Engine intermittantly running rough

dtcerrato · September 17, 2021, 09:33:06 AM

The bus engine has started running intermittantly rough. A change I have made which may have created this condition is aux. electric fuel pump relocation from the engine house to the fuel tank compartment (ie: push instead of pull on majority of fuel line) and the addition of another spin on filter/water separator & plumbing at the fuel tank location. Question: I want to add a pressure guage in fuel flow. Where is the best place for the guage? I'm thinking between the secondary engine fuel filter & the cylinder head fuel manifold? What would be normal fuel pressure at this location for 671? I've also got the added fuel tank compartment fuel/water separator & plumbing on radar. I'm in the process of looking for air bubbles on outlets of primary & secondary engine fuel filters if in the case there's leakage in any of the added fuel lines at fuel tank compartment. Any observatory comments would be appreciated. The condition is intermittent so governor isn't suspect (yet) as I changed governors also.
I should know better than to do multiple changes/upgrades between road tests.

Lin · September 17, 2021, 04:11:10 PM

Could you have gotten some contaminated fuel?

Utahclaimjumper · September 17, 2021, 05:36:58 PM

It IS very difficult to " go back to what you did last" when what you did is multiple in nature..>>>Dan

dtcerrato · September 17, 2021, 05:50:29 PM

I put a pressure guage between the engine mechanical fuel pump and cylinder head fuel inlet. Da book states normal fuel pressures at this location. Engine starts fast when I rev it to over 1800 rpm the pressure guage climbs to 44 psi then drops to zero & stays. I replaced both primary & secondary fuel filters with the same results - engine seems to be running no load a little smoother but where is the fuel pressure? Tomorrow I'm gonna feed the fuel pump from a 5 gal. jug to see if there's better pressure...
The fuel looks to be okay.

Bill Gerrie · September 18, 2021, 05:32:29 AM

Dan
Here is my schematic of my fuel pump at the tank. A check valve lets the fuel bypass the pump once the engine is going but when the pump is operating the check valve forces the fuel to the engine.

dtcerrato · September 18, 2021, 06:25:06 AM

Thanks Bill
I like the inline check valve instead of bypass valves. Does the added check valve increase flow resistance in the suction line? There is a fuel check valve at the rear engine bulkhead. The manual states it prevents static fuel from flowing back into the tank.

Quote from: Bill Gerrie on September 18, 2021, 05:32:29 AM
Dan
Here is my schematic of my fuel pump at the tank. A check valve lets the fuel bypass the pump once the engine is going but when the pump is operating the check valve forces the fuel to the engine.

dtcerrato · September 18, 2021, 08:27:26 AM

Well from a portable fuel supply at the engine it runs fine with plenty of fuel pressure so back to the drawing board on the pos diy at the permanent fuel tank area I brain stormed. Don't ya just hate it when sheet like that happens...

Bill Gerrie · September 18, 2021, 06:13:41 PM

Dan
Look at Bus Grease Monkey reply for the check valve

luvrbus · September 18, 2021, 08:32:22 PM

Check valves should be on the return line ,if you install a priming pump it has to be a flo thru type or it will restrict the flow of fuel and stumble bad,I install those pumps on the primary filter after the filter,a good Autex pump has no problem pulling the fuel from the tank but you don't buy one for 20 bucks

dtcerrato · September 18, 2021, 08:54:23 PM

Today I ran the engine from a portable container 5' from the engine mechanical fuel pump. The most psi I could get between the fuel pump & the cylinder head was 44psi for a short then slipping down to 30psi. When I moved the portable container to about 20' away the PSI was almost lifeless. The DD engine manual 671 fuel pressure spec is 30 to 45psi @ 1800rpm and 45 to 70psi @ over 1800 to full throttle. So now I suspect the mechanical fuel pump is tired. I'm changing it out with a remanufactured DD reliabuilt fuel pump tomorrow and run through the same conditions to see where the PSIs are. There is a .80 restricting orifice in the return port and a straight clear line from fuel pump to portable diesel container.
My new configuration on the aux. electric fuel pump at the main fuel tank area will be a complete bypass system so unless I need the aux pump for an emergency purge or fuel prime - it will be out of the main engine fuel circuit.
Another bus nut told me of a similar situation on his 8V71 and a fuel pump change out remedied the issue.
We'll see...

luvrbus · September 18, 2021, 09:04:07 PM

You waste money but it's your money doing the valving by pass MCI uses the flo thru pump from the factory with no valves ,I never have valve one in 30 years,lol but people told me I needed valves

Bill Gerrie · September 19, 2021, 04:51:42 AM

The reason I ran a fuel pump to prime the system every time I started the engine was to correct a glitch in the DDEC IV computer program. Every so often the engine would stumble and blow a lot of smoke on start up. It would do this for a bit but if I shut the power off and restarted it would clear up. The pump helps but it still does this every once in a while. The pump primes the fuel circuit while the DDEC computer does its self check then once the engine starts the fuel pressure switch cuts it off. The pump is a high volume pump ($400) so no cheap one. It is used in high performance Cummins pick up trucks where I have seen 1600 HP at the rear wheels. The check valve allows the fuel to flow unrestricted once the engine starts and the pump is off and stops the fuel from flowing back to the tank while the pump operates.

dtcerrato · September 19, 2021, 06:51:59 AM

We have a flow thru electric fuel pump that's been issue free for several years now. It's a $12 Chinese knock off. The reason for the brass fitting bypass ($40) is to keep cheap Chinese imitations out of vintage quality oem fuel plumbing. The main reason for the aux. electric fuel pump is to feed the Chinese diesel heater fuel tank in the forward compartment next door. Contrary to wasting $$$ is tight wading on imitation knock offs in limited income retirement. There's places we cut corners but not pertaining to the oem systems and others we go all the way like the Magnum inverter. I just hope the oem fuel pump replacement is a fix for this hiccup.

luvrbus · September 19, 2021, 07:39:08 AM

Most of the time you check the return volume .9 GPM to check for bad fuel pumps,the gear pump is amazing 1 from 2-71 will supply a enough fuel to run a 16v71

dtcerrato · September 20, 2021, 06:52:01 PM

Got the fuel pump changed out. This remanufactured pump is putting out 12 to 15 more psi and the engine is running great but even this new pump doesn't like flow resistance in the suction line so we're configured with a true bypass where unless the aux. electric fuel pump is needed for emergency purge or prime the fuel line plumbing is identical to oem. By oem design there is a fuel line check valve in the supply line on the engine bulkhead just before the primary fuel filter. Glad this happened in the driveway & not otr.