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Very high negative fuel trim at idle

 
Old 03-10-2019, 07:00 AM
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Default Very high negative fuel trim at idle

I'm having very high negative fuel trims at idle and just off idle. This causes momentary surges in power on takeoff occasionally. Cold start is fine. Once the O2 sensors kick in I smell fuel and see the STFT going way negative and the LTFT following suit. -10 to -19 on STFT with LTFT quickly following those numbers. When cruising without a lot of stop and go the fuel trims even out and reach 0. Fuel mileage is terrible. 21-23 highway. Used to get around 26. I have already done all the research but would like some professional input. Below are the things I've already done.

New map sensor. Scan tool readings seem fine.
New o2 downstream - Bosch unit 2 years old.
New TB gasket cleaned entire throttle body
Checked for vacuum leaks in the engine area.
Replaced PCV
Replaced IAT
Scan tool reports all sensors appropriately
No CELs

My engine is freshly rebuilt by Inline pro. It was doing this before it was rebuilt. It wasn't as bad possibly due to the CAI I had installed. I now have the stock air box. Just FYI, I had a cam gear bolt break while warming my engine one day. I stopped the engine as soon as I heard the rattle of the head of the bolt. I pulled the valve cover and found the issue, had it towed to Inline pro. The pistons and valves were caked with carbon. I just had them do a full rebuild even though it ran fine.79k miles on an '01 AP1.

The only thing I see left to do is check the EVAP system I just haven't gotten around to it. I'm assuming a huge vacuum leak. My second assumption is a bad fpr but there isn't any fuel coming out it. Although I did catch a hint of fuel smell from the hose and when I sucked on it. Inline Pro said they flowed and matched my injectors so I'm assuming they are not leaking. My last assumption is a lazy O2 sensor. I believe I replaced it with a Bosch unit. I have a Denso upstream and downstream units coming tomorrow. I'm so sick of throwing money at this car. Any help to steer me in the right direction would be appreciated. 20 hours of research hasn't yielded much progress.

Last edited by kikkomon; 03-10-2019 at 07:01 AM. Reason: Small edit
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Old 03-11-2019, 06:31 PM
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Bumping my thread with some new info.

Swapped fuel pressure regulator and damper with another used unit. No changes in fuel trim. Still reading -10 to -24 on STFT and LTFT is following it quickly. LTFT got up to -12.
My new Denso O2 didn't come today like it was supposed to so instead I pulled the old one out and it was covered in soot. I cleaned it up and stuck it back in. The car seemed to run even worse. Sometimes it leaves soot on the ground behind my tailpipes when it;s sitting there idling. I also noticed that on cold start this morning before the O2 sensors are heated up it's running much richer than it should. You can hear the gurgling and popping in the exhaust. Sounds neat but it's definitely not right. Any ideas?
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Old 03-11-2019, 10:48 PM
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I would double check those injectors.
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Old 03-12-2019, 03:05 AM
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Yeah double check those injectors
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Old 03-12-2019, 06:25 AM
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How do you guys suggest I check them short of buying another set?
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Old 03-12-2019, 09:19 AM
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Check if anyone is leaking, press on a short piece of hose over the top and apply some pressure from a can of brake clean or similar.
You could maybe use compressed air as well, but don't use to high. Stock fuel pressure is 40-50psi.
Or send them away for a flow test, you said Inline pro did this so maybe ask them about a protocol, they might have mixed them up or something.
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Old 03-12-2019, 09:31 AM
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I still think its an 02 sensor. You "cleaned" the existing and then it ran worse.
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Old 03-12-2019, 10:34 AM
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another suggestion would be to check your valve clearance. I know on my car I was having high negative fuel trim and found that the exhaust valve were tight. After doing a valve clearance the fuel trim return to normal.
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Old 03-12-2019, 02:18 PM
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^ Good call,^
if you installed the head yourself..?
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Old 03-12-2019, 04:24 PM
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I doubt it's the valve clearance but I'll check and readjust them. Inline pro did a full rebuild on the engine which included their stage 1 head overhaul. They did the valve adjustment. Please note that this was happening before the engine was rebuilt. Inline Pro did a great job on my engine. It sounds better mechanically than any other s2k I've had previously. Fit trim and tight like my women.

I changed the upstream and downstream O2's today. Reset the ECU. Did the idle learn procedure. No dice. Very high negative fuel trims only at idle. Still some surging of power just off idle in the low rpms most likely due to overly rich conditions. Smell of fuel out the exhaust. Outside of the low rpms the car freakin screams. Seems to have more torque than before my rebuild especially between 4-6k. There isn't much of a kick anymore at vtec. It's a much smoother transition. I attribute this to the increased torque in the 5k rpm range.

Inline Pro said they'd have another set of matched and flowed injectors tomorrow for me to swap out. They've been somewhat helpful although I understand that they are very busy in that shop. The place is filled with s2000's. Can barely move around in there when they have all the cars inside. I believe there are only 2 options left.

1) Leaky injectors
2) Unknown high pressure fuel pump in the tank providing too much fuel.
3) Whatever someone in this forum can point me towards. ;-)

Thanks guys for your contributions. I stress out on this car day and night. It's all I think about all day. Once the problem is solved I'll be so relieved. Any ideas you guys provide will be much appreciated. I'll check my valves tomorrow.

Last edited by kikkomon; 03-12-2019 at 04:25 PM. Reason: typo
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