900 RPM roughness (running) 93 7.3 F350

Gotta reach out again to my fellow IDI experts. My engine has a rough / weird running at 900 RPM. Sort of feels like missing a cylinder, but don't think it is. No excess smoke at any time during operation. Actually feels like something off balance. Motor & Truck shake but at soon as I go above or below 900 RPM, it's gone whether I'm driving or not. Here's some refresher background info. 93 F350 DRW IDI non turbo. 276000 on the clock, fuel system (low pressure / high pressure / injectors only have 60k. Idles fine and smooth. Cranks fine. (no long period of time and great cold weather cranking). Motor was pampered the first part of it's life. Original injectors and pump lasted until 199K and even then the only reason it was replaced was due to smoking when cruising at 65mph or off and on during idling and poor fuel mileage. Have had no issues at all until I noticed this weird operation about a month or two ago. I don't drive the truck very often. I did noticed that I'm having a small leak at the heater probe on the filter, but that seems to go away when the motor is warm. By small leak, I literally mean a small observation of moist fuel, not dripping. I did question air penetration, but I would assume this was cause rough engine operation specifically at idle, and or difficulty in starting???? Other than that the truck runs flawless and still get's decent fuel mileage (avg 15mpg). Any advice or suggestions would greatly appreciated.

Is the issue occurring hot or cold?


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Man there is such a long list of what ifs. I would start very simple. What the temp out side? Old fuel? Jelling? Air filter fuel filter? Loose connect at fuel pump? Loose water temp sensors/switches? Battery voltage? Loose connections anywhere?

I was originally headed toward cdr maybe an oil pressure issue, all the while still thinking; and ive changed my mind i had a 91 idi tht after driving it a few months did the same thing. It was a single injector malfunctioning, it would miss about a consecutive 3-4 times then have enough pressure still inline to get a full pop, any higher or lower rpm was fine and there was no smoke. I replaced injector and tht was it. Problem gone. I would adjust ur idle set screw to the point the truck shakes and do i line crack test. If u need to kno how to do that i would happily oblige to inform, let me kno. Good luck.


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Is that much torsional vibration good for any motor?

No, but luckily a crack test can be performed in a very short period of time. And it is a lower rpm so i wouldnt be worried, 1150-1200 and up i wouldnt recommend.


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I'm thinking a inferred temp gun on the exhaust manifold and find the cold hole and you have the *******.

Point taken


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Sorry, I guess my post may have been a little mis-leading. In frequent use of the vehicle actually doesn't mean sit's long enough for fuel issues. Sorry about that. Just meant, It's not a daily driver, but get's used enough to keep things flowing.
The problem happens hot or cold. Simply put when I crank it, after the warm up solenoid kicks down I can push the pedal to put the engine at 900 RPM and there it is. It literally feels like a 454 gas engine with a high performance cam. Lower or higher on RPM and the roughness is gone. The thing that is confusing, is that if were an injector, I would think I'd see smoke problems or something and if it was air penetration I would think it occur at any RPM? That being said, why is it only at 900 RPM. And just for further clarification 900 is approximate. Our needles are sort of wide so I guess it would be more like 877rpm - 942rpm is where the roughness is located. Below or above that, all is good. The other reason it's of concern, is that this is the slow moving rpm when sitting in this freaking Atlanta traffic at 5pm so it's pretty annoying on the day's that I have to drive my truck. Helidiesel - Not sure what you meant by batt voltage, or loose connections. It's an IDI. No computer / electronics on motor, but I did check that as a normal maintenance item, specifically since I was on a road trip towing my jeep recently.??? On a non related note, like your jeep. My crawler is a 95 yj on 35's SOA, 8.8 rear, 44 front, SYE kit, winch, CV shaft, and a host of other projects going on. Preparing for club Rubicon trip next year in Cali. Again, thanks in advance for any an all input.

What I'm trying to say is what is your battery voltage in general. Or any loose connections at the battery's or ground ends. Weak battery's do crazy a thing even with out computers. Low voltage is always a problem. Nice jeep. Here in fl that's my only hill in the picture!! I hate it. What lead me to diesel here in fl!!

If the injector is only starting a slight leak it is possibly to not leak so much tht cylinder compression Still has enough heat to burn it, and if theres a couple u cld experience some serious shaking. And again this is only a possibilty.


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Helidiesel mayb correct too with his voltage theory, to test his theory would probably deal directly with the ip. Though we have no computers we arent going anywhere without electricity to the ip. Set up a voltmeter in the connection on top of ip closest to the grill and rev to 900rpm to test his theory. What do u think heli?


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Sounds possible to me, at 900rpm might be the right vibration frequency to shake the fuel solonoid conection on top of the IP.

does the roughness clear up when you rev up the motor?
if so, your problem is in your injectors. you managed to get crap stuck in one of them.
these trucks aren't rocket science. voltage, CDR, torsional vibration won't affect these motors. a lazy injector will screw up your day.

I'm also thinking a bad injector. I may have missed it but how many miles are on the injectors. Usually they wear out around 125,000 to 150,000 miles. Same with the injection pump. This forum trys to recommend buying only new BB code Delphi injectors. Very few shops actually rebuild injectors because parts are not avaialable. The few parts that can be found are more expensive than a new injector. So how is a shop rebuilding and pop testing an injector and selling it cheaper than a new one.. Well short answer is they aren't. But money talks when its in short supply and buying cheaper parts forces us to do the impossible.