Help Troubleshooting - Lope / Surge at Start

[bumblinstumblin]

Hey Fellas

I've been traveling around the country in a '84 F250 with a pop-up camper on the back for the better part of three years. I have the 7.3 swapped in with a banks turbo. ZF5 manual trans. I also have an electric fuel pump swapped in. Currently hanging out in Jacksonville doing a few repairs on the truck / camper.

I've been stumped by a lope / surge that occurs intermittently, but most frequently during a cold start. It does not happen every time and it rarely happens once the engine is warm. It goes away immediately when I give her gas, and has never occurred at any time aside from start up. Here is a video.

I've been doing some research on diesel forums and on Youtube and it seems this type of problem presents itself in many ways, and the solutions are equally varied. I've seen lope / surge problems fixed by replacing an ICP sensor, tach sensor, neutral safety switch, dual mass flywheel, chasing down an air intrusion... the list goes on.

Does anyone on this forum have guidance for me? Does anyone have experience troubleshooting this type of issue, and could offer some advice on the best place to start?

Thanks.
Bumblin'

 

[Nero]

Sounds like it could be intermittent air intrusion, especially since it's only doing it at idle.
All the sensors and stuff? This is a purely mechanical engine, to tach sensor, icp, ect will cause it.
The usual culprits are going to be the injector return line caps and o rings. Even if they don't look like they're leaking, they could be sucking air in when it's cold. Had a similar issue on my truck, turns out when the previous owner swapped the return caps, they didn't do #1, it was still original from 1988... I changed it last summer.

 

[Black dawg]

I have had small air leaks cause that. Pay attention to if the truck is parked nose up or downhill when it does or doesnt have the problem.

Have also seen a sticky metering valve cause the same thing. If you rev it up and take your foot off the pedal, does it drop down below idle speed and then come back up.

 

[bumblinstumblin]

Sounds like it could be intermittent air intrusion, especially since it's only doing it at idle.
All the sensors and stuff? This is a purely mechanical engine, to tach sensor, icp, ect will cause it.
The usual culprits are going to be the injector return line caps and o rings. Even if they don't look like they're leaking, they could be sucking air in when it's cold. Had a similar issue on my truck, turns out when the previous owner swapped the return caps, they didn't do #1, it was still original from 1988... I changed it last summer.

Thanks for the reply, Nero!

Quick question on your second sentence - Did you mean to say "this is a purely mechanical engine, so the tach / icp / etc will not cause the issue"?

Did your truck have a mechanical lift pump? I was doubting the fuel lines because an electric pump pushes instead of pulls, seems like it would compensate for smaller air leaks.

Thanks.
Bumblin'

 

[bumblinstumblin]

I have had small air leaks cause that. Pay attention to if the truck is parked nose up or downhill when it does or doesnt have the problem.

Have also seen a sticky metering valve cause the same thing. If you rev it up and take your foot off the pedal, does it drop down below idle speed and then come back up.

Thanks, Black dawg.

I'll pay attention to uphill / downhill today and tomorrow, same with idle speed post-rev. Then get back to you.

Also want to swap front / back tanks to see if that could help isolate the issue.

Thanks.
Bumblin'

 

[Nero]

I have a mechanic lift pump, but just because you have an electric one doesn't mean it primes the system completely before firing off.
And yes, you can literally run the engine off 1 wire. No tach, no coolant sensors, no nothing. So the stutter would most likely be something fuel related.

 

[Brian VT]

And yes, you can literally run the engine off 1 wire. No tach, no coolant sensors, no nothing.

And that's the beautiful thing about these engines.
Of course, you still need a way to heat the glow plugs and turn the engine over to start it. But, after that, good to go on 1 wire.

 

[Stu Bailey]

And that's the beautiful thing about these engines.
Of course, you still need a way to heat the glow plugs and turn the engine over to start it. But, after that, good to go on 1 wire.

That’s what Bill Cosby juice is for

 

[Booyah45828]

I'd pop test the injectors. You might find one isn't sealing and drips. While you've got them removed, install new return lines, caps, o-rings, and olives. See russ @typ4 for purchasing russrepair.com

 

[chicken bones]

I recently put new injectors / Pump & return lines on my Truck and I have the same problem. I was not inpressed with how easy my return lines went onto the plastic cap or the clamps for that matter. I'm going to replace my mechanical lift pump next. if your running OEM fuel filter head , check the fuel filter heater plug, i had to change the o rings on mine in the past. Some people delete the plug all together.

 

[bumblinstumblin]

Fellas! Apologies I've been away. I'm balancing a few projects that need to be complete before leaving on a trip next week. I had my camper down to the studs when I first posted and she's come a long way..

I've been driving the truck around a decent bit and have a few updates. Switching to my rear tank completely clears up the issue. No lope or rough start at all! Any guidance on where to look? Hopefully easier to troubleshoot closer to the fuel tank instead of the engine block.

There is no difference in lope whether I'm parked up, down, or neutral when going to start the truck.

Thank you all for the help and advice.

 

[Black dawg]

Did the issue happen regardless of fuel level in that front tank?

If fuel level didnt seem to have any correlation, I would be checking fuel line from tank valve to that front tank. Seems like the 84 still has rubber hoses clamped at tank and at tank valve. I would inspect those hoses and check the tightness of the clamps. I have seen these hoses let air in through the hose..... generally will be some sign of wetness on the outside of the hose or connection.

 

[KansasIDI]

Sorry to rob your thread but my 86 6.9 does something similar, but it only did it on a hot start off of the front tank. Switch to rear and no surging. But now the truck will not run on the front tank for more than a few minutes, then it will die and not restart until you switch to the rear tank. I am guessing that the FSV is what was doing that on my truck, it seems to make a bit more power and blow a little more smoke off the rear tank.