Engine/Transmission (1994 - 1998) Another Fuel Pressure Thread

[JohnsonJim]

See if I can get you the cliffs notes here.



95 6bt swapped into my 88 Chevy V30 Crew Cab.



Engine loses power/misses under load at about 2000 rpm sustained after a couple minutes.



Usually have to pull over to the side of the road and engine will die.



Engine will start right up and then run fine... until you are pulling load up a hill at sustained rpm. Then the jerking missing happens again.



*built fuel pressure gauge... pressure was at 10psi.



*Replaced fuel filter



*replaced lift pump... was leaking from weep hole



*replaced over flow valve with adjustable one.



NOW fuel pressure at 12-15psi.



*adjusted OFV all the way in. Fuel pressure at idle is now 20.



*took truck camper and trailer for a run again... same hill same rpm speed gear etc... the missing/jerking happening again... not as bad... but loss of power... slow down to side of road... engine dies. Starts right back up again... . runs fine til you put a load on it again.



*I have replaced all the fuel lines and double checked all the clamps. I have pressurized the tank and sprayed the connections with windex... no air leaks...



*my next try is going to bypass the tanks and draw fuel from a Gerry can and see if the pressure is better.



*BTW... fuel gauge is at the injection pump banjo with a torktech snubber.



ANY thoughts on the low pressure... and could that be causeing my 'stumble/shaking/stuttering' under load?



Thanks



Jim

 

[JohnsonJim]

Forgot to add... fuel heater has already been removed... and it was also hot out. 80 plus. My fuel lines run perpendicular to the exhaust... about 6" away. COULD there be a heat issue and would I need to have a heat shield?

 

[hsmith]

Still may be a pressure issue. Mine never drops below 24 psi and that is at idle. If it starts right back up then it is not losing prime, air getting into the fuel system, etc.



Why are your fuel lines on the exhaust side of the engine?

 

[JohnsonJim]

Lines are perp as they cross over not parrallel. Ex on. Pass side. Fuel lines at valve on pass and cross over to driver and run up frame. Posted from my phone... . so what about the low fuel press at idle?

 

[fest3er]

Almost definitely fuel. If it runs fine otherwise, then you have adequate compression and air. Those are the three things a diesel needs to run.

Does the tank have adequate fuel in it?

The *lift pump* has a weep hole?

Re-check your fuel line fittings; one or more may be letting air in. If you didn't replace them, check the fuel lines on the engine itself; they may be a source of air.

Check your P-pump's overflow valve. Fuel pressure on a P-pump should almost always be 22+ (in that vicinity). The lift pump is capable of supplying 35PSI (I think); the overflow valve regulates the actual pressure.

I'd install a heat shield in any case. 6", even perpindicular, is closer to the exhaust than I'd like fuel lines to be, especially when the exhaust temp approaches 1000F.

 

[JohnsonJim]

Almost definitely fuel. If it runs fine otherwise, then you have adequate compression and air. Those are the three things a diesel needs to run.



Does the tank have adequate fuel in it? yes... both tanks full... this happens on either tank.



The *lift pump* has a weep hole? Yes they do... the early pumps did. .

Re-check your fuel line fittings; one or more may be letting air in. If you didn't replace them, check the fuel lines on the engine itself; they may be a source of air. Have checked... other than the steel lines from filter to pump



Check your P-pump's overflow valve. Fuel pressure on a P-pump should almost always be 22+ (in that vicinity). The lift pump is capable of supplying 35PSI (I think); the overflow valve regulates the actual pressure.



As mentioned. . brand new tork tech adjutable overflow valve. adjusted all the way in and only 20psi.



I'd install a heat shield in any case. 6", even perpindicular, is closer to the exhaust than I'd like fuel lines to be, especially when the exhaust temp approaches 1000F.

 

[DRAG DIESEL]

Does the tank have adequate fuel in it? yes... both tanks full... this happens on either tank.

Both tanks? Do you know how much the "switch valve" can flow?

 

[JohnsonJim]

Both tanks? Do you know how much the "switch valve" can flow?

That I do not know... it is the same valve that is used in the Chevys and the Fords with dual tanks. (gassers). This afternoon after work I will try and bypass that as an option as well. I have a short list of things to try that will get me that nice #2 smell on both arms! LOL



I mean is it possible though that I have a faulty Cummins (new) LP that is only half ***** working?

