Hello Ford fellas. I joined this forum for your expert opinions and general knowledge. Diagnostic help, tips and tricks.

Here's the story: I just purchased a 1991 Ford F 350 Superduty XL 7.3L idi Ambulance. I'm using the vehicle to take my band on tour. Also I plan on living in it for a while. This is my first time ever having a truck worth working on. I just finished changing the oil on a few hours ago. First oil change done by myself ever too. That should give you an idea of my mechanical expertise. Not much there.

The truck was no start when I bought it. ($1500 by the way. Not bad for a full ambulance right?) Hadn't been started in well over a year. Put new batteries in it. Still no start. Seller said it had a bad starter solenoid so I bridged the terminals on the relay with a wrench and after a little cranking the beast fired up. Before we could drive it home the thing needed a little power steering fluid but after that it drove ok for the ten mile trip home.

Here's the issues. Starter relay obviously needs to be replaced, idles over 1K rpm till the engine warms up. Then it drops down to about 500 to 650 which is around the right range. Just a little low I think. Granted that was before I changed the oil (which I'm still trying to verify I did correctly). I purchased a PSD oil filter for it which I read was supposed to fit these trucks and were also a better choice because they could hold an extra quart of oil. But in practice the filter did not fit. I spent 30 minutes trying to fit it. The infernal thing just never caught the threads. Kept bumping against the oil pan and suspension. I'm wondering if it didn't fit because it's the wrong filter or because my truck is an ambulance? I'm pretty sure the ambulance is equipped with an air ride suspension which could maybe put components in the way of the filter. But also maybe not. I don't know yet. The PSD filter was not going in though.

More Issues: I don't really know what a "knock" sounds like but after I changed the oil I heard what I assumed was an "engine knock". Honestly I don't remember if the same sound was there prior to the oil change but while I was running the engine after the change the sound was enough to frighten me into shutting the engine off right away. next: I'm having trouble finding the fuse box in the cab and under the hood. Got the Chilton book about these trucks and it said the fuse box would be by the pedals or near the instrument gauges. Found nothing in either spot. I really need to find that because none of the gauges work. No speedometer, no gas gauge. The only thing that seems to work is the tachometer.

I'll leave it at that for now. I have a ton more questions though. I've never had a project truck before and I'm really excited about it. My father taught me how to troubleshoot computers but didn't give me anything about cars other than "maintenance is cheaper than repair". So... here I am. Clamboring my way through an oil change and asking for help.

 

First off if its an F350 its NOT a superduty (wasn't until 99 that F250 and F350 were called Superduties, prior to that what is not an F450 was the only superduty), as far as the fuses go figuring they didn't do anything funky for the ambulance conversion the panel below the steering wheel comes off with 4 thumb screws and behind it is the fuse box. The PSD filter works on probably 80-90% of the vehicles but for whatever reasons like you noticed some it just plain don't fit on. Idle sounds about right, high idle should be 1000-1200 and low idle around 600-650 (I think).

Oh and last but not least welcome to FTE

 

well, welcome to FTE

for the starter solenoid, about half of the new ones are bad right out of the box, and have been known to get stuck on from their first use, so make sure to be prepared to pull the battery cable the first few times you try it on the new solenoid.

for the PS filter not fitting, must relate to the ambulance package, so use the one it calls for. not a big deal.
on that note, i once met a man in a jeep who had the wrong filter installed on his rig, it rubbed on the frame, tore a hole in the filter, and really screwed him over, so its good for anyone switching to the bigger filter to make sure it has room around it.

for the knock you're hearing, i obviously haven't heard how intense it is, but there are a few levels of what it could be, not all of which are bad.
diesel clatter - a clatter/rattle, but not quite a knock. this is a good sound that these engines make. its strongest when cold because the timing is further advanced when cold.
injector knock - a failed fuel injector can knock, and can be identified either by using a stethescope (proper listening device) or some crude version thereof, or by loosening one injector line at a time to see if it goes away for a certain injector
valve/lifter noise - results from dirty/stuck lifters which fail to take up the slack at the rocker arms/valves. low oil pressure can also cause this. can often be cleaned up by adding a bottle of seafoam or similar product to your oil to free things up.
rod knock - a heavy knocking from the bottom of the engine as a result of damaged bearings. requires major engine work to solve.
so having said all that, you're probably looking at one of the lighter knocks up there, but if you were to post a quick video of it, that would help us confirm it.
on that note, if you didn't fill your oil filter full of oil before installing it, that will produce several seconds without oil pressure, which can cause some scary noises.

 

Thank you fellas. I've been confused with this superduty thing. The truck is certainly 91. But it says F superduty XL on the side. Maybe someone just replaced the old F-350 logo with an F superduty? Bizarre.

That thing about rod knock scared the crap out of me because the sound was from the bottom of the engine I thought. Not mine but this is the sound. At least very similar. Comments seem to say bad injector. I'm fixin' to run injector cleaner through it. If that doesn't work I'll look into replacing the injector. I wouldn't mind replacing all eight just to learn more about the fuel system. I do not believe it's rod knock though since that's bearing related. The knock came on after I did the oil change. I'm certain it wasn't there before. So if the bearings went bad between now and then, that would be absurd.

 

For a 91 there was a F-Superduty (later became known as an F-450) easiest way to tell for sure figuring its stock is count the lug nuts, 8=F250/350 10=F Superduty. Superduties will also typically be geared higher to easier handle the extra weight (think it was commonly a 4.86 or 5.31 but I could be wrong there) For an ambulance I can honestly see it going either way as far as F350 for Superduty is concerned (sorry pet peeve of mine as back in the IDI era their really 2 completely different trucks)

Good luck figuring out the knocking Ill let those with much more IDI experience here figure that one out (Im used to medium duties so while similar still a much different ballgame still)

 

10 lugs. F superduty is what I got. Now, when I bought batteries for it I purchased batteries for a 91 F 350. And what's worrying me is every time I turn the key to run I hear a loud POP. Is that normal for these trucks? Little concerned I have the wrong batteries in it.

 

You can't have the wrong batteries, or at least it wouldn't be causing any popping noises, if you had wrong batteries it might just not crank fast enough or they might just not fit in the battery trays.