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Old 07-31-2008, 11:29 AM   #1 (permalink)
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Sierra 4.8 in a buggy - no power to fuel pump

I have a 2000-ish 4.8 liter in a jeep buggy and have been having some weird symptoms with the fuel pump.

A few months ago, the buggy died on the trail. We no buzz from the fuel pump. tried running 12v direct and nothing. Finally tore in to it yesterday and swapped out the fuel pump(using a TJ in-tank pump). It turned out the old one was bad. Then we went to fire it up but no power. Ended up being a loose main ground wire so we fixed that.

Once that was taken care of, we turned the key over and... no power to the fuel pump. The engine would turn over just fine but no buzz from the fuel pump. My buddy flicked the key on-off-on-off several times to "try to" reset the computer. Somehow that did something and the fuel pump started working again. It fired right up. Later that afternoon I hooked up the trailer to get the Jeep out somewhere to pull it off the trailer. When I went to start the Jeep, no power to the fuel pump again. I followed similar steps listed above and it worked. fired the buggy up.

Once it was running, I was turning the steering wheel back and forth(to see about my broken trac-bar) and then the engine died. Tried to restart, and fuel pump is not working again. haven't checked to see if it's getting 12v or not yet but seems weird.

Anybody have experience with GMC V8s that might know why it keeps dying on me? I'm going to "try" to find how the fuel pump wire ties in for power to see if it's grounding somewhere. Is there something in the computer that would not send voltage to the fuel pump if something is wrong?

I bought a ODBII reader last night to see if I had any codes but when I plugged it in, it tried reading and then gave me "LINK ERROR" so no luck getting codes. The computer was reflashed to remove all SMOG and extra validations. not sure if that's confusing the reader or not.
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Old 07-31-2008, 07:03 PM   #2 (permalink)
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I have a 2000-ish 4.8 liter in a jeep buggy and have been having some weird symptoms with the fuel pump.

A few months ago, the buggy died on the trail. We no buzz from the fuel pump. tried running 12v direct and nothing. Finally tore in to it yesterday and swapped out the fuel pump(using a TJ in-tank pump). It turned out the old one was bad. Then we went to fire it up but no power. Ended up being a loose main ground wire so we fixed that.

Once that was taken care of, we turned the key over and... no power to the fuel pump. The engine would turn over just fine but no buzz from the fuel pump. My buddy flicked the key on-off-on-off several times to "try to" reset the computer. Somehow that did something and the fuel pump started working again. It fired right up. Later that afternoon I hooked up the trailer to get the Jeep out somewhere to pull it off the trailer. When I went to start the Jeep, no power to the fuel pump again. I followed similar steps listed above and it worked. fired the buggy up.

Once it was running, I was turning the steering wheel back and forth(to see about my broken trac-bar) and then the engine died. Tried to restart, and fuel pump is not working again. haven't checked to see if it's getting 12v or not yet but seems weird.

Anybody have experience with GMC V8s that might know why it keeps dying on me? I'm going to "try" to find how the fuel pump wire ties in for power to see if it's grounding somewhere. Is there something in the computer that would not send voltage to the fuel pump if something is wrong?

I bought a ODBII reader last night to see if I had any codes but when I plugged it in, it tried reading and then gave me "LINK ERROR" so no luck getting codes. The computer was reflashed to remove all SMOG and extra validations. not sure if that's confusing the reader or not.
I guess you didn't do the swap. I would trace and ohm the wires out. It may just be a bad connection. You also need to see how its wire as well it might just be wired wrong. The PCM will kill the pump if it gets a low oil pressure reading, it thinks it has rolled.
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GM Gen III/IV V8 swap info

TECH: Gen III/IV "LS motor" to Early GM Auto Trans
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Old 08-01-2008, 12:18 AM   #3 (permalink)
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Someone just said the same thing earlier today regarding the low oil pressure reading turning off the fuel pump. I'll check the oil and clean as many ground wires as I can find.

You're right, I didn't do the swap. Would have been a lot easier to troubleshoot if I knew how it was wired and how the computer was flashed.

Thanks for the help.

Coincidentally, I let it sit overnight. Went out today thinking I needed to bypass the computer to get the pump to run and walla, it started right up. If it starts acting up again, I'll wire the fuel pump directly rather than going through the computer(and pray it's still getting spark). at least till I can get a read on the computer or track down the short.
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Old 08-01-2008, 07:36 AM   #4 (permalink)
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You also mentioned that turning the wheel killed it, like said it could be the low oil pressure switch or you may have a problem with the ignition switch/harness inside column. Wouldnt hurt to do a tap test on the PCM, may be a bad connection/corrosion inside the PCM too.
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Old 08-01-2008, 12:25 PM   #5 (permalink)
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Thanks for the input. I do have some problems where engine RPMs drop a lot when steering/braking at the same time(hydro brakes, hydro assist steering, HD pump). drop in RPMs might be causing the low pressure and computer to kill the pump. Anyone know where the oil pressure switch/sensor is on this beast? Suggestions on how to test it?


