Diesel Place banner

1 - 19 of 19 Posts

·
Registered
Joined
·
115 Posts
Discussion Starter #1
Help, the water in fuel light has come on after filling up.
I have tried the procedure for draining and it worked the first few times, however now no diesel comes out of the drain hose. I replaced the filter and tried the procedure for bleeding air, and got no fuel out of the hose. What is going on? Please help.
 

·
Registered
Joined
·
5,245 Posts
The lift pump that supplies the filter bowl may not be working. It is relayed off the Oil Pressure Switch and this arrangement is known for problems. Once the weak contacts of the OPS fail the lift pump isn't powered and no fuel pressure. If the lift pump connector has voltage, it may be the pump itself.
 

·
Registered
Joined
·
115 Posts
Discussion Starter #3
ok, but the truck still runs fine. Does that make any sense?
 

·
Registered
Joined
·
5,245 Posts
If the lift pump is not opperating the truck may stall or studder, right after start-up or during a hard acceleration due to low fuel pressure in the fuel lines (pre-injection pump).


It will be the OPS or the lift pump, but more likely the OPS, due to it's being failure prone due to a weak amp rating. A relay can be wired to carry the amp load instead of the OPS, easy insurance.
 

·
Registered
Joined
·
115 Posts
Discussion Starter #5
How do I fix it, I'm very mechaniclly inclined on SBC but this is my first diesel. Where is the OPS? And how do I go about wiring the relay? What load relay and where do I get it?
 

·
Registered
Joined
·
294 Posts
No it does not make sence. If the truck runs that clearly says the lift pump is receaving voltage from the oil pressure switch and you should clearly get oil from the drain line if you open the petcock while the truck is running. The lift pump produces 4+ lbs of pressure and that would send fuel through the pitcock and drain line if the cock is open unless something has cloged the hose.


In order for thr truck to run the oil filter has to be at least half full of oil as the injectiion pump is feed by an outflow tube that is in the center of the filter chamber and half way up the inside of the filter. The drain line comes off the bottom of the input side of the filter and thus below the level necessary to supply the injection pump. This is to insure and dirt or water that comes into the filter is settled out at the bottom and before the filter.


I would remove the filter, suck out the fuel from the chamber once the filter is removed. Put a stopper in the outflow tube to insure no dirt from the bottom of the bowl or anything that may come back out of the drain hose can not get into the outflow tube. Then try to blow compressed air back throught the drain line while the petcock is open. This will clear the line or at least prove it is clear. Then reinstall the filter and try the drain test again.
 

·
Registered
Joined
·
5,245 Posts
It would make sense to check and make sure the drain line is clogged. On top of the fuel filter is a pressure release (bleed valve). If you turn the bleed valve with the engine on and fuel comes out the top, you have pressure and the drain line is clogged. If the truck dies ( lack of fuel) you can test the lift pump connector (under the driver side frame rail) for volts( ignition on). If the lift pump is getting power, it's not the OPS, because it switches voltage to the lift pump. If the connector shows no voltage, it is more than likely the OPS ( located at the rear of the engine behind fuel filter and wires). I used a 30v HD accessory relay. It is wired so that when the OPS closes its contacts the relay is switched. The grey wire form the OPS connector goes to the lift pump. To utilize the relay it is cut in two and the end from the OPS connector isused to switch the new relay. the other end of the grey wire goes to the output on the realy so that when it is switched, the lift pump is powered. A splice must also be wired to the common of the relay from the orange wire off the OPS connector. This wire enables the relay to power the lift pump and the OPS is only the switch that closes the relay contacts.
 

·
Registered
Joined
·
1,444 Posts
If the lift pump isn't working, it can take about 30 seconds for the engine to die at idle with the bleeder open.


A '98 should already have a relay in the system. If you turn the key to the on position, you should hear the pump run. (it is mounted on the frame under the vehicle about as far back as the front door. (on the driver side)


It looks exactly like the gas filter on that GM gas vehicles use but has some wire coming out of it.
 

·
Registered
Joined
·
294 Posts
Yes the lift pump will come on when you turn the key on as a priming function only. Once the engine is started and reaches about 5 lbs of oil presure the oil pressure switch closes and become the feed for the lift pump and computer drops the relay out of the circuit. This is a safty feature that shuts off fuel if the engine stops and the oil pressure drops below the set point.


If you want to relay the oil pressure switch the diagram is on my site, noted in my signature,
 

·
Registered
Joined
·
1,444 Posts
I thought that they added a relay to the OPS circuit in 96. if so, it is most likely just the pump that is bad and not the OPS.
 

·
Registered
Joined
·
6,945 Posts
Still some question as to what function the relay plays in the OBD-II trucks.


After OBD-I -


PCM drives the relay at key-on, holds thru crank.


Engine starts, OPS takes over at rising oil pressure, relay drops out.


If PCM sees no output from Crank Position Sensor, meaning no crank, relay drops out.


PCM will pick up relay again when CPS output indicates crank has begun.


OPS takes over when engine starts.


OBD-II got Lift Pump Circuit Failure input, with DTC, which monitors voltage to pump. OPS fails, or Relay fails, or broken wire, etc = DTC


Manuals are ambiguous in logic description, and so far no one has proven PCM picks up relay at incident of LPC failure (OPS).


Specifically, the relay has been there since '92 NOBD-Zero - picked up by the start circuit, '94 OBD-I picked up by Start, and '96 OBD-II picked up by the PCM.


