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Old 01-11-2009, 07:00 PM   #61
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Re: 99 4.3 Rough Idle, Low Temp, Bad Gas Mileage

I can't say I really feel the miss. Its more like just a low idle that shakes the truck. Also cut open the fuel filter and it really looked pretty clean despite the fact that it was hard to blow through.
I don't understand why low fuel pressure would cause a miss fire on one cylinder and not all of them.
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Old 01-11-2009, 09:21 PM   #62
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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.
Sounds like u got pressure regulator for 94 cpi sys or bad pump.
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Old 01-12-2009, 08:29 AM   #63
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Re: 99 4.3 Rough Idle, Low Temp, Bad Gas Mileage

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Originally Posted by dr_lee29
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 decal.
I haven't read the codes but I'm betting it will be a misfire on cylinder 4.

If the fuel pressure regulator is good and the fuel pressure is that low the fuel pump is not supplying enough pressure and volume.


Low fuel pressure will not squirt the injectors good causing the engine to run lean fuel mix and misfire.


You need to get the fuel pressure up to par for it to run good.
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Old 01-12-2009, 08:35 AM   #64
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Re: 99 4.3 Rough Idle, Low Temp, Bad Gas Mileage

Yes its looking like a bad pump. I don't know what else could cause the drop in pressure short of a pinched line and they all look fine. I tested the direct fuel pressure and I got 90 psi. I think thats a little low, it was cold when I tested so maybe it would drop off even more as it heated up. My testing hook up isn't the best yet so I'm exactly sure of the pressure.
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Old 01-12-2009, 08:42 AM   #65
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Re: 99 4.3 Rough Idle, Low Temp, Bad Gas Mileage

Whats the easiest way to get at the fuel pump? I changed a pump in an older S10 several years ago and we dropped the tank but were told that it would have been better to take off the bed and go from the top down. It really looks like it will be a pain to get the fuel lines off the tank in order to drop it off the truck frame. Any suggestions?
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Old 01-12-2009, 09:52 AM   #66
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Re: 99 4.3 Rough Idle, Low Temp, Bad Gas Mileage

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Originally Posted by dr_lee29
Whats the easiest way to get at the fuel pump? I changed a pump in an older S10 several years ago and we dropped the tank but were told that it would have been better to take off the bed and go from the top down. It really looks like it will be a pain to get the fuel lines off the tank in order to drop it off the truck frame. Any suggestions?
It is more or less the help and equipment you have.

If you can set the bed off easy that may be the way to go on it.

But either way get a AC-Delco/delphi fuel pump.
About 25$ more than cheap aftermarket junk pumps.
Lot of problems with aftermarket pumps.
Good luck and let us know how it goes.
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Old 01-13-2009, 08:33 AM   #67
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Re: 99 4.3 Rough Idle, Low Temp, Bad Gas Mileage

Found out this morning that my battery is low. I've known this for a while but the truck starts just fine so I didn't do anything about it. Could it be that the low battery is causing the low pump pressure? I think more importantly could the low battery be causing the low pressure at idle?
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Old 01-13-2009, 10:14 AM   #68
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Re: 99 4.3 Rough Idle, Low Temp, Bad Gas Mileage

Quote:
Originally Posted by dr_lee29
Found out this morning that my battery is low. I've known this for a while but the truck starts just fine so I didn't do anything about it. Could it be that the low battery is causing the low pump pressure? I think more importantly could the low battery be causing the low pressure at idle?
No
Unless the fuel pump is not getting good ground or voltage.
If fuel pump is getting 12-13 volts it should pump proper pressure.
Doe the battery check weak.
What is the voltage reading on it and what is the charging coltage?

What is the fuel pressure readings at idle and speed up?

Low battery or low fuel pressure usually causes hard/long cold starts.

Besides low fuel pressure problem.
You have two strange problems on low engine running tempt and the miss on no 4 cylinder.

All three need checked out and repaired.

Low fuel pressure can cause engine miss but the miss on no 4 cyl may be more than low fuel pressure.

I would do more testing on that miss first.

You need a good engine capable scanner to aid testing on the miss.

Is the miss full time at idle?

Is that injector losing injector pulse.

Pull plug wire to no 4 and check for good hot blue spark.
Does pulling plug wire change engine idle?
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Old 01-13-2009, 10:13 PM   #69
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Re: 99 4.3 Rough Idle, Low Temp, Bad Gas Mileage

Pulled the gas tank tonight. I hope to get the pump in later this week. If this doesn't work, my next diagnostic test is going to be testing the number of dynamite sticks that can fit in the bed. If it were spring, I would have already tried the diagnostic test of parking next to the tree during a thunderstorm.
Thanks for the help.
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Old 01-14-2009, 09:16 AM   #70
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Re: 99 4.3 Rough Idle, Low Temp, Bad Gas Mileage

Quote:
Originally Posted by dr_lee29
Pulled the gas tank tonight. I hope to get the pump in later this week. If this doesn't work, my next diagnostic test is going to be testing the number of dynamite sticks that can fit in the bed. If it were spring, I would have already tried the diagnostic test of parking next to the tree during a thunderstorm.
Thanks for the help.

You are welcome and let us know how it goes.

Retest fuel pressure and post back after pump replacement.
Good luck
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Old 01-14-2009, 10:32 AM   #71
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Re: 99 4.3 Rough Idle, Low Temp, Bad Gas Mileage

Guys I'm a little worried about this. I looked up the fuel pump online today at Napa and found the following specs:
Attributes:
Product Features:w/ Its Line Of OE Replacement Fuel Pumps & Modules
Delphi Leverages More Than 50 Years Of Fuel System Design & Manufacturing Experience Daily To Service The Needs Of The Aftermarket. In Fact
Delphi
Pressure Rating:61.6 psi
Gallons Per Hour:22.8
Fitting Type(s):Straight Tube - Supply & Return Fuel Line Is Push-On
Then Is Clamped
Fitting Size(s):9.5 mm
7.9 mm
Manufacturerelphi

The fuel pressure rating says 61.8. Shouldn't I expect something much higher than that? If it can make direct pressure of 95-100 and suppose to produce 66 before start?
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Old 01-14-2009, 10:32 AM   #72
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Re: 99 4.3 Rough Idle, Low Temp, Bad Gas Mileage

oops I mean it says 61.6 on this pump, shouldn't I expect more?
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Old 01-14-2009, 12:10 PM   #73
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Re: 99 4.3 Rough Idle, Low Temp, Bad Gas Mileage

The ratings and specs are confusing.
Even airtex and Delphi have different ones and different applications/pumps are different.

The gallons per hr was probably figured at 61.6 lbs of pressure on the one you looked at.

But flow rate/gallons hr. varies with pressure and line size.

Delphi Shut off/relief/full pressure is usually 95/105 on Delphi pumps for the later V6 engines.

Airtex pumps drop the rating down to 80-90 lbs.

Running pressure is controlled by the fuel pressure regulator.
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Old 01-14-2009, 12:15 PM   #74
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Re: 99 4.3 Rough Idle, Low Temp, Bad Gas Mileage

Here is another fuel pressure problem you might find interesting.
http://www.automotiveforums.com/vbul...d.php?t=937249
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Old 01-14-2009, 03:58 PM   #75
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Re: 99 4.3 Rough Idle, Low Temp, Bad Gas Mileage

WHAT! Delphi pump = $404.99
Autozone pump = $287.99
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