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Ahubbard

Return Fuel Problem/ Misfire

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Hey guys,

 

I have a 1994 EM7-300 PLN fully mechanical motor in my truck. I am having a problem with the fuel. It is not as noticeable when the truck is cold in the mornings but after I work the truck for a while it gets pretty distinct. It will do it when it is cold but sort of clears up quickly, when warmed up I can set the throttle lock at 1200 and it keeps doing it continuously.

 

The engine shakes/misfires around 1200 rpm. It sort of sounds like it is running out of fuel. Smooth at idle, runs good above that rpm. I put a gauge on the supply line going into the injector pump and have about 20psi at all rpm. Not sure what I should have there.

 

After work Friday I set the throttle lock to 1200 and cracked open injector lines. They all chugged the motor good but #1 not as much. I swapped #1 and #3 injectors and that did not change. Cracking #1 still sounded like less of a chug. I started to think that maybe because #1 is farthest away from fuel supply that it wasn’t getting enough fuel.

 

While checking things over again I see air bubbles in the return line. I have a clearish plastic/rubber hose on there and I can see through. Actually there doesnt seem to be air in the return from the head to the back side of the pump. But there is air in the short line form the back side of the pump to the front where the return check valve is. If I link that line off while the truck is running at 1200 it seems noticeably smooth up. Is that because I am cutting off those air bubbles or does linking that off raise the pressure and help keep it from running out of fuel?

 

Anyway I am going to put new return lines on today and make sure that they are good and tight on the fittings. Also, I see that the return fitting on my side of the pump is a check valve with a spring in it. What is on the back side in by the engine where those 2 return lines meet? It is like a Y fitting where the return line comes from the head then a short line comes around to the front. Is there some sort of check valve in that too?

 

Any knowledge of what I am looking at would be great!! Also any ideas of thinks I am not thinking to look at. I want to start with bigmacktrucks first! Any shops around I think will throw in injectors then if that’s not it try a new injection pump. I don’t have the $$ to be just guessing at big parts and downtime. I want to get it diagnosed first. Thanks Guys!!

 

 

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Oh man I meant Kink the return line not link it. Damn autocorrect lol

 

This line has air

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What is this ?

 

77b676437d20c39b87d773f9508a23c5.jpg

 

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After further inspection that 2 prong banjo fitting just has a small orifice behind it. Nothing seemed blocked there. I looked harder at the return line from the head and am not really getting any fuel coming back through that line. Just the first couple inches have fuel in it. But that lower line flows a ton of air through it.

I ran the suction line into a pail of diesel and then also ran it with the outlet line from the lift pump direct to the injection pump. No change either way. Is my lift pump or flowing enough or should I be thinking something with the injection pump?


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Posted (edited)

Don’t have the pressure spec for that engines fuel lift, someone does, sounds about right, at low end of good spec, to me off the top of my head. 

In the old days they bridged each injector for internal spill fuel returning to tank. Now you just have that one circuit plus a head crossover tube. Those little tubes have a tendency (actually are notorious for) to leak and let fuel drain away from the pump back to tank and lose prime. Cracked stand pipe in the tank is also a possibility for air leak under suction. Fuel is swaying in the tank all day, against the stand pipe, trying to flex it. Time can take a toll.

If your maintaining 20 psi at the pump rail your lift is providing enough volume of fuel.

if you have a system pushing air through you will want to also consider replacing all fuel lines before the transfer lift pump. After pump they leak fuel with a hole in line, before transfer they suck air into system with a hole in line... and drain back causing hard start. 

Be cautious about using certain hyd lines as fuel line replacements. Hyd psi lines are not engineered to experience suction, suction lines are. It’s surprisingly easy to pull the inner liner of a hyd psi line into a drawn-collapse due to engineering. 

Edited by Mack Technician

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Just now I pulled the return line off my head. I put my thumb over the line to keep fuel from feeding back from the pump. It cleared up the air in that lower line. I am getting no fuel out of the front head. There is pressure there when I put my thumb over but no fuel.

I took the little crossover to tube off from between the heads. Ran truck with it off. Back head consistently spills fuel out and front head has nothing. Unless I put my thumb over it then I get a big shot of air and a small surge of fuel when I let my thumb off.

Is it possible to have a bad injector or cup that is letting compression in the system causing my air problem in the return lines and pump?


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Don’t give much thought to injector cups on your engine. God bless it, you got a good engine Volvo didn’t screw up. 

Think leaking copper fire ring @ nozzle tip.

