1998 S70 Negative LTFT and P0171

Sandawgk5 · Post by **Sandawgk5** » 21 Dec 2014, 23:57

So, I have been fighting this car for about 6 months trying to get it to clear the codes so I can smog and register it. I have replaced all of the vacuum lines as well as the evap lines above the fuel tank and for the canister purge and vent valves. The problem I am having is the ECM is pulling fuel by driving LTFT -20 to -25% it takes about 2-3 test drives of the drive cycle to set the code and in that time LTFT will drive from 0 to -20 something %. Idle is smooth and right at 840 RPM Vacuum at idle is 20" and boost WOT is ~9# Fuel pressure is 36# at idle and ~56# at 3K and 5# boost. MAF is clean and indicates smooth and hovers right around .570 lb/min at idle and upstream O2 cycles between .14V and .8V.

I am really stuck on this mainly why is the ECM pulling fuel? IT seems like I might be leaking boost and causing a rich condition but looking at my live data it drives STFT double digit lean when coasting like approaching a stop sign. I am not one to throw parts at a problem but I don't know where to go next.

Any help or advice is appreciated.

Thanks

Ira

draser · Post by **draser** » 22 Dec 2014, 12:27

Pretty contradictory readings having negative fuel trim and a lean code. What is your short fuel trim at idle, engine hot? The front o2 sensor is it narrow band, and does it switch rich to lean within a sec? If you look at the plugs, do they look smoked, brownish clean or white? Not sure about maf reading level, is your engine a 5cyl about 2.4, 2.5 liter?

Sandawgk5 · Post by **Sandawgk5** » 22 Dec 2014, 14:48

I have a 2.3L T5 the plugs are a chocolate looking light brown, clean and not wet. The front O2 seems to be switching according to my scanner. STFT at idle is 0 but bounces + and -, if I reset fuel trims LTFT will stay at zero until I go for a test drive. I make sure it is in closed loop and at operating temps before I start the drive cycle. Cleaning the MAF did not change the readings.

Watching my scanner it is really weird as the STFT really drives hard negative on deceleration and that is what seems to be driving the LTFT. There is a slight stumble when I stab the throttle but I have not been able to find any leaks. Unplugging the MAF has no effect on the idle, IAC will idle higher when unplugged, fuel filter is new and fuel pressures seem fine.

I really don't understand what is causing the fuel trims to lean out. I am sure that is why I am getting the lean code but what is driving them low. Could an intercooler leak/boot leak cause this issue when only under boost?

Post by **jimmy57** » 22 Dec 2014, 17:33

The maf signal bothers me a bit. It is on the high side but the idle LTFT being ok is puzzling if it is in fact high.

Look for intercooler and hose leaks. Pressure will blow out air that raises MAF signal and ECM computes fuel for that air and then O2 sensor comp drives it lean.

Sandawgk5 · Post by **Sandawgk5** » 22 Dec 2014, 21:10

I think my intercooler has a leak. Both tanks are covered in oily dirt around the crimp areas.

All of the hoses were tight but they look like this and are super soft and pliable.

Also I let it warm up today and checked the Upstream O2 at idle and it would bounce from .12 to .9 back to .12 up to .6 down to .3 up to .5. It was not rich and then lean it was kinda all over the place. Could it be a combination of boost leaks and a slow O2 sensor?

2nd has anyone ever had any luck reattaching the intercooler mounting tabs? both of mine were bolted in but the intercooler was suspended by the hoses only the brackets fell right out when I unbolted them.
I think this fix has gotten a bit more expensive. I am still into the car for under 2k. an intercooler and hoses will push me over but not by much.

Thanks for the help so far.

Ira

Sandawgk5 · Post by **Sandawgk5** » 22 Dec 2014, 23:42

I also noticed looking at some new intercooler pictures, my weep hole is cracked and about 1/2" in diameter vice the little 1/8" one in the pictures. I wonder if this is causing low air under boost which in turn causes a rich condition to pull fuel.

Post by **jimmy57** » 23 Dec 2014, 09:49

yes, a 1/2 inch hole would lose a large percentage of the air at normal cruising throttle opening.

Post by **cn90** » 23 Dec 2014, 09:54

You can get these parts (turbo cooler) as used at local junk yard or ebay.

Sandawgk5 · Post by **Sandawgk5** » 23 Dec 2014, 15:10

Here is the size of the hole in the bottom of the intercooler. As you can see by the part number it is considerably larger than the factory hole and has a crack running out from it.

Post by **jimmy57** » 23 Dec 2014, 17:06

Get a plug of some sort and plug the hole and recheck the LTFT after a drive. That will tell you for sure if the hole is it.

I have used short pieces of rubber hose one inside the other with a screw or bolt run into the center to expand it. 3/8 fuel hose is 1/2 od and 1/4 vac or fuel hose is about 3/8 od to fit inside. Push in the 3/8 into hole and then lube the 1/4 and push it into the 3/8 hose. Then screw a 1/4 inch lag screw or regular thread bolt to expand it.