1995 Accord ex. maintenance history - the head gasket,vacuum lines clutch and timing/balance shaft belts, tensioners and water pump were replaced in 01/2012 prior to ownership . (the rest are all things I have done since I purchased the car in 6/2012.)
all 4 rotors/pads
right half shaft
air filter
shocks
valve cover gasket and plug seals
reset valve lash
map sensor (denso)
O2 sensor (denso)
cap, rotor, and wire-set
Plugs (NGK)
battery 7/12
inner and outer tie rod ends
right upper control arm and ball joint
front wheel bearings
Cleaned EGR passages
fuel filter
distributor shaft seal
vtec solenoid gaskets

A couple weeks ago i got a CEL for code 3 manifold absolute pressure circuit.

I back-probed for voltage to the reference (got 4.94v.) and the signal wire KOEO and got 2.86v. I then started the car and back probed the signal at idle and my voltage had dropped to 0.8v. According to my manual the voltage should range between 2.5v and 4.5v as manifold vacuum decreases with opening throttle. As i open the throttle the voltage stays below one volt unless i burst the throttle to 2000 - 2500 rpms in which case it will rise to the 1.2v - 2.2v range and immediately return to .8v (even when i hold throttle open after burst).

I applied vacuum to the sensor with a handheld pump KOEO and it ranges from 2.8 with no vacuum to .5 with 25in/hg decreasing at a rate of .4v per 5in/hg

If I probe the signal wire KOEO with the map disconnected i get 4.6v

I borrowed a map sensor from another car and repeated all the tests with identical results.
Vacuum gauge reads 21in/hg steady, vacuum lines are all in good condition and routed correctly.

Low voltage = high vacuum right? so what would cause low initial voltage, and no rise in voltage with decrease in vacuum?

Also it has probably been 8 months since i have done any work to it other than routine maintenance..

 

best thing to do is use a vacuum pump and manually add vacuum to test however these things rarely go bad...another from the junkyard should get you going again

 

as I stated in the op, I did a bench test with handheld vacuum pump on the original and second map sensor. both had the same readings. this leads me to believe that the problem lies somewhere else.

 

either intermittent open or ecu is bad

 

how do I test?

 

first thing's first, check the vacuum hoses for cracks/leaks...look at all the hoses associated with the map sensor

for the electrical test, put your test lead on the signal wire and wiggle around the connections and see if you lose voltage

 

Vacuum lines are all in new condition, vacuum test reads 21in/hg steady (connecting to the vacuum ports on end of throttle bore), electrical readings don't fluctuate with harness/connector movement. I applied vacuum to each of the four vacuum lines running off the throttle body vacuum ports, two have a very slow leak down.. the IAR control solenoid (the IAR check valve and and reservoir hold vacuum with no leak down) and the EGR control solenoid seem to be the culprits. I tested both solenoids to ensure proper operation and both switches apply vacuum at the appropriate RPM range. (Very slow leak down, -1in/hg every 10 seconds or so and the same at both switches) checked all vacuum connections around the switches with propane to no avail. (I disconnected the map sensor prior to the propane test so the ecm wouldn't compensate for leaks). So the question is, can this tiny amount of unmetered air affect my map signal to this extreme? (I unplugged a vacuum house from the throttle body port at idle to see if a lot of unmetered air would cause a significant difference, and the ecu instantly adjusts rpm back to idle speed)

 

since the map sensor tests fine I would think maybe you're reading the code wrong or perhaps the ecu is bad

 

what do you mean reading the code wrong...?

I replaced the iar and egr solenoids with a couple that held vacuum at the junkyard. my idle signal voltage reads the same .8v - .9v. however now as I open the throttle I get a rise in voltage relative to throttle advance, that no longer drops back to the initial .8v. seems to me the ecm is not the issue either..

 

Off the deep end, why not. Is the o-ring on the MAP sealing? I will admit I just read the highlights of this thread.

 

using your hand pump you should be getting these readings

0 -> 2.8-3.0

5 -> 2.3-2.5

10 -> 1.8-2.0

15 -> 1.3-1.5

20 -> .8-1.0

25 -> .3-.5

 

o ring is new

 

re-tested today. That is what I'm getting. i found the specs I've been looking for in my haynes.. Could be just wrong i guess? I reset the ecm after i put in the iac and iar solenoids and have not seen the code 3 again. I'll post again if anything new arises.

 

if it comes up again you might need to try another ecu