88red4cyl Posted July 22, 2008 Report Share Posted July 22, 2008 Sorry for yet another thread, but I'm trying to diagnose the fuel mileage problems on my 99, and I found something of interest.. When I was driving this evening, I noticed a whooshing sound that sounded like a vacuum leak when I got into the gas.. I thought this was odd, because I had never noticed this sound before.. So I came home and tried to find the source of this noise.. I took the cover off and revved the engine using the throttle cables, and I tracked my noise to the pass side of the engine.. Upon closer inspection, I noticed my MAP sensor (I think, it's the one on top of the engine right by the alternator) and its housing moved up every time I got into the gas and the whoosh appeared to come from under it... It kind of has me a bit concerned.. Knowing me, it's probably something perfectly normal that I've just never noticed before, but I thought I'd get your opinions to be sure.. Thanks! Quote Link to comment Share on other sites More sharing options...
BXX Posted July 22, 2008 Report Share Posted July 22, 2008 This is one your LTZ w/3800??? You need a new intake plenum.. Odds are, there is the typically crack where the MAP sensor plugs in and you are obviously getting a vacuum leak. You can remove the MAP and check things out and maybe the o-ring decided to die or something.. Quote Link to comment Share on other sites More sharing options...
88red4cyl Posted July 23, 2008 Author Report Share Posted July 23, 2008 This is one your LTZ w/3800??? You need a new intake plenum.. Odds are, there is the typically crack where the MAP sensor plugs in and you are obviously getting a vacuum leak. You can remove the MAP and check things out and maybe the o-ring decided to die or something.. Yeah, it's the LTZ.. So it's common for it to crack? Where do I look for the crack at? Sorry for the (probably) dumb question, I just have no clue what I'm looking at.. Quote Link to comment Share on other sites More sharing options...
BXX Posted July 23, 2008 Report Share Posted July 23, 2008 Yes, the SII 3800 Intake plenum is known for cracks, coolant, and vacuum leaks.. If you dont know what you are looking for, take it to a compentant mechanic. Not being a jerk, but if you cant really track down a crack or a vacuum leak on a plenum, you shouldnt be changing one.. Wherever the whoosh or hissing noise is coming from, there is your leak. Pull the MAP out and inspect for cracks/damage in the oil ring and the sensor itself.. Quote Link to comment Share on other sites More sharing options...
88red4cyl Posted July 23, 2008 Author Report Share Posted July 23, 2008 Okay.. I pulled out the MAP and had a look at it.. There is a tiny rip in the o-ring where the sensor plugs in at, but it doesn't look like it goes all the way through, only abou half way.. I did see one thing of possible concern.. I don't know if you can see from the pic, but there is a small gap between the plenum and the housing for the map sensor.. I couldn't get my camera to take a good close up pic, but it's there.. I tried pulling the housing off to inspect for warpage, but it's connected to some sort of spring assembly on the inside.. Could that be a problem, or is there supposed to be a gap there? Other than that, I didn't see any cracks on the plenum anywhere near the map... Quote Link to comment Share on other sites More sharing options...
AWeb80 Posted July 23, 2008 Report Share Posted July 23, 2008 Makes Me Wonder. My Bro's 2K Gp Gt Gets 22 Mpg On The Highway, He Might Have Some Issues I Need To Look At Too Quote Link to comment Share on other sites More sharing options...
88red4cyl Posted July 23, 2008 Author Report Share Posted July 23, 2008 Makes Me Wonder. My Bro's 2K Gp Gt Gets 22 Mpg On The Highway, He Might Have Some Issues I Need To Look At Too I would definetely take a look.. This car used to get 29 mpg (at 75 mph)on the highway before everything simultaneously took a shit on it.. Do you happen to know if the gap is any cause for concern though? I hear a big whoosh right after I pull the throttle cable hard, and the sensor and it's base move up.. When I looked into the hole I saw a plunger and a spring, so maybe it's supposed to move up to release pressure? I really have no clue.. Quote Link to comment Share on other sites More sharing options...
AWeb80 Posted July 23, 2008 Report Share Posted July 23, 2008 No. It's Probably Running Rich Because Of That. Again, You Have No Codes Being Thrown? Quote Link to comment Share on other sites More sharing options...
88red4cyl Posted July 23, 2008 Author Report Share Posted July 23, 2008 Nope.. Not a one... No. It's Probably Running Rich Because Of That. So you think it's not anything to be worried about, or do you MAP moving around is causing it to run rich? Again, I apologize.. I really have no clue if that is the normal operation for this sensor or not.. I've never had to mess with a MAP sensor before.. Quote Link to comment Share on other sites More sharing options...
