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Old 06-18-2022, 07:55 PM   #1
KT1986
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Default MAF may be dying, 4th time's the charm?

Ive got another MAF that appears to be dying, or so it seems according to the Volvo LH 2.4 green book. According to the book, its supposed to read 2.5 to 4.0 ohms of resistance between pins 2 and 3, but Im reading 19.25 M ohms of resistance on these pins according to my meter.

This is the 3rd MAF Ive put in this thing, and I bought the Volvo Genuine part from FCP this time, thinking the Delphi MAFs I had bought prior were just craptastic. My fuel economy is crap, 14 to 15 mpg, and I know these cars arent the best for mileage, but damn. My buddy's 96 silverado with a 350 gets better mileage.

I dont dog on the car. Sure, once and a while I get a little frisky with the gas pedal but I keep that to a minimum since premium hurts the wallet. Also, interestingly, when my alternator died, before I figured it out, my mileage rose to around 18 mpg, but at the expense of a dead battery. Though I was getting the 14 to 15 before the alternator died, so I dont think that had anything to do with it.

Also, I noticed that my idle when cold, say first thing in the morning, seems to be smooth and steady but after the engine warms up and outside temps go up, the idle isnt steady and is real low at first at initial start. Ive replaced every vacuum line, cleaned every ground, and checked the connections to everything and nothing seems to have helped at all.

When I first got the car, i was able to at least get 18 before the first MAF died, then its been this back and forth with good mileage, MAF dies or semi works, 14 to 15 mpg. Is it possible the ECU is killing these or am I just having a string of bad parts?

I have a 1991 744 Turbo, B230FT LH 2.4, all stock
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Old 06-18-2022, 09:55 PM   #2
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You'll want to do some testing. There is a platinum wire burn off function that happens about 45 seconds after you shut off the car. If that function doesn't work. Crud will build up on the platinum wire and it will fail after a little while. You need a voltmeter or test light and back probe the pin on the mass air sensor. Sorry I forget which pin it is but it's in the manuals.

The burn off is done by the ecu and if it's not happening and the wiring is good then you want another ecu to try.
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Old 06-19-2022, 12:11 AM   #3
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There is no possible and conclusive test on an air mass meter using a multimeter. To test one you must send it and hope for the best
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Old 06-19-2022, 01:59 PM   #4
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the cleanflametrap method has been the best maf "test" method iv'e come across. The voltage on bad or worn mafs is gets too low at powered rest. https://www.turbobricks.com/forums/s...74&postcount=6
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Old 06-19-2022, 02:20 PM   #5
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Yes, Arts method is good. Volvo has an old TSB on testing the MAF with a voltage test like Art has on his site.
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Old 06-21-2022, 07:51 PM   #6
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The voltages I get are 1.4v with ignition on, engine off and 2.4 volts when at idle. I remember reading somewhere its supposed to be 2.3v, but Im not wholly sure that little extra is an issue or not.

I bought a used Bosch MAF off ebay, when it gets here ill put it in and see what happens. Worst that happens mileage stays the same and I have a spare. Then off in a different direction to see whats up.
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Old 06-21-2022, 08:10 PM   #7
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If the engine block ground(s) is crusty it can fry AMMs, inexplicably. That can be a weird one to diagnose/isolate.

As can a bunch of blowby from the oil trap box huffing around the corrugated intake hose (N/a models, rubber hose on 7/9Turbos). on an old blowby motor or marginally sludged up oil separator box.
Pre-heat t-stat stuck hot or tar balls from a filthy airfilter in a dusty/road construction environment will also do them in.
As can moisture/non PH neutral soaps used at the coin op car wash/used car lot type places pressure washing the engine bay or that part with soap containing lye or hard water getting into the film resistors on the circuit board of the amm...
As can vibes/getting yanked on from marginal motor mounts or taking the car off jumps/gravel stage rally use

90+ 7/9 Turbo cars have the sound deadening foam in the top of the airbox lid that can get sucked into the AMM in little bits cause it to fail prematurely.

Haven't had any luck with anything but the OEM Bosch replacement or Bosch Exchange reman in W.Germany AMMs.
2-6 months or DOA is pretty typical of the 'rebuilt' &/or china-repop AMMs...

After trying a 'known good' amm & it's still using excess fuel, keep looking/doing other tests as you say?

Driving gently(ish) (but not obsessively so keeping the boost explicitly below 0-1psi at all times or anything silly...get up to speed/merge in with traffic like a normal person), 19-20mpg combined was about all I was able to get out of my automatic 700Turbos in good shape. 24-27 highway maybe.
22-23 combined driving gently(ish) in the 5-speed 1990 740 in good shape. 27-31mpg freeway.
Really bad traffic heat soaked with the A/C on, 16mpg wasn't unheard of in the stock otherwise healthy automatic 7/9Turbo cars, all city.
Kind of remarkable the range for a car that only has/had? ~162HP when new (& I guess a bit more torque/grunt to get up to speed with the turbo, obviously?)
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Last edited by Kjets On a Plane; 06-21-2022 at 08:22 PM..
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Old 06-22-2022, 10:03 PM   #8
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Quote:
Originally Posted by Kjets On a Plane View Post
If the engine block ground(s) is crusty it can fry AMMs, inexplicably. That can be a weird one to diagnose/isolate.

As can a bunch of blowby from the oil trap box huffing around the corrugated intake hose (N/a models, rubber hose on 7/9Turbos). on an old blowby motor or marginally sludged up oil separator box.
Pre-heat t-stat stuck hot or tar balls from a filthy airfilter in a dusty/road construction environment will also do them in.
As can moisture/non PH neutral soaps used at the coin op car wash/used car lot type places pressure washing the engine bay or that part with soap containing lye or hard water getting into the film resistors on the circuit board of the amm...
As can vibes/getting yanked on from marginal motor mounts or taking the car off jumps/gravel stage rally use

90+ 7/9 Turbo cars have the sound deadening foam in the top of the airbox lid that can get sucked into the AMM in little bits cause it to fail prematurely.

