# MAF or fuel/pump related?



## Angel97 (Feb 2, 2011)

Symptoms - seems like fuel starvation under load. Idles smoothly, and runs fine under low/normal load, but cuts out up hills etc. No CEL. No black smoke - it just dies in all 4.
Have replaced filter as first step - no difference.
Used parts supplier has a bin full of used pumps for about US$60 each, and doesn't sell many or hear of many failures, so he suggests that it's the AFM rather than the pump or relay etc. He could be right, but it doesn't seem like the AFM to me, as it's running very well/smoothly under low load, not hesitating or missing on acceleration, and only starts cutting out when it's been under load for 10 seconds or so.
I think MAF signal voltage is looking okay , at warm idle, it's steady at around 1.4v, signal voltage increases and falls smoothly, it's steady at about 2.5v @ 3,000 rpm. But, I also hear that the AFM unit failures are related to internal dry solder joints, so it might test okay but fail on road test.
On the other hand, the pump also seems to be operating as normal, can hear it when the ignition turned on, and it's cutting out after about 5 seconds. (not sure if that initial 5 second run is just based on a timer circuit though - the fact that it cuts out "as normal" might be meaningless wrt fuel pressure)
I don't have a fuel pressure gauge, but the fuel hoses feel much softer at idle than I'd have expected if they were at about 34psi (FSM spec), but perhaps I'm wrong.
Any suggestions for "next step"?


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## jdg (Aug 27, 2009)

Fuel pump will cut off after 5 seconds or so after the key is turned and only stays on if a trigger is received from the distributor, so you're on track there.

Fuel pressure at idle will be less than at W/O/T as the pressure regulator maintains a pressure differential between the intake manifold and fuel rails.
W/O/T = manifold pressure of near ambient (i.e. 14psi or so), that plus about 30 psi or so gives you a fuel rail pressure of about 44psi.
At idle, you should be pulling about 18 inches of vacuum, meaning a manifold pressure of about 5 psi, add in the regulated 30psi, and you get about 35psi fuel rail pressure at idle.
I guess the main thing that matters there is that the pressure moves the gas pedal.

For the MAF, get it good and warmed up, let the idle settle out for a bit, then tap on the MAF, heck beat on it. If you've got a bad connection in there, a good beating should make it show up.

Could be the fuel pump (kinda doubt it for your reasons).
Could be a bad fuel pressure regulator (kinda doubt it also 'cause they sound pretty reliable).

I'd be more leaning towards a bad pickup in the distributor and/or the ignition module inside the distributor...neither of which are easily swappable unless you're a handy guy with tools/pieces/parts. Or maybe even something as easy as a distributor cap/rotor/button assembly.
Have you knocked out a good tune up lately?


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## Angel97 (Feb 2, 2011)

Thanks for your quick reply.


jdgrotte said:


> Fuel pump will cut off after 5 seconds or so after the key is turned and only stays on if a trigger is received from the distributor, so you're on track there.


Yes - I'm leaning towards it not being pump etc. I can feel the pressure in the lines when ignition is turned on, lifted the pump cover, and can feel the line "pulse" when I rev it, and now after it starts missing, it will continue missing at lower load/rpm (wasn't doing that before), but I can't get it to miss at all with no load. While the lines feel "softer" than I did expect, 30-40psi isn't that much, still "pinchable" between thumb and forefinger.



jdgrotte said:


> For the MAF, get it good and warmed up, let the idle settle out for a bit, then tap on the MAF, heck beat on it. If you've got a bad connection in there, a good beating should make it show up.


Yes, but dammit - it runs perfect at idle - 3,000 rpm no load. I wondered if I should put a long lead to multimeter on the signal, and see if I'm getting erratic reading when it's missing/starving. I've sent a note to a guy who reconditions these MAF units, for an opinion whether they can fail intermittently but still give in spec signal voltages at idle/2500 rpm.



jdgrotte said:


> Could be the fuel pump (kinda doubt it for your reasons).
> Could be a bad fuel pressure regulator (kinda doubt it also 'cause they sound pretty reliable).
> 
> I'd be more leaning towards a bad pickup in the distributor and/or the ignition module inside the distributor...neither of which are easily swappable unless you're a handy guy with tools/pieces/parts. Or maybe even something as easy as a distributor cap/rotor/button assembly.
> Have you knocked out a good tune up lately?


