6.9 IDI dies off throttle and weak at idle

[seanres]

I've got a 1983 6.9 IDI F350 with the 4 speed manual. When I downshift or if I shift from a gear to neutral and let off the throttle too fast the engine will die pretty consistently. I recently got it up and running properly and just got it on the road. Some of the recent changes I think could be affecting this are the diesel being treated for algae and not yet switching filters and a new IP that has not yet been timed. When it runs it has mostly gray/black smoke but still some white, not as bad as before the IP and injectors swap.

I am thinking that it has to do with a power issue as even when the engine is warm it can die after startup on idle if I don't give it throttle. It feels weak at that lower RPM, the idle is currently set to 700rpm. I have to be aggressive on the throttle when going into gear otherwise I have issues with it dying. Any suggestions on what I should be checking or is this the nature of the beast with shifting? First time driving manual in a while. I will be doing a fluid change on the transmission but not sure what else I should be looking for besides timing

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[onetonjohn]

Not starting great either. Is it always that hard to start? I'd start with glow plugs and fuel. Did you just get the truck? Any known history?

 

[seanres]

I've had it for a year, and don't know much besides the mileage and what I can tell has been modified.

I've got new glow plugs, IP, injectors, return lines (~1 y/o), electric fuel pump, fuel lines, water fuel separator, fuel filter. I have a glow plug controller bypass and confirmed the function of the glowplugs and that they're good. Its a dual tank and one tank is new but I've been refilling the old tank. In the clear fuel pump filter housing I could see orange indicators of algae. I used bio kleen biocide to treat the fuel in both tanks and then have just been running it as is. It has always been hard to start, but this is the best its ever been. I have a clear line from the electric fuel pump and the filter housing and see no air from there.

Since the last post, I now can't even get it to start with freshly charged batteries! I did adjust the timing but it started yesterday without issues in that video... not sure what else could have changed.

I think I will buy new filters and fill up the new tank to see if that makes a difference. Otherwise I have no idea what could be stopping it from starting!

 

[david85]

Could be a restricted fuel filter, but I would also feed it some ATF in the fuel. I had this happen years ago where the engine would stall when letting off the accelerator. Sometimes it would catch itself and lazily come back up to idle RPM. I put a liter of transmission fluid in the fuel, and it cleared the issue after about 2 hours of driving. Might not be so easy a fix for all cases but its cheap and harmless to try it.

And for what it's worth, diesel fuel additive might not work the same as ATF. I'd still try it even if you did use other additives.

 

[WrenchWhore]

Man! That's probably one of the fastest cranking IDI's i've ever seen! How's the compression?

 

[seanres]

Compression seems OK but have not measured it with a gauge. I am going to try running it off of a fuel can to see if its something to do with the tank or fuel pickup and then pick up a fuel filter next week to see if that helps. Fuel is the last unknown besides everything else that I switched to new. I'll try adding the ATF as well, I have not been using any additives for the rest of the tank besides what I used to treat the algae

 

[seanres]

I just tried to run it off of a fuel can and still crank no start!!! Really scratching my head as to what my issue is. New GP, new fuel, it ran just 2 days ago and now it won't even start!

I'll be picking up a new fuel filter and seeing if that has any effect but not sure where else to go from here given what I've replaced already.

 

[WrenchWhore]

At this point I think it would be wise to list all the parts you've changed and what brand they are. I ask because in 2025 we have seen a large influx of what I call "brand new broken". Counterfeits everywhere. Namely the glow plugs. Listing those, possible pictures of the packaging can help us assist better. The next info I think would be more history related to the truck. What's the actual mileage and can it be trusted. As you know these trucks roll over ever 100k on the cluster. Any ideas on previous owners and how many actual miles it was driven? Have you checked the engine ID numbers to verify it's the same motor? That trucks been around for about 42 years and a lot can happen in that amount of time. I mean i've only had my 87 for 3 -4 years and i've already put another rear axle in and another motor. If all the parts you've install were done correctly then we need to start looking into timing and my personal favorite...compression. If it doesn't have any compression it's not going to want to fire off easy. I'd 100% to a compression test to make sure you're not throwing parts at a tired motor...

 

[seanres]

At this point I think it would be wise to list all the parts you've changed and what brand they are. I ask because in 2025 we have seen a large influx of what I call "brand new broken". Counterfeits everywhere. Namely the glow plugs. Listing those, possible pictures of the packaging can help us assist better. The next info I think would be more history related to the truck. What's the actual mileage and can it be trusted. As you know these trucks roll over ever 100k on the cluster. Any ideas on previous owners and how many actual miles it was driven? Have you checked the engine ID numbers to verify it's the same motor? That trucks been around for about 42 years and a lot can happen in that amount of time. I mean i've only had my 87 for 3 -4 years and i've already put another rear axle in and another motor. If all the parts you've install were done correctly then we need to start looking into timing and my personal favorite...compression. If it doesn't have any compression it's not going to want to fire off easy. I'd 100% to a compression test to make sure you're not throwing parts at a tired motor...

