Here's the screenshot. I decided to spray the MAF sensor down thoroughly with MAF cleaner. Drives great but still have no idea why -20 LTFT. STFT goes even more negative throughout the day.

Mileage is about 10-11L/100km or 21-23 mpg.

 

Is yours in Oz land 1.8L? At idle it should be approx 1.8 grams then I believe. That is about 20% difference there from 2.2. Which o2 sensor did you replace? It says in your screen shot MAF = 2.4 now. The upstream is called a wideband O2 sensor aka Air/fuel ratio sensor I believe.

 

Is yours in Oz land 1.8L? At idle it should be approx 1.8 grams then I believe. That is about 20% difference there from 2.2. Which o2 sensor did you replace? It says in your screen shot MAF = 2.4 now. The upstream is called a wideband O2 sensor aka Air/fuel ratio sensor.

I'm in Aus but the car is a Japan build (pre Oct from memory?).

Both the MAF and the O2 are Denso. Yeah I call I O2 to simplify things but you're right it's a wideband sensor. On these Jap builds though they only have one sensor. I'm not sure why but I've confirmed it with a number of people + looked for it myself in the engine bay tracing it out of the engine to the exhaust (including removing the shield at the back).

Pardon our interruption...

The weird thing is that I've got a cheap Chinese MAF sensor as something to test and it gives me excellent numbers. The trouble is that it's gives me rattling timing chain noises while the Denso doesn't so I'm trusting the Denso more 🤷‍♂️.

 

I’m willing to bet that you have a leaking fuel injector o ring. In a rich condition, you’re consuming more fuel. Lean condition is the opposite and points to a vacuum leak in front of the maf sensor.

You don’t have a downstream sensor because you don’t have a cat.

 

I'll buy some injector O-rings and come back. That's the one at the top of the injector and not the big donut that attaches at the nozzle?

 

Replace both. After a long time, they flatten and glue themselves. Or break apart.

 

Forgot to mention, check your fuel injectors too. They may be leaking internally.

 

That part I find hard to do. Do you know what's the easiest way? I've tried YouTube before but still a bit confused.

 

Use 12v battery and pulse the power. If you use constant power, it will break. The pulse will mimic the actual operation in the engine. You’re looking for all orifices to spray. When it doesn’t spray out, then it’s clogged or leaking.

Here’s the basics video.

 

Thanks so much! I'll get done as soon as the repair kits come 😁 This might take a weeks time or two.

 

Do you know if an exhaust manifold leak could cause a rich condition at idle?

I am pretty sure it can as it can affect the A/F ratio seen by the sensor

 

Here's what my stats look like after the Engine Oil Flush and Oil+Filter Change. The interesting thing about this is in Park - usually my car runs at a significantly higher rev on Park. And yet, MAF is running fine. I'll be working on the fuel injectors next.

 

Well, I cleaned the injectors and put new kits in but it still is the same mileage/numbers.

Two things that have improved include smoother driving and less noise (with new timing belt tensioner)

I had found one injector that didn't seem to spray too well - it seems to not be able to stay open when I gave it charge.

Is it worth buying a cheap injector on eBay to see if it improves?

 

What’s the condition of the coil packs and spark plugs? Do any of them smell of fuel?

Have you checked for any vacuum leaks?

 

I'll double check when I get home. I replaced the coil packs a while ago with 2nd hand parts and I have sprayed carb cleaner to try to detect leaks.

What manifold would cause -ve?

 

Is it cable throttle or electronic? Maybe throttle body is gummed up from the engine flush.

 

It's 2005 Jap build so cable. I've wiped it down before, but I can check.

 

Have you ever adjusted the throttle cable since owning it?

Have you checked the IACV?

 

Yes. In fact, I adjusted it after the fuel injectors were serviced. I made sure that it just had enough slack to not lift the butterfly valve when idle.

The IACV valve may be on its way out. In the past, I would experience RPM surges. It eventually went away when I sprayed carby cleaner. Right now, the RPM sit comfortably at 675 RPM and does not deviate much from that.