MAF sensor question

[Mark R]

Vehicle is 2006 Lexus IS 250 with 150k miles.

I've just been playing around with my scan tool, and I''ve had some readings which make me wonder if my MAF sensor is borderline.

There are no codes or anything like that. However, I've got some borderline driveability issues e.g. when the car is cold it makes the accelerator insanely sensitive (the car will surge forward with a light touch on the gas, before settling down to make progress as expected).

The thing that makes me a bit suspicious is that my LT fuel trims are quite low: about -15% at idle, -6% at 2500 rpm idle and about -3% at highway cruise.

The other thing that is a bit odd is that the scan tool is showing that the ECU isn't cutting fuel on the overrun, instead during overrun, the ST/LT fuel trims drop to about -18%/-15%, and the AFR drops off the bottom end of the sensor range (<9). Eventually when the rpm drops to about 900 rpm, the ECU cuts fuel for about 2 seconds (AFR shoots off the top end of sensor range > 20) before the engine hits normal idle speed.

MAF at idle is about 3.2 g/s. This sounds a bit high to me, but I wonder what others think for a 2.5 l v6.

I'm guessing that the MAF is overreading at low air flows, which is making the car a bit feisty in open loop mode and blocking the fuel cut because the ECU thinks that the throttle is still open.

Was just wanting some ideas before I drop $150 on a new MAF sensor.

 

[Jimzz]

Have you tried cleaning it yet? Most parts stores carry the proper cleaner. 1 can will do several MAF sensors.

Have you cleaned out the intake and intake valves? I think that motor is a early DI motor so the intake is known to get dirty.

 

[Meghan54]

Have you tried cleaning it yet? Most parts stores carry the proper cleaner. 1 can will do several MAF sensors.

That's the first thing I'd try. Simple and cheap to do. And I'd especially suggest that if you were foolish enough to be running a K&N filter or any other oiled cotton/fabric type filter.

 

[Mark R]

I gave the MAF filter a clean out with a can of sensor cleaner a few k miles ago, just as part of routine maintenance when I did the plugs, air filter, intake cleaning, etc.

I might give it another go and see if it makes any difference to the numbers.

I did think of taking a look at the valves when I did the plugs. However, it was a difficult enough job to get the top half of the manifold off to get access to the plugs, and getting access to the valves would have meant taking off the rest of the manifold, as the bottom half contains the swirl flaps which obstruct access to the valves.

Lots of bolts, lots of wires and lots of gaskets to replace.

 

[tortillasoup]

you might be able to clean the maf wires with a q-tip if you're super careful. The cleaners work but they don't get all the dirt off. You need to sometimes mechanically clean off the gunk that gets on that wire.

 

[disappoint]

I don't think a MAF would over read the airflow if it were dirty, usually they under read the air flow when they get dirty. Also you recently cleaned it which reinforces that it's probably not from being dirty.

Have you changed the O2 sensor(s) yet? I think they only last about 90k and you're at 150k so you must have.

 

[tortillasoup]

Well typically (but not always) if the o2 sensors in one of the banks is dying, they'll read lean which will cause the car to run rich. When you have a long term fuel trim as high as -15% that severely narrows the causes of the problems since that means the o2 sensor is detecting a very rich fuel mixture. If a o2 sensor detects lean like when its failing, then it adds fuel. It's pretty uncommon for the computer to DETECT a RICH condition and for the solution to be replacement of the o2 sensor.

After OP cleans his MAF sensor, he should find out the valve lash and see if it's within spec. If it is and there is no adjustment possible, then aside from a multiple cylinder misfire that is very light not to be detected or trigger a CEL, the most likely issue would be then a sticking/clogged fuel injector(s). To further the point about it being fuel injector related, the fact that this issue is worse at idle and better when cruising suggests that some injector(s) aren't able to close all the way and are leaking fuel, screwing up the balance.

