CIGLexus

iTrader: (1)

I've been through most of the threads on this. I know what parts can possibly be faulty. However, I'd like to see if we can guess what it is before we find out. I recently bought this car and it was fine when I bought it, but after changing the alternator and fixing the VVTi seal leak, it has changed it's behavior.

CODE : PO171 System too lean
2000 Lexus GS300 287k miles
idling at 500rpm
This car appears to not have been tuned up or serviced in a long time (I bought from original family owners).

Long Term fuel trim at +46% while the short term is still trying to add more fuel. The lean condition is only at idle and low RPM. After 3000 rpm it goes to normal.

1 - I swapped out the intake hose with a known good one, just in case it was cracked. I also cleaned the throttle body it was pretty black. This helped the idle and the car ran slightly better, but still +40 long term and engine light. I also sprayed all around the intake manifold and TB to find a vacuum leak and nothing apparent. Idle increased to a nice stable 650rpm.

2. I applied the full time +12v mod to the fuel pump. I wanted to see if maybe it was weak at idle and this would help. Now the LT fuel trim is around +33%, some improvement and so far the code is not showing up. The midrange feels much smoother and its running a little rich until you go to idle and you can see the fuel trim slowly go from -10 to +10% the longer you idle.

3. The air filter was old and dirty so I popped in a new one. I also threw in some lucas fuel treatment in case the injectors are clogged/dirty. This helped further, now the LT fuel trim is +29% and it is no longer trying to add more fuel. The low end to top end power band is smooth and the car is running much better. But, there is still a problem, obviously, so I want to find it.


The next obvious step is to clean the MAF, but it does report a good 3.8g at idle which is I believe what others are seeing in normal operation.

What do you think? I am still leaning towards fuel pump as it as solved many others problems in other threads. But, if the pump is bad, why would it be plenty rich in the top end?

I do not think it is the o2 sensors because if you clear out the ecu by removing the battery, the car will have trouble idling. This is consistent with running lean until the ecu eventually adds more fuel. Even with no o2 sensors installed, it should idle smoothly from a fresh battery reset.

Mntabs

Clean MAf and check ALL vacuum hoses. Should be a diagram on the passenger side under the hood by the fuses. I have a 1998 gs 400 with 206,000 miles. Same codes plus knock sensor. On mine it was the hoses on the drivers side that go to the charcole canister and one to the power steering. Hope this helps. You could also hook up a fuel pressure guage to the fuel rail and check the pressure to see if your fuel pump is putting out enough fuel.

CurlyG

Buy/borrow a smoke test machine, and smoke test your intake.

CIGLexus

iTrader: (1)

I forgot this is the older setup. I am used to my 2001 which has far less vacuum lines.
UPDATE:
Checked the PS line, it's not that.
Checked EVAP VSV hose, no leak.
Cleaned MAF, reads the same at idle as before.

dwoods801

I had a long bout with code P0171, I had always thought if the car ran good then the fuel pump would be good and if it was bad, the car wouldn’t even start. I eventually found out my fuel pump had just gotten weak with age and couldn’t keep the correct fuel pressure in the rail. So it would try and up the fuel at the injector which is what causes those high LT fuel trim numbers. The fuel pump was a quick and straight forward job, they are very affordable on eBay. Just try get a Denso one, or another high quality equivalent brand that is specifically for your car. I initially tried to go with one from a parts store and they wanted a fortune. It was an “Airtext” that was more of a universal that was supposed work, but didn’t have a good track record in the GS. It’s always advisable to get the bests parts u can afford. So hopefully u don’t have to do the job again. If you have all the freeze frame data from the ob2 reader, I can confirm if it’s the fuel pump, because the O2 sensors will not be showing any anomalies.

CurlyG

I found a Denso 950-0107 Fuel Pump Kit on Amazon that worked perfectly. It's currently listed for $114.

dwoods801

Well hopefully that is in your budget? If you can swing it, I would recommend ordering it. If it’s not in the budget right now, or your not convinced it’s going to solve your DTC. A budget solution would be to try and find a 2nd hand replacement in a Pull-it-Yourself type wrecking yard. They are probably available for under $25 (probably less than $10). This will also give u a chance to practice removing it, to make sure you have the tools and skills to accomplish the repair. And give you time to save up the cash for the brand new Denso.
I would find a how-to video on YouTube, then locate the closest donor vehicle available. Make a list of any other parts that you would like to get if available for your car. Take time to look up the prices for everything at the specific yard you’ll be going to. Take photos of anything you would like to replace so you have a picture to compare with the new parts and the old ones. You want to certain to replace old parts with newer better ones. And u want to try and make sure everything is identical before buying a slightly different part that has a different design after certain years. Get the parts marked for the warranty before you leave. They usually give u a 30-90 day exchange warranty if you need to return anything.
U want to find out what year and model vehicles are available as donor vehicles in your area. And if there is any difference between yours and the donor vehicle, find out if they are going to have any compatibility issues. Even if they should have identical parts, make sure to do a visual comparison between the replacement parts and the old ones, before installing them. If there is any difference they could still be usable, you just want to identify any possible problems in case you have any issues later on.

CIGLexus

iTrader: (1)

I have an OEM fuel pump handy, not the assembly, just the pump. I was just trying to find other solutions before I go there.

dwoods801

if you already have a reliable unit to swap it with, I would definitely start there and confirm or rule out the fuel pump. If the DTC doesn’t come back during the first dozen miles or so and you’ll know you found the problem and can always purchase a new one later. It’ll be about the easiest fuel pump you’ll find to R&R. I had to get a gasket separately cause I was getting an evap code, but that was the only issue I had when replacing mine.
https://www.ebay.com/itm/7716914010-...-127632-2357-0

dwoods801

Did u have any luck finding a solution?