m1008 wierd fuel issue, help please.

[tyro]

hello, i have a 86 1008 thats been great to me (bought it in sep. 50,500k) it recently developed an odd fuel issue: truck starts and runs great up to about 45mph (+/- 10mph) when the cold weather high idler kicks on. the odd thing is it usually kicks on when i'm on flat ground or going down hill, when i go up hill it kicks off. the truck never wants to stall as if from lack of fuel. the truck has a new fuel filter, i've replaced or blown back all of the lines and there is not one bubble of air coming out of the injection pump.
could this be a bad lift pump even if it is not leaking fuel?
any thoughts on this would be much appreciated.

 

[abarkus]

the high idle is controlled by a sensor on the pass side head in the back and has nothing to do with the fuel

 

[tyro]

huh...well on that note if this sensor went bad would it cause the high idle to turn on and off only at high speeds?? another question: i run heated WVO (160F) could this cause the sensor to behave sporatically. i didnt think sunning grease made a difference because my truck does this whether i'm running on diesel or grease. it actually runs better on grease. so what would cause this sensor to wig out or what would cause my truck to act exactly like the high idle is turning on and off?

 

[Westech]

that happened to mine too.. I changed the injection pump and it stopped. Never touched the sensor.

 

[acmunro]

Are you sure the solenoid is turning on and off ? Or is the truck just surging ? If it is surging, I think it sounds like fuel starvation.

 

[tyro]

i'm not sure if the celenoid is turning on and off. i just disconected the sensor and took it for a ride and i had the same issue. i pulled the return valve. fuel was clean and unremarkable and the ball was still there. it does "surge" forward as i am backing off of the throttle or as i crest a hill but as i begin to go up hill fuel is suddenly cut and that feels exactly the same as if i begin to drive the truck before its warmed up and the high idle cuts out while i'm driving. it feels like someones flipping a switch whether the truck surges forward or slows down. if it is starving for fuel would the I.P. try and compensate and is that what i'm experiencing? also...i live in maine and it does get very cold here. i had a problem with gelling earlier in the season and i have had this problem since but i put diesel 911 in ran the tank empty and i have been using anti-gel all winter it was in the high 40's and i stlll had the problem. the problem seems to get worse as the tank gets lower but its hard to say.

 

[acmunro]

I have a truck I was having problems with air getting into the system. I converted it to an electric fuel pump. That mod fixed the air problem, but now I have a surging issue at speed. I feel the pump is not high enough volume. I also encountered this with a civi 6.2 truck that the previous owner converted to an electric pump. If you are not getting air in your system I would lean toward a weak lift pump in your case. Good Luck

 

[tyro]

thanks for the info. i have what i believe to be a 8psi electric pump which i'll put in tomarrow and see if that helps. i forgot to mention that the truck idles perfectly. again, thank all of you for your help. i'll get back to you tomarrow and let you know what i discovered.

 

[gaunt1969]

mine does the same thing and have come to the conclusion its the ip

 

[acmunro]

thanks for the info. i have what i believe to be a 8psi electric pump which i'll put in tomarrow and see if that helps. i forgot to mention that the truck idles perfectly. again, thank all of you for your help. i'll get back to you tomarrow and let you know what i discovered.

How many GPH is the pump ? The trucks I speak of idle and run around town great. It is out on the highway at top speed they seem to be running out of fuel at the high end. I would love to know the GPH rate on the newer normally aspirated 6.5 Civilian trucks that used the frame mounted fuel pump. My next step is to try one of those pumps to see if it corrects the problem. Good luck and I will be looking forward to hearing what you find.

 

[abarkus]

Mine did that. A lack of power at driving speed put start and idel great. I though that it must be sucking air wrong fuel filter
waste oil is hard on filters

 

[Somemedic]

Waste oil w/o proper filtration will kill an ip faster than you can say "shallow grave"...

I would take a strong look at that lift pump and the hoses/connections between the fuel tank and the ip. Good hoses with no cracks and good connections wont allow air into the system.

Unless the filter sock is clogged or blocked in the tank maybe...?

 

[tyro]

there is a seperate filter for grease in my truck and i can switch between diesel and grease while driving, so even if the grease filter was clogged the truck should in theory run on diesel as long as there is no leaks or clogs in the common line (i made sure of this today). i put an eletric pump in the truck and it seemed slightly better but really no change. could a clogged return line cause this issue?

 

[tyro]

just thought of this...could this be an I.P. timing or adjustment issue? it only seems to surge at speed when i'm backing off the throttle.

 

[tyro]

abarkus what did you do to fix it?

 

[tyro]

Truck now surges at idle on grease, not on diesel. New grease filter and I'm back to the mechanical pump because there was no difference with the electric one. I'm getting minor white smoke. Does all this point to ip? And if so does anyone have one for cheap cuz I have no money and I need this truck. Thanks.

 

[tyro]

found the issue. and intermittent air leak where the rubber fuel line attatches to the inlet of the I.P. it only sucks air when i get into the high rpm's. i think this because i put a clear line coming out of the pump and i get no bubbles at idle. when i rev the engine at idle i get a "misting" of bubbles. when i check it after a drive there is one fat bubble sitting in the top of my return line so i can only assume this is my problem.

 

[yetti]

Hi guys i am new here but I worked at a Chevy dealer doing diesels for 11 years. if I may have you treated your fuel with a Bio Guard additive yet? I have seen some system get so over loaded with scum in the tank they can't pull enough fuel even with a huge lift pump running piggy back. had it happen to me several times. also I have seen the fuel tank vents get plugged bad. older trucks get mud daubers nests and other mud sucked into the vents on the tanks. if you ever popped the cap on the tank and it "sighed" or sucked air when you did the vents plugged. quick check is to run without the fuel cap.

also on the surging issue; if the pump is eating itself the output valve on the pump to the return line plugs. if it does, it raises the head pressure to about 10psi like its in advance mode then the pop offs don't open in the head. when it does that it will start and then die or it runs like its full advanced, at least till the pressure leaks down again. if you think your is doing it the remove the out put valve fitting and replace it with an open fitting and a chunk of clear line. it runs fine without the cold start advance then you need a fresh pump.

hope this helps

 

[gaunt1969]

also on the surging issue; if the pump is eating itself the output valve on the pump to the return line plugs. if it does, it raises the head pressure to about 10psi like its in advance mode then the pop offs don't open in the head. when it does that it will start and then die or it runs like its full advanced, at least till the pressure leaks down again. if you think your is doing it the remove the out put valve fitting and replace it with an open fitting and a chunk of clear line. it runs fine without the cold start advance then you need a fresh pump.

Can you elaborate. I am not shure I am getting this

 

[skyrider]

HI, I have has the same mater many times but just because the fuel level (LOW) in the tank....
I am motorbike mechanic and I have work a lot on this amazing machine!! I thought the only thing about your problem is that, just check it out with full fuel tank level..
xtreambike@gmail.com

Babis
M1008 A1 1985"
GREECE