Cleaning the fuel injectors on this car would not be a bad idea at all!!!

 

[Mark R]

After OP cleans his MAF sensor, he should find out the valve lash and see if it's within spec. If it is and there is no adjustment possible, then aside from a multiple cylinder misfire that is very light not to be detected or trigger a CEL, the most likely issue would be then a sticking/clogged fuel injector(s). To further the point about it being fuel injector related, the fact that this issue is worse at idle and better when cruising suggests that some injector(s) aren't able to close all the way and are leaking fuel, screwing up the balance.

Cleaning the fuel injectors on this car would not be a bad idea at all!!!

I'll let you know what happens once I sort the MAF out.

I've been thinking about valves and fuel injectors. The trouble is that they are both hellacious jobs. You basically need to disassemble the entire high-pressure fuel system to do either of them, and that needs special tools and a huge number of replacement seals and O-rings.

Lexus do not have valve lash as a scheduled maintenance item, probably because of this (at least not in the European service manual; it's listed as "audible" valve inspection every 50 k).

As for injector cleaning potions, I've used a variety of those, and not noticed anything, but I didn't have particularly high hopes for that. Things I haven't tried are induction spray cleaning, and injector cleaners that deliver solvents directly to the fuel rail (or in this case to the inlet of the HPFP); but these are expensive, and I don't particularly fancy their chances at solving the problem.

 

[tortillasoup]

The system on your car is a combination of port injection and direct injection. If there is any consolation, one thing you might consider is cleaning just the port injectors and leaving the direct injection system alone entirely. There are some accounts that the port injection is used exclusively at idle (likely to prevent carbon buildup if the car gets lots of city driving/idling) so that would explain quite easily why you'd have poor trims at idle. Since port injection is at a low pressure it kinda makes sense that it would be more prone to clogging vs. the high pressure direct injection system.

 

[Jimzz]

The system on your car is a combination of port injection and direct injection. If there is any consolation, one thing you might consider is cleaning just the port injectors and leaving the direct injection system alone entirely. There are some accounts that the port injection is used exclusively at idle (likely to prevent carbon buildup if the car gets lots of city driving/idling) so that would explain quite easily why you'd have poor trims at idle. Since port injection is at a low pressure it kinda makes sense that it would be more prone to clogging vs. the high pressure direct injection system.

The 250 is Direct Injection only. The 350 motor has dual injection (DI and Port Injection)

 

[tortillasoup]

The 250 is Direct Injection only. The 350 motor has dual injection (DI and Port Injection)

You're right! Shit that sucks for op...

He should do some light reading in this thread:http://www.clublexus.com/forums/is-...se-engine-carbon-build-up-merged-threads.html

Good website for more information on your lexus that and Lexusownersclub.com

 

[Mark R]

I cleaned the MAF sensor again, and it made a bit of difference. The trims went from about -15 to about -10 at idle (with a logging driving range of -3 to -14).

MAF at idle was about 2.9 grams. Service manual says normal range is 1.9 - 2.9 g/s.

I decided, what the heck, and got a new sensor.

Trims are now at zero (with a logging range of +3 to -3), and noticably more fuel cut. MAF at idle with the new sensor is 2.4 grams.

 

[tortillasoup]

Keep the old sensor and inspect it. I don't know how your MAF sensor is designed but on some vehicles, it's difficult to access the "back side" of the MAF wire which is the part that gets most exposed to the air stream. so when you clean the front of the wire, you're actually cleaning the back side (the side of the wire that doesn't get exposed to the incoming air stream).

I'm glad to hear getting a new MAF resolved your issue.

 

[Mark R]

That's exactly how it's designed! Thanks.

 

[bryanl]

O2 sensors aren't used for cold engine operation.

I believe all MAF sensors are heated enough for the first few seconds to burn off deposits, making them largely self cleaning.

There's been no mention of the throttle position sensor, which can make the gas pedal more or less sensitive than normal and alter A/F.