block learn
Thread Starter
Instructor
Joined: Nov 2005
Posts: 210
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From: fall river MA
block learn
i borrowed my buddies snap on scanner tonite because my car has had some intermiting surging at idle. there were no codes stored but my block learn went down to 98 . i checked my fuel pressure which was 38psi with vacuum hose disconected. sprayed starter fluid on the intake and all vacuum hoses to check for leaks. i recently did a full tune up . the o2 sensor was going into closed loop. what causes the blms to go rich or lean. any advise would be greatly appreciated.
Racer
Joined: Aug 2004
Posts: 379
Likes: 170
From: Buffalo Grove IL
Odd--not sure why your BLM values would be so low with stock injectors. Perhaps you have a leaky injector or two?
I'm sure others will chime in with some good suggestions, but I'd suggest verifying that the injector resistances are good (16-17 ohms) and also read your sparkplugs to see if any look out of whack.
If the injector and plugs look good then hmmm.... vacuum leaks would cause your BLMs to go up, not down, so I wouldn't suspect those.
Ben
I'm sure others will chime in with some good suggestions, but I'd suggest verifying that the injector resistances are good (16-17 ohms) and also read your sparkplugs to see if any look out of whack.
If the injector and plugs look good then hmmm.... vacuum leaks would cause your BLMs to go up, not down, so I wouldn't suspect those.
Ben
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Thread Starter
Instructor
Joined: Nov 2005
Posts: 210
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From: fall river MA
i did check for exhaust leaks. i tightened header bolts and checked entire exhaust. it's funny, it does not do it all the time and sometimes worse than others. i'm puzzled. if it did it all the time it would be a vacuum leak or something. it has not thrown any codes. i'm going to pull the o2 sensor tomorrow to take a look at it.
Intermediate
Joined: Apr 2007
Posts: 26
Likes: 0
From: Niles MI
Injector Balance Test?
Have you tried an Injector Balance test? That should tell you if you have one sticking or leaking. 
There is a special tool you will need but it proves quite useful once you have it. It is GM multech specific.
There is a special tool you will need but it proves quite useful once you have it. It is GM multech specific.
Intermediate
Joined: Apr 2007
Posts: 26
Likes: 0
From: Niles MI
Field Test Mode
Another thing you can try is putting it in Field Test Mode. If you have the Snap-On Brick (mT-2500) you should be able to OBD1 Gm vehicles into ths mode. I think it is found in functionality tests. When you put it in this mode it bypasses all computerized parameters, kinda eliminates the computer. GM had issues with PCM's and this allows you to eliminate it. If the vehicle acts normally (problem goes away) you could suspect the PCM. It's just a real quick and easy test you can do with the brick.
Le Mans Master
Joined: Feb 2006
Posts: 8,753
Likes: 9
From: Louisiana
The BLM should have not gone down that low unless the chip has been tuned for it. I belive 108 is the lower limit in a stock chip. The fuel pressure sounds low with the vac line off. Usually (depending on vaccum pressure) the fuel pressure will increase by about 5 psi with the vac line off. That means you are at about 33 psi with the vac line on.
Check for exhaust leaks before the O2 sensor, check that the AIR system is diverting the air to the catalytic converter, and check the vac lines around the engine again. Vac leaks will introduce unmetered air into the exhaust stream and cause the O2 sensor to send a lean signal to the ECM. The ECM will add more fuel to compensate, thus a rich condition.
Check for exhaust leaks before the O2 sensor, check that the AIR system is diverting the air to the catalytic converter, and check the vac lines around the engine again. Vac leaks will introduce unmetered air into the exhaust stream and cause the O2 sensor to send a lean signal to the ECM. The ECM will add more fuel to compensate, thus a rich condition.
Le Mans Master
Joined: Dec 2000
Posts: 7,839
Likes: 22
From: San Diego, Ca
Just the opposite - unmetered air is picked up by the O2 as a lean condition - BLM's are above 128. The ECM adds fuel and it runs too rich. You'll see at the tailpipe, but not on a scan - BLM's will be lean, not rich. With this result, the ECM should be reducing pulse width, not adding it. That extra fuel can only come from a couple of sources - the injectors, bum regulator, or a broken evaporative emissions system.
Safety Car
Joined: May 2006
Posts: 4,290
Likes: 140
From: Salem NH
The BLM should have not gone down that low unless the chip has been tuned for it. I belive 108 is the lower limit in a stock chip. The fuel pressure sounds low with the vac line off. Usually (depending on vaccum pressure) the fuel pressure will increase by about 5 psi with the vac line off. That means you are at about 33 psi with the vac line on.
Check for exhaust leaks before the O2 sensor, check that the AIR system is diverting the air to the catalytic converter, and check the vac lines around the engine again. Vac leaks will introduce unmetered air into the exhaust stream and cause the O2 sensor to send a lean signal to the ECM. The ECM will add more fuel to compensate, thus a rich condition.
Check for exhaust leaks before the O2 sensor, check that the AIR system is diverting the air to the catalytic converter, and check the vac lines around the engine again. Vac leaks will introduce unmetered air into the exhaust stream and cause the O2 sensor to send a lean signal to the ECM. The ECM will add more fuel to compensate, thus a rich condition.
-- Joe
Safety Car
Joined: Feb 2002
Posts: 4,212
Likes: 10
From: Sun City West Az
Just the opposite - unmetered air is picked up by the O2 as a lean condition - BLM's are above 128. The ECM adds fuel and it runs too rich. You'll see at the tailpipe, but not on a scan - BLM's will be lean, not rich. With this result, the ECM should be reducing pulse width, not adding it. That extra fuel can only come from a couple of sources - the injectors, bum regulator, or a broken evaporative emissions system.
How old / milage are on the O2's? I"d start there to be sure the ECM is recieving a true value from the O2's.
Safety Car
Joined: May 2006
Posts: 4,290
Likes: 140
From: Salem NH
Correct. However his BLM is @ 98. The ECM is not in control (BLM below 108). Therfore the O2's are living above 450 mv seeing a rich exhaust stream. The short term intergrator can't adjust and the long term fuel trim is being skewed to the left as a result. the ECM tries to remove the extra fuel to compensate but is beyond the computer's control.
How old / milage are on the O2's? I"d start there to be sure the ECM is recieving a true value from the O2's.
How old / milage are on the O2's? I"d start there to be sure the ECM is recieving a true value from the O2's.
-- Joe
Safety Car
Joined: Feb 2002
Posts: 4,212
Likes: 10
From: Sun City West Az
I too thought that 108 was the bottom figure for the ECM according to the FSM. Apparently not. With a 98 reading the ECM clearly is not in control of the A/F mixture. That's why I *suggested* that he needs to be sure the ECM is seeing a true value of the A/F being reported by the O2's.