Engine Uncertain MAF voltage

PonyGTrider · Sep 16, 2019

Hi all,
Now a long time since my last post.

I’ve been running onto an apparent rich condition for a while, and now I’m trying to get to the bottom of the problem.
To start there are no Trouble Codes
Nice and clean injectors
Good pressure regulator
Fuel pressure at 39 psi.

Right now doing MAF sensor test and O2 sensors to be done.

MAF sensor test completed and there is what I consider a high voltage output at idle (1.2 volts)
The Chilton’s book call for 0.6 volts at idle.

Is my MAF voltage out of range?
If I start her from stone cold there is some hesitation and takes one to two minutes for the idle to stabilize. And if take off quickly after that and and give a medium/high
acceleration, I heard and feel some popping in the exhaust pipe and starts bucking so have to ease on the throttle. Also some raw gas smell (Have to mention that the smog system and the secondary cats are not there anymore)

Like I say, I will test the O2 sensor voltage at the PCM harness, but does anyone tell me what is going on with my MAF? The voltage increases to over 3 volts... And I go thru a full fuel tank amazingly quick ☹
NO ERROR CODES
Any ideas? Thank you all!!!

Noobz347 · Sep 16, 2019

What are your other mods?

PonyGTrider · Sep 16, 2019

Noobz347 said:
What are your other mods?

PonyGTrider · Sep 16, 2019

I have a TFS 306 with 190lph pump, Kirban AFPR, 65mm TB, stock 19 lbs injectors paired with its 55mm MAF tube and stock sensor....

Noobz347 · Sep 16, 2019

Try re-clocking your mass air meter.

PonyGTrider said:
paired with its 55mm MAF tube

Does this mean that using a mass air housing that makes use of a removable sample tube? If so, this is likely your issue.

"Dirty Air" across the sensor often causes gremlins that will make you pull your damned hair out. Try reclocking the sensor to see if it improves. If not, try swapping in another meter, even a stock one with a stock housing, just to see if things smooth out.

Sample tubes in Mass Air Housing are a personal pet peeve. I've seen way too many that just suck suck suck for it to be a coincidence. I'm certain that I have an old video around here somewhere when I shot a few off of the top of a fence post.

jrichker · Sep 16, 2019

PonyGTrider said:
Hi all,
Now a long time since my last post.

I’ve been running onto an apparent rich condition for a while, and now I’m trying to get to the bottom of the problem.
To start there are no Trouble Codes
Nice and clean injectors
Good pressure regulator
Fuel pressure at 39 psi.

Right now doing MAF sensor test and O2 sensors to be done.

MAF sensor test completed and there is what I consider a high voltage output at idle (1.2 volts)
The Chilton’s book call for 0.6 volts at idle.

Is my MAF voltage out of range?
If I start her from stone cold there is some hesitation and takes one to two minutes for the idle to stabilize. And if take off quickly after that and and give a medium/high
acceleration, I heard and feel some popping in the exhaust pipe and starts bucking so have to ease on the throttle. Also some raw gas smell (Have to mention that the smog system and the secondary cats are not there anymore)

Like I say, I will test the O2 sensor voltage at the PCM harness, but does anyone tell me what is going on with my MAF? The voltage increases to over 3 volts... And I go thru a full fuel tank amazingly quick ☹
NO ERROR CODES
Any ideas? Thank you all!!!

No code 11 (two flashes when the computer dumps the codes)? A perfectly functioning system will always get a code 11, it is the everything is OK code. The 11 code is computer passed its internal self test.
If you don't get an 11, you have computer or wiring problems.
Please check and repost.

PonyGTrider · Sep 16, 2019

Well I posted twice NO ERROR CODES. Code 11 absolves the ECM from any errors.

I’m a little confused about the O2 voltage readings. I red that O2 sensors should be switching between 0.2 V to 0.9 V, and yet I just red a suggestion than they should go as low as 400 milivolts.
I just posted my readings... So are they out of specs???

jrichker · Sep 17, 2019

PonyGTrider said:
Well I posted twice NO ERROR CODES. Code 11 absolves the ECM from any errors.

I’m a little confused about the O2 voltage readings. I red that O2 sensors should be switching between 0.2 V to 0.9 V, and yet I just red a suggestion than they should go as low as 400 milivolts.
I just posted my readings... So are they out of specs???

You would be surprised how many times I have been told NO ERROR CODES and they didn't even get the code 11. No code 11 is usually the result of internal damage to the computer caused by connecting the test point to the wrong connector.

If you got a code 11 and no code 41/91, then the O2 sensors are good.

Be sure to use an analog meter or a DVM with a bar graph reading below the digital numbers on the display. Digital only display means the DVM averages the voltage instead of giving an accurate real time measurement of a constantly changing voltage.

PonyGTrider · Sep 17, 2019

No removable sample tube, I meant I have a 55mm stock MAF tube and sensor. Sorry for the confusion

PonyGTrider · Sep 17, 2019

jrichker said:
You would be surprised how many times I have been told NO ERROR CODES and they didn't even get the code 11. No code 11 is usually the result of internal damage to the computer caused by connecting the test point to the wrong connector.

If you got a code 11 and no code 41/91, then the O2 sensors are good.

Be sure to use an analog meter or a DVM with a bar graph reading below the digital numbers on the display. Digital only display means the DVM averages the voltage instead of giving an accurate real time measurement of a constantly changing voltage.

Ok I’ñl Try to do that, and no error codes only code 11.

What about the MAF high voltage readings?? Any ideas?

vristang · Sep 17, 2019

Not sure I would trust the Chilton on the test voltage value... 1.2v is right in the ballpark of where the TPS sits at idle (both are 5v scaled), so that value doesn't seem too outlandish. especially if an aftermarket intake is in place... Is that an intake and heads? not familiar with the 'tfs-306'
Has the MAF sensor been cleaned recently?