I also just got a note back from the folks that built and drove this rig previously. They had 2 ignition switches go bad on them already and expect this might be a similar issue. I'm planning on 12voltguy.com building me a new ignition/switch panel anyway so that should remove ignition issues from the equation.
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Old 08-01-2008, 02:16 PM   #6 (permalink)
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Thanks for the input. I do have some problems where engine RPMs drop a lot when steering/braking at the same time(hydro brakes, hydro assist steering, HD pump). drop in RPMs might be causing the low pressure and computer to kill the pump.
Depending on the tune a lack of VSS will cause this might check the tune.
The oil pressure sender is behind the intake sticking straight up off the engine block.
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Anyone know where the oil pressure switch/sensor is on this beast? Suggestions on how to test it?
After further research, the stock truck PCM doesn't really use the switch to shutoff the fuel. The earlier LT1 did!

Most people I have seen use a oil pressure switch to turn off the fuel pump during a swap through the fuel pump relay. So depending on install it might and might not be the case.

It sounds to me like its a voltage or wiring problem. What size wires and is there a fuel pump relay?


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I also just got a note back from the folks that built and drove this rig previously. They had 2 ignition switches go bad on them already and expect this might be a similar issue. I'm planning on 12voltguy.com building me a new ignition/switch panel anyway so that should remove ignition issues from the equation.
What kind of switch is it currently?
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GM Gen III/IV V8 swap info

TECH: Gen III/IV "LS motor" to Early GM Auto Trans
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Old 08-01-2008, 09:49 PM   #7 (permalink)
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painless ignition/switch box.

I'll ask about the relay to see if they installed that feature.
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Old 09-19-2008, 10:11 AM   #8 (permalink)
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Haven't checked on whether they installed a fuel pump relay or not. The wiring harness was a "Current Performance" harness. Computer reprogram was done by FastChip.

I put a hole in the oil pan on my last trip out(after this original post). I just got the oil pan replaced and the MAF and throttle body cleaned up. I also fixed some exhaust leaks that were located right next to the O2 sensors. And I replaced the ignition. The engine fired up and idled perfectly. Sounded great. I only let it run for a couple minutes and then shut it off. Idle problem solved, probably.

I took it down to the carwash to get the leaky oil cleaned off the engine and then took it home. when I tried to fire it up again, the fuel pump wouldn't turn on. fidgetted with a few wires(map sensor, oil guage, tps, knock sensor). then turned the ignition over again and it started right up and idled well. pumped the gas while it was running(with engine cold) and it backfired a few times, acted like it was dumping too much fuel or was pre-igniting or something, and then after a couple pumps of the gas pedal, the engine shut itself off. when trying to turn the ignition over again, I think the fuel pump turned on but the engine wouldn't start(spark turned off) turned ignition off and then back on, fuel pump would not turn on either.

This is an identical problem I was experiencing when I cracked the oil pan. I don't know if it originally recognized low oil pressure when I was pumping the gas and since the engine was stalling due to too much fuel it recognized low oil pressure and shut off? but the computer is now pissed. I bought an ODBII reader but there's something wrong with the ODBII port so I can't get codes.

I'll try cleaning the ODBII port to see if that fixes the problem.

I found the oil pressure sender but that's just an aftermarket clear tube sender to the guage cluster. There's also supposed to be a oil pressure SENSOR on this thing but trying to match the visuals on alldata to the engine, can't seem to find it(it's probably right in front of my eyes, alldata shows it being on the driver's side of the intake manifold about 1/4 to 1/2 way up towards front of engine?
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Old 09-19-2008, 06:36 PM   #9 (permalink)
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This is an identical problem I was experiencing when I cracked the oil pan. I don't know if it originally recognized low oil pressure when I was pumping the gas and since the engine was stalling due to too much fuel it recognized low oil pressure and shut off? but the computer is now pissed. I bought an ODBII reader but there's something wrong with the ODBII port so I can't get codes.
They don't have a low oil shutoff like the LT1's unless someone installed one based off an oil pressure switch.

Sounds like you have wiring problems!
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GM Gen III/IV V8 swap info

TECH: Gen III/IV "LS motor" to Early GM Auto Trans

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Old 09-21-2008, 07:06 PM   #10 (permalink)
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Thanks. that's leading me down an unfriendly road but one I'll just have to take. I was hoping it was one of the sensors causing the problem.

I also noticed my 12v+ switched wire on my ODBII connection(at least I think that's what it was, bottom right corner connection) was corroded pretty bad. It was the only one corroded. I wonder if my bad ground problem before(battery connection to frame was loose) was doing its job corroding my 12+ and 12- connections. I took care of the main grounds but I'll look for others. will also pull harnesses and wires to check for other problems.

