
I have some anecdotal evidence to support your observations. I have been complaining for sometime now that my car was "not running right" i.e. just as you described and dynoed 5-7 rwhp understock. A buddy of mine is not using his car at the moment, so I switched MAP sensors....no more problems.

 

Here is another food for thought...

My car is completely stock except for gauges. The defi bf boost gauge has a memory function where once in a while the max will go past atmospheric pressure where it ranges from ~.2 to ~1.5psi till I reset it again.

 

just for reference, the car UL is talking about is mine. my dyno chart was crazy erratic before he cleaned it up with the VAFC. i've only had the car since February, so i wasnt really aware that anything was "wrong". i checked all the wires where i'd previously tried hardwiring the VAFC, everything was covered in soider so that was fine. last night i did the "MAP-whack" and the car did seem a bit peppier this morning (but that could be placebo effect).

i'll try to switch out the MAP this weekend with a friends and see if the car responds any differently. and then i'll move onto the spark plugs because they also looked pretty sickly.

ahhh isn't trouble shooting fun?!