If I had an OBD-II, I'd pull the plug on the OPS with engine running to see if the PCM actually pulls in the Relay.


But I don't.


So I won't........






memory reflash..Edited by: gmctd
 

·
Registered
Joined
·
1,444 Posts
gmctd said:
Still some question as to what function the relay plays in the OBD-II trucks.


Since OBD-I -


PCM drives the relay at key-on, holds thru crank.


Engine starts, OPS takes over at rising oil pressure, relay drops out.

My 95 and 93 both work the same. The lift pump won't run till the key is turned to the crank position. It doesn't run in the key on position. (to get the pump to run without the engine running, I have to put it in drive and hold the key in the crank position)
 

·
Registered
Joined
·
115 Posts
Discussion Starter #13
Thanks to everyone, the lift pump was bad. Replaced for $80.00 after a $95.00 service trip to find that out. Still had enormous amounts of water in the tank. Spent all day dropping and cleaning and draining etc. etc. etc. The lift pump, fuel pump for us former gas guys, is just an electric fuel pump that primes up with the key on. It doesn't pump unless there is a demand. I'm not sure about the power source but that's ok.
FYI
Found out today that the IP is under warranty from GM for up to 120,000 miles. or 11 years.
 

·
Registered
Joined
·
5,245 Posts
Winchester,


I'm sure you'll feel more familiar with the 6.5 diesels after having a few problems and then seeing them resolved. Three years ago I was reluctant to try and fix anything myself, for fear of making problems worse, but it's a great learning experience to read the possible solutions and then try them out.


My '93's lift pump comes on with the key on (not crank), the '96 as well ( I think I finally found why it's not working on the '94 when I was searching for bad connections. One of the wires off the junction block was grounding out).


GMCTD,


Are you saying to unplug the OPS connector on a '96 and see if the lift pump is still powered?
 

·
Registered
Joined
·
6,945 Posts
QM The manuals state that the OPS is backup for the Lift Pump Relay, so most 'gurus' argue from that position.


Another area states that the LP relay pulls in to supply fuel during crank, before oil pressure builds. When the engine is running, the OPS supplys battery to the lift pump.


Only way to prove which is correct is to pull the OPS connector to see if PCM will pick up the Relay
 

·
Registered
Joined
·
294 Posts
This is copied from 1996 gm shop manual.


When the key is turned On, the PCM energizes the fuel pump relay which sends voltage to the lift pump. The lift pump remains On during the glow plug cucle, If the engine is not started immediately after the glow plug cycle, the PCM shuts the fuel lift pump OFF and waits until the engine starts. As a backup system to the fuel pump relay, the fuel lift pump can also be turned ON by the fuel pump/oil pressure switch. When the engine oil pressure reaches 28kPa and the fuel pump relay does not complete the cirtuit, the fuel pump/oil pressure switch will close and complete the circuit to run the fuel lift pump.


The term "AS A BACK UP" is either a mistake or a very poor representation of how the circuit works. The relay is only on during the glow plug cycle. Also note the statement " WHEN THE ENGINE OIL PRESSURE REACHES 28 kPa AND THE FUEL PUMP RELAY DOES NOT COMPLETE THE CIRCUIT". Again a very poorly written statement that should have said " WHEN THE ENGINE OIL PRESSURE REACHES 28kPa AND THE FUEL PUMP RELAY IS NO LONGER COMPLETING THE CIRCUIT.


These two poorly written statements have lead a number of individuals to missunderstand how and what the oil pressure switch does. If you understand electrical drawings you would clearly see that these statements are not representive of what is going on.


It is not uncommon when manuals are translated from China or Japan to see such mistakes.
 

·
Registered
Joined
·
6,945 Posts
I agree, Howie.


I think what throws everyone is some verbiage about the input the PCM uses to monitor the Lift Pump circuit.


My advice, when polled for such, has always been to unplug the OPS with engine running. The ALDL plug has a Lift Pump Test circuit connection, which can be monitored for switched\relayed voltage to the pump.


I like many things about the 'new' format manuals, but there's a lot to dislike, also.
 

·
Registered
Joined
·
5,245 Posts
Listen to this,


The wire I found grounding itself from the fuse block('94 k3500) is the fusable link (E1 on ECM) to orange wire that feeds the fuel pump relay. It wore its plactic sheath off on an exposed body bolt coming through the firewall before the fuseable link. The fuel pump works when the OPS builds enough pressure to switch it on but it doesn't work during the glow cycle. It seems to me that the ECM has been damaged in some way by this because now that the fuseable link to E1 is fixed, the liftpump still doesn't come on in the glow cycle.


Would a new chip fix this? This is the same truck that the crank sensor is showing as off, crank reference is 0.0 and cam pulses 0. I'm wondering if this one short could be the cause of all these sensors not giving their readings due to ECM damage.


GMCTD,


I would think that you would have an idea of how to start looking to find out one way or the other. I looked at all the conections between the engine wire harness and PCM wire harness and all was clean as a whistle. I could pull out the glove box and see what has volts with the engine running, It's all a big mess of wires to me but i can probably stare at it long enough to make some since of it.
 

·
Registered
Joined
·
294 Posts
quantum mechanic


I think we have gotten away from the original topic of water in the fuel. If you have a current problem how about starting a new post and detailing that problem. A clean start.
 
1 - 19 of 19 Posts
Top