One other question - if the exhaust pipe connected to the exhaust manifold was a smaller diameter then the outlet on the manifold, could that cause backpressure and do something nasty to the engine? Maybe cause the o2 sensor down on the exhaust pipe to read poorly or cause exhaust to get pushed back in to the heads? I think the OD on the exhaust pipe was something like 3/4" to 1" smaller diameter than the outlet on the passenger's side header.
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Old 09-21-2008, 07:41 PM   #11 (permalink)
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One other question - if the exhaust pipe connected to the exhaust manifold was a smaller diameter then the outlet on the manifold, could that cause backpressure and do something nasty to the engine? Maybe cause the o2 sensor down on the exhaust pipe to read poorly or cause exhaust to get pushed back in to the heads? I think the OD on the exhaust pipe was something like 3/4" to 1" smaller diameter than the outlet on the passenger's side header.
No not really. The stock truck manifolds are like 2.25'' or so, which most replacement short headers are that, I believe Just shooting from the hip!
As I don't have one in front of me to mesure.


It will just hurt performance. I would at least have dual 2.25'' or dual 2.25'' into 3'' exhaust.
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GM Gen III/IV V8 swap info

TECH: Gen III/IV "LS motor" to Early GM Auto Trans
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Old 09-22-2008, 09:38 AM   #12 (permalink)
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I just got back from a shop(GM Specialties in Roseville, very helpful). They were able to read the engine with their test tool. The O2 sensors are not turning on. The system has a couple of 4-wire O2 sensors(internal heaters) so they should have read almost immediately but he said they're only showing as running at half voltage.

He also hooked up a fuel pressure tester on the rail and we watched the fuel pressure build while the pump was on(keyed on position) but immediately after the pump turned off, the pressure dropped to 0. He said it should have stayed pressurized. It was also running at about 52-53psi regulated. The alldata stated it should hold at least 26psi for at least 1 min after the pump turns off. Alldata also showed it should have been running 55-62psi regulated.

So... #1 - I have a fuel pump problem. I'm going to go back through the searches on the board to see what folks put in for an inline pump. That way if and when it screws up again, I don't have to pull the pump out of the tank(major PITA). and will fab up something to drop a regular hose/filter in the tank.

#2 - as stated on here before(a couple of times), definitely have a electrical problem. I don't know if the O2 sensors are not reading because FastChip turned off all O2 sensor readings when they flashed the computer or if it's just something in the wiring harness so I'll have to find someone around here that can check/tune stripped down engines(no smog, no tranny checks, 2 o2's instead of 4, etc). I got a link to someone on another posting. Regardless, the ignition is still intermittent turning on the fuel pump so there must be something wrong with the wiring. Time to pull the harness, look at all the connections and check ohms on all connections, then plug it all back in.

Thanks a lot for your feedback. This is my first time really digging in to a ECM based engine and I appreciate all the input along the way because it's unchartered territory for me(I love old carburated engines, just not for wheeling).

I'll post up if/when I find something.
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Old 09-22-2008, 12:00 PM   #13 (permalink)
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He also hooked up a fuel pressure tester on the rail and we watched the fuel pressure build while the pump was on(keyed on position) but immediately after the pump turned off, the pressure dropped to 0. He said it should have stayed pressurized. It was also running at about 52-53psi regulated. The alldata stated it should hold at least 26psi for at least 1 min after the pump turns off. Alldata also showed it should have been running 55-62psi regulated.



#2 - as stated on here before(a couple of times), definitely have a electrical problem. I don't know if the O2 sensors are not reading because FastChip turned off all O2 sensor readings when they flashed the computer or if it's just something in the wiring harness so I'll have to find someone around here that can check/tune stripped down engines(no smog, no tranny checks, 2 o2's instead of 4, etc). I got a link to someone on another posting. Regardless, the ignition is still intermittent turning on the fuel pump so there must be something wrong with the wiring. Time to pull the harness, look at all the connections and check ohms on all connections, then plug it all back in.
Might have to ground the O2 sensors themselves. Some times they don't ground good through the exhaust system. Make some jumps to ground the body of the 02 to a good ground.

I would also find someone with HPtuners that can look at your tune and make changes if needed.
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GM Gen III/IV V8 swap info

TECH: Gen III/IV "LS motor" to Early GM Auto Trans
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Old 09-22-2008, 02:25 PM   #14 (permalink)
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How many of these swaps have you done BO185? You have some great tips on this

but a question, create some jumps? Do you mean get a ring washer that fits over the threads of the O2 sensor and run a ground wire off that direct to a good ground(body/frame/block)? Not sure what "jumps" means.
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