runs fine only in closed loop.
runs fine only in closed loop. My LS1 is put away so I haven't been on here in ages but I asked a question in a sierra forum and those guys probably don't even know what closed/open loop is.
But my 99 Sierra (5.3 bone stock) has had an annoying issue for the last couple of years and I finally have a fire under my butt to get some things sorted out (kids have been a big distraction .i.e life/time suckers).
So if the engine is cold is bogs a little bit. The colder out the worse it is. Once it's up to @ 160-180 it runs fine. So I figure it has to be something sensor related that either isn't there is closed loop is doesn't really effect closed loop.
There are no codes and I don't see anything that stands out on the scanner. I don't want to drop 100 buck to unlock the ecu since I am leaving it stock but my matco scanner I see almost everything live and nothing looks odd but maybe there is something I am overlooking.
I have had two new fuel pumps in the last couple of years and filters as well. (the first pump the check valve failed to I had to replace it again). Cleaned the MAF.
02s are old but I would think that would cause an issue when warm not cold.
Hoping someone has seen this before and and it's simple that I am overlooking.
But my 99 Sierra (5.3 bone stock) has had an annoying issue for the last couple of years and I finally have a fire under my butt to get some things sorted out (kids have been a big distraction .i.e life/time suckers).
So if the engine is cold is bogs a little bit. The colder out the worse it is. Once it's up to @ 160-180 it runs fine. So I figure it has to be something sensor related that either isn't there is closed loop is doesn't really effect closed loop.
There are no codes and I don't see anything that stands out on the scanner. I don't want to drop 100 buck to unlock the ecu since I am leaving it stock but my matco scanner I see almost everything live and nothing looks odd but maybe there is something I am overlooking.
I have had two new fuel pumps in the last couple of years and filters as well. (the first pump the check valve failed to I had to replace it again). Cleaned the MAF.
02s are old but I would think that would cause an issue when warm not cold.
Hoping someone has seen this before and and it's simple that I am overlooking.
I am trying to figure out which parameters are diff between open and closed loop. Sprayed the intake down looking for intake leak and found none. that was something I reluctantly checked because I don't think a vac leak would be diff between warmed up and cold. Plus the intake has never been off.
And I am not sure I can check the fuel trims on it. I haven't unlocked the pcm. I don't want to spend the 100 bucks to unlock it as I am leaving this one stock. My taghoe is unlocked and my LS1 is inlocked but not my beater truck.
I guess I could hook my wide band up to it.
TECH Addict
Joined: Mar 2013
Posts: 2,628
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I told you where to start....... don't assume anything......1st thing you need to understand is how the pcm uses the fuel trim data when the engine is cold. Checking for vacuum leaks is tough because cold it could be leaking all over the place.....hot it could be sealed up tight. I've been down this road a few times with LS1 engine using the nylon intake. Hook a fuel pres gauge up cold start the engine and watch the fuel trims. It will tell you exactly what is going on. I'll bet all the intake bolts FEEL tight but their not.
TECH Resident
Joined: Jul 2007
Posts: 967
Likes: 53
From: Winter Haven FL
I'm going off memory here so don't quote me. Open loop should be essentially speed density which relies on intake temp, manifold pressure and throttle position. A lazy sensor or a small vacuum leak can throw things off, especially the map sensor reading. Closed loop brings in mass air and O2 which can compensate for the lazy sensor or vac leak.
First check is for vacuum leaks. If you don't find any then watch the map sensor in real time and look for a nice steady reading and a change that corresponds to changes in rpms.
First check is for vacuum leaks. If you don't find any then watch the map sensor in real time and look for a nice steady reading and a change that corresponds to changes in rpms.
TECH Addict
Joined: Mar 2013
Posts: 2,628
Likes: 30
The fuel trims can effect the cold fuel if I remember correctly its been a while but if the fuel trims are positive in closed loop that extra fuel also gets added to open loop operation. If the trims are negative fuel is not taken away in open loop.
Sometimes stock ECU is set up to go Into closed loop colder than you think. Then the O2s trim way positive cause they are cold.
It could be that at 170 the sensors finally warm
Up enough to quit dumping fuel.
It could be that at 170 the sensors finally warm
Up enough to quit dumping fuel.
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That makes a lot of sense since the sensors are stock ad the truck has 240K on it.
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Verdad Gallardo I'm going off memory here so don't quote me. Open loop should be essentially speed density which relies on intake temp, manifold pressure and throttle position. A lazy sensor or a small vacuum leak can throw things off, especially the map sensor reading. Closed loop brings in mass air and O2 which can compensate for the lazy sensor or vac leak.
First check is for vacuum leaks. If you don't find any then watch the map sensor in real time and look for a nice steady reading and a change that corresponds to changes in rpms.
First check is for vacuum leaks. If you don't find any then watch the map sensor in real time and look for a nice steady reading and a change that corresponds to changes in rpms.
Thanks
Was that a stock intake that had never been off? I;m still going to check it later on probably on the weekend but I was just curious.
Teching In
Joined: Feb 2016
Posts: 37
Likes: 1
Just had a similar problem on my ls1. Cold start was tough barely wanted to stay on. Once it got warmed up things were fine aside from an idle search that I’m still tracking. Anyway. Hooked up a homemade smoke machine ($10) and my intake gasket was leaking. Swapped out the gaskets and I was good to go.