Hello .. asking for some advice regarding what to me sounds like a valve rattle under load, but only at high engine rpm (foot on the gas and accelerating just before shifting to the next gear), and a contiunous rattle at highway speeds anywhere above 60mph. During light acceleration, hardly a tap, but the hotter the engine gets and the harder I step on the gas, the rattle gets louder.

My van is a basic '78 E150, 300six, Carter 1bbl carb, standard 3-speed column shift .. rattle starts in 2nd gear @35mph .. if I stay in 2nd gear and keep my foot on the gas, the rattle continues loud to about 40mph, silence between 40&45mph, then around 45mph the rattle starts again. Same rattle when in 3rd gear beginning @60mph and above, with no letup .. at today's highway speed limits, this doesn't fly with me.

I've heard and read plenty about carbon buildup on valves and in combustion chambers .. have dumped pint after pint of the 'sea' stuff down the carb .. with zero effect on the rattle.
Of the van's nearly 150,000 miles, at least 140,000 of them are long distance highway miles, so how could carbon buildup be an issue.
Last winter I took all of the lifters out to check for cupping wear on the bottom, and all are still perfectly flat, so a worn cam/lifters is not the issue.

The distributor vacuum advance is working just fine .. careful small experimental adjusts to the left/right with a 1/8" allen wrench were noticeable during acceleration.

I could be mistaken, but it seems like one day the engine was running great as it always has, and the next day it was making that noise.

Right now I'm thinking maybe weak centrifugal advance springs could be causing the issue? .. I'm no mechanic, so that's just a guess on my part.
If distributor mechanical advance springs were weak, or maybe one of the two were broken, what would the symptoms be?

TIA for any advice...

 

I forgot to mention .. in '78 the E150 could still run on either leaded OR unleaded gas, as long as the octane was a minimum of 89 .. this engine has always run just fine with unleaded 89 octane gas .. fwiw, fillups with 93 octane don't solve the noise issue.
It was the E100 series that became 'unleaded fuel only' in '78, and the E150 became 'unleaded fuel only' in 1979.

 

I'm having the same issue/noise with my 300. Curious to see what you find.

 

Does it make this noise when you rev up the engine while you're in neutral (ie. your driveway)? Can you get under the hood and bring up the RPMs by hand and listen for the source of the noise?
Or does it only do it while you're driving? Are you sure it's an engine noise and not a transmission noise?

I would think that a broken or bad distributor advance spring would simply cause it to not advance correctly, so your timing would be retarded and just make your engine weak and gutless. Not necessarily cause a noise.

 

Have you tried retarding the initial [static] timing? The vacuum canister controls when the vacuum advance occurs, not the degree of maximum vacuum advance.

As you wind out the engine [in any gear], the engine vacuum will be rising, so some vacuum advance will be happening. This plus perhaps too much static timing as well as the centrifugal advance due to higher RPMs might be too much [total].

Have you ruled out vacuum leaks? You mentioned temperature... is it staying within a normal range? Do you still have the EGR valve in place?

Just throwing out ideas...

 

Actually, come to think of it, if you had a broken distributor spring, the weights would be fully out. The springs keep the weights from advancing too quickly. You'd be very advanced on your timing, which would cause lots of pinging and could damage your engine. However, if this is the case, then you'd notice it more and lower RPMs since at higher RPMs, the weights would be out all the way anyway.

Although difficult, you can inspect them. Pull the cap and rotor and there are small holes in the plate that you can look through with a flashlight. Turn the engine to rotate the shaft so you can see them. Pulling the distributor out would make this a LOT easier though.

 

You can try to rotate the rotor and see if the springs return it. You wouldn't have to necessarily pull the distributor.

 

Isn't the rotor held in place by the shaft and the distributor/cam gear?

 

Thanks so much for the replies .. based on the answers, I've already ruled out my 'broken distr advance spring' theory..

I hope you don't mind my addressing each of your questions separately...

@FrenchtownFlyer .. great tip on that simple check to see if the rotor springs back after a slight manual rotation .. mine snaps back as it always has with a strong spring resistance, so I'm leaving the distributor alone .. that advice saved this non-mechanic the panic of having to remove/reinstall the distr..

@AbandonedBronco .. no spark knock at low engine rpm, no noise while revving to high rpm in neutral .. the rattling noise starts only when I'm giving it the gas at high engine rpm and UNDER LOAD (like just before shifting gears, on an uphill incline, and maintaining highway speed above 60mph). The more steep the uphill, the louder the rattle, but ONLY @high rpm. On the other hand, mild acceleration on a level road, on a downhill slope, or with a stong tailwind, the noise is almost nonexistent.
The noise seems to be coming from the front of the engine somewhere .. the transm is directly under/just to the right of the driver's seat. Pretty sure no noise from it. What in the trans/clutch/bell housing assembly could make a noise like what I described?

@1986F150six .. yep, I've tried the 'retard initial timing' procedure already .. initial timing spec on this engine is 10degrees BTDC .. set it back in two degree steps at a time from 10 to 8 and all the way to TDC with test drives after each adjust, with negligible improvement, particularly on the highway over 60mph .. rattlerattlerattle
I bought the van new in early '78 during the transition from leaded to unleaded gas .. this one was built for leaded gas, but can also use unleaded gas due to the hardened valve seats, as long as the fuel octane is min 89. Plain old-style motor .. no EGR valve and no catalytic converter .. the only vacuum lines are for the single distr vacuum advance port, the PCV valve, and the air cleaner housing temp control valve that regulates the flapper valve at the front of the air intake housing.
Temp gauge usually reads on the low side of the normal range on that old instrument panel, same as it always has. By engine 'hot', I meant 'at normal operating temp'
As for a vacuum leak, where would I first want to check? .. all of the vacuum hoses are new .. distr vacuum advance and the valve in the air cleaner are solid. Surface corrosion has the manifold bolts pretty much 'frozen' in place, so I had no luck trying to budge them to try to snug them more.

