MAF voltage

[GodziRRa]

Hello!
I've been trouble shooting my car lately for some cold start problem, and found out that my MAFs are giving me different voltage readings! For example, with the key on the bottom MAF is gving me 0.25v while the upper MAF will only read 0.18. But at idle the difference will be significant. MAF 1 will be around 1.25v, and MAF 2 around 0.80 only!

Any of you guys know what is the correct MAF voltage values w/ the key on and at idle?

[Skym]

Cold start is either dirty or faulty IACV or ECU tune needs adjusting (common) if your engine has exhaust, etc. Factory tends to run start up tables rich and engine struggles to idle. You should smell fuel on startup which gives it away.

Also engine running rich is the cause of rough idle, especially when there's airleaks as o2 sensors pickup engine running lean, so ECU runs engine rich and engine splutters, misses, then ECU runs engine lean again and then repeats. I was told it can be worse if sparkplugs are gapped down from factory gap.

That's normal for GTR MAF's to have different voltages. ECU should find the middle ground between them to use as part of formula to workout load figure to select cells on fuel, ignition maps, etc.

MAF voltage varies at idle, especially if there's a airleak near MAF's or near vacuum hoses that supply air to idle valves.

[GodziRRa]

I see. One mechanic told me that a GT-R's MAF voltage should be the same. That's why I'm concentrating on the MAFs at this time. I don't think that an air leak in the MAF is my problem now since w/ just the key on the my MAF voltage is already different from one another. And yes my car is running extra rich lately. The mechanic added that this causes my hard cold starting and rich running. This just happened lately, and I haven't done anything significant in the car. It already has the 3.5 exhaust, and new AAC valve...

How about you guys what's your MAF voltages?

[markc32]

skym is right the readings on both mafs are diff on my car and my car has no issues on cold start...

my mafs read .80 to 1V at idle fully warmed up and approx 2.00V at 2000rpm driving i hit 3V at 4000rpm...ecu will read in the middle for a/f ratio

urs is kinda close .8 n 1.25v so the ecu will see .9v or so for both...i havent tried reading voltage with ign on only...ill check for u later but to me just clean ur maf if u havent n use .8mm gap plugs u mite want to check ur spark if u have a weak spark u can run rich anytime

[Skym]

AACV (Auxilary Air Control Valve) is different to IACV (Idle Air Control Valve). Both are idle valves and both are used during warmup. IACV uses engine heat, voltage to power motor to close valve (sometimes valve can stick when dirty due to excessive oil in blowby from engine PCV system or turbo failure). AACV is controlled by ECU and controls idle rpm when engine is warm and when auxilaries like aircon, light's, etc are used.

O2 sensors are another cause of running rich. Unplugging them should tell you if they are the cause.

Faulty coilpacks or anything knocking on engine are things which create high knock count, ECU switches to knock maps that are richer.

[GodziRRa]

Thanks guys!
@ Skym, is the IACV the cold start valve under the plenum? Anyways, as for the coilpacks they're fairly new also (splitfire), same w/ the coilpack harness and igniter. Bad O2 sensor will cause the car to run rich, but can it also cause a coldstart problem?

@ markc32, yes please do check if it's alright.

[Skym]

I think IACV is mounted on throttlebodies under intake plenum (close to engine due to it needs heat to work) on RB26 (hard to get to). Mainly IACV or airleak is the cause, but ECU tune is another. O2 sensors are not looked at until coolant temp reaches around 30 degrees.

With ECU tune problem, keep cranking the engine over (gets pistons pumping, sucking air into engine for idle valves) for about 5-10sec or until you feel engine struggles to crank and then let go of key and it should start fairly normally. If that happens, ECU tune is probably too rich.

What happens is cranking uses first time startup fuel table (ignition key to on position) on ECU and letting go of key uses after start startup fuel table on ECU. The after start table is usually richer and the cause of bad starting, but both tables are way too rich. Also you'll notice when you turn engine off after starting engine, then turn engine on again, engine starts without problems as I think ECU uses the cold warmup table that's leaner by 20-30% or so.

