Single cylinder knock?
Hi, all - Frequent lurker, first time poster.* I'm not a mechanic by trade, but I do a fair bit of car/truck repair and have worked on diesels
in the past.* That said, I've never had to troubleshoot something like this and I haven't found any posts that seem to relate this sort of issue.
First of all, this is a 96 Ford F250 7.3 with 118,362 miles w/ automatic.* It is bone stock with single-shot OEM injectors.* I do not know
the overall history on this truck, so I don't know if it's ever gotten new injectors, etc.* This is my buddy's truck, and I'm helping him out
since he's a single dad with 2 kids and an all-around good guy.
Recently he had the lift pump replaced.* Shortly thereafter, a knock showed up out of the blue.
The engine starts and runs well, and it doesn't seem to be smoking in any unusual way.* The owner says it seems to run like normal, with the
exception of the new knock.
The oil was replaced about 1,000 miles ago.
The knock seems to be coming from the driver side, and it sounds like it is from a single cylinder.* It occurs at idle, when revved up, and
when the engine is decelerating from a higher RPM, in gear or out of gear.* It does not appear to be present at first start, but it shows up
after the engine has run for a few minutes or has been revved up.
Checked for engine codes, no engine codes found, no CEL.
Completed the contribution test 4 times, passed every time.
Completed injector buzz test 3 times, and it passed each time.
Completed KOEO and KOER tests, results are good.
HPOP pressure is above 500PSI at idle, and somewhere around 3,000PSI at higher RPMS, according to the scanner.* I have not confirmed this
with a manual gauge yet, but I'll try to get firm numbers from a scanner tonight.
Checked engine blow-by and found nothing significant.
The fuel pressure reading on the scanner (I believe) was 50 to 70 PSI.* I need to confirm this by gauge, but haven't found a 90-degree
adapter to make it work yet.* I'll try to get firm numbers from a scanner tonight.* Suggestions on where to find the fuel pressure adapter?
I looked in Snap-on's RWD and OTC's catalogs and I didn't see it..
Pulled an oil sample and had it lab tested.* No evidence of fuel or coolant in the oil, nothing out of the ordinary so far as the makeup
of the oil, and no significant levels of bearing materials, iron, boron or aluminum evident.* I'll attach the oil test if needed.
I checked all 4 UVC connectors (harness side) and found the one for cylinders 6 and 8 to be damaged at the glow plug terminals.* My buddy
described a cold starting issue, so I'm not surprised.
I removed the driver side valve cover and visually inspected the valvetrain, harnesses and UVC wiring.* Nothing out of the ordinary found.
I disconnected injectors one-by-one, and the loss of each injector seemed to produce a consistent drop in RPM.* The knock persisted, regardless
of which injector I disconnected.
All four injectors are ejecting oil from the spouts, though I can't tell if they are pulsing evenly compared to one another.
I checked the injector retaining bolt torque on 2,4,6 and 8.* The first test at 60 in-lb came out ok.
I raised the torque to 120 in-lb (Snap-on Techwrench) and re-checked torque on the injectors.* I found that the bolts started turning around
75 in-lb, which would suggest that none of the injectors were torqued sufficiently.* Does injector torque normally relax over time?* I doubt it,
but I'd be interested it getting your input.
Going forward, I think I need to remove the passenger side valve cover and perform the same tests, as well as to remove all 8 glow plugs.
Once I've removed the glow plugs, I'll probably look in the bores with a boroscope.
If I find that the passenger side injectors are also under-torqued, I think I'll need to remove the injectors for a further look.* Do you agree?
If I find soot on the injectors (making it evident that the copper seal hasn't sealed), is injector replacement an automatic next step, or
should I reseal them and re-try?
Has anyone seen a mechanical issue (such as a bad piston pin, etc.) or valve issue on a truck with such low mileage that isn't used for towing?
Is there anything you can think of that will help me to arrive at a firm conclusion as to the cause of the knock?
This has been tough for me.* Not due to the brain damage of having many variables, but the realization that I may lack certain (expensive)
tools which may be needed to really diagnose this.* Thank you in advance for any help or suggestions you can offer.