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Old 01-08-2009, 10:41 AM   #46
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Re: 99 4.3 Rough Idle, Low Temp, Bad Gas Mileage

The temp has not been fixed. I drove it today to work and it never got to a proper running temp. But this seems like a secondary problem because the idle is rough immediately upon starting the truck. Of course the low temp could be the cause of the bad gas mileage though. I just can't figure out what would make the engine run cold. The PCM says its cold (165 last I checked), the gauge says its cold, the fact that I can't get the cab roasting hot says its cold.
I'll try to check the fuel pressure tonight. I'm a little confused though. How do I check the direct fuel pressure? I know there is a valve on the fuel lines on top of the block where I can attach a gauge, but where (how) do I block the return line? Do I do this at the tank or on top of the block (these are metal lines)? Then for the rest of the testing I assume I attach the gauge to valve on the metal lines and run the gauge under the hood to the windshield?
Again, thanks for your help.
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Old 01-08-2009, 10:49 AM   #47
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Re: 99 4.3 Rough Idle, Low Temp, Bad Gas Mileage

Quote:
Originally Posted by dr_lee29
The temp has not been fixed. I drove it today to work and it never got to a proper running temp. But this seems like a secondary problem because the idle is rough immediately upon starting the truck. Of course the low temp could be the cause of the bad gas mileage though. I just can't figure out what would make the engine run cold. The PCM says its cold (165 last I checked), the gauge says its cold, the fact that I can't get the cab roasting hot says its cold.
I'll try to check the fuel pressure tonight. I'm a little confused though. How do I check the direct fuel pressure? I know there is a valve on the fuel lines on top of the block where I can attach a gauge, but where (how) do I block the return line? Do I do this at the tank or on top of the block (these are metal lines)? Then for the rest of the testing I assume I attach the gauge to valve on the metal lines and run the gauge under the hood to the windshield?
Again, thanks for your help.
The tempt problem needs to be fixed for it to ever run right.
If it has a good thermo stat there is no reason for the temp not to come up to 195 degrees.
Also you will need fuel pressure up to par.

Direct fuel pump pressure can be checked by hooking a gauge onto the fuel filter in line or blocking off or plugging the fuel return line.
Down between frame and engine there should be a rubber part of fuel and or fitting where you can block it off.
Let us know what the fuel pressure test show.
Good Luck
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Old 01-08-2009, 06:09 PM   #48
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Re: 99 4.3 Rough Idle, Low Temp, Bad Gas Mileage

Well I haven't checked the direct fuel pressure because I don't have a fitting to get the gauge on the fuel filter and I can't find a rubber line to pinch off the return line.
But I did check the fuel pressure on top of the block. Its about 54 at start up and hovers around 50-53 at idle. When I shut off the truck it jumps to 60 and stays there for at least 15 minutes. I really expected a higher pressure than these readings.
I drove around with the gauge and at about 2000 rpm I get 54-56 psi, if I tromp the gas I can get about 59-60 but never higher and it won't stay at that pressure very long before it gets back down to 55-56. When I let off the gas pedal it drops to 50 then creeps up to about 52.
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Old 01-08-2009, 06:30 PM   #49
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Re: 99 4.3 Rough Idle, Low Temp, Bad Gas Mileage

Quote:
Originally Posted by dr_lee29
Well I haven't checked the direct fuel pressure because I don't have a fitting to get the gauge on the fuel filter and I can't find a rubber line to pinch off the return line.
But I did check the fuel pressure on top of the block. Its about 54 at start up and hovers around 50-53 at idle. When I shut off the truck it jumps to 60 and stays there for at least 15 minutes. I really expected a higher pressure than these readings.
I drove around with the gauge and at about 2000 rpm I get 54-56 psi, if I tromp the gas I can get about 59-60 but never higher and it won't stay at that pressure very long before it gets back down to 55-56. When I let off the gas pedal it drops to 50 then creeps up to about 52.

Yes fuel pressure is way to low.
It is not going to run right to you get the pressure up.
And could be the engine miss on no 4.
A bad fuel pressure regulator can lower pressure.
Need the return line locked off to test fuel pressure regulator.
But only true fuel pump test is to block off return or test direct fuel pressure.
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Old 01-08-2009, 06:34 PM   #50
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Re: 99 4.3 Rough Idle, Low Temp, Bad Gas Mileage

Well on second thought, Haynes actually says I should expect 58-60 without the engine running and 3-10 below that at idle so I'm within the good range?

By the way, I replaced the distributor cap, the old one did have some corrosion on the posts so it was probably a good thing to replace but it didn't fix the problem.
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Old 01-08-2009, 06:53 PM   #51
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Re: 99 4.3 Rough Idle, Low Temp, Bad Gas Mileage

Quote:
Originally Posted by dr_lee29
Well on second thought, Haynes actually says I should expect 58-60 without the engine running and 3-10 below that at idle so I'm within the good range?

By the way, I replaced the distributor cap, the old one did have some corrosion on the posts so it was probably a good thing to replace but it didn't fix the problem.

Mr Haynes is full of crap on fuel pressure specs.

Your factory specs is 60/66,
Engine off you need 65 lbs of fuel pressure.
Engine cranking you need 65 lbs.
Engine under load you need 65 lbs of pressure.
Engine idle or no load the pressure regulator will cut it down to around 61 -62 lbs.

