leathermaneod

Hey guys,

I've got more problems .I have had my fuel system apart multiple times recently and every time I get it back together the fuel pressure is doing something different. At first I attributed this to air in the system, but I have checked it over and bled the air and found no leaks. I also recently took two 300+ mile trips with no change from start to finish. I would think if it was air it would work its way out in that time. It also makes no difference if the fuel tank is full or not. Right now i am getting about 20 psi idle, which I would be happy with, however, maintaining 65 mph on the highway it drops as low as 13 psi! I could be wrong but i don’t think there should be that much fluctuation. Especially considering thats not even WOT, only about 15 psi of boost at 65 mph. The even bigger mystery is that just the other day I had the fuel system apart and put it back together and I had 22 psi at idle and only dropped to about 18 psi on the highway. It was great! Right after that the truck developed a leak in the water drain valve on the filter housing and i took it apart to replace it. When i put it back together and bled the air it wouldn’t maintain pressure. Since then i made the 2 long trips i mentioned earlier with no change. On the second trip i also had a “Dead Pedal” incident. I don’t think that is related but I'm not sure. I was doing about 55 with the cruise on and suddenly i lost power. I downshifted after a few seconds and suddenly the pedal worked again but i had a check engine light. I pulled over and pulled a P0121 code, which, i believe, is low voltage at the throttle position sensor. I don’t think these two things are related unless my ECM is going bad and so not putting out enough voltage to TPS and the FASS lift pump causing it to not maintain pressure very well. I don't think the gauge is the issue because it is mechanical. Fuel goes from the shrader valve on the vp44 to an isolator to the gauge. I have read the blue chip vp44 diagnostics on the dead pedal and I don't know for sure if the pedal was totally dead or just had a dead spot. I didn't know to check and i was on the highway when it happened. However, i don't think it could be a bad vp44 because it is less than two years old and is US remanufactured. So my questions are, should i replace the APPS? I can get one from Geno's for about $160. I don't want to spend money I don't need to obviously. Could a bad ECM possibly be causing these issues? Is there any way to test voltage to the lift pump to see if thats causing it to not keep pressure up? Could the FASS just be worn out? it is about 10 years old....I also have a big line kit and fittings from vulcan on the way to replace all the Banjo bolts from the inlet side of the fuel filter to the vp44 just incase it is sucking air somewhere.
Any input or ideas will be greatly appreciated!!

leathermaneod

So I have an update on my fuel pressure. Driving the truck around today and fuel pressure is a solid 20psi at idle but I took the truck to WOT in 4th gear, hit about 30 or so psi of boost and fuel pressure dropped all the way to 11 - 12 psi. Any ideas? anyone else experience anything like this? I haven't had anymore dead pedal episodes yet but i ordered a new TPS from Geno's anyway. That and my fittings from vulcan should be here by the end of the week.

Crallscars

Dirty filter, bad lift pump are the first 2 things that come to mind.

Another possibility is the tank pickup may be partly clogged, you'd get pressure, but no volume.

leathermaneod

Is there any way to test the lift pump? Like measure the resistance in it or check voltage to it or anything? I know the filter is good because I just changed it.

KATOOM

Every time you open the fuel system you introduce air which is difficult to fully purge and will cause erratic pressure readings. How long it takes to fully purge the system can be a few miles to a few days depending on how much air, the volume of the fuel pump, and if air is trapped in the gauge line. You cant bleed the air out of the system either as its not the same as purging air at the injector lines in order to start the engine.

20 psi at idle is fine and yes, a little fluctuation is fine but it sounds like maybe either the isolator isnt working correctly due to air or diaphragm function, or the FASS pump has a bogus pressure regulator. A regulator working properly will not allow such pressure variations at idle. Maybe a phone call to FASS is in order.