I wondered about the regulator, but from what I've read, you get different symptoms from usual mode of failure - ruptured diaphragm.

Pulling the distributor apart doesn't bother me too much. I replaced the leaky internal oil seal in this one about 20,000 miles ago, and it's clean as a whistle now. Could still be the problem - but I'm leaning toward it being a dry solder/intermittent MAF fault.

Motor is fairly sweet, reasonably new plugs, air filter, fuel filter, ignition leads, timing checked (has never needed adjustment anyway) etc etc.


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## jdg (Aug 27, 2009)

Angel97 said:


> now after it starts missing, it will continue missing at lower load/rpm (wasn't doing that before)


Leaning me towards an ignition problem that's getting worse.



> Yes, but dammit - it runs perfect at idle - 3,000 rpm no load.


You hit the nail on the head with that statement right there....No Load.
Under no load, engines will run fine if they've got other faults (rotten plugs, bad towers in the dist. cap, cracked caps, old rotors, etc.) If you use a long lead on the multimeter, I'd almost say you'll read more noise than actual signal anyways.
As far as 'reconditioned MAFs'...False economy in my mind. They're either good or bad...no in between. I wouldn't use a 'reconditioned MAF'...Maybe a used one from a junkyard car if it was cheap enough...that's about as far as I'd go.



> I wondered about the regulator, but from what I've read, you get different symptoms from usual mode of failure - ruptured diaphragm.


True...on that note, I was kinda leaning towards a sticky diaphragm...if it ever happens. Who knows...



> Pulling the distributor apart doesn't bother me too much. I replaced the leaky internal oil seal in this one about 20,000 miles ago, and it's clean as a whistle now. Could still be the problem - but I'm leaning toward it being a dry solder/intermittent MAF fault.


I'm not. The ECU is pretty good about picking up goofy MAF readings and setting codes for it. If you can find yourself a high quality scanner, you might look for 'pending codes'. Those are codes that are just about ready to, but haven't yet, satisfied conditions to trip the CEL. Ya never know...



> Motor is fairly sweet, reasonably new plugs, air filter, fuel filter, ignition leads, timing checked (has never needed adjustment anyway) etc etc.


But how about the cap, rotor, and the rest of that stuff? Goes right along with the ignition stuff. If you could find another distributor, after the normal 'tune up' stuff listed above, that's where I'd be heading.
About the only other thing I can think of would be the crank sensor. Pull it out, clean it off (sometimes they get metallic fuzz on the tip that screws it up), and put 'er back in and see what happens.


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## Angel97 (Feb 2, 2011)

Thanks again for prompt reply.

Yes it could be distributor rotor, ignition/electrical related, but I'm still guessing it isn't - and is the MAF. Motor is actually GA15DE (1.5) mainly similar to GA16DE apart from the obvious, but no CPS (crank). MAF is in/on top of the throttle body, but same type. Model also has no OBDII, so a simple scanner can't be used. MAF is renowned for failure due to dry-solder joints between pcb and socket. Failure on different pins may have different symptoms, plus it's quite likely to be intermittent. I finally found full photos/description of this MAF, actually quite easy to disassemble, check, clean, and resolder if needed. Will check TPS while I'm at it.

This Youtube shows the problem:




This PDF shows disassembly:
http://www.micra.com.au/technical-articles/cg13de-throttle-body-resoldering-guide.pdf


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## Angel97 (Feb 2, 2011)

Final post on this (I hope).
Pulled cover off MAF, resoldered all terminal connections, to be doubly sure resoldered all component / lead connections on PCB, cleaned "hot wire" and temp sensing resistor with carb cleaner and soft artist brush, replaced cover and sealed with RTV. 
Problem (severe missing under load) solved. As MAF signals were within spec when tested (several times), it was probably a dry solder joint giving intermittent signal, only showing up when the engine was hot, and for not long enough to trigger a CEL.


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