Help with a Nissan UD diesel

[boltonranger]

The truck is at my work. It is a 1991 UD1800.
4 cylinder diesel box truck. Six wheeler.
I realize this is not a boat etc. but I was hoping you diesel fellows might help.
It is left to me to fix the truck - and I'm a diesel newb.
Symptoms:
Intermittent hard starting.
It will often start if left to sit.
Only once did it quit at speed.
Today it quit when the driver let the clutch out to move it.
Wouldn't restart though it tried and died after catching.
My observations:
There is a fuel water separator which is between tank and engine.
We (driver and I) tried to drain by opening valve at bottom of bowl.
When drain valve was opened; air was sucked into bowl rather than fuel or water draining out. This happened after much cranking; and could be duplicated. If left open long enough the bubbles traveling upward stops and a very small amount of liquid drains without fully emptying the bowl. (I can see it's still full.) I removed the fuel cap from the 30 gallon fuel tank to alleviate a tank vacuum condition but it didn't help.
My conclusions:
I think there should not be a suction at the drain. I think that not only that but also that the bowl should be willing to drain off when the valve is opened. I also think that the fuel inlet line to the separator must be clogged somehow either by a dirty screen in the tank itself; or by a stuck check valve in the tank or dirt in the line etc. (The fuel filter in the engine bay is new; was changed recently to hopefully stop this trouble but did not.)
My questions:
- Is it a common thing for this type of truck / tank setup to have any sort of fuel strainer inside the tank at the pickup?
- Is there a check valve normally found in the fuel pickup line in the tank?
- Should not fuel be free flowing from the separator bowl drain when opened?(regardless of any water presence.)
- Am I correct that the separator drain valve should not be admitting a stream of air bubbles into the bowl?
What can I troubleshoot on my own?
Are my above conclusions correct?

I'm grateful for any help you can offer.
br

 

[LippCJ7]

Re: Help with a Nissan UD diesel

I would apply air pressure back into the fuel tank from the seperator, take the gas cap off and see if it flows backwards freely. Take a cone shaped air nozzle and push it into the fuel line and see if you have free flow

 

[rbh]

Re: Help with a Nissan UD diesel

Yup, I am inclined to think you have air leak as well.

change filters, double check any O-RING's. (top them with diesel first)
Are the mating surfaces clean????

Crack the injector lines to get air out.
Cross fingers

Can you see any place between the filters and the injectors, including injector pump were it may be leaking?? wet spots

 

[boltonranger]

Re: Help with a Nissan UD diesel

Yup, I am inclined to think you have air leak as well.

change filters, double check any O-RING's. (top them with diesel first)
Are the mating surfaces clean????

Crack the injector lines to get air out.
Cross fingers

Can you see any place between the filters and the injectors, including injector pump were it may be leaking?? wet spots

Wait - I'm saying that when I OPEN the bleeder valve on the bottom of the separator it sucks in air. iow it is under vacuum after cranking the engine.
There are no leaks.
-br

 

[rbh]

Re: Help with a Nissan UD diesel

Wait - I'm saying that when I OPEN the bleeder valve on the bottom of the separator it sucks in air. iow it is under vacuum after cranking the engine.
There are no leaks.
-br

OK, understood.
plugged filter makes the most sense then, unless the fuel pump is before the water separator filter and it is not putting out enough???
put a gauge on the output side of the fuel pump and see what it is putting out.

(not a diesel tech, just work on my own when they brake down)

 

[DJ]

Re: Help with a Nissan UD diesel

If an older diesel dosn't start, you have NO FUEL. Could be air, as suggested and I agree with.

Start at the tank and start looking forward for bad connections and/or bad gaskets on a fuel/water seperator.

 

[LippCJ7]

Re: Help with a Nissan UD diesel

Heres the problem though guys he has replaced the fuel filter recently, yet there is still a vacuum at the seperator, I would force air through the fuel line between the seperator and the fuel tank to see if there is any obstruction there keep in mind there should be a fuel pump somewhere in line too so disconnect that and check air flow. depending on where the fuel pump is you may also need to check flow between the tank and the fuel pump(if the pump is on the frame rail). If the pump is in the tank then I would just replace it. I am also not a diesel tech but I slept at a holiday inn express several times this year.

Diesels have very similar fuel systems to cars so don't overthink this, from the tank you should have a fuel pump then fuel fuel seperator then filter then injectors/rail with fuel lines connecting them, also I have seen where the fuel filter is before the pump. yes there can be a check/roll over valve but its not something thats going to create vacuum so I don't think it could be part of your problem. Diesels do require high pressure fuel delivery since diesel fuel is harder to push through injectors it has a higher density then regular fuel(thats how it was explained to me), I am leaning towards your fuel l pump going south but it could also be a plugged pinched fuel line as well. Since you say your a diesel newb is why I would use pressurized air to figure some things out since its not likely that you have a fuel pressure gauge to use. another thing you can try very quickly is to disconnect the fuel lines at the injector rail and check for fuel pressure(have someone else crank the motor) there but remember if the fuel pump is working properly the fuel is going to come in a real hurry so some clear tubing to route the fuel to a spare container would be adviseable. You can also do this at the seperator, fuel filter and the output of the fuel pump. you said that the diesel has quit after running and that for the most part rules out electrical so fuel delivery is most likely where your issue is.

