Rough idle, erratic shifting help please.

Jon Schwenke

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Hey guys, I'm in the final stages ( I hope) of diagnosing some issues I've been having, and need some input from the more experienced. 1992 f250 7.3 idi NA. I have a loping rough idle, worse when at operating temp feels like air intrusion, but I'm pretty certain it's not. After high idle disengages the RPMs are 700 ish. The RPM gauge bounces ever so slightly in time with the surges. In addition, sometimes when at operating temp my transmission will freak out, throw a code (vss related) and then continue to waffle between which gear it wants to be in with the occasional position of none. I'm not sure the issues are related, but I'm laying it all out there. The engine starts well cold and hot (better hot). No visible fuel leaks, no visible air in lines as I plumbed in clear tubing after lift pump, after filter housing, at tank return line, and top of IP at overflow. Minimal smoke at startup, no bubbles in coolant reservoir, doesn't lose coolant.

It could be IP failing as its maybe 150k mi+, but it starts well and doesn't seem to lose power when driving. I'm leaning towards pump timing (if it's IP related) or possibly a bad torque converter. Maybe exhaust leaks? Would that cause rough idle? Please help!!! After reading all of this tell me what you think and where you think my efforts are best focused, and/or if there is anything else I should check / test.

List of mods, repairs, tests (not all were done to chase this problem)

New reman injectors (pop tested within acceptable specs 110 - 125 bar)
new return lines (includes o-rings and caps)
new fuel filter
new air filter
clear fuel lines to check for air bubbles ( no bubbles)
new and properly adjusted neutral safety switch
tested cam shaft sensor
tested vss in diff
20331 Dura-Lift electronic fuel pump
cracked each injector line while running (all get fuel, all change idle condition)
Adjusted TPS to be within spec
changed trans fluid / filter, filled with Napa dex/merc
New throttle cable
New water pump
 
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Jon Schwenke

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I see you have electric fuel pump, do you know what your fuel pressure is?
I haven't tested it. I will. The pump is a Facet duralift 20331 / 40222. Rated at 9-11.5 psi at 33 gph. I'll check it at the filter head, either at schraeder valve or (deleted) injector return line, yes? What is the accepted range of psi please? Would I expect these symptoms from too high or too low a pressure? Thank you! https://www.amazon.com/gp/product/B00PYAW10I/ref=ppx_yo_dt_b_search_asin_image?ie=UTF8&psc=1
 

Big Bart

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Some thoughts
1) Changing the injectors generally changes the timing. You mentioned it idles smoother with the high idle on. There is also a timing advance that is on at that time. Perhaps when it is on, your timing is closer to where it should be. I would start by checking your timing.
2) The fact you have a rough idle could suggest other things. I would also do a compression test to see if you have any valve or ring issues. I watch the 1st, 2nd, and 5th compression stokes and record. I look to see if the cylinders build and max out around the same psi.
3) If your timing is good, your compression is good, your fuel pressure is good. I would send the pump for rebuilding and the injectors with it for testing. The recommended rebuild mileage is 100,000 miles and you are well past that.
4) The tranny issues if at or near idle speeds it could be the tach surging with the engine causing some of your issues. But the tranny does not control the engine idle. But to your point if something came loose in the torque converter perhaps it could cause issues (Binding) but not likely surging. So I do not think your converter is your surging issue.
5) Once the truck runs good then you can chase down the tranny issues.
 

Jon Schwenke

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Some thoughts
1) Changing the injectors generally changes the timing. You mentioned it idles smoother with the high idle on. There is also a timing advance that is on at that time. Perhaps when it is on, your timing is closer to where it should be. I would start by checking your timing.
2) The fact you have a rough idle could suggest other things. I would also do a compression test to see if you have any valve or ring issues. I watch the 1st, 2nd, and 5th compression stokes and record. I look to see if the cylinders build and max out around the same psi.
3) If your timing is good, your compression is good, your fuel pressure is good. I would send the pump for rebuilding and the injectors with it for testing. The recommended rebuild mileage is 100,000 miles and you are well past that.
4) The tranny issues if at or near idle speeds it could be the tach surging with the engine causing some of your issues. But the tranny does not control the engine idle. But to your point if something came loose in the torque converter perhaps it could cause issues (Binding) but not likely surging. So I do not think your converter is your surging issue.
5) Once the truck runs good then you can chase down the tranny issues.
Thanks Bart. The truck ran almost the same before the new injectors, but now little to almost no smoke on startup. There were two that were leaking very badly and popping just @ 100 bar. Maybe it's a bit smoother, but I can't really say for sure.

Timing - I think the truck runs smoother at high idle because the surging is less noticeable at higher rpms, but that doesn't mean the timing isn't off. What is the recommended method for checking the timing? With the strobe and clamps? I'll have to dig into that. I did check out the marks on the top of the IP and IP gear housing. It appears "advanced" by maybe 1.5 to 2mm for what that's worth. I'll check the compression. I thought since it was starting good that wouldn't be a factor, but.... the truck does have 358k on it, but I think the PO had a reman pump and new injectors in at 229k and a reman engine and trans put in around 230k if I'm reading the receipts right.
 

Big Bart

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Message Brian VT he found a eBay seller selling military surplus DTI units for $160. These are $400 new from DTI.

I also suggest you spend $90 more on the DTI timing light specifically for this unit. You can order direct from DTI.


Once you get it start a new thread and we can walk you through doing the timing.
 
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