O2 output is very tough to measure correctly with a DVM, which is why jrichker suggests relying on the ecu for that test. I wouldn't even bother using a DVM on the O2's...

PonyGTrider · Sep 17, 2019

I understand what you’re saying about the O2 sensors. As far as the MAF sensor is going, the resistors are nice and clean. Since I’m using a K&N filter and even though I use very light oil coat on it, I clean the MAF sensor as often and careful as I can to remove any potential contaminants from it.

The sensor itself is an old piece but then if the ECM diagnosis come as 11, then the sensor supposed to be working fine. Could a sensor be working on the borderline of its acceptable specs inducing a higher fuel consumption without triggering the expected ECM error, or it is just a crazy idea?

On the other hand and just myself thinking loud, if the sensor based on the chart on the manual.

Engine Condition / Signal Voltage
Idle. 0.6 Volts
20 mph. 1.1 Volts
40 mph. 1.7 Volts
60 mph. 2.1 Volts

If those values are close to reality, and having my MAF voltage 1.2 Volts at idle, does it make any sense to think that looks like at idle is sending the signal as if it was at 20 mph which woul consume more fuel, but she is still at idle?

Please be patient with me about my hypothesis

Also I heard that some bad sensors won’t trigger a fault until they have no signal and are completely broken. Could that be the case on a sensor like the MAF, which is my case?

Thank you all guys for your patience and help

vristang · Sep 17, 2019

I suppose the MAF could be an issue the way you describe. I don't recall the thresholds that the ecu uses to throw codes...
Don't suppose you have a spare sensor laying around?

Noobz347 · Sep 17, 2019

The different voltages you listed are largely irrelevant unless they are either out of scale or inconsistent.

Maybe @a91what has a transfer function for an OEM meter laying around that he can post?

a91what · Sep 17, 2019

well of course I do.... however this information will be courtesy of @Decipha website.

MAF Transfer Curves / EFIDynoTuning

now to simplify some information.... .6 volts from the stock maf is around 14 g/sec this is very normal.. your saying you have 1.2V at idle on the stock maf that is around 45g/sec and in no way normal, this would be a gross over-fueling.. BTW the MAF scale is not linear it is more of a logarithmic scale. now 'at idle' is a also suggestive... what is your idle speed? 400millivolts would in fact indicate a rich condition from the O2 but as stated above they are hard to read with a DMM..

PonyGTrider · Sep 17, 2019

Thank you all for staying with me on this issue.
Unfortunately I don’t have a spare MAF sensor, so I will have to get a new one.

That is a great information about the MAF transfer curves. I appreciate so much you share them. And my Idle speed is about 850 - 900 rpm

You also mentioned that the MAF scale is not linear and really it looks like in this particular case is very close to it... 1.2 V @ idle, about 3.3 V @ WOT. And yes it makes sense that somehow based on those numbers signaled to the ECM resulting in over fueling the system.

Now to prove this hypothesis a new MAF sensor is required.
First to see if the new sensor provides lower voltages at idle and under acceleration.
And second, to prove if this is the culprit of my hypothetical rich condition, improving my mpg at the end.

Thank you all!!!

Decipha · Sep 17, 2019

if you have tfs heads on a 306 then you will peg the 19lb injectors

if you have the stock injectors and stock maf then you should not have fueling problems.

At idle your maf voltage should be around 1.2-1.5 maf volts depending on your cam. No way its at 0.6 volts you must be measuring it wrong as the engine would be lean.

since you have an adjustable fpr verify the base pres is 40 psi at idle with the vac line off, make sure theres no fuel in the vac line either

the most usual cause for vehicles running rich is due to old o2 sensors. If they arent recent then replace them first.

the o2 sensor enable wire on the back of the head usually causes issues too. Make sure its secured to the head with a clean connection as it is the ground reference for the o2s.

PonyGTrider · Sep 18, 2019

... I’ve heard some people making 370 hp on 19 lb injectors, but probably you’re right.

I’m running stage 1 cam and no I’m not measuring wrong the MAF voltage. I got 1.2 V at idle and almost 3.3 at WOT. I was expecting 0.6 V at idle.

I’ve got almost 41 lb vacuum off at the AFPR and 39 lb vacuum on, no broken diaphragm, and the O2 sensors ground is bolted clean and solid.

Boostedpimp · Sep 18, 2019

PonyGTrider said:
... I’ve heard some people making 370 hp on 19 lb injectors, but probably you’re right

lol 370hp with 19lb injectors... Sorry gonna have to call BS on that one

The theoretical maximum for 19lb injectors @ stock pressure is 330 flywheel bhp (~280whp) at 100% duty cycle with an AFR of 13:1 and a BSFC of 0.48

Given that information and the fact that one of my previous cars we got 287 to the wheel on the dyno with the car really lean. The duty cycle was pretty much static even having the fuel pressure cranked up a stupid amount.

Noobz347 · Sep 18, 2019

Boostedpimp said:
lol 370hp with 19lb injectors... Sorry gonna have to call BS on that one

The theoretical maximum for 19lb injectors @ stock pressure is 330 flywheel bhp (~280whp) at 100% duty cycle with an AFR of 13:1 and a BSFC of 0.48

Given that information and the fact that one of my previous cars we got 287 to the wheel on the dyno with the car really lean. The duty cycle was pretty much static even having the fuel pressure cranked up a stupid amount.

Oh, it can be done. Just not at 46 psi. At one time, I pushed mine upwards of 90 psi and supported a blown 302 to nearly 400 RW with 225L HP Walbro, and an FMU.

This was by no means ideal and things got considerably better when I swapped to 42 and kept pressure much closer to stock levels. The whole fuel curve improved.

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