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Login to post Forums » Grassroots Motorsports » 5.0 Ford knock, knock, knockin to come in..

June 24, 2008 7:28 a.m. Xceler8x Reader

Guys, I have a 5.0 Ford question. It's in a 1991 F150.

When the motor starts you hear a loud knock for about 3 seconds. I'm assuming that's the valvetrain. The oil pressure rises to normal, even when hot. After the motor is running it sounds great. No knocks or strange noise. Idles really well after hunting in rpm for a minute.

After this you'll start down the road. While under load it's quiet and sounds as a V8 should. When you put in the clutch to switch gears, and the engine rpm drops, you will hear what sounds like rod knock.

I thought rod knock was constant. Idle, fast rpm, under load, off load, etc. Is this true? Also, if it isn't rod knock are there any other ideas as to what's making that noise? I thought it might be a stuck lifter that was losing oil pressure once the rpm's fall off for a gear change. Then I thought it might be a valve spring that goes "slack" when the engine goes off load.

Thoughts?

Thanks fellas.
June 24, 2008 7:48 a.m. Jensenman SuperDork

What you describe sounds very much like rod knock in its early stages. Main bearing knock is generally there all the time but is a deep thump kind of sound. Rod knock will become more constant as the clearances open up. At that point you can pinpoint the cylinder by pulling plug wires one at a time, when you pull the wire for the offending cylinder the noise will lessen dramatically.

Rod knock in its early stages can generally be heard with no load on the engine by goosing it and listening as the RPMs fall off. It's a fast hard solid sounding knock, generally valve knock is much slower and has a much lighter sound to it.

Timing chains will make noise under the same conditions as a rod but it's generally described as a rattle rather than a knock.
June 24, 2008 8:01 a.m. John Brown SuperDork

Actually it is likely it is piston slap or wrist pin noise. Both are common with the 5.0L.

The only real resolution is refinishing the cylinders and replacing the pistons.
June 24, 2008 8:37 a.m. Dr. Hess SuperDork

In my experience (based on two observation points), rod knock can be heard more after the motor is warmed up. Knocking sound when first starting up cold is likely a bad hydraulic lifter. Put a quart of Rislone in it and see if it gets better.

Your best solution is the old gas cap trick: Unscrew gas cap, roll Toyota Truck under, screw gas cap back on. You sometimes have to replace the gas cap.
June 24, 2008 9:01 a.m. confuZion3 HalfDork

Dr. Hess wrote: Your best solution is the old gas cap trick: Unscrew gas cap, roll Toyota Truck under, screw gas cap back on. You sometimes have to replace the gas cap.

Is it just me or does that not make any sense?
June 24, 2008 9:16 a.m. Jack SuperDork

If you put the risolone, or other cleaner in, follow the directions, with regard to taking it back out too.

This is the first I've heard of a gas cap solution. I'd always heard it as a radiator cap solution.

ConfuZion3 - It's you.

Jack
June 24, 2008 9:55 a.m. confuZion3 HalfDork

Jack wrote: If you put the risolone, or other cleaner in, follow the directions, with regard to taking it back out too.

This is the first I've heard of a gas cap solution. I'd always heard it as a radiator cap solution.

ConfuZion3 - It's you.

Jack

Unscrew gas cap, roll toyota truck under, screw gas cap back on. I don't understand the second step: roll toyota truck under.
June 24, 2008 10:01 a.m. JFX001 HalfDork

confuZion3 wrote:
Jack wrote: If you put the risolone, or other cleaner in, follow the directions, with regard to taking it back out too.

This is the first I've heard of a gas cap solution. I'd always heard it as a radiator cap solution.

ConfuZion3 - It's you.

Jack

Unscrew gas cap, roll toyota truck under, screw gas cap back on. I don't understand the second step: roll toyota truck under.

Put the body of the Ford on top of the Toyota chassis, then it would be a reliable vehicle. ;)

It's an old joke.
June 24, 2008 10:02 a.m. Keith SuperDork

He's just being cute. Basically, junk the F150 and replace it with a Toyota pickup.

Although if you chose a 1989 Toyota pickup with the 3.0, it would be a downgrade.
June 24, 2008 10:23 a.m. confuZion3 HalfDork

Oh. I get it. Roll the toyota truck under the gas cap. OK. I'm dumb.
June 24, 2008 12:38 p.m. Xceler8x Reader

I can't believe I got thread jacked by a bad joke.

Thanks for the info guys. I was thinking on purchasing the truck as a tow'er for the Miata.

I could also agree with the lifter thing short of the knocking coming back during a drop in rpms. That seems to be rod knock as Jensenman suggested.
June 24, 2008 12:50 p.m. 914Driver HalfDork

Hijacked by a bad joke? Here?

Since you don't own the truck, it doesn't matter. If you already owned it, would it be worth all the time and expense to repair,or even swap a motor? Nice truck for the money?

Dan
June 24, 2008 2:21 p.m. Dr. Hess SuperDork

What are you calling a bad joke? That's exactly how I finally fixed a 67 Chevy. Also how I fixed that Mercedes, except with a Camry instead of a Truck. Both needed new gas caps too. Sounds like a rod then. New (used) motor.
June 24, 2008 4:09 p.m. confuZion3 HalfDork

Or do what we have been doing with our F-150 for years with the same problem. Just keep using it. It'll probably get worse - ours did. But it still works just fine for some reason. It ticks, knocks, and makes all sorts of strange noises, but none of them seem to slow it down.
June 24, 2008 4:23 p.m. Hocrest New Reader

A friend of mine has that noise in his 96 f-250 with the 5.0. It's been exactly the same for the many years he owned the truck.

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