Thread: Lost Pressure

'03 GTP. 138kmi. Open cone air filter, 180* t-stat, res. delete, flowmaster 40s, otherwise drivetrain is stock. I used to pull 7+psi boost at will, and would peak at over 8psi regularly. Last several months its gotten gradually worse, to the point that I only get 6-6.5psi boost.. Also, I've gotten a couple P0440 codes. Fresh/clean fluids, air filter, pcv valve, autolite 606s. I've tried spraying carb cleaner around, a few times, with no affect. So couple easy questions, before I throw $ away on all new vacuum lines or a mechanic diagnosis. Before we start, let's pretend vacuum lines are not leaking. 1) what else could cause loss of vacuum/boost (and likely the P0440), eg plenum gasket? evap vent solenoid? evap purge solenoid? bad vapor canister? bad gas cap? bad fuel tank filler neck? BBV solenoid filter? I'm grasping, but it would be a huge help if I could eliminate some possible problems in my trouble shooting. 2) What is a good vacuum pressure for a warm idle, or in P, in inch/Hg?

Thanks a ton in advance.

-19 to -20 in/hgs, in park, warm idle.

My LIM was causing me to have a VAC leak.

i had a po441, and po440 a while back, so i changed the evap solenoid on the engine, and the map sensor, the codes both have gone away.

some say the map can cause this code too. and being the map is what reads boost, 2+2= 4 right?

Thanks guys. I've been scanning and tinkering. Sometime at a warm idle in P I get -19.3 to -19.6 in/hg, sometimes its -18.4, but usually it sits right at -19.0 in/hg. I've read LIM gasket can result in loss of pressure, but my oil and coolant look good, and I don't see any leaking fluids. Going to do my best to clean engine and engine bay this weekend. Hopefully you can help with a little follow-up. 1) How do you clean the MAP sensor? 2) Is the BBV solenoid filter an inlet or outlet? Thanks again!

Hold on a second, you have looked up the codes haven't you? If so.. you are aware of the issue that is causing your boost to show 1-2 pounds down.

The problem that's causing the issue is right there between the seat and steering wheel. It's you, fix the known issues. It's amazing how cars run better when they don't have problems.

PS, please give yourself a noogie for posting a "if I didn't know I had issues, what might cause my boost to be lower". Plus.. if you had a vacuum/pressure leaking LIM gasket, it wouldn't have to be visibly leaking anything but air. I have a hell of a time seeing air, you?

Originally Posted by BillBoost37

The problem that's causing the issue is right there between the seat and steering wheel. It's you?

Dang, when I first read this, I thought, Bill must have started shaggin' one of my ex-wives... Then I realized that cynicism doesn't always translate well thru forum. When I said, "lets pretend", its b/c 1st thing everyone says is, you have leak in vac line, but vac lines are good, been checked, double checked, triple checked, that's not where loss of pressure is coming from.

Originally Posted by BillBoost37

Hold on a second, you have looked up the codes haven't you? If so.. you are aware of the issue that is causing your boost to show 1-2 pounds down.?

Yes, I looked up code, hence list in OP. But, no, I don't knowwhat the problem is, and I would pay for professional diagnosis before I just start buying parts; gas cap, vent solenoid, purge solenoid, vap canister, fuel filler neck, and I guess MAP sensor and LIM gasket too..... I posted here in hopes of being able to identify the problem, or at least eliminate some possibilities.

Originally Posted by BillBoost37

Plus.. if you had a vacuum/pressure leaking LIM gasket, it wouldn't have to be visibly leaking anything but air. I have a hell of a time seeing air, you?

Actually, I thought a bad LIM gasket would generally allow oil and coolant to mix, creating a sludge, or that the carb cleaner spray would cause engine to rev or stall (flood). So thanks, I learned something.

A little more, I've had car for just over 2 yrs, and evap purge solenoid was obviously new at that time, fwiw. With more scanning, Vac seems to stick occasionally--in P, it will sometimes sit at -20.5(ish) in/hg for 1-2 seconds, then pop to -18.3in/hg, like it flutters? then finds/sits right at -19.0 in/hg. That seems to me more like a sensor/electric issue than a physical leak in the system. But obviously, IDK... much of anything.

I'm still hoping to learn more.

I'm back, after more reading/research/information, many scans, and new worries.

