Lift Pump questions

[mike.morris]

I am getting ready to install a filter / lift pump assembly pre-OEM and I have a few questions. First of all, I want to thank everyone who has worked on the fuel filter issues. I have learned more than I ever thought possible about fuel systems. On to the questions.

1. When wiring in a lift pump, I see that several installs are going to the oil pressure switch for triggering the pump. What is the reason for that? Along those same lines, could it be wired into an ignition hot circuit, or do we have to wait till the engine is running for the pump to come on?

2. What happens when the lift pump fails? Am I paranoid in thinking that I should plumb in a bypass line that can be turned on with the flick of a couple of valves, or am I just throwing money away on the valves?

3. Those of you with lift pumps, what are your backup measures in case the pump fails?

4. I have a Racor vac gauge that I dont mind mounting permanantly in the engine compartment, but I don't know where or how I would plumb this gauge into the system. Any advise from those of you who are doing vac / pressure checks?

Thanks in advance for all the info.

Mike

[chuntag95]

Mike,

Here are the answers to your questions in my opinion. Tommy and Bill will set me straight if I mess up. :D

1. The oil pressure switch allows you to only run the lift pump when the engine is running and drawing fuel. The system does not have a free flowing return and you don't want to pressurize the system or hurt your pumps. There are several "ignition" hot circuits that stay on with the engine off with the RAP feature. All of the ignition hot circuits are in the cab :( so your wiring would have to come from there vs. directly from the alternator stud or jump start lug. Believe it or not, it is safer and easier to use the oil pressure switch.

2. It depends on the pump you use. If you follow the design of Tommy, Bill and myself, you will have a bypass with a valve to set up your pressure. In the event of pump failure, the one JK sells will allow fuel to flow through it AND you have the bypass open already. If you use a different pump that is not a flow through, then you should add a bypass for pressure and failure. You would want to open the valve all the way if the pump failed.

3. See above. Pump still flows when failure occurs and bypass allows flow both ways.

4. I have not had good luck with a gauge in the engine compartment. The heat of the engine messes with the gauge's accuracy. I have mine on the bleed side of my Mega filter. I have a valve that I can equalize the pressure in the system to atmosphere and verify the gauge reads zero. Then I do my measurements. If I had a Kent Moore or a fitting for the Schrader to use the gauge I have, I would do that instead and leave it in the console. I would plug it in when I wanted to check out the system only.

I have 2 tests I do. First is a pump on pressure. I am at 0.5 psig at the mega, so I should be close to zero at the injector pump. Second, I have a pump off test to measure restriction of all the filters. I am currently at 3-4" Hg at idle.

I also had the gauge on top of the engine for a while in a setup post Mega. It was accurate when the gauge was cool, but I got +5 psig or -5" Hg variation due to temperatures. That's why it's in front of the air box on top of the Mega now. It still see's the heat, but no where near as much as sitting over the turbo. :eek:

Sorry for the novel on 4, but like the scorpion, it is my nature. :rolleyes:

Chris

[a bear]

1. When wiring in a lift pump, I see that several installs are going to the oil pressure switch for triggering the pump. What is the reason for that? Along those same lines, could it be wired into an ignition hot circuit, or do we have to wait till the engine is running for the pump to come on?

The oil pressure is a safety feature which would kill the pump in case of an accident resulting in fire.

2. What happens when the lift pump fails? Am I paranoid in thinking that I should plumb in a bypass line that can be turned on with the flick of a couple of valves, or am I just throwing money away on the valves?
3. Those of you with lift pumps, what are your backup measures in case the pump fails?

If you use a flow through pump it should give ample flow to keep you going untill you install a replacement. The bypass would allow you to tone down the pressure for what I hope will equal a longer pump life (not proven) and give an additional port while waiting on a pump. I think JK plans on plumbing a pump W/O the bypass and just let it pump w/ full pressure of about 5-6 PSI. I'm sure he will test it and post the results as to if it would provide an adequate fuel supply if a failure happens. I think someone told me the 6.5 ran W/O the bypass and with no issues.

4. I have a Racor vac gauge that I dont mind mounting permanantly in the engine compartment, but I don't know where or how I would plumb this gauge into the system. Any advise from those of you who are doing vac / pressure checks?

Myself and Chris have tried the liquid filled gauges with accuracy problems resulting from the heating of the glycerine in the gauge. I think Bill Plock is using a dry gauge that is working. I am presently using a home made slack tube manometer that I set at 1' of fuel head or .34 PSI.
The ideal place to mount your gauge would be the last available port before the engine. At the clean side port of your last filter would be ideal.

This is my personal setup. There are many others here that are working. A few members are taking the interest in sending in samples which should provide a good data base for the various designs which should be of interest to a lot of members. I also will be sending a sample in to AV Lube. Just waiting on a connection. :D

[ 07-11-2003, 06:13 PM: Message edited by: a bear ]

[jbplock]

What Chris & Tommy said ... :D

My Setup (http://community.webshots.com/album/77018086bLHHHC) also has a (NON-liquid filled) Vac/Pressure Gauge (http://community.webshots.com/photo/77018086/78007628dTiRDx) (from MSC, pic taken with engine off shows 2inHg vac) mounted on the megfilter bleed port and so far it has been very stable (hot/cold) and tracks my Kent-Moore at the schrader valve. The resolution near zero on the low cost bleeder gauge is poor so I can't accurately say what pressure differential is between the bleeder and the schrader. I currently have my needle valve adjusted for approx 0.3 psi at the schrader and when I look at the low cost gauge it's close to 0.

One more thing, you could probably get by with out the OP switch relay. However the lift pump is an inductive load, so switching it on and off will cause some arcing on the OP switch contacts (with out the relay). How much arcing? and will it damage the switch? is debatable. The OP switches I found were not speced for a DC inductive load. As for me, the ultra-anal type who has three fuel filters and a oil bypass filter, :eek: :rolleyes: I went with the 30 amp relay (http://community.webshots.com/photo/77018086/78006127xwErdP).

Truck is running great… no gas no bleeding .. I also ordered a fuel test kit from George today so I’ll be posting the triple filter results when I have them.. smile.gif

[ 07-11-2003, 06:27 PM: Message edited by: jbplock ]