OBD-II reading boost under load, no turbo or sc, what's up?

[KeMBro2012]

One of my coworkers installed a short ram in his '00 last week and we went for a test drive with my OBD-II hooked up. Part way through the drive, I looked at the vacuum readout and noticed that, under load, he was generating anywhere from 2-16 inches of boost. Like I mentioned in the topic, there's no turbo or supercharger involved here, so there's only one thing I can think of that could be causing this, which I've already suggested to him. I don't want to mention what I think it is until I hear from a few other people (I don't want him to be able to say I influenced this is any way when I show it to him).

If anyone wants to chime in with possible causes, I'm all ears (and honestly hope to find out I'm wrong, the fix isn't a job I necessarily want to do if avoidable).

Other info that may be of interest:

• Fuel trim (short and long term) seemed to be in spec. I don't have the numbers handy right now to post. I suggested that he disconnect the battery for 5min to reset the long term trim, simply because of the new intake, but I'm not confident that this will solve this problem.
• The last couple minutes of the test drive, the engine idled very rough. He says it's fine now, I haven't been in the car since, so I'm going on his word.
• He's had a 35% drop in gas mileage, from 33MPG to a hair under 22, which is more than I'd attribute to a new, properly designed, intake with a decent filter (it's a Weapon R intake designed for this model/year, with a K&N filter installed on it).
• He replaced his worn spark plugs with NGK Iridium plugs a week prior to installing the new intake
• We installed a new suspension last week, he still hasn't had an alignment since then (that would account for some of the MPG loss, but I don't hink 35% sounds reasonable)

Thanks for taking a look

Edit: Added a few items to the list, that he just reminded me of.

[Bennie]

If I'm following you correctly, and we are assuming that like all NA engines, there is no positive pressure present in the intake, then it sounds like he has a bad MAP (manifold absolute pressure) sensor. Since that's what reads pressure and you are getting impossible numbers I don't know what else it would be. The car wouldn't run right if the MAP sensor is malfunctioning either.

[KeMBro2012]

When you say "wouldn't run right", what symptoms would we be looking for?

In addition to apparently burning more gas, it takes off a fair bit faster than it did before the intake, so it seems to be running "right". I did verify that he's not seeing any misses and the rough idle only happened that one time when I was in the car.

I'm gonna look like an ass now, because I just got done telling him the MAP doesn't fail in that way, when it apparently does.

Edit: To clarify, would it throw a code or light up the CEL?

[Bennie]

I haven't had a MAP fail in that way before & it's hard to diagnose this stuff from a keyboard but I'd definitely take a good look at the MAP. We can go off the fact that there is not positive pressure in the intake and they MAP says there is.

IIRC the only MAP I've actually had to replace was on a Bonneville and it shorted out the 5V supply so the fuel gauge & oil pressure gauge didn't work. It was really wierd.

The rough idle & bad mileage would be symptoms of a bad MAP. The car will still run with a bad MAP just not perfect. What year is the car? I could plug my scanner in tonight and see what a normal reading should be.

The importaint thing is getting the car fixed. If you get the car fixed you won't look like an ass. right?

[KeMBro2012]

We're looking at a 2000, which I've been comparing to my 99. He's got VVT and a MAF, I have neither, but neither should affect MAP readings.

The rough idle was for 30sec or so, one day last week, it's not an ongoing problem.

Normal at idle should be on the 20-24in and he sees 22-23in at idle, as do I, so that seems right. It shoots to anywhere between 24 and 29in when he lets off the gas after revving, returning to 22-23in as the engine settles back to idle. Pretty normal compared to my 99, as well as every other vehicle I've ever had any form of vacuum gauge connected to.

In my 99, I see vacuum drop to between 4 and 10in on acceleration (or revving in park or neutral). In his 2000, the needle just keeps going, out of the vacuum range and into the boost range, I've seen as high as 16lb. Mind you, this is via OBD-II and not a proper vac/boost gauge. I'll have to see if he still has the one I gave him, so we can test that way, as well.

[Bennie]

Originally Posted by KeMBro2012

We're looking at a 2000, which I've been comparing to my 99. He's got VVT and a MAF, I have neither, but neither should affect MAP readings.

The rough idle was for 30sec or so, one day last week, it's not an ongoing problem.

Normal at idle should be on the 20-24in and he sees 22-23in at idle, as do I, so that seems right. It shoots to anywhere between 24 and 29in when he lets off the gas after revving, returning to 22-23in as the engine settles back to idle. Pretty normal compared to my 99, as well as every other vehicle I've ever had any form of vacuum gauge connected to.

In my 99, I see vacuum drop to between 4 and 10in on acceleration (or revving in park or neutral). In his 2000, the needle just keeps going, out of the vacuum range and into the boost range, I've seen as high as 16lb. Mind you, this is via OBD-II and not a proper vac/boost gauge. I'll have to see if he still has the one I gave him, so we can test that way, as well.

Hey, perfect! swap them out and see what happens. They are interchangeable from 1998-2002.

[KeMBro2012]

That's gonna be up to him, if I can get him to agree to swap back after the test, hahahahahahahahaha

If he still had the vac/boost gauge, I'd prefer to try that first; I don't like futzing with fragile parts that don't need to be futzed with. It wouldn't be good if something happened to my sensor (or his, if it's not the problem) during either of the swaps.