Lean Fuel trims - two year problem solving journey (solved) (Page 1/1)
Odin MAR 09, 01:03 AM
After 2 years of chasing lean fuel trims on my 3800SC 88 GT, I managed to solve the problem. I wanted to post to help anyone else out who might be having the same issue.
Stuff done-
Replaced fuel pump
Replaced o2 sensor
Replaced all vacuum lines
Replaced FPR
Replaced MAF
Smoked motor for leaks multiple times - no leaks.
Tried carb cleaner around the LIM - no change in idle.
Smoked exhaust system for leaks near o2
Checked voltage to MAF and looked for any potential wiring issues.
Checked voltage to o2.
Replaced battery and battery cables
Replaced EGR and EGR tubes
Replaced MAF screen in TB
Replaced TPS and IAC
Tried a secondary PCM

Checked fuel pressure, compression and everything else and everything checked out within spec.
Engine was seeming to burn a bit of oil on takeoff so replaced old valve seals - which solved that problem. They were dry and hard even though the motor is low mileage.
With all of this - still lean long term trims and no change there. Engine had a miss every once and a while at idle, changed wires and plugs as well as ICM, connector and coils. Trims would normalize anytime I was on the throttle.
Had what seemed to be tiny brown contaminants in coolant (extremely minimal and even hard to notice). Thought I was in for doing head gaskets. Replaced all seals on the tranny side of the motor. Did one of those block leak tests looking to see if it was oil in coolant BT-500 test kit. Test checked out fine. Pressure tested cooling system and it was holding pressure for over half hour with no change. Brown specs in coolant turned out to be bars leaks which had been put in prior to my ownership. Ended up flushing coolant.

Car never had any coolant in the oil. Oil always looked good. It also didn't have the burning coolant smoke in the exhaust or sweet smelling exhaust (indicative of a failed LIM gasket)
I was still pretty baffled and considered replacing the motor.
Car still had the same lean issue.
Got HP tuners to do some scans, had terrible latency issues with the generic software you can download for free.

I ended up pulling the SC off and LIM off. Gaskets were the original plastic ones. I could see some slight leakage near one intake port as well as some grime build up. This was very minimal but could see the gaskets were on their way out.
Replaced all of the gaskets on the top end of the motor and used the metal ones from Felpro. Replaced the SC gasket and the O rings. Replaced elbows with aluminum ones and new o rings. Replaced valve cover gaskets.
Put it all back together and had a lifter take about a minute to pump up. Thought maybe I should have replaced lifters while I was down there. After a minute or so the car ran like a champ.
Scanned and scanned. No issues now and my fuel trims are where they should be. Car is running absolutely amazing and seems like it has more power. Fingers crossed I don't see lean trims again and the LIM/SC gaskets seem to have solved the issue.


I just wanted to say thanks to Ryan (Sinister Performance) for my annoying emails and lousy scans from the generic software prior to HP tuners and he was a great help with the initial work -through on eliminating potential issues that eventually led to fixing the issue.

Needless to say - it seems like I have a motor now that has had most things replaced and is hopefully good for quite some time. If you have not replaced your LIM gaskets your car will thank you. Hard to believe GM put those in there in the first place they are complete garbage.

-Markus

[This message has been edited by Odin (edited 03-11-2024).]

jelly2m8 MAR 09, 02:37 AM
Errrr anyone that follows a 3800 knows about the only thing to kill and mess up is those 3800 intake gaskets.....


Glad you found your issue....
Raydar MAR 10, 01:27 PM
Thanks for sharing your solution. Lot's of people come in here and ask questions, and then just disappear.
We never know if what was said was of any value...