 

[rbh]

Re: Help with a Nissan UD diesel

Remember there are 2 pumps on a diesel, main lift/fuel pump and the injector pump

 

[LippCJ7]

Re: Help with a Nissan UD diesel

Remember there are 2 pumps on a diesel, main lift/fuel pump and the injector pump

jesus how did I forget that, ok buy a new lift pump bet me this solves your issue.

 

[thefairlaneman]

Re: Help with a Nissan UD diesel

How long has it been since the fuel injectors were cleaned and tested? After being sure you have clean fuel going to the engine do a fuel pressure test.Also it wont hurt to do a COMPRESSION TEST. A worn out engine can do the things you are experiencing also. You got to start at the basics.

 

[windsors03cobra]

Re: Help with a Nissan UD diesel

Sounds like a bad lift/transfer pump, on many vehicles that is an in tank electric pump.

 

[LippCJ7]

Re: Help with a Nissan UD diesel

Makes sense too injector pump is trying to pull through the lift pump, explains the vacuum perfectly, excellent catch!

 

[boltonranger]

Re: Help with a Nissan UD diesel

Well fellas I DO thank you much for all the replies and thoughtful comments.
Here's where I am:
I pulled fuel inlet line off the separator.
Blew in with my breath and heard all the bubbles in the tank.
easy to make them too.

Since the order of things is: tank...separator....fuel filter...injector pump

I then assumed that their was a block in the separator housing since the inlet line hose was clear.

Removed the separator from the frame of the truck and put on the bench.

Disassembled the inlet and outlet fitting and found inlet fitting had a bunch of crap in it.

Cleaned and reassembled the separator.
I filled the bowl about 2/3rds full before I put it back on the frame to see how quickly it filled up and had the driver crank it over.

After a few moments fuel began to enter the bowl. A good stream which after it got to about 3/4 full slowed WAY down to a trickle.

Got truck to start and ran for several minutes until all the air was out and running smooth.

Shut truck off and tried to re-start. It started.

Did this twice more. The last time it would not restart.

Opened the drain valve under the separator again.
Once again there was vacuum and bubbles.

Driver played around with the truck for 20 more minutes until he got it to go and then left with his delivery, fingers crossed.

I'm thinking there is crap in the separator inlet fitting again.

The fact that I could pull the separator inlet hose (not the fitting) and blow bubbles in it initially makes me think that there is no lift pump - am I wrong?
--br

 

[rbh]

Re: Help with a Nissan UD diesel

There should be a lift pump????
Ok the pump can be pre or post the filter, either electric and mounted on the frame rail or in the tank, or possibly mechanical and mounted to the side of the block.
If you follow the line back to the tank there will be a wiring harness with,
I bet 3 wires, 1 hot for pump, 1 ground and one for the sender unit.
If just 2 wires then just sender, check frame rail, and follow through to engine.

and crud built up in inlet of filter, not good, what is in the tank.
(were the pieces rubber or plastic like a pumps impeller???)

 

[boltonranger]

Re: Help with a Nissan UD diesel

Okay. The only other thing on the tank is a fitting with a black and green wire I assume sender unit.

The crud was non-descript reminded me of decomposed leaves or stuff in the bottom of a pond. May have been rust etc.

Tracing the hoses from memory the lift pump IS at the engine;
I think mechanical and on the block - I will verify later when (IF) driver returns.
Maybe I can take some pics. Did all the work this am in the RAIN. Ugh.
-br

 

[dockwrecker]

Re: Help with a Nissan UD diesel

I had a Mercury Cyclone Spoiler in high school with a 429CJ that did this sort of thing. I had bought the car from a wrecking yard. Got it all fixed up and it would die and run intermittantly. Changed filters, blew out lines, etc. Finally pulled the tank out of desperation and found a red shop towel inside that had wrapped itself around the fuel pickup. Would flow fuel for awhile, then plug up. Then I remembered I'd bought the car without a gas cap...

 

[LippCJ7]

Re: Help with a Nissan UD diesel

Okay. The only other thing on the tank is a fitting with a black and green wire I assume sender unit.

The crud was non-descript reminded me of decomposed leaves or stuff in the bottom of a pond. May have been rust etc.

Tracing the hoses from memory the lift pump IS at the engine;
I think mechanical and on the block - I will verify later when (IF) driver returns.
Maybe I can take some pics. Did all the work this am in the RAIN. Ugh.
-br

yeah but now your a diesel pro like the rest of us!! let us know

 

[boltonranger]

Re: Help with a Nissan UD diesel

AN UPDATE:
So the next time it acted up I took the fuel inlet off the water separator.
No blockage - able to blow bubbles into the fuel tank.
(However there was suction of bubbles again at the drain in the separator when I opened it.)
Next the Driver re-attached the line to the inlet fitting this time and rotated the hose slightly.
There had been an upward loop right before the inlet.
He made it go out to the side - so now the fuel flow / hose are level.
Thought maybe me fixed it.
Nope.
Died on him later (on the highway!!) Same problem.

Boss is letting it sit now 'cause no money.

I have one last idea:
to siphon via the fuel hose into a bucket and see if my siphon stops when it shouldn't.

Like if for instance there is something in the tank making a problem; either a rag or a sticky check valve or something. I dunno.
Thanks for the help fellas I hadn't forgotten you.
Just busy with my "real" job.
Merry Christmas/HH to all.
-br