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· Registered
28 Posts
Discussion Starter · #1 ·
Hi all of FSB forum,
I replaced my fuel filter the other day on my 1988 XLT with the 5.0 and the E4OD manual. In order to get better access to it, I jacked the car up on a stand. It wasn't excessively high, but it was uneven from side to side. I ran the car for a minute or two total while it was up. When I brought it back down I noticed what I believe to be squeaky belts. We used spray grease and they went away for a few minutes before coming back. I took it for a test drive, and after a few miles I noticed that I could hear at least three distinct knocks in the engine bay. I have one rod knock usually that comes out around 2000rpm, but its not too bad. These sounds were terrible. It sounds like gunfire now, with a background of screeching. How could this have happened??

I stitched together a quick video showcasing the sounds. The first clip is how it ran before, the rest are after.

· Super Moderator
33,938 Posts
Other than a rod knock;
A high carbon buildup in the combustion chambers & on the tops of the pistons. Can be cleaned up with
MotorVac carbon cleaning service or using products such as Seafoam or GumoOut Carbon Cleaner, etc.

Fuel Quality; Do you use non Top Tier gasoline?

Is engine overheating?

Vacuum leak, See my Vacuum leak test in post #11
Excerpt; "...One way to do a quick check is to grab a vacuum gauge. Some parts stores will loan you a gauge with refundable deposit.
The vacuum gauge should read between 15 and 22 in-Hg depending upon the engine condition and the altitude at which the test is performed. SUBTRACT ONE INCH FROM THE SPECIFIED READING FOR EVERY 1,000 FEET OF ELEVATION ABOVE SEA LEVEL.
The reading should be quite steady. .
When engine is rapidly accelerated (dotted needle), needle will drop to a low (not to zero) reading. When throttle is suddenly released, the needle will snap back up to a higher than normal figure.
When vacuum leaks are indicated, search out and correct the condition. Excess air leaking into the system will upset the fuel mixture and cause conditions such as rough idle, missing on acceleration, or burned valves. If the leak exists in an accessory unit, such as the power brake, the unit will not function correctly. Or Air Conditioning when in MAX mode may switch to Defrost..."

Overadvanced Timimg;
How to properly set the timing on a 302 or 351 by sackman9975 @ How to properly set the timing on a 302 or 351

Read your spark plugs. The wrong heat range plug can cause detonation as well as preignition. If the insulators around the electrodes on your plugs appear yellowish or blistered, they may be too hot for the application. Try the next heat range colder spark plug. Copper core spark plugs generally have a broader heat range than ordinary plugs, which lessens the danger of detonation.

The Exhaust Gas Recirculation (EGR) system is one of the engine's primary emission controls. Its purpose is to reduce oxides of nitrogen (NOX) pollution in the exhaust. It does this by "leaking" (recirculating) small amounts of exhaust into the intake manifold through the EGR valve. Though the gases are hot, they actually have a cooling effect on combustion temperatures by diluting the air/fuel mixture slightly. Lowering the combustion temperature reduces the formation of NOX as well as the octane requirements of the engine. If the EGR valve is not opening, either because the valve itself is defective or because its vacuum supply is blocked (loose, plugged or misrouted vacuum hose connections, or a defective EVP solenoid), the cooling effect is lost. The result will be higher combustion temperatures under load and an increased chance of detonation..."
Source: by

For upper engine noises related to valvetrain, the noise will happen at camshaft speed, which is half the speed of the crankshaft or the rpm indicated on a tachometer. For example, if your engine is idling at 600 rpm, the camshaft is rotating at 300 rpm. As such, a valvetrain noise will occur five times every second at this rpm. Valvetrain parts that can be sources of such noise include the camshaft, lifters, pushrods, rocker arms, springs, guides, and the timing chain. Noises associated with the upper engine and valvetrain will increase in frequency with rpm, but will still be half of the engine (or crankshaft) rpm.

