2003 MCS, 159K miles, 15% SC pulley, IE exhaust. Everything else basically stock.

Car surging when attempting to idle, from near stall then surges up to about 2K rpm. Loud whine which I believe is the supercharger. Got a check engine light, with these codes:

p1109 Manifold Air Pressure Too High in Deceleration

p1242 Secondary Upstream Manifold Air Pressure Sensor Too High in Deceleration

Could this be the bypass valve? Other ideas??

Stupid question: Also happened to note the exhaust manifold to be glowing red hot. I guess I've never looked under the hood when it was fairly dark outside and the engine running. Is this normal??

Thanks in advance.

 

Speculation time.

If the codes actually mean what they sound (yeah...) then it sounds like you car is putting to much air into the engine when it shouldnt be. You can still decelerate if the car starts cutting fuel but doesnt cut air, you will just go lean, very lean actually. Lean mean hot, which would explain the red exahust manifold (which I have never seen on mine). To me it sounds like an air leak somwhere between maybe the TB and the rest of the engine, or a TB that doesnt close properly.

Of course, there are some problems with this theory... I think on theses cars if you take your foot off the gas while decelerating it doesnt partially, but completely cuts fuel, meaning no combustion takes place, meaning no lean condition, meaning no glowing exhaust manifold. Then there is also the fact the TB has its own set of codes for when it malfunctions...


See, speculation, I have no idea if that theory I just came up with is any good really. Anyway, an air leak is certainly possible, I had one myself (but with ver different symptoms...)

 

Thanks for input Some Guy.

I changed the BPV and cleaned the TB. Problem resolved. Running smoothly and not throwing any more codes.

 

clogged cat?

 

Yeah, got a big hairball out!

Naw, it was the bypass valve.

 

Was the bpv spring snapped? How dirty was the valve in the throttle body?

 

So the hairball was in the BPV??

What was it apparently made of? would love a pic or more info.

Otherwise was the BPV OK?

Did you change it just because, or was it broken?

Just very curious to add more info to my history files.

YD

 

Sorry, the hairball thing was a (pretty weak) joke. "Clogged cat", hairball, felines get hairballs.....anyway.....

The BPV spring was weak (quite a bit weaker than the Detroit Tuned replacement I installed) but not snapped. The throttle body throat was pretty dirty but I can't say that it was bad enough to significantly affect function. Seemed like the TB valve moved and closed OK prior to cleaning.

The reason I decided to focus on the BPV was that after my original post I went out and manipulated the little external arm on the BPV with the engine running and it appeared as though it wasn't closing completely at idle and when I pulled it completely closed the engine ran smoother.

Interestingly (and perhaps completely coincidentally) my cruise control is working again since the BPV replacement.

 

just me but i thought the bypass valve was open at idle (high vac.) and closes as vac. goes down and boost goes up ? hence the spring.

 

I believe you are right Herbie! My bad. Honestly I don't remember which way I had to move the plunger/arm to get the car to idle normally, but it was apparent that the problem was with the BPV.

 

okay so stuck closed caused the manifold pressure to contradicteverything else giving you the codes!! cause you were building boost.... naw wait that aint it ; cause others have tied it closed with no codes . i still say check the exhaust .

 

I just had exactly the same problems, only I had just installed a boost gauge. I obviously suspected my work first. I had thought that the vacuum line had fell off the fuel pressure regulator causing the poor mixture and glowing manifold.
When I went in to disconnect boost line I found I had pinched the intercooler boot. I re-installed everything correctly, took it out and give it the beans. No red hot manifold and a nice smooth idle.

Andy

 

So I have the same problem going on and I just changed the bypass valve and it’s still doing the same thing. Got any other ideas?

 

Could be erroneous readings. You could clean or replace the MAP (mounted near the thermostat) and TMAP (mounted on the front of the intake manifold) sensors.

 

(I’m Praying 🙏🏾 and hoping this will help someone)
So I did a clutch upgrade to my 04 mini s and after putting it back together I ran across the same idle issue with the p1109 and p1242 code. I tried everything that I was reading on the web from checking the TP, MAP, SC BOOTS, BPV, Throttlebody, Map sensor tube.. ETC ETC.. and still No cure for my 2 codes from hell. I must of assemble and this disassemble the car at least 10 times hoping I would run across a loose plug or some sort of disconnection. Started to get frustrated because I knew that car ran 👌🏾 and this was not an issue before I did the clutch change. Four minute I even thought that maybe I had damaged a wire.Finally I decided to do the Old carburetor cleaner vacuum leak check. (you know spray brake cleaner or carburetor cleaner as the engine is running to see if you hear any change in the motor ..This is when I found that my supercharge gasket (The Green one up front (in the pic below)



that fits in the black plastic tube connecting to the BPV WAS NOT SEALED FOR SOME REASON. Also There is A 10mm Bolt that is supposed to secure it to the engine block. The tab which is plastic on the end of the tube was also broken causing the tube to be loose. See tab it in the pic 👇🏾

So excited And thinking that I had found the problem I jumped online and bought a replacement gasket. 🛑 DON’T DO IT! There is No need ‼️ After taking apart the supercharge port I found that someway somehow I had loosen the port and it only needed to be readjusted Tightly. So Five days later and countless amount of hours my two codes from HELL were gone. THANK GOD!!!