Open Loop Problem?
Thread Starter
Instructor
Joined: Dec 2008
Posts: 228
Likes: 1
From: Atlanta GA
Open Loop Problem?
Gang,
Looking for some guidance on what appears to be an open loop issue. The car stumbles when torque is applied. This occurs either when cold or for the first few minutes after it is started when at operating temperature. Idle and starting are fine.
It is a pretty much a stock 94 LT1 w/103K. I have done a lot of basic maintenance to it which has fixed a lot of my other problems including plugs, wires, tps, filters, injectors, etc. The opti was supposedly just replaced by the PO but who knows.
I am still working on mastering the FSM (a LOT of info there).
There are no codes (paper clip method), passes emissions fine.
Any suggestions? I was thinking of picking up the oddball cable to do some data logging on it but have not pulled the trigger yet on that.
Thanks!
-David
Looking for some guidance on what appears to be an open loop issue. The car stumbles when torque is applied. This occurs either when cold or for the first few minutes after it is started when at operating temperature. Idle and starting are fine.
It is a pretty much a stock 94 LT1 w/103K. I have done a lot of basic maintenance to it which has fixed a lot of my other problems including plugs, wires, tps, filters, injectors, etc. The opti was supposedly just replaced by the PO but who knows.
I am still working on mastering the FSM (a LOT of info there).
There are no codes (paper clip method), passes emissions fine.
Any suggestions? I was thinking of picking up the oddball cable to do some data logging on it but have not pulled the trigger yet on that.
Thanks!
-David
Instructor
Joined: Jan 2010
Posts: 174
Likes: 4
From: Austin_Palm Bay_Houston Texas_Florida_Texas
So you are saying that once the engine is warmed up that it runs fine, right ?
If this is truely only an "open loop" problem, then the 2 main inputs used by the PCM for AFR is your MAF and MAP sensor. These are the only sensor input used by your PCM during "open" loop to derive a ballpark air to fuel mix of 14.7:1
Maybe your MAF or MAP sensor is flakey and only shows up during open loop. During closed loop, the feedback from your O2 sensors may be enough to compensate for the faulty or flakey sensor....
Anyway, I would test the MAP sensor first, then the MAF.
If this is truely only an "open loop" problem, then the 2 main inputs used by the PCM for AFR is your MAF and MAP sensor. These are the only sensor input used by your PCM during "open" loop to derive a ballpark air to fuel mix of 14.7:1
Maybe your MAF or MAP sensor is flakey and only shows up during open loop. During closed loop, the feedback from your O2 sensors may be enough to compensate for the faulty or flakey sensor....
Anyway, I would test the MAP sensor first, then the MAF.
Melting Slicks
Joined: Feb 2006
Posts: 2,753
Likes: 15
From: backwoods upstate ny
nope..coolant temp sensor is critical for 'cold start' and 'cold drive-away'...many are 'off calibration' but no code will display...check cold and at operating temp using scanner or DVOM/compare with chart in Serv Manual before replacing
Le Mans Master
Joined: Aug 2007
Posts: 8,735
Likes: 412
From: WV
This is probably going to need a scanner hooked up to figure it out..WW
Instructor
Joined: Jan 2010
Posts: 174
Likes: 4
From: Austin_Palm Bay_Houston Texas_Florida_Texas
On a cold startup, disconnect your MAF and the engine will stop. Disconnect your MAP and you will hear a discernable change in idle. These actions will however set error codes. But the OP's problem is a very subtle one, and I do agree that a scan tool is his bet bet on identifying the problem.
-Al
Last edited by Al Borman; Jan 18, 2010 at 03:14 PM.
Thread Starter
Instructor
Joined: Dec 2008
Posts: 228
Likes: 1
From: Atlanta GA
Thanks everyone for the suggestions. Looks like a scan tool is in my future.
To answer the questions, yes, she runs great warm. It does have the problem when warm if i turn off the car and then start it again. It was my understanding that it would be in open loop in that situation as well which is what is making me look at open loop issues.
I will update folks once I get more info.
-D
To answer the questions, yes, she runs great warm. It does have the problem when warm if i turn off the car and then start it again. It was my understanding that it would be in open loop in that situation as well which is what is making me look at open loop issues.
I will update folks once I get more info.
-D