 

[MLee]

Try the gerry can, and get away from the crossing of the fuel line to the other side. Is the fuel line 3/8th? or 1/4 from the tank? maybe not enough fuel is being delivered @ high demand. Aslo if you have any rubber pieces in the fuel line that are original, maybe these have de-graded and are plugging the fuel delivery.

 

[JohnsonJim]

Here we go.



hose from filter to the gerry can. 20 psi.



changed out the OFV to the stock one... back to 14.



I can prime the lift pump button and it releases at 27psi.



Tork tech over flow valve should be factory set at 30-32.



when I rev the engine up... I may see 1-3 psi change.



stumped, other than bad LP... . that is NEW

 

[DRAG DIESEL]

Forgot to add... fuel heater has already been removed...

When you removed this, did you remove the spacer behind the LP that holds the heater?

 

[JohnsonJim]

When you removed this, did you remove the spacer behind the LP that holds the heater?

No... only the heater assembly... the strainer is still there.

 

[hsmith]

Pinch off the return line and see what it does to the pressure.

 

[JohnsonJim]

been ther done that. slow rise... not a quick spike.

 

[Nyoest]

I *think* the lift pump uses separate plungers for priming and running. Either that or your cam lobe is worn something terrible.

Ive never seen under 25 psi on my 96 even with the fuel plate removed wot at 65mph. If pinching off the return doesn't change it you either have a restriction of incoming fuel, bad pump or bad cam.

 

[JohnsonJim]

sorry for the delay in getting back to this thread... kids football season is underway and I have been out.



OK... I took the pump and strainer off with the steel lines... submerged in water and pressure tested. . NO leaks.



I stretched the spring and added a bic pen spring tot he orig OFV... and got 30 for short time and then fell to 10. I think the seats are shot in it.



remember the tork tech adjustable will get me 20 adjusted all the way in. I can get 27 before it releases pressure by pumping the prime button... it takes a ton of pumps before i get that pressure though.



fuel pressure the same from either the tank or the gerry can.



does nayone know what the 'lift' spec is on the cam lobe. looking in thehole does not look like much of a lobe at all... more like an essentric. I could try and get a dial indicaor down there and measure the 'lift'... if i knew a spec.



IF the FP lobe is dead... woudnt i have a problem with the intake and exhaust lobes too?



I hate to drop 600 for a fass or an air dog.



Is there any possibility that there is a problem with the injection pump? I have heard problems with the 94-5 with warped barrels and such... .

 

[Nyoest]

One single cam lobe can wear damn near completely off and the rest will be fine. It's probably not your problem though. If you can get thirty for a while then it drifts down it's in either the supply or pump a worn lobe would never have enough lift to create the flow.

Where are you measuring pressing at? I'll try to get volume test data for you tomorrow if I can.

 

[fest3er]

... does nayone know what the 'lift' spec is on the cam lobe. looking in thehole does not look like much of a lobe at all... more like an essentric. I could try and get a dial indicaor down there and measure the 'lift'... if i knew a spec. ...

You should be able to estimate the lobe height: measure how much the tappet moves in and out of the LP. That'll be greater than the absolute maximum it *could* be.



My '98 SM says:

• 53. 962mm: journal minimum diameter
  
• 47. 040 (intake), 46. 770 (exhaust): minimum height
  
• 35. 5mm: lift pump lobe minimum diameter
  

They may mean height for the lift pump lobe. But the lobe could be an 'offset' circle unlike a valve lobe. A 'round' surface might provide a more steady stream of fuel; it doesn't need the sudden movement of a valve lobe.



If the LP lobe is worn, you should've seen comparable wear on the LP tappet.



You know, you *could* temporarily install an electric LP from a 24 valve. Or any inexpensive electric 35 PSI fuel pump rated for diesel fuel. Temporary won''t hurt anything *that* quickly.



Something else to consider. Check the fuel filter's o-rings, et al. There's a small chance they could be 'failing' at higher pressure/temperature and preventing proper pressure from being made.

 

[Nyoest]

Lift pump output with lines unhooked should measure . 70 qts for thirty seconds of cranking. I usually get a paint mix cup they are cheap and marked well. You should have no more than 3 psi drop a across the filter.

 

[BigPapa]

... the lobe could be an 'offset' circle unlike a valve lobe...

The "lobe" is an eccentric. I've never seen one wear to the point that it caused the pump not to function. I'm going to say that you're looking at a bad pump or a bad overflow valve.



Have you pinched the return line off as suggested? Results?



I don't know of a "cheap" electric lift pump capable of supplying the needed fuel pressure for 12V.



Try pinching the return line. No improvement, replace the pump.



Scott