390 w/Serious Knock Under Load

dlowenst · #1 (**permalink**) 09-08-2006, 11:41 AM

I've got a '71 F-100, 390 motor (~15K miles), Holley Model 4160 carb (P/N1850-S 4bbl 600 CFM), stock dual vacuum advance distributor (w/Pertronix electronic ignition & matching new Flamethrower coil), and Edelbrock Performance 390 manifold. Timing is at 10 degrees, set with vac advance disconnected & plugged.

The motor is knocking badly under any kind of load (including even very modest acceleration, and at constant throttle at anything over 45 mph on a flat road w/empty bed). The problem came on suddenly (wasn't there at the start of a recent 45 mile drive, and was happening consistently by the end of that drive). It sounds fine at idle and when revved in park (not under load). The engine does not miss under load, only knocks.

The truck has been running perfectly for a year, and has had no recent engine work of any kind until this problem started suddenly. Here's what I've done to try and diagnose/resolve it:

- Ran multiple tanks of 92 octane gas, to make sure it wasn't a bad fuel issue. No change.

- Checked distributor. Seemed worn so I replaced with a new one (including dual vac advance). No change.

- Replaced Pertronix with set of new points. Ran great for 5 miles and then the problem returned exactly as before.

- Replaced vacuum advance hose (only the front one is connected). No change.

I'm pretty sure I can tell the difference between rod/bearing knock and detonation, and this certainly sounds like detonation (although I don't have any previous experience with Ford motors, so I could well be wrong).

Any help or suggestions would be very much appreciated.

Dave

Redmanbob · #2 (**permalink**) 09-08-2006, 01:32 PM

maybe a lifter pumping up ?, bent a pushrod ? just thinking..a real mechanic will pop in soon enough

rusty70f100 · #3 (**permalink**) 09-08-2006, 01:56 PM

Try disconnecting the vacuum advance entirely, plugging the lines, and then driving it to see if the problem goes away. It wont run very good like that, but at least it'll give you an idea where the problem is.

Also pull a couple spark plugs and look at them. I like pulling number 1 and 8 to check fuel distribution and mixture. Are they white, black, tan, oily? Plugs can tell you a lot.

bertha66 · #4 (**permalink**) 09-08-2006, 01:58 PM

Your vacumn advance is a dual? Is it hooked to the side that is pulling and not pushing? Try unhooking the vac line, pull the dist cap and suck on the vac line to make sure that it is pulling the plate. The vacumn advance can be ajusted on some with a allen wrench in the hose fitting. Are you getting vacumn from the port on the carb, ported vacumn and not manifoild vacumn.

aMoneypit · #5 (**permalink**) 09-08-2006, 03:23 PM

Just for the hell of it, do a compression test. Sounds like a blown gasket between cylinders......aMP

PS: you're not running 50 or 60 degrees of timing are you?........

baddad457 · #6 (**permalink**) 09-08-2006, 09:40 PM

That so called "dual advance" isn't. The outer nipple is the advance. The inner is a retard. Use only the outer nipple. I also agree with Rusty's advice about reading the plugs.

PerfDistIgnition · #7 (**permalink**) 09-12-2006, 10:01 PM

If you don't get to the bottom of your problem, see our custom curved distributors at:

www.PerformanceDistributors.com

There are many advantages of getting away from point style ignitions.

dlowenst · #8 (**permalink**) 09-17-2006, 12:35 PM

Quote:

Originally Posted by aMoneypit

Just for the hell of it, do a compression test. Sounds like a blown gasket between cylinders......aMP

This seems like it could be the problem. Results of the compression test are as follows:

1: 190
2: 195
3: 60
4: 70
5: 180
6: 175
7: 170
8: 170

All the plugs (except 3 & 4) are clean and tan. Plugs 3 & 4 seem a bit wet with fuel but not oily. The truck is not using any oil, and it's not overheating. There is no evidence of antifreeze in the oil. Nevertheless, the 3 & 4 compression test results are obviously a problem. Does this sound like a blown head gasket between cylinders?

P.S. Considering that this motor only has 15K miles on it, I'm also concerned about the variation of 170 to 195 psi on the others as well. Thoughts?

Krewat · #9 (**permalink**) 09-17-2006, 10:33 PM

170-195 isn't too bad...

Yes, sounds like the 3-4 head gasket is leaking between cylinders. Blowing fuel/air into the other cylinder, and then igniting almost nothing. Compression ratio: 0:1

It floods the other cylinder, and after that, no spark.

Krewat · #10 (**permalink**) 09-17-2006, 10:39 PM

Oooh, a few other things:

If there's that much of a leak between cylinders in 3/4, the rest of the compression check is suspect anyway.

If you pull the heads yourself, chase all the head bolt holes with a tap, lube with light machine oil, re-chase, clean, lube, chase again, lube again, etc. Make sure you'll get a good torque reading when reassembling.

And, as to what happened to the block surface between 3 and 4, well, take pictures

dlowenst · #11 (**permalink**) 09-17-2006, 11:07 PM

Thanks very much for the replies. Everything you say makes sense, but can you help me understand why this situation (the blown 3/4 gasket) results in the extreme knocking I'm experiencing? Is it that you end up with vaporized fuel in the cylinder out-of-synch with the spark ignition? If so, can this really lead to detonation with only 60 psi of compression?

Krewat · #12 (**permalink**) 09-18-2006, 10:42 AM

I guess it depends on which cycle the 3/4 cylinder is in. One compresses, pushing air/fuel into the other cylinder. What's that cylinder doing at that point? Too much thinking

And who knows, when the cylinder fires, maybe it's lighting off the other cylinder at the wrong time. When do you have a lot of air/fuel in the cylinder? When your foot's to the floor... so there's not a lot of mix going to the other cylinder under light loads.

But, I'd hate to see you replace the head gasket and all the other work required just to find out that a main bearing is gone.

Guys?

rusty70f100 · #13 (**permalink**) 09-18-2006, 11:56 AM

The two low readings strongly point to a blown head gasket between cylinders. If #4 is lighting off #3, this has the potential to be very damaging, and is the source of your knock.

If it was just one cylinder that was low, or two not next to each other, then I might think it was something else, like a valve or rings. But this is just too convenient.

I wonder if you couldn't remove the #3 or #4 plug, and have someone rotate the engine by hand. See if there's a hissing noise coming out of the plug hole when the adjacent cylinder is on the compression stroke.

dlowenst · #14 (**permalink**) 09-19-2006, 08:51 AM

Thanks for the responses guys. Sounds like the next step is to open up the top. I will report back.

Dave

dlowenst · #15 (**permalink**) 09-21-2006, 12:24 PM

The plot thickens, and it's not pretty....

I decided not to tackle the head gasket myself, and instead brought the truck in to a trusted local shop. They confirmed the compression test results but also did a leakdown test that showed major valve leakage in all cylinders, that they strongly believe is due to seat recission. Their diagnosis is that when the motor was rebuilt (15K miles ago), it was done without using hardened seats and gear. Their conclusion is that I need two complete rebuilt heads.

Assuming this turns out to be an accurate diagnosis, anyone have a really good source for complete heads for this motor?

Thanks,
Dave