1st, follow Bill's veiled wisdom. After enough googling and youtubing to confuse myself, I went back to alldatadiy. The P0440 is a 2-part test, failure of either triggers a P0440. 1) tests evap system ability to achieve pressure by closing the normally open evap vent solenoid and opening the normally closed* evap purge solenoid to apply intake manifold pressure to evap system; 2) tests evap system ability to hold pressure by closing evap purge solenoid and timing the rate of pressure loss. Normally closed* because evap purge solenoid is PWM and is expressed by PCM as a % of open, fully closed is 0%, fully open is 100%.

Now it makes sense to me how a loss in engine pressure would result in P0440, even if the evap system was perfect. And how a bad evap purge solenoid could allow engine pressure to escape. So what... I still don't know what my problem is, and still not willing to just throw $ at it, guessing. I decided to drive it until the SES came on again, try more scans, have something for a pro to work with. Sure enough, last week, P0440. Then, on Sunday, the SES went off. Later that day, I was watching my boost/vac gage after parking & it sat at 21, then snapped down to 18. My mechanical autometer gage reads surprisingly close to my android torque readings, so I trust it.

Recent scans with torque, at warm idle in P, were generally; b/v 19.0 in/hg, intake manifold absolute pressure 5.2 psi, evap 15.7%.

This morning, at warm idle in P; b/v 10.1 in/hg, intake manifold absolute pressure 9.7psi, evap 14.9% .
This eve, at warm idle in P; b/v was 6.6 in/hg, intake manifold absolute pressure was 11.5 psi, evap was 17.3%.

A bad LIM gasket won't raise intake pressure, right. Seems electrical to me, sporadic, a vac would be steady/constant, right.

Any help? What do I try first? Weird, but seems to being running good, Is it safe to drive????

Also wondering, what's a good intake manifold absolute pressure, warm idle in P and good peak/max for stock L67?

Thanks!

LOL.. sorry to torture you and not see the post again to guide you a bit more.

If you want to know if your evap is the issue causing pressure differences, you could block off the TB where it goes in and leave the evap open right there so no tank pressure builds. Typically though, if the evap code pops up, you have an issue in the evap system. Inspect the lines from tank to TB, pull your rear left wheel well and look over the evap can. Get the free obvious inspections out of the way.

The loss in pressure you see on the scan, doesn't mean it's not right. It could be the soleniod purging tank pressure, or more load hitting the engine. Say your defroster/ac is cycling, that'll drag down rpm slightly and bring down vacuum.

Bill, thanks! It's been a long, cold winter so I haven't done too much w/ it. New gas cap, evap purge solenoid & lines. Tried to find vacuum leaks again w/ both carb cleaner & propane. CRC maf cleaner on maf & map sensor. B-12 throttle body cleaner on iac valve. Scans are the same, maybe more of the 18.4 in hg & less of the 19 in hg. Seems to be idling higher more, had a bad rough warm idle twice, 11mpg city, but no more codes. I'm back @ it. Bill, going to follow-up throttle body suggestion. First, I'm cleaning it on principle, gasket should be here tomorrow, decent forecast for Saturday.

IMAG0348_zpsufpgw8if.jpg

It cleaned up nice, but the head/plunger had a bit of play/wobble sideways, but not in/out. Is that kind of play normal or is it time for new oaf valve too?

Hold on, that's not a pic of my oaf valve. It's my dirty IAC valve...

Sorry for typo & auto correct.

I got another P0440 and started over with Alldatadiy, google, and youtube and things like when Bill said, evap codes are typically issues with the evap system made more sense. I decided it was the vapor canister but I got all confused with part numbers. GM redesigned the part so true OER is hard to find, GM and most others require wire harness adapter for an additional $25-$35 and these wire harness adapters were not readily available. So I decided to pay for more convenience of return at local autozone and take a chance on a fit. Vent valve is super easy, except my bracket bolt is completely rust and I stripped it proper. Ended up just bending the bracket to get valve off. Autozone part was new design and didn't fit w/o harness adapter which I don't have b/c it's not stocked. So I give my original valve a few good blast of canned air and reinstalled it. Instant improvement.

Not sure it its actually fixed, but I spent last five months with 10.9-11.5 mpg city and in last 2 days/60 miles same driving it's at 15.3 mpg. Vac/Boost are strong, and best of all the car loves it, just running smoother and stronger. Sprint is finally here. Finally time to start some exhaust work!

New; gas cap, pcv valve and grommet, vacuum lines, evap purge valve and hoses, cleaned and/or tested map and maf sensors, iac vavle, and air filter--and all it took was a little air in the right spot. All in, still cheaper than the $150 diagnosis the shop quoted.

Thanks.