The dead pedal experience is when you're driving along and all of a sudden the throttle pedal doesnt respond to changes in throttle position but seems to do nothing. I'm not sure this is what you experienced since you said the truck lost power while the cruise was on. That sounds like an APPS issue. Remember these engines are drive by wire so there's no direct cable connection to the injection pump. Throttle position is relayed electronically through the APPS. An ECM problem isnt described like what you have.

I'd suggest trying to reset the APPS first and also check the AC voltage coming from the alternator. If its too excessive then it can cause strange issues and/or cause the APPS to fail. Lastly, if you want an APPS, I understand the TIMBO one is supposed to be a good choice. I didnt look to see what Geno's has though.

You need to change how the fuel pressure gauge is connected. The VP created hydrodynamic fuel pulses which transmit back through the fuel line to the fuel pump and any FP gauge. These pulses are damaging and will eventually destroy an electric FP gauge sender or a mechanic gauges mechanic. An isolator isnt capable of buffering those pulses either. You MUST have a snubber in that supply line to the gauge and I think Autometer even has one specific for the Dodge Cummins VP application too. Sometimes dependent on how the system is plumbed you could even benefit from two snubbers. I'd suggest pulling fuel pressure from the outlet side of the fuel filter housing instead of directly from the VP. At least you're that much farther away. If you still can, see if Vulcan can supply you with the "tapped" 90* barbed fitting which will allow you to pull fuel pressure from the fuel filter housing.

leathermaneod

Once again thank you for so much helpful info! Im not sure if i was quite clear on one or two things though so i want to clear them up quick. The fuel pressure fluctuations i described are while driving not at idle. not sure if that changes anything. It seems that the lift pump is not able to keep the pressure up while under load.

The dead pedal experience continued after the cruise shut off for at least a few seconds. I pressed the pedal after it shut off and got no response. Then i downshifted (if i remember correctly, I was trying to figure out what to do because i was in the middle of a busy highway with no where to go). After the downshift I had throttle control again but I had the check engine code I mentioned earlier. I had no issues the other 150 miles home or at all yet today. I did order a new APPS from Geno's though because I want to rule it out right away so i don't end up stranded on another long trip. Also, I have not tested the alternator but it is less than 5000 miles old and a good quality Bosch one so hopefully thats not it.

As far as the gauge plumbing goes, I thought that a mechanical gauge w/isolator did not need a snubber!? I sure hope mine isn't ruined already Fortunately Vulcan is already sending a tapped 90. I was planning to use it right at the VP like before but ill put it on the other end at the filter like you said. What exactly does a snubber do the the isolator doesn't? and why don't you see the spikes of pressure from the vp on the fuel pressure gauge?

leathermaneod

I just ordered an $8 snubber from Geno's so hopefully ill get it about the same time i get everything else from vulcan. The only issue is its 1/8 NPT and I'm not sure what size the tap is on the elbow from Vulcan. Hopefully its 1/8th NPT lol

KATOOM

You're welcome and thats what the forums are for.

Yes, I understand the fluctuation is while driving but I was pointing out that for the idle pressure to vary is an indication that the regulator isnt holding the same pressure. Air in the system will do that too though so again, maybe FASS needs to hear about this.

The dead pedal problem wont throw a code since what causes it is poor electrical connections inside the PSG from crystallized solder. If the APPS is failing it will throw a code. But do check that alternator asap since even one right out of the box can have a poor diode.

A snubber is basically a very small orifice which allows just enough volume to pass through for accurate pressure readings but drastically reduced pressure spikes. An isolator is merely a way to "isolate" the gauge from raw diesel running into the cab. So how the isolator works is by running a fuel line to the isolator which then pushes on a diaphragm. On the other side of the diaphragm is fluid (coolant works best) which runs up to the gauge. When the fuel pushes on the diaphragm the coolant pushes on the gauge. Isolators work but they're very finicky and can easily give false readings if there's ANY leaking on the coolant side line to the gauge or if the coolant line isnt filled properly resulting in too much air per the amount of fuel. Some people dont care to deal with isolators and run their gauge line into the cab. Doing so is up to you.

