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1994 isuzu trooper: you cant chop the throttle..intake..fuel pump…

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I have a 1994 isuzu...
I have a 1994 isuzu trooper with 192,000+ miles on it. it takes awhile to start and idles rough for a bit then smooths out you can't chop the throttle or it will start to pop back through the intake kind of like a plugged cat but it does not have a cat on it anymore. the fuel pump was changed with filter also the pressure regulator was changed. the guy i got it from had it at a mechanic said fuel pressure was at 25 instead of the 40-42 it should have even after the new pump was installed and I changed the regulator but it didn't fix anything. plugs/wires have been changed also. can't get it to rev out all the way.
Submitted: 8 years ago.Category: Car
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Answered in 22 minutes by:
6/27/2010
Auto Mechanic: PAVLIN, Technician replied 8 years ago
PAVLIN
PAVLIN, Technician
Category: Car
Satisfied Customers: 3,487
Experience: 20 YEARS HANDS ON TRUCKS AND CARS/EMISSION INSPECTOR LICENCE
Verified

Hi, if you really have low fuel pressure, then vehicle will not run properly.

recheck fuel pressure,

if you can find MAF sensor from another vehicle, replace it with yours for testing. MAF sensor can cause symptoms like that.

also stuck open EGR valve will do that.

EGR valve can be plugged for testing purpose. just plug exhaust passage to it and drive the car, if gets better that is the problem. MAF sensor is hard to diagnose, It is best to be replaced with good known one. But it is better to try before you spend money on it.

 

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Customer reply replied 8 years ago
ok i dropped the tank to check power to the fuel pump it was around 12 volts also blew compressed air through the feed and return fuel lines and still only had about 20 pounds of pressure. I took some New fuel line ran it from the fuel pump to the fuel rail it still only had around 22 to 24 pounds of pressure so i'm guessing the new pump that was installed is bad or the wrong one. as far as the egr valve that was also cleaned out. I did notice when i unplugged the MAF sensor it didn't really back fire through the intake but it still wouldn't rev up so maybe a combination of MAF and low fuel pressure. In the end when i put it all back together so i could move it till i could get a new pump to work on it i did get 30 pounds in the fuel rail but that's still low from 40 to 42 i hear it is suppose to have..
Auto Mechanic: PAVLIN, Technician replied 8 years ago

Those are the specs of fuel pressure.

so looks like you are ok with that, now you have to check MAF I would suggest.

I know it is not easy, but that how it is in our business. So try your bets to find Maf replacement.

1994 Isuzu Truck Trooper (2 Door) V6-3165cc 3.2L DOHC

Fuel Pressure: Specifications

Fuel Pressure

At Idle, vacuum to regulator ............................................................................................................................................... 25 - 30 psi (190 - 200 kPa)

At Idle, no vacuum to regulator .......................................................................................................................................... 41 - 46 psi (280 - 320 kPa)

Pavlin

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Customer reply replied 8 years ago
ok let me try typing this up again as the first time the site messed up and lost what i typed. Ok even at idle with the pressure in spec at around 28psi now when i remove the vacuum line off the brand new regulator the pressure does not go up at all at idle. also when i had the tank dropped and ran new line from the fuel pump straight to the fuel rail bypassing everything key on engine off i only got 25 psi. so that just tells me that the fuel pump is weak even though it was suppose to be new it looks new could be new but bad had that before with electrical parts. But i do think that there is more to it that just a bad pump something electrical. You can't give it more than a 1/4 throttle and it starts to pop and backfire not sure if plug wires were crossed or not but i looked at the dates on the reciepts from fuel pump and new plugs/wires and the pump was the first thing that was replaced so probably not. when i unplugged the MAF sensor it didn't make much difference this time and unplugging the TPS really made it run bad so that seems to work. I have a manual coming as advance couldn't even get a manual for it. let me know what you think about the fuel pump.
Auto Mechanic: PAVLIN, Technician replied 8 years ago

Ok, personally I do think you have fuel issue. I am more to maf sensor or EGR leaking exhaust gases to intake or 02 sensors and restricted exhaust.

 

EGR you can plug the passage from exhaust and this will show if different or not.

02 sensors you can observe with volt meter

exhaust you can disconnect and rev the engine and see the difference.

MAF is the only part that has to be replaced to test if it is good.

 

Pavlin

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