Haven't had any luck with anything but the OEM Bosch replacement or Bosch Exchange reman in W.Germany AMMs.
2-6 months or DOA is pretty typical of the 'rebuilt' &/or china-repop AMMs...

After trying a 'known good' amm & it's still using excess fuel, keep looking/doing other tests as you say?

Driving gently(ish) (but not obsessively so keeping the boost explicitly below 0-1psi at all times or anything silly...get up to speed/merge in with traffic like a normal person), 19-20mpg combined was about all I was able to get out of my automatic 700Turbos in good shape. 24-27 highway maybe.
22-23 combined driving gently(ish) in the 5-speed 1990 740 in good shape. 27-31mpg freeway.
Really bad traffic heat soaked with the A/C on, 16mpg wasn't unheard of in the stock otherwise healthy automatic 7/9Turbo cars, all city.
Kind of remarkable the range for a car that only has/had? ~162HP when new (& I guess a bit more torque/grunt to get up to speed with the turbo, obviously?)
I did replace the breather box, flame trap and associated hoses already as they were either broken, hard or just not in good condition, also replaced every vacuum line under the hood and cleaned all the grounds in there too. Ive had more issues with grounds biting me in the ass before so it seemed prudent to go after them.

I replaced the hose that goes from the AMM to the turbo with a silicone one from IPD as the original was NFG, would collapse under hard acceleration and choke it. as far as the pre-heat t-stat, mine does not have that, cause I wondered about that myself and pulled the airbox apart to look for it, didnt see one at all. The foam in the airbox is still there, though. Its not falling apart and still whole so there's that. The foam is in the bottom of mine, below the air filter.

I wash the car by hand so i try not to use high pressure, save for the mold that has started to accumulate near the fuel door and one little part of the hood. I dont have a good AMM yet to test with, its in transit now. Should be here friday with any luck. Motor mounts were one of the first things I replaced as they were..... non-existant. One was in literal chunks and the other just split and lost its oil. Had an oil leak that contributed to one of them failing, dripped oil on it and it just eventually rotted it away.

When i first got the car, i was getting 18-20 mpg, but after the first AMM failed, its been up and down since. First replacement lasted 6 months, second one a year, current one is about 6-8 months, ill have to look and see when i bought it from FCP. My highway mileage is about what you stated when the original AMM was in the car, same with combined. Now? yea.... not even close. Worst Ive had yet is 13 but I was romping it, lived in FL at the time and had to dodge idiots left and right.

I dont use the AC in my car, even though it works perfectly fine, previous owner replaced pretty much everything to make it work, gets damn cold too. The hit in mileage was pretty bad with that thing on. 2 at 40 mph was what i used, lol

Last edited by KT1986; 06-22-2022 at 10:18 PM..
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Old 06-23-2022, 01:11 AM   #9
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Quote:
Originally Posted by KT1986 View Post
According to the book, its supposed to read 2.5 to 4.0 ohms of resistance between pins 2 and 3, but Im reading 19.25 M ohms of resistance on these pins according to my meter.
The original Volvo/Bosch MAFs are a hot-wire sensor, where there is literally a really thin heated platinum wire in the air stream. If you take the screen off the MAF, you can see the wire strung between 3 of the 4 little posts in the inner venturi cylinder (you may need a good light and a contrasting background to see it). The wire in the hot wire MAFs can get contaminated, so a periodic high temperature burn-off cycle is needed.

The newer style of MAFs uses a couple heated precision thick-film resistors in the air stream. These may look like square lumps in the airstream, or a single ceramic blade in the air stream. These MAFs are pretty robust and don't need a periodic burn-off cycle.

I'm wondering if the replacement MAFs are now using a hot-film [resistor] sensor instead of the original hot-wire sensor. This could explain your 19M resistance measurement.

Can you remove the screen and look inside your MAF to see if you can see the tiny hot wire, or if it has a ceramic blade instead?

[BTW FCP has a lifetime replacement guarantee that should cover the MAF]

Edit: here's a pic of an ebay hot-wire MAF showing the 4 posts and the thin wire between them. If it doesn't look like this, it's not a hot-wire MAF.
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Last edited by bobxyz; 06-23-2022 at 09:03 AM..
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Old 06-24-2022, 07:54 AM   #10
KT1986
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Yea the inside of my current MAF looks nothing like that, its a green PCB with what looks like a couple of resistors, one hanging in the air attached to the PCB by its leads. The board has one or two traces etched into the PCB, cant really tell without getting it out. I know the two components i can see on the board are probably not resistors, at least not both of them.

The replacement one coming in today is the bosch hot wire style. The last two dephi units i had iirc are like my current one in setup on the inside.

Ill see if i can get a pic of my volvo unit i got from FCP once i get the bosch one in, its nothing like the bosch unit. I plan on getting the current one exchanged with FCP since it does have that lifetime guarantee thru them.
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Old 06-27-2022, 07:19 PM   #11
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Got the "new" MAF installed and it did make a difference, the hunting at idle is gone and it seems to run a bit smoother. Time will tell if the fuel economy picks up but with the way it runs, it seems like it would. Got the car up to 70 and didnt seem like it was really struggling to get there, just got up to speed quickly and didnt have to floor it to get there.

Seems like the aftermarket sensors arent that great, even the volvo part I got. Oh well, live and learn.
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