But for the fuel pressure to regulate fuel pressure right you need 95 -100 lbs full fuel pump pressure to deliver right regulated pressure and and fuel volume.
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Old 01-08-2009, 07:25 PM   #52
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Re: 99 4.3 Rough Idle, Low Temp, Bad Gas Mileage

Still having trouble finding a rubber line to pinch, as far as I can tell these are hard plastic then metal lines from the tank to the spider.
Does it tell us anything that the pressure is just too low and doesn't bleed off after the engine is shut down? Does that lean towards a pump and/or a filter as opposed to the pressure regulator?
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Old 01-09-2009, 08:20 AM   #53
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Re: 99 4.3 Rough Idle, Low Temp, Bad Gas Mileage

One more thing, would fuel pressure cause a big drop in gas mileage?
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Old 01-09-2009, 09:11 AM   #54
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Re: 99 4.3 Rough Idle, Low Temp, Bad Gas Mileage

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Originally Posted by dr_lee29
One more thing, would fuel pressure cause a big drop in gas mileage?
Yes low fuel pressure will caus the computer to richen up fuel mix.
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Old 01-09-2009, 09:21 AM   #55
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Re: 99 4.3 Rough Idle, Low Temp, Bad Gas Mileage

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Originally Posted by dr_lee29
Still having trouble finding a rubber line to pinch, as far as I can tell these are hard plastic then metal lines from the tank to the spider.
Does it tell us anything that the pressure is just too low and doesn't bleed off after the engine is shut down? Does that lean towards a pump and/or a filter as opposed to the pressure regulator?
Blocking return line separates fuel pressure regulator from weak fuel pump.
With Return line blocked and fuel pressure comes up to around 95 lbs the fuel pressure regulator is bad.
If no more pressure with it blocked /pinched off the fuel pump is weak.
If you can not find a place to block/pinch/plug off return line.
You can hook a gauge onto the in fuel line at fuel filter to check direct fuel pump pressure.
Sometimes a rubber hose on the end of into filter line will help connect a gauge direct to it.
It should have around 95-100 direst pressure from fuel pump.
But do not run fuel pump at full longer than required to test it.

Pump has to be capable of full pressure and fuel volume to hold regulated pressure to engine.

The later 4.3 engine have to have right pressure and fuel volume to run right.
Even sometimes a couple of lbs low will give problems.
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Old 01-09-2009, 09:08 PM   #56
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Re: 99 4.3 Rough Idle, Low Temp, Bad Gas Mileage

Alright, I checked the direct fuel pressure. I rigged up a fuel line directly to the output side of the fuel filter and turned the key on, I got 90+ PSI. I was afraid to leave it too long, but I think I could have had a little higher.
So I replaced the fuel filter since I was down there already. The old filter was pretty hard to blow through, I'd say the pump was having a hard time. After I put the new filter on, I tested the pressure on top of the block. This time the gauge needle was shaking violently between 50-55 psi, even when I gave it gas, the pressure was about 56-58 and shaking like crazy. Is this the pressure regulator symptom?
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Old 01-10-2009, 09:25 AM   #57
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Re: 99 4.3 Rough Idle, Low Temp, Bad Gas Mileage

Quote:
Originally Posted by dr_lee29
Alright, I checked the direct fuel pressure. I rigged up a fuel line directly to the output side of the fuel filter and turned the key on, I got 90+ PSI. I was afraid to leave it too long, but I think I could have had a little higher.
So I replaced the fuel filter since I was down there already. The old filter was pretty hard to blow through, I'd say the pump was having a hard time. After I put the new filter on, I tested the pressure on top of the block. This time the gauge needle was shaking violently between 50-55 psi, even when I gave it gas, the pressure was about 56-58 and shaking like crazy. Is this the pressure regulator symptom?

Yes it is a good indication of bad fuel pressure regulator.

I would repace it and retest fuel pressure.

Cut open old fuel filter and check for a lot of black stuff in it.
A lot of real black stuff can be from a fuel pump going out.

Fuel pump pressure test should be done on a fuel pump that has run for 20-30 minutes to.
Some pumps will pump good untill hot and then stard getting weaker pressure.
Let us know how it goes.
Good Luck
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Old 01-11-2009, 03:20 PM   #58
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Re: 99 4.3 Rough Idle, Low Temp, Bad Gas Mileage

Alright, new pressure regulator in, buttoned back up. On start up the truck raced for about 10 seconds then came back down to a low rough idle. I drove it around the block and the light came back on.
Fuel pressure
Key on, engine not running 59
At idle 51
Vacuum 18 cm Hg at idle, 21-22 @2500 RPM, drops to 0 with accel, goes to 24-25 with decel.
I haven't read the codes but I'm betting it will be a misfire on cylinder 4.
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Old 01-11-2009, 03:22 PM   #59
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Re: 99 4.3 Rough Idle, Low Temp, Bad Gas Mileage

I forgot, the fuel pressure doesn't bleed down quickly but the fuel pressure isn't high in the first place.
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Old 01-11-2009, 05:42 PM   #60
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Re: 99 4.3 Rough Idle, Low Temp, Bad Gas Mileage

Quote:
Originally Posted by dr_lee29
I forgot, the fuel pressure doesn't bleed down quickly but the fuel pressure isn't high in the first place.

Fuel pressure is still way low.
Run some more test on fuel pump.
Check voltage and ground to fuel pump
Run fuel pump for 20-30 minutes and see what the pressure does.

Is the miss on no 4 cyl a dead miss?
Can you feel the miss on engine?
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