I made a previous post and thought I had resolved my issue with a new set of glow plugs https://www.oilburners.net/threads/6-9-idi-endless-hard-starts.94431/#post-1157676

In order of what I've replaced/changed from newest to oldest, I have new:
- Water fuel separator, GOLDENROD 496 (this past week)
- Fuel filter, Motorcraft FD-811 (before algae treatment, have a new one coming in)
- Braided fuel lines all the way from the tank to the IP
- Glow plugs, Motorcraft ZD28 from rock auto, ohm checked before going in
- Electric fuel pump, Facet 40285 Dura-Lift 12v Fuel Pump
- Hard lines from IP to injectors, amazon
- Injection pump, locally rebuild in northeast. I've had it running with marks aligned and slightly advanced, but ran into these running issues before I could use the timing equipment that I have. At the moment it is back to the marks being fully aligned.
- Injectors, R&D IDI Performance
- Glow plug controller bypass w/ relay, confirmed to be functioning
- Return lines, bronco graveyard I think (now 1yr since install)

All fuel lines and fittings after the fuel filter housing are new until the return line union near the glow plug controller. I have put in clear line from the electric fuel pump to the fuel filter housing to check for air and have no air coming in. I have new fittings to replace the rest of the older return lines from the IP to the 4 way which is still partially hard lines and will insert a clear line in that portion as well. The return line cap from 1st cylinder back to the outlet of the fuel filter has also been capped with a vacuum cap and clamp.

The truck has ~32,000 miles on the odometer and from what I can tell it is most likely 132,000 at max based on registration history from early 2000s, but can't tell beyond that. Same motor. After swapping fuel filters and trying to run off the external can again I'll be picking up the compression kit and seeing if that shows anything meaningful.

 

[WrenchWhore]

Ah yes I remember that thread. That's a bummer this is still giving you trouble. You say it had hard starts when you first picked it up. How long did this truck sit with the previous owner before you started working on it?

I'm interested what the compression test comes back as. It isn't the end all be all since you haven't really be able put it to work work (towing, long distance driving) but it's great info to have especially if it comes back decent. I'm sure i've said this a bunch of times in the past but these motors are work horses and definitely like to be under load. They aren't best for short drives even though you can. I'm just hoping you can get it on the road, up to temp, flog the heck out of it, and free up any stuck rings that may be giving issues. I'd be tempted to run some ATF or MMO in the oil to see if it knocks/loosens up any junk from it sitting most of it's life.

 

[seanres]

When I bought it the truck was running but with a lot of white smoke, it was being used as a farm truck on a property for wood for about a year and before that was on the road back in 2021. I got the truck in 2024 and it was smoking lots and could not start without ether which brings us to today where I am still having similar issues....

I used MMO on the cylinder walls to unstick the rings about a year ago which seemed to help. This Saturday I was driving on the road for about a half hour or so driving and it was feeling better as time went on, but then by the end of the day after it cooled off I had this no start issue which is worse than its been in the past. Got up to third gear but was mostly taking my time in 1st and 2nd trying to get used to the truck so I can take it on a longer trip

 

[WrenchWhore]

Great to hear. Seems like you're headed in the right direction of figuring it out. Keep us updated as things unfold.

 

[ihc1470]

Just because you have new glow plugs, controller, etc. does not mean that they are working. It takes what 15-20 minutes to run through and check the system. Be the first thing I would do if it was mine. I personally use an amp meter to mearsure how much those plugs draw in the system. The whole system will draw 200 amps maybe a little more. Each plug is around 25.

 

[IDIBRONCO]

Since the symptoms are exactly the same as they were before replacing all of those things, I'm starting to wonder if maybe the IP gear timing is off. Maybe someone in the past removed the gear housing for some reason and didn't get the timing right when it was put back. I'm not saying that it's your problem, but I wanted to throw it out there.
Is there any way that you could check with the last owner to see how it ran the last time it was on the road? Maybe that's the reason it was mostly used for what sounds like off road and short drives.

 

[seanres]

Just because you have new glow plugs, controller, etc. does not mean that they are working. It takes what 15-20 minutes to run through and check the system. Be the first thing I would do if it was mine. I personally use an amp meter to mearsure how much those plugs draw in the system. The whole system will draw 200 amps maybe a little more. Each plug is around 25.

I do not use the GP controller, I bypass it with a relay which is connected directly to the battery. I've checked the ohms of each glow plug to be ~1ohm and the voltage as I turn them on with the momentary switch and I can see they get 12v. I have not checked the amperage, did you use the amp meter on the wire to the GP to check?

Since the symptoms are exactly the same as they were before replacing all of

those things, I'm starting to wonder if maybe the IP gear timing is off. Maybe someone in the past removed the gear housing for some reason and didn't get the timing right when it was put back. I'm not saying that it's your problem, but I wanted to throw it out there.
Is there any way that you could check with the last owner to see how it ran the last time it was on the road? Maybe that's the reason it was mostly used for what sounds like off road and short drives.

I have videos of it running from the prev owner and it always smoked which I attributed to IP, fuel turned up too much or injectors. The IP was original but was timed or adjusted at some point as one of the three mounting nuts was missing, but not sure what was done beyond that. So maybe I should remove the timing cover and check the gear alignment to see where the mark is if I can't find any other issues?