I really doubt your gauge is damaged by the pulses because its something which happens over time, not instantly. The pulses are so rapid that its hard to notice them but sometimes you can see a slight bounce of the needle or hear buzzing. What the pulses do it literally beat the mechanics until they dont move correctly anymore. I actually had ISSPRO retrofit my gauge with a built in snubber and I added a second one on the line to further smooth things out. If you're worried about gauge accuracy then make sure you leave one of the shrader valve test ports somewhere in place. That way you can use a test gauge any time you want. Lots of people fail to think about doing that during their install and then when their gauge readings are questionable, there's no way to check.

And the 90* elbow does have the 1/8 NPT threads.

leathermaneod

How do you go about checking the alternator? Also, do you have any recommendations for a good diesel thread sealant? And finally, do you know if the fittings from Vulcan that are used to replace the banjos on the vp44 and filter housing will fit in place of the return line banjo at the back of the head? I'd like to eliminate all banjos while I'm at it as will as that T in the return line. I know the threads will work, I'm just not sure if there is enough clearance for the firing. Have you or anyone else done this?

KATOOM

The best kind of thread sealant I've used on engines is from Loctite. I'm NOT talking about the thread lock either. The sealant is just like the plumbers goop stuff you can get at the hardware store but its fuel resistant and comes in a white tube. I'm sure you can get something similar at any auto parts store as it doesn't have to be specifically from Loctite, but as long as its fuel resistant. Just dont use Teflon tape as that stuff doesn't like to turn in more than one direction and will leak. But.....that said, its VERY IMPORTANT that you dont get ANY thread tape or sealant into the fuel system otherwise you'll quickly plug and/or harm the injection pump and injectors. So stay away from the first couple leading threads of the fittings just to be safe.

The thread size for all the fittings is 12x1.5 and I'd highly suggest NOT messing with the return line and the overflow valve. All you need to do is replace the banjo's on the supply side and use larger diameter line. The return side is just fine and messing with the overflow will only result in problems you dont want to deal with.

Here's a video on how to check the alternator too.

leathermaneod

Thanks again! I may have missed it in the video but I'm not quite clear on where to hook up the multimeter leads to perform that test?

As far as the return line goes I have already replace the return check valve a while ago with one from Geno's to rule out it as a possible cause of my fuel pressure issues. I was only talking about the banjo at the back of the head and the T fitting, however, I will do as you said and leave it alone for now at least. I thought I had heard of people removing all banjos to eliminate the possibility of air leaks.

Also thanks for the advice on the thread sealant.

KATOOM

No problem.

The multi meter reads from the alternator output. If you're uncertain, there's plenty of videos on Youtube which should help out too. Moparman is just notorious for helpful videos on the Dodge trucks.

As for the overflow valve, yes there are replacement ones you can purchase. There's even adjustable ones, which I'd stay away from. I was just making sure you didnt plan on getting rid of that overflow valve altogether since it is a needed part. You can replace the banjo on the back of the head if you want but you'll find there is no room for anything wider than that banjo fitting. Plus, there's simply no reason to mess with it.

leathermaneod

Oh ok that's what I was afraid of. I won't touch it unless I have a problem with it.

leathermaneod

ok so looking on youtube I'm seeing some people test the AC ripple by just hooking the multimeter to the battery and some are mentioning testing from the output like you said. Which is the most reliable? and if you test at the alt output where do you put the ground? just on the alt case or negative battery terminal?

Also, I was reading and a lot of people are getting 30mv when performing the test. Is that due to a cheap/faulty meter? or performing the test improperly?

KATOOM

I would check directly from the source, the alternator. Maybe this link will help. I know its titled for TC locking/unlocking issues but the APPS and TC both suffer from excessive alternator AC noise, which means this page also applies to what you're after. Mopar1973Man's Dodge Cummins Articles - Torque Converter Lock/Unlock Issues