Causes of Negative Fuel Trims
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From: Livonia, MI
I'm having very similar issues on my LNF right now... it will cold start at idle just fine until it gets to around 168* coolant temp (is this when closed loop begins roughly?) and then STFT's take a large dip to the negative around the -15 to -20 range. I'm not certain that coolant temp really plays a part in this either, its just what I noticed out of the one log I have of it acting up. I've sprayed every vac line with a carb cleaner and cannot find any leaks anywhere. I've replaced the primary o2 and it was not the issue. During this cold start time, it will pull -19 inHg which seems normal, but then once it starts acting up its in the -11 to -13 inHg range which is a fairly low vacuum number to be idling at..
Seems similar to what went on with the OP's car...
little backstory, the car was fully bolted just over a year ago but is now on bone stock everything. the car ran just fine for the 2 times I had it out this summer, then when i took it to school in august it just went downhill fast during the 3 hour drive and its been in my garage since end of August. I've only put maybe 500 miles on the car since march, so its been sitting alot. No idea how it suddenly just runs like garbage. I have logs of it as well, its back on stock tune and still doing the same thing as my 22psi tune it had been running for the last 35k miles. I know the valves need cleaning, but that would not explain how it runs perfectly fine during the initial cold start.... I'm at a loss
Seems similar to what went on with the OP's car...
little backstory, the car was fully bolted just over a year ago but is now on bone stock everything. the car ran just fine for the 2 times I had it out this summer, then when i took it to school in august it just went downhill fast during the 3 hour drive and its been in my garage since end of August. I've only put maybe 500 miles on the car since march, so its been sitting alot. No idea how it suddenly just runs like garbage. I have logs of it as well, its back on stock tune and still doing the same thing as my 22psi tune it had been running for the last 35k miles. I know the valves need cleaning, but that would not explain how it runs perfectly fine during the initial cold start.... I'm at a loss
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From: Rochester, New York
Mine was the HPFP. The dealer replaced that and I reflashed the GMS1 tune and shes good to go now. My pump was still covered under the powertrain and I bought my car in December 08. I never thought the pump would give me those problems. With it replaced and reflashed my fuel trims are consistant and never dipping below -4%
I'm having very similar issues on my LNF right now... it will cold start at idle just fine until it gets to around 168* coolant temp (is this when closed loop begins roughly?) and then STFT's take a large dip to the negative around the -15 to -20 range. I'm not certain that coolant temp really plays a part in this either, its just what I noticed out of the one log I have of it acting up. I've sprayed every vac line with a carb cleaner and cannot find any leaks anywhere. I've replaced the primary o2 and it was not the issue. During this cold start time, it will pull -19 inHg which seems normal, but then once it starts acting up its in the -11 to -13 inHg range which is a fairly low vacuum number to be idling at..
Seems similar to what went on with the OP's car...
little backstory, the car was fully bolted just over a year ago but is now on bone stock everything. the car ran just fine for the 2 times I had it out this summer, then when i took it to school in august it just went downhill fast during the 3 hour drive and its been in my garage since end of August. I've only put maybe 500 miles on the car since march, so its been sitting alot. No idea how it suddenly just runs like garbage. I have logs of it as well, its back on stock tune and still doing the same thing as my 22psi tune it had been running for the last 35k miles. I know the valves need cleaning, but that would not explain how it runs perfectly fine during the initial cold start.... I'm at a loss
Seems similar to what went on with the OP's car...
little backstory, the car was fully bolted just over a year ago but is now on bone stock everything. the car ran just fine for the 2 times I had it out this summer, then when i took it to school in august it just went downhill fast during the 3 hour drive and its been in my garage since end of August. I've only put maybe 500 miles on the car since march, so its been sitting alot. No idea how it suddenly just runs like garbage. I have logs of it as well, its back on stock tune and still doing the same thing as my 22psi tune it had been running for the last 35k miles. I know the valves need cleaning, but that would not explain how it runs perfectly fine during the initial cold start.... I'm at a loss
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Joined: 12-22-08
Posts: 8,356
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From: Rochester, New York
Last edited by padlock; Oct 28, 2014 at 01:47 PM.
The oscillation is normal. What cam and timing tables are you running? Did you tweek anything in the OS and OT tables?
Its odd that the fuel pressure isnt registered prior to the trims going odd, my car enters closed loop operation way before 168*.
Its odd that the fuel pressure isnt registered prior to the trims going odd, my car enters closed loop operation way before 168*.
I pulled the EVAP solenoid, and verified that it was functioning. I could not blow through it, so according to everything I've read/seen it should be good.
I'll be checking the boost solenoid and cleaning my MAF next, along with bypassing the small vacuum box below the manifold. Maybe there was a small section the carb clean did not get to where its leaking. bypassing it should answer my questions.
After further thoought, I do have my catless downpipe on yet. would the secondary o2 going bad have any affect on what I'm seeing (even though I delete the code)?
The secondary might, I never know with the Bosch ECU what table makes reference off other tables we dont have access to, the code you have shut off is just emissions not full function of the sensor. Do you still have Powells oil separator on (PO of my car installed it and hand tightened all the AN fittings together...major air leak)
The secondary might, I never know with the Bosch ECU what table makes reference off other tables we dont have access to, the code you have shut off is just emissions not full function of the sensor. Do you still have Powells oil separator on (PO of my car installed it and hand tightened all the AN fittings together...major air leak)
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