6.9 dies suddenly, then starts up shortly thereafter

Howdy, our Bullnose 420cid diesel is in pretty good shape. ZF5 conversion. The injection pump (IP) was rebuilt 30k ago. Fuel filter OK. Ignition switch 2 years old. It's a daily driver now for one of my sons while he builds up his Bricknose dually.

Today she died three times while driving at low speeds. Both times she started right back up. The third time he had to wait a few minutes, by the side of the road. He made it to his destination (volunteer fireman) while I was on my way there. We swapped trucks, my other son drove the Bullnose home, and she had full power, didn't act up at all.

These aren't symptoms of a clogged fuel filter, but I ordered a few spares anyway.  The only thing I can think of that could cause this on a diesel IDI is the ignition switch, which is new, or the fuel cutoff solenoid on the IP. I'm leaning for the latter. Tommorow in the light he'll inspect its wiring.

I looked for that solenoid on Rock Auto but no go. We do have a good NAPA nearby and I'll check its price tomorrow. If it's less than $50 I'll just put one in, unless we spy an obvious wiring issue tomorrow. I think we'll try a jumper from 12v to that solenoid as a test workaround.

The only other thing that pops into my mind is that it’s getting air in the system somewhere.

Like the fuel filter.

Or the IP, even though it’s rebuilt.

If it's getting air, it won't "start right back up". It'll take lotsa cranking time. Low speed stalling or stalling when you take your foot off of the accelerator quickly is the first sign of a failing pump - especially if it runs well under load.

The FSS has a solid 12v and its terminals are now cleaned and copper buttered.  But it doesn't have a distinct click when I touch the terminals. Almost a buzzing, and every 6th time or so it's silent.

I think my FSS is dying.  I hope Accurate Diesel has them, because NAPA doesn't and I dislike the Furd Stealer. IH is a mess these days when you want parts for any motor that isn't in a tractor. "Oh no, you have to call Fred at Case in Kalamazoo, he can help you...." Then Fred says "What? A pickup truck? We only have tractor parts." and so on.

Update:  Accurate Diesel has that solenoid in stock for $59. Tim and his crew are lifesavers for IDI stuff.

I dont know anything about disel engines, but this sounds to me like classic ignition module problem especially if you have the tfi module.  Just throwing that out there, it happened to me and it was then we decided to relocate the module to the fender wall.

George, a 6.9 IDI diesel dosent have a distrubiter, or for that matter an ignition system. Some folks look at the injection pump and think it is a distrubiter because of the 8  fuel lines to the injectors, but in fact all they do is carry fuel.

If you don’t hear a crisp ‘click’ when you touch power to the FSS then that is probably your culprit. Air intrusion would be accompanied by running rough and surging while driving. The other thing that can/will cause stalling (and immediate re start) is an obstructed fuel return line. Back pressure retards the IP timing to the stall point. You will notice the diesel clatter getting quieter and quieter and power falling off until it stalls. To test, you can disconnect the return line by the firewall and run it into a bottle and see if the problem goes away. I once drove several miles home like this, pulling over every so often to empty the bottle.

Steve, i wasn't sure, i know nothing whatsoever about a disel engine, just though i'd throw it out there

Update: We installed the new FSS and clearly the old one was on its last legs.  Nice crisp click now on actuation.

But now we're having trouble getting her restarted. 3 injector lines cracked and we're still getting splatters of air after cranking for long enough for my eldest the FT wrench to say 'stop, the starter's new'

Now she's charging up overnight, tomorrow we're going to eliminate the water separator on the upper driver side of the firewall. It looks as old as Moses and I hear tell that it can be a source of big air in the fuel.

She sure acts like air in the fuel.

My other son just joined the forum, he drives this Bullnose daily, and I told him to read this thread and bone up on the water separator issue. Our newer IDIs don't have one, and from what I remember they are nothing but trouble.

Howdy,
 I'm RenoHuskerDu's son, tomorrow I will rip out the water separator and replace it with some fuel line. We'll see if that fixes the problem, and keep you updated.

Welcome!  Glad you joined.  Maybe you can sort out the old man.  

Seriously though, how 'bout going to the New Members Start Here folder, read the guidelines, and then introduce yourself.  We ask people to do that to ensure they've had a chance to see our guidelines as we hold people to them and wouldn't want you to get surprised.

He will do that. Thanks.  On the 6.9 today we found the water separator already bypassed. Currently we're installing a section of transparent 3/8" RC fuel line as a debugging tool, looking for bubbles.

One thing escapes me. If air gets in the fuel return line, and all that fuel is just on its way to getting dumped back in the tank, then how does that cause air to get into the fuel fed into the IP?  I must be missing something.

In my experience, air intrusion on the inlet side of the IP comes with rough running and power surging. Intrusion on the return side doesn’t really affect how it runs but can make it a bear to start. The air does not have to make it back to the IP, it just needs to enter the injector which prevents it from firing. Usually this happens around the O-rings in the plastic caps, or old return lines that are cracking or not tight on the cap barbs. If your truck has a return hose coming from the fuel filter head to the return line system, cap it off and delete it. It’s not necessary and is another infamous point for air to get in. Its’s a high point and drain back to the tank sucks air in if it’s not perfectly tight.

Is this the superfluous return line you mention?  All my return lines look pretty new. The PO was a heavy diesel mechanic and he had the IP rebuilt, looks like the lines were new.

Yes, that looks like the one i capped off on mine. It may or may not be an issue for you, but it is a problem spot like the old water separators. The return lines do look brand new, but a twisted or badly installed O-ring can still cause you grief. If everything else looks to be in order, you might need to go back and pull the caps to double check the install. It’s highly advisable to use Viton o-rings, and install them with petroleum jelly so they don’t roll when you push the caps on. It’s not that hard to nick or cut one. They are affordable enough it might be a good idea to have a new set on hand:

https://www.ebay.com/itm/Set-of-18-Ford-6-9-7-3-IDI-Fuel-Injector-Return-Cap-Viton-O-Rings-E6TZ-9229-AA-/192517074499

Done deal on the O-rings, thank you.

I cannot quite fathom why that top return line is there at all. That line feeds fuel into the IP. And yes if it were to leak that would let fuel leak back down to the tank. I'll close it off.

I'm going out to check that out on our 7.3 Bricky tomorrow also. It's always a moderately long crank start.

Quick update on progress.  A new FSS did not fix the problem but the old one was clearly dying.  Still acts like air in fuel lines. Spitting some air out of cracked injector fittings on long crank sputtery no start.

Checked the evil water separator and found it already bypassed. Decided to insert a section of clear hose there as a debugging method. But on attempt, one of those rubber hoses cracked in two.  Ah hah.

Found that replacement fuel pump at NAPA is only $35, so bought and installed that today.

Decided to abandon the whole fuel line loop up to upper driver firewall, and cross over with new tubing at the frame rail under driver seat.

Will update later today with results. Had to take a break to work on a 7.3 SD and update its tunes.

Good luck!  

We'd somehow bent the arm on the FSS when we installed it. Or it came bent. We're not sure, because #2 son took it apart and didn't take pictures or check for goolagtube videos first.  Truth be told, there's not much IDI knowledge out there anymore. But anyway, that little arm that holds the valve open was bent and not holding the fuel valve open enough. Enough to start a little and sputter, but not run.

Glad you found that!  It can be really, REALLY frustrating when you have all new parts and it still won't run.