Lower (or bottom end) noises, generally appear as a knocking sound, deeper in tone than something from the top of the engine. The bottom engine noise will occur once per engine revolution. If your engine is idling at 600 rpm (and therefore that is the crankshaft rpm), the sound will occur 10 times per second. Lower engine noise sources include the crankshaft, connecting rods, pistons, main and/or rod bearings, flywheel/flexplate, and various excessive clearances such as a mis-bored cylinder causing piston slap. Like upper engine noises, lower engine noises increase in frequency as rpm increases, but they can also be affected by engine load and temperature as well. A knock might be heard in gear under load, but not in park at idle. by Motor Trend

  • Engine Noise
  • Excessive main bearing oil clearance.
  • ADJUST clearance or REPLACE main bearing.
  • Seized or heat damaged main bearing.
  • REPLACE main bearing.
  • Excessive crankshaft end play.
  • ADJUST end play or REPLACE crankshaft.
  • Excessive connecting rod bearing oil clearance.
  • ADJUST clearance or REPLACE connecting rod.
  • Heat damaged connecting rod bearing.
  • REPLACE connecting rod bearing.
  • Damaged connecting rod bushing.
  • REPLACE connecting rod bushing.
  • Worn cylinder.
  • SERVICE or REPLACE cylinder block.
  • Worn piston or piston pin.
  • REPLACE piston or piston pin.
  • Damaged piston ring(s).
  • REPLACE piston ring(s).
  • Bent connecting rod.
  • REPLACE connecting rod.
  • Malfunctioning hydraulic valve tappet.
  • REPLACE hydraulic valve tappet.
  • Excessive hydraulic valve tappet clearance.
  • ADJUST clearance or REPLACE hydraulic valve tappet guide.
  • Broken valve spring.
  • REPLACE valve spring.
  • Excessive valve guide clearance.
  • SERVICE clearance or REPLACE valve guide/stem.
  • Malfunctioning or damaged cooling system.
  • Malfunctioning or damaged fuel system.
  • REFER to Powertrain Control/Emissions Diagnosis Manual OBDI or OBDII.
  • Leaking exhaust system.
  • SERVICE exhaust leakage.
  • ●Improper drive belt tension.
  • .adjust
  • Malfunctioning generator bearing.
  • Listen w/ mechanic's stethoscope
  • Loose timing chain.
  • ADJUST or REPLACE timing chain.
  • Damaged timing belt tensioner.
  • REPLACE timing belt tensioner.
  • Malfunctioning water pump bearing.
Replace pump

· Registered
1,954 Posts
I’d remove the belt too just as a Hail Mary. But that sounds like whatever knock you were living with before decided it’s had enough. It’s possible the timing is just coincidental with you putting it on the jack.

· Registered
362 Posts
Check for a busted motor mount. If you jacked up the truck at a weird angle and the motor mount was toast, the whole works might have shifted and something is banging.

· Registered
2,654 Posts
I like festus's idea to rule out other possible contributors. You might use a mechanic's stethoscope, or a rubber hose held to your ear to localize where the noise is coming from. Be sure to use all precautions, so that you do not get caught up into moving parts and into the engine.

· Driving Stuff Henry Built
*90xlt,351w,e4od,1356m*79,400,C6,205,19donors*73,400,np435,d20j twin
12,321 Posts
I'd probably go to the old shadetree trick where you hold one end of a hose near your ear (next to your ear, not plugged directly up to your ear), & move the other end around to see if you can pinpoint the noise.

· Registered
28 Posts
Discussion Starter · #8 ·
I’d remove the belt too just as a Hail Mary. But that sounds like whatever knock you were living with before decided it’s had enough. It’s possible the timing is just coincidental with you putting it on the jack.
My grandpa thinks that the screeching is actually from a piston that is not getting oil, rather than the belts. He thinks that it is revealing other knocks from faulty rod bearings. I'm leaning towards the coincidental hypothesis.

· Registered
1986 Eddie Bauer 5.0EFI AOD Full length headers Y pipe into single 3" Magnaflow 3" factory exit
1,931 Posts
A rod knock only does one thing, and that one thing is, it gets worse, & usually pretty quickly if you run the motor.

I am in agreement with the hail Mary belt removal, which eliminates anything external, and tells your just how screwed you actually are.

· Premium Member
6,385 Posts
Did you check the 85-89 secondary fuel filter housing? My 89 had a fuel filter in there that appeared to be from the 90s that was plugged solid. My symptom was back firing under load and dropping down to 0 rpm. No engine noises.

Try replacing your harmonic balancer. That could be it. I also had an 88 bronco that looked identical to yours same color but a base model that had an engine knock. I think the previous owner ran it out of oil. The next owner drove it for atleast a year still vibrating and knocking every time the rpms dropped. Try a motorcraft oil filter and some 20w50 oil to see if the noise changes. You can try some marvel mystery oil too maybe if it has sludge. Essentially if you can't find the problem just keep driving it and find another 5.0 or 5.8 to swap in. PS go roller motor with gt40 heads. 96-97 5.0 Explorer Mountaineer Long block is ideally the engine you want. Swap the distributor gear for a roller one.
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