Engine/Transmission (1998.5 - 2002) Fuel delivery restriction

[John Berger]

This is perplexing. I've installed another new lift pump. I'm reading 8 psi at cruising speed inlet, 12 psi outlet. This was the same thing I was seeing with the old lift pump, but lower pressure readings: 6-7 inlet, 10-11 outlet side. This is the second new pump that did this, so my thinking is that I've got a restriction somewhere before the lift pump. I swapped the senders around to make sure that wasn't the problem. Same thing. Lower readings in than out. I'm measuring readings at the inlet test port on the canister housing and at the outlet banjo fitting on the canister housing. When I installed my second lift pump two years ago, pressure readings always read the same on both sides, with hardly any fluctuations between the two. My fuel gauge has been very unreliable for quite some time now and makes me believe I've got a problem at the tank. What might I want to look for to solve this problem? I've got a bad feeling I'm going to have to drop my tank. Is there anything after the fuel filter housing that could cause such a difference with the pressure numbers?

 

[PROSTAR190]

Try cracking you fuel cap. I remember a TSB about early 24v trucks did not have the vent on the top of the tank installed. My truck des not have a vent I leave my cap loose. To lazy to drop tank remove plug and install a vent hose. Not sure if this applies to 99's

 

[Gary - K7GLD]

John, I have 3 of the Carter type pumps - 2 of the 4600's, and an old model used for my fuel transfer setup and in-bed tank - careful inspection of the inlet/outlet ports of those pumps reveals substantial variations and difference in machining/manufacturing that on ONE of them, really potentially restricts fuel flow. In fact, in a thread from some time back I commented on that and posted a picture of the restriction, my main point at the time being, why spend lotsa money on larger lines and fittings, when you're STILL left with other serious restrictions in the system that largely defeat their purpose...



Wonder if you mighta got one of those poorer machined pumps... ?

 

[Bob Wagner]

I am having the same problem on my truck. The dealer wanted to drop the tank and check for restrictions.



I said no way and left.



If you have a service manual, there is alot of good information regarding the complete fueling system.



I will try to post some page numbers later.



I still believe that my lift pump is on its way out

 

[kaffine krazed]

Are you taking the fuel pressure readings pre and post fuel filter?



To test for restriction before the lift pump take one of the banjo bolts with a test fitting and install it on the inlet of the lift pump and hook up a vacum gauge to it. The book spec is 6inHg but it has been updated they don't want to see more the 3inHg.

 

[Steve Campbell]

John....

if you have the Carter lift pump... . I couldn't determine by your profile... . there is a fine screen in the top of them to "pre-filter" the fuel. You might take the top off and check that!!!

 

[John Berger]

I have the factory lift pump and I'm measuring pre and post filter. The lift pump I replaced was two years old. When I installed it it read 14-15 psi inlet and 15-16 psi outlet. I watched as it got weaker over time and the readings always stayed the same or within 1 psi of each other. Last year, the inlet pressure started reading lower than the outlet pressure. At first it was 1 or 2 psi, but for the last few months there's been a 4 psi difference all of the time. That's what led me to believe that there may be a restriction before the lift pump. Gary, you may be right about the pumps being manufactured inconsistently, but I would like to know for sure. I probably need to drop the tank and replace the fuel gauge sender.

 

[kaffine krazed]

The inlet to the fuel filter is reading lower than the outlet of the fuel filter? If that is the case get a mechanical gauge a verifiey operation of your gauges because something isn't right.



If the outlet of the fuel filter is reading lower than the inlet of the fuel filter by more than a couple of psi it's time to replace the fuel filter.



If the restriction was before the lift pump both pre and post filter reading would be low.

 

[Whitmore]

Sounds as if you may have some junk plugging the intake in your fuel tank, some members have reported of plastic shavings around the screan.



cheers, Kevin

 

[illflem]

gauge problem

kaffine krazed hit it on the head. I don't think it's possible to have higher pressure on the outlet vs. the inlet, it has to be your gauge. I know you switched senders but think trying a simple temporary mechanical gauge will set it straight

 

[John Berger]

If the gauge is a problem, why would it function properly from one sender and not another? I would think a malfunctioning gauge would malfunction regardless of the sender it's receiving signals from. Maybe both readings are bad.

 

[LarryM]

John, It's hard to believe that the pressure post filter could be higher than the pressure in. Even if the restriction is before or after the pump, but prior to the filter it should read right. Try this... your fuel guage is electric and relies on voltage from the senders through a switch to your guage, (I'm assuming that you have one guage and use a switch to select the sender). Even though you switched the senders, you may have a high resistance connection in the wiring, or your switch may have developed a little resistance in the contacts over time. Check it out with a mechanical guage both in and out to put my theory to the test, or eliminate the wiring/switch problem altogether. Hope you find the problem, let us know what you find. By the way, I havn't talked to you in a while, I used to be stationed at TRF and saw your truck around a lot, we talked on a post or two awhile back, anyway, good luck

Regards, Larry

 

[John Berger]

Hey Larry, I remember. Hope things are well. Any ideas on where to get a cheap mechanical gauge? I can't seem to find anything I want in this little area that I live in. The Internet or a trip to Jacksonville is usually in the cards.

 

[illflem]

A 30 psi water pressure gauge availible at most any hardware store will work fine as will the fittings to connect it to the same location as your senders.

 

[John Berger]

I bought a mechanical gauge and hooked it up to the inlet port and although the needle vibrated like crazy with the truck running, the reading stayed over 10 psi. I bumped the engine to run the lift pump and the gauge read a steady 15 psi inlet. I guess I need a new gauge or a new switch. Or both. Funny thing is, I probably didn't need a new pump. The old one would run a steady 7 psi, most times. It never dropped below 6, even when towing. I guess I got a good spare.

 

[illflem]

I've never had much luck with electrical fuel gauge senders lasting long on diesels. Big problem is when they fail they can trick you into thinking they're ok. Finally gave up and went with mechanical.

 

[John Berger]

What kind of mechanical gauge can you get that would allow you to monitor fuel pressure and not introduce fuel into the passenger compartment. I was under the impression that no such beast exists.



I tried a new switch and that is definitely not the problem. Same readings. Any ideas on a reliable gauge/sender combo?

 

[illflem]

Several manufacturers have FP gauges that include an isolator to keep fuel out of the cab, pricey though.

I personally have no problem bringing low pressure non-combustible diesel into the cab. For years I've run rigs with hot higher pressure motor oil in the cab running a mechanical oil pressure gauges without a problem. I've installed at least a dozen mechanical non-isolated FP gauges in Rams without a hitch, many others here will echo the same.

 

[John Berger]

Geno's sells the Di-Pricol gauge with an isolater. The price is a bit high, but it's not exorbitant. I think I might replace the senders for now. If I go with a new gauge, I'll pay the extra money for the isolater type setup. I just don't feel comfortable with diesel fuel in the cab. And I really don't want to drill a hole in the firewall that's big enough to accomodate the fuel lines I would need to run. Thanks for all the advice.