I tested my fuel pressure on the fuel rail and I am only getting 16 psi with the motor running. If I shut off the motor and turn on the key the pressure will only go to 18 psi and the pump shuts off. I dropped off the fuel tank and put on a gauge direct onto the outlet of the pump and it put up 39 psi. I checked my fuel line for damage but didn,t see any. Any ideas why at the pump I have good pressure and its bad at the fuel rail or is my pump shot. It got a new pump (not sure of the miles but I think with in the last 15000 20000 miles- Will look it up).

 

You should be getting 49PSI ±5 psi at the rails. It's physics that explains the pressure drop from the pump to the rails along with other anomalies such as a leaking injector.

The pump output at the pump will be higher than at the rails. The same thing can be had with a regular garden hose and, for each foot out from the source, the pressure decreases due to friction, temperature of the fuel, etc.

At the source, it will always measure higher. (assuming the lines are the same internal dia all the way down the line)

 

cmckenna, would blowing compressed air through the line at the rail with it disconnected from the pump help out any?
or maybe clamping a rubber plug on the fuel pump side and shooting compressed air into the line at the rail (with a rubber tip to get a good seal) to check for leaks?

leaking injector is a good theory too, how would you be able to tell without removing all the injectors?
then again, since leahyhfarms removed the tank to get at the pump then i guess pulling the rail wouldn't be too much more trouble. with new o-rings before removal of course.

 

Compressed air is of no use unless you've got a gas such as helium and a leak detector and even at best, you still won't be able to spot it by eye as air is a gas well, technically it's a liquid but, you understand what I mean.

The leaking injector condition is easy for me to find and, there's two ways / methods for that. One is to simply pull the rail while under pressure and check for dripping. Simple as that. But you can't do that on some rails like mine because, there's no mechanical retention that keeps the injectors in as mine are a compression system. It uses compression to keep it all in place.

Anyway, the second way involves simply watching your pressure with the engine off. If it drops rapidly and there's no external leaks, you can bet on a leaky injector or a bad check valve at the pump. One of two things there.

But, there's also another method which involves electrically activating each injector one at a time with a 12VDC source but, I don't like telling people how to do it because of the risk possible damage to the injector plus, if there's an external fuel leak say, at the fuel rail o-ring, then you've got a chance of fire if there's a spark. So, I won't mention that method.

 

Is the truck running?

 

Yes the truck is running but I think it should more in powerful because if I was beside a little 183CID 4 cylinder turbo diesel suv here, the the little 4 cylinder would beat me off the line and up thru the speed. I have the plenum pulled off at the moment to do the plenum gasket. I removed all the plugs a short time ago and they were all clean (no fouling). When I tested the pressure at the pump the pressure dropped slowly ( over about 5 minutes) to about 18 psi and more or less much held there. Would you guys reccommend using thread kock on the plenum bolts. Is there any need to use or would it be a bad idea to use a little RTV on the plenum gasket? Thanks for ideas so far.

 

Are you getting a Hughes or APS plenum plate? Follow the directions if you are. If you are just replacing the gasket using the stock plate, good luck. Doesn't matter what you use on the gasket, it will blow again.

 

Where would be a good place to buy a Hughes or APS plate. Please keep in mind where I am so I am a bit limited in some of the parts that I can get. At the moment I just going to put back the stock plate. How many miles is the gasket likely to last.

 

if you do reuse the stock plenum plate, look closely at the length of the pan bolts. its been said they are a hair too long, and bottom out in the hole. and use the best gasket you can find (felpro).

if you can't ship from US to Ireland, then take your belly pan to a machine shop or metal shop, and buy a piece of 1/4" alumiunum plate and make your own. you could have a machine shop cut it out fancy like, or just cut it with a jig saw. use your stock pan as a pattern. its just a flat piece of aluminum.

http://www.hughesengines.com/Index/p...d&partid=22078



hughes says to use medium strength blue thread locker on the belly pan bolts.

 

Do you see that? There's your problem right there. Your fuel system is not right at all.

Per OEM spec, it's supposed to hold at a MINIMUM, 20 psi for 15 minutes.

SO, either your check valve on your pump is not working, or, you've got a wicked leaky injector, or, you've got gross leak - that is not right at all- not even close to OEM SPEC.

This is also why you've got no power at all. It's your fuel pump that's not delivering- this is why it lacks power. THe fuel, for whatever reason, is not flowing like it's supposed to.

Now, how it works, is, your pump has a check valve and, even in freezing cold temps, it will drop pressure over time down to zero from a hot off state. As the fuel constricts due to both the fuel lines and rails cooling, this in turn drops the temp of the gas which in affect, constricts thus dropping pressure down to zero. However, this happens over a much longer duration than five minutes.

Conversely, when cold, when ambient air temp starts to warm up past the point of the temp of fuel, the gas now begins to expand thus increasing the pressure in the lines up to full 50psi rating or, very close to it depending on ambient air temp and temp of the lines.

Now, if your dropping pressure over that short of a time period, (five min) something is wrong.

I think it's time to look at your fuel pump and lines as well as look for a leaky injector/s as well as gross leaks.

Gross leaker- you'd smell it. I have ten bucks on the pump. Low power=bad pump output and check valve is unable to hold pressure while operating.

That's what I think. See, if you had a gross leak, you'd still get power or, most of it anyway and, you'd be smelling gas all the time. Leaky injector= hard starts when hot.

Hard starts when cold and lacks power = bad pump / check valve / regulator