AWeb80 Posted July 23, 2008 Report Share Posted July 23, 2008 Well, Moving Around Isn't Normal. Quote Link to comment Share on other sites More sharing options...
88red4cyl Posted July 23, 2008 Author Report Share Posted July 23, 2008 Well, Moving Around Isn't Normal. That's what I figured... Would it be a vacuum leak when it moves, or would it be pressure escaping from inside the engine? It only does it on the initial rev, and then it sets back down enough that I don't really notice it.. Also, how do I replace the base that it sets on.. It seems to be attached to that spring and plunger deal... Quote Link to comment Share on other sites More sharing options...
BXX Posted July 23, 2008 Report Share Posted July 23, 2008 You wont have a light mainly because you LTFT and STFT are compensating... I bet if you read live data, you would see the fuel trims way outta whack Quote Link to comment Share on other sites More sharing options...
88red4cyl Posted July 23, 2008 Author Report Share Posted July 23, 2008 You wont have a light mainly because you LTFT and STFT are compensating... Sorry for the dumb question, but what are those? Also, what should the fuel trims be for live data? Quote Link to comment Share on other sites More sharing options...
BXX Posted July 23, 2008 Report Share Posted July 23, 2008 You wont have a light mainly because you LTFT and STFT are compensating... Sorry for the dumb question, but what are those? Also, what should the fuel trims be for live data? Umm, sorry but I didnt know you didnt know.. Its not really something I can teach you over teh internets and with a few paragraphs... It basically has to do with the computer pulling fuel or adding fuel compared to its base maps and the LTFT are more based of what it has learned over some time.. The STFT are what its doing at that moment... STFT = Short Term Fuel Trims LTFT = Long Term Fuel Trims You would need a good scan tool to view the live data and help you make a diagnosis after that.. But since you are hearing a leak, then thats your problem.. Quote Link to comment Share on other sites More sharing options...
88red4cyl Posted July 23, 2008 Author Report Share Posted July 23, 2008 You wont have a light mainly because you LTFT and STFT are compensating... Sorry for the dumb question, but what are those? Also, what should the fuel trims be for live data? Umm, sorry but I didnt know you didnt know.. Its not really something I can teach you over teh internets and with a few paragraphs... It basically has to do with the computer pulling fuel or adding fuel compared to its base maps and the LTFT are more based of what it has learned over some time.. The STFT are what its doing at that moment... STFT = Short Term Fuel Trims LTFT = Long Term Fuel Trims You would need a good scan tool to view the live data and help you make a diagnosis after that.. But since you are hearing a leak, then thats your problem.. Thanks! I really do appreciate you explaining it.. I want to learn as much as I can.. So what needs to be replaced to correct the problem? Another question.. Why does the base that the MAP is connected to turn? I can't seem to get it off, but there must be some reason for it turning... Quote Link to comment Share on other sites More sharing options...
BXX Posted July 24, 2008 Report Share Posted July 24, 2008 You need to replace what is actually causing the leak... Im not sure about the MAP removal issue.... I cant really help you out without actually being there working on the car. Its been awile since I tore into a 3800 Quote Link to comment Share on other sites More sharing options...
88red4cyl Posted July 24, 2008 Author Report Share Posted July 24, 2008 You need to replace what is actually causing the leak... I think what's causing the leak is the base the map attaches to.. I can't seem to find a name or anything for the part, or how to remove it... I'm not even 100% sure that isn't part of the plenum to where I'd have to buy a whole new plenum to fix the gap.. I really appreciate the help though.. Maybe someone else who has a 3800 will chime in.. Quote Link to comment Share on other sites More sharing options...
88red4cyl Posted July 24, 2008 Author Report Share Posted July 24, 2008 All right.. So I finally found my Haynes manual for the car, and I discovered that the map sensor sits on top of the pcv access cover, and the pcv in underneath this.. Could the pcv be the cause for the vac. leak/pressure escaping/whatever, or would it be the map sensor? Quote Link to comment Share on other sites More sharing options...
88red4cyl Posted July 25, 2008 Author Report Share Posted July 25, 2008 I did some searching online about my 'vacuum leak'.. I found this on automotiveforums about the 3800: You'll also want to inspect the bushing which seals the MAP sensor stem to the intake plenum. If the bushing is deteriorated and leaking, the MAP reading will be consistently low. I think the bushing they're talking about is the same thing that air is leaking around on mine.. Does anyone know if this is something that commonly goes bad on the 3800's? Or do I have some kind of bigger problem making the pcv cover move up when I rev it? Quote Link to comment Share on other sites More sharing options...
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