[GodziRRa]

I think IACV is mounted on throttlebodies under intake plenum (close to engine due to it needs heat to work) on RB26 (hard to get to). Mainly IACV or airleak is the cause, but ECU tune is another. O2 sensors are not looked at until coolant temp reaches around 30 degrees.

With ECU tune problem, keep cranking the engine over (gets pistons pumping, sucking air into engine for idle valves) for about 5-10sec or until you feel engine struggles to crank and then let go of key and it should start fairly normally. If that happens, ECU tune is probably too rich.

What happens is cranking uses first time startup fuel table (ignition key to on position) on ECU and letting go of key uses after start startup fuel table on ECU. The after start table is usually richer and the cause of bad starting, but both tables are way too rich. Also you'll notice when you turn engine off after starting engine, then turn engine on again, engine starts without problems as I think ECU uses the cold warmup table that's leaner by 20-30% or so.

I tried another ecu today mate, but still the same starting problem. So ecu tune is not the problem. And on thee other ecu the upper maf still has the lower voltage. Checked the harness also... seems everything is in order. What do you think about CAS mate?

Yep the cold start valve is under the plenum and is very hard to reach, needs to take the whole plenum off. Will a bad cold start valve still make the car run extra rich after warm up?

No error codes by the way.

[Skym]

Using another ECU won't solve ECU tune problem, as has the same table problem. Fitting Nistune, adjusting tune can fix it.

Faulty IACV can cause high idle when engine is warm. When dirty it causes starting problems.

CAS can cause hard starting problems when engine is cold or hot and get CAS error code on ECU. But since you said no error codes, then CAS is not the cause.

Sounds like faulty o2 sensors. The o2 sensors should be checked and changed if necessary every 40,000-60,000km.

If you hold rpm at 2000rpm, should notice o2 sensor voltage fluctuate from 0V lean to 1V rich. I think o2 sensor voltage should do this in 5sec from lean to rich, then 5sec from rich to lean.

[GodziRRa]

Ok I'll check o2 voltages tomorrow. About the error codes, isn't it the ecu won't have any error codes as long as sensors is still giving signal? Like for example, I won't be having error codes on a malfunctioning coolant temp. sensor, since it is still functioning but is giving false reading.

Thanks for the ecu tuning explanation. Though I think it is not my concern at the moment since I haven't done any mods to the car when this problem began.

[Skym]

Also check TPS voltage, as that affects fueling (transient enrichment).

Sometimes ECU doesn't pickup on faulty sensors. Found that out with TPS on my car, as had to check at TPS plug via multimeter.

Coolant temp sensor can make engine run rich, as ECU doesn't move off the warmup tables. But would have coolant temp sensor error code on ECU.

Sometimes it can be as easy as a intake hose joiner popping off on FMIC.

It's just a process of elimination.

[GodziRRa]

I think I know what the problem really is now... I remembered this problem also started when I also started using 100 octane fuel. Since this fuel will be more SLOWER to burn, I think it causes my cold start problem since my timing isn't adjusted accordingly w/ the new fuel. And what I thought to be the car running extra rich is really not the case, but actually it isn't burning the high octane fuel efficiently resulting to dirty plugs.

I'll check my timing tomorrow and see how it goes.

[Skym]

I noticed you get the same starting problem with E85 fuel. There's a start up ignition timing table. Nistune would help, as can fix startup tables.

[GodziRRa]

Yeah. I also did my research and other cars or motorcycles alike here are experiencing this problem.

[GodziRRa]

Well... unfortunately it is not the problem. I think it really has to do w/ the low MAF voltage in the rear. I tested it again with my scan tool today. When I step on the gas pedal @ idle, the low MAF voltage also risen up above 1 volt but still not as high as the front MAF... but when you release the pedal the rear MAF will drop to like as low as 0.40 V then will climb up to around 0.75 V, while the other just falls to around 1 volt. And when that happens you can feel the engine shudders a bit, at sequence w/ the drop of the rear MAF voltage.

I tested another MAF, another ecu, used a test light to check for signal and grounds of the MAF plug... still no luck.