Hello everyone,

I am new to the forum and also new to Powerstokes. I just bought a 2000 F-350 7.3L Shortbed, Supercab, Automatic, Lariat with only 87,000 miles on it (single owner, garaged its whole life). I have had it for about a month and have noticed a strange vibration/knock when coming to idle/stop after an extended run period (such as coming off freeway). I can feel it worst in the floorboards. I have read up on the notorious 8th cylinder cackle, but the wierd thing is that it only does it in drive. At a stop if I switch to neutral it goes away, which makes me think it could be a flex plate. Transmission was serviced 7 yrs ago and had a billet TC put in. In that 7 yrs though only 6,000 miles was put on rig. The knock is intermittent and does not do it every time, however, when it does it is VERY noticeable. The truck is bone stock except for a Banks wastegatr, I believe it has an aftermarket exhaust, and a Banks TransCommand. The exhaust seems dangerously close to crossmember but I don't think that would be it when it only does it sometimes when truck is warm and coming to a stop only in drive.

To have a flex plate removed it would be around $800. My trusted transmission guy says there is no way to know for sure unless he pulls it. If that's not the case, then that would be a wasted $800 that I could put to a hutch mod or canaopy. Thanks for your help, hopefully I can figure this out. Other then that it is a great truck!

 

Is there any chance you can get an audio recording of it and post to YouTube. A lot of new 7.3 members tend to think the engine has a knock to it when in actuality the engine sounds just fine.

EDIT: Retiredsparky has a great idea too. Post up where you are located and chances are that someone on here will be able to refer you to a good 7.3 diesel mechanic.

 

I think you need to find an independent shop that has a lot of experience in the 7.3. The knock you describe sounds more like an internal engine problem. A good mechanic that is honest will be better able to disable one injector at a time and run the engine under load to determine which cylinder is the bad one.

It is possible that a loose injector could cause a delayed injection event. If the solenoid valve is slow to close or open this can effect the injection timing. A weak injector spring will delay the injector shutoff and increase the length of time the injector is pushing fuel into the cylinder.

Then there is the possibility of a rod or main bearing with excessive wear, a broken piston, broken connecting rod, etc. This may not show up all the time when the engine is running.

Bottom line, it is hard to diagnose a knock over the internet, but there are many on this site who can offer their more experienced and valid opinion.
Larry

I second what Sous says above.

 

When you mention floorboards I think of the front sway bar end links going bad, as it is often described that way. Take a look to see if the rubber has rotted away in the link ends.

OTOH relating the knock to the trans sounds like a different problem. Is the knock related to engine speed, road speed, same stretch of road, etc? 2WD or 4WD?

Don't spend $800 on a wild goose chase. We'll help you figure out what ails her. Then you can spend the money on mods!

 

Thanks for everyone's fast reply. I will try and capture the sound next time it happens. Even when the conditions are right (running for awhile on highway, and then sitting at idle) it does not happen every time. And it only happens when in drive with brake on meaning I can't safely crawl under the truck.

Sorry, I forgot to mention that it is 4x4. And I did just replace the sway bar endlinks and bushings, they were toast. This helped but not with the knock I am describing. I live in Stanwood, Wa, I might take it to Dynomite Diesel and have them do a "once over" on the truck. With low miles and the way it was used I thought an injector would be out of the realm. I guess crazier things have happened! Still wondering if it could be the exhaust...

 

Yes exhaust does make sense. Since it is 4X4, you should be able to crawl under and watch the exhaust while the tranny is in drive and see if the torque of the engine is shifting that pipe close to something it can bang on. A driver you trust with foot on brake and park brake set. Maybe a faulty or loose motor mount?
Larry

 

The ONLY driver I would trust would be nose of truck up against a concrete wall.

 

Here is a video of a bad injector with the truck in drive - pushing against a cement bollard in a parking lot. The only time I could make the noise is when the truck was really warmed up and in gear.

 

And were you able to narrow down which injector it was?

 

Removing the valve covers allows you to access the injectors and unplug one at at time.

 

This was the early days... I took it to the dealership and they used a breakout box to isolate the offending injector.