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I think I am pretty set that I have a problem there and I should pull injectors and see what’s up. They do have 25,000 hours on them. I just didn’t want to jump to conclusions and not diagnose. I feel like if taking that return line off the head eliminates the air cycling around my injection pump then I probably have narrowed it down to injectors. Too bad...I should have planned ahead. Last winter would have been a good time to do injectors. Dump truck suppose to be running in summer!!!!

Thanks for the help I will keep you posted what I find. I figured I had a lot of information but I didn’t know exactly how to use it. Guys in here always seem to know their stuff!!


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25,000 hours on the injectors!!!! Man I still love Renault’s PLN... 

Your not lacking anything on your analytics. Everything you’ve said so far, down to cylinder closest to return being affected by combustion air intrusion makes perfect sense. Be sure you put a closer on this thread. 

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Ok... here we go.

 

I took a trip to Antrim Diesel today. Kevin checked my injectors and didn’t find one good one. Lol. MAYBE number 6 was decent but still bad. #1 and #2 were terrible. Think just streaming fuel on the pop tester. Yikes. Guess I got my use out of those. Traded them in for a reman set he had on the shelf that have a little better flow.

 

Came home this afternoon and installed them. That solved my return fuel problem. Nice clear fuel with no bubbles in the return line. Took that running out of fuel sound away. But I still have the shake/chug at 1200 rpm. That’s a little frustrating. Can’t figure out why it is at only one specific rpm. Smooth as silk at idle. And smooth above that..even at 1250 for example. Wonder if this could be in the pump. Someone else suggested a cracked line between the pump and injector somewhere. I didn’t have them totally out I just backed the nuts out and pulled then back an inch to clear the injectors when I switched them. I should have picked kevins brain a little more about it at Antrim. But with a 4 hour ride one way I guess I got a little hurried to get home and install my new toys.

 

I wish I could post a video here so you could hear it.

 

On the bright side the new injectors are great! Test drive shows a noticeable power gain and I feel like (other than 1200 rpm) the engine has less vibration. Took my turbo boost pressure from about 28 psi to 35. Gotta watch the pyro harder now!!! Probably will have to get a bigger turbo... might as well have the pump worked over while they check it..... lol

 

ANY MORE IDEAS?! I am going to work the truck tomorrow. Gonna have to keep running it. I am just not sure where to go from here.

 

 

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1200 rpm? Sounds like it could be a resonant ride issue with the pump?

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1200 rpm? Sounds like it could be a resonant ride issue with the pump?

I am not sure what this means


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Like governor fluctuating at this rpm?


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Yes, resonant ride in the pump feels kinda like the fuel rack in the pump is bouncing back and forth, usually right around that rpm. 

I remember it being a big pain in the ass in the early/mid 90s at the dealership I worked at. Sent out many pumps for this problem, but rarely was it ever fixed.

I remember Mack coming out with some kind of contraption that mounted to the front of the pump to fix it, but man its been a long time!

Does yours almost feel like you are bouncing the throttle real fast at that rpm? 

And usually if the engine had a strain on it, you wouldn't feel it. Only happens when there is steady rpm with little to no strain on the engine.

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Yes I will get that feeling when I drive. Almost feels like the spring is too light on the throttle return even though it’s not. I will let my foot of the throttle some and lay back in to correct that sometimes.

When this does it at 1200 it almost is like a horse galloping. Rhythmic and kinda shakes the motor.


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1 minute ago, Ahubbard said:

Yes I will get that feeling when I drive. Almost feels like the spring is too light on the throttle return even though it’s not. I will let my foot of the throttle some and lay back in to correct that sometimes.

When this does it at 1200 it almost is like a horse galloping. Rhythmic and kinda shakes the motor.


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Yup sure sounds like resonant ride. Unfortunately I'm not sure there's much you can do to fix it. We've sent pumps out before, some were better, most weren't. I'll do some digging about that contraption Mack came out with.

Maybe someone else here knows of a fix?

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My problem definitely isnt under load. It actually does it most prominently when I rev the motor up past 1200 and then let it back to 1200. Or for example when I come to a 35 mile per hour zone and don’t downshift if my foot is lightly pushing the pedal as i coast through it starts the gallop. Is there potential for this condition to damage anything if I run it? I am hoping to get through my busiest season and get to October/November before I take the pump off and send it to the shop.


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No, it won't damage anything at all. It's just annoying.

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Probably little hard on drive train, doing that bucking

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I appreciate all the help guys. We will get this fixed!!! I would love to get any info people have on a fix to this resonant ride problem... makes it easier when I talk to pump shop if I have an understanding about things.


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14 hours ago, 1965 said:

Probably little hard on drive train, doing that bucking

Keeps torsion springs of clutch center plate in constant motion. Clutch life is usually determined by the spring life. 

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