 

Check the 2 nuts which hold the carburetor to the intake manifold. They tend to vibrate loose and the result is a vacuum leak.

Pinch the vacuum line going to the power brake booster to see if the engine speed changes. Do the same for the line going to the PCV valve. [I just reread your last post and you do not mention having power brakes].

Spray carburetor cleaner around the interfaces between the intake manifold and the engine while idling. If the speed changes, a vacuum leak has been found. Be careful of moving parts and don't let the fan blow the cleaner into your eyes.

 

Hrm.... The symptoms really do sound like pinging, which is caused by too much advance under load.

You said you set your timing to 10, but have you checked for a slipped harmonic balancer? The vulcanized rubber that holds the outer ring onto the inner ring starts to lose its grip after time and the outer ring will slowly slip. Meaning, your timing mark is no longer accurate and 10 TDC very well could be 15 - 20. Or, if it's bad enough, the mark could even be on the opposite side of the pulley.

To test for it, remove your #1 spark plug and use a socket (15/16") to turn over the engine. You can look down in the spark plug hole and watch the piston rise up. Another trick is to put a long screwdriver into the hole and watch it instead. Rotate the engine until it's at its peak. Now look at the timing mark. It should be pointing at the 0 mark (or at least close to). If it's not, then your balancer has slipped and needs to be replaced.

 

Trying to think of other possible causes... it is speed/rpm related, so could something have come loose [exhaust system, transmission inspection plate, etc.] that might rattle at different frequencies?

 

@Bronc ... Last winter when I checked the lifters, etc, the rocker arm nuts were loose and the plugs were out, so it was real easy to manually turn the engine just with the fan blades .. with the timing mark on TDC @ cylinder#1, I took a look at where the piston top was .. if the plug hole were big enough I could have stuck a finger in there and easily touched it .. guessing for now that no issue with the harmonic balancer, but thanks for mentioning the screwdriver test, will keep that in mind the next time I have the #1 plug out. So far, as long as I drive the van on a regular basis and don't leave it parked for a month @ a time, the engine starts and is running even before the starter cranks it over once .. it's always started easy like that .. likewise at shutoff with the engine hot, never dieseling/run-on (whatever it's called).

@F150six ... you are correct about there only being a brake booster .. I overlooked that in my prior post .. basic '78 E150, no power brakes, no power steering, no AC .. only options I ordered beyond base van were the high output heater and the capn's chairs.
Finally braved the heat here today and drove it on an errand to warm up the engine for the 'pinch hose' vacuum check .. all seems fine, no change in idle .. I didn't connect the vacuum gauge, but last winter when I hooked it up, I seem to remember the reading @ almost 19 with not much waver in the gauge needle, and no needle vibration. This engine has a very smooth curb idle when warm. Original specs via the sticker on the side of the valve cover call for curb idle adjustment @ 600-650rpm with accessories off, but I set it slightly above that with headlights & heater fan on.


I hadn't mentioned that, last winter, I also replaced the original carb with a rebuilt one because there were symptoms of a stuck carb float .. stumbling idle/nearly stall, particularly when stopped on an uphill incline (at traffic lights, my steep driveway, etc) .. hadda pretty much keep on revving the engine to keep it from stalling.

Fwiw, the exhaust heat control valve in the exhaust manifold is working ok and fully to open when the engine is hot.

The 'new' carb solved the issue, and now it's idling fine again.

For approx the two years before the old carb really crapped out with the float issue, I'd noticed an unusually strong rich & burnt fuel smell from the exhaust .. rear door 'flip'windows open, anytime decellerating from highway speed had always made it easy to get a whiff of the old familiar 'clean' exhaust smell .. the sudden uncharacteristic strong exhaust smell had me concerned from the start, but I didn't know what to make of it .. until the carb float idle issue.

Years ago when the van was new and the highway speed limit was 55MPH, I'd routinely get 23-25MPG, then when the speed limit was increased to 65MPH, 19-20MPG was the norm even with the unleaded 89 octane ... only after the burnt/rich exhaust odor started, did my MPG kinda suddenly drop to around 16-17MPG highway .. it bugged me, but I had no clue as to the cause. City mpg was always and remains @ 13-15MPG.

Any chance the old carb dumping excessive fuel for so long could have lead to such big deposits on the valve stems/piston tops that the deposits could be causing the the pinging? Maybe the 'sea..' stuff isn't quite strong enough of a solvent to clean them?

Dad had a simple little contraption back in the '70's for his LTD that used distilled water, regulated via a vacuum-controlled valve, that introduced small amts of H20 into the carb while driving, that allegedly 'steam-cleaned' the valves/combustion chambers .. problem is, the water bottle needed to be constantly refilled during long road trips across country, and he refused to have to continuously spend the money on the water. He sold it to somebody else.
Any good chemical cleaners out there that might work ok without my having to remove the cylinder head/valve assembly?

 

Sure would like to find the answer to this.

I have been all over my six with a stethoscope and have found nothing that sounds similar. Kinda tough though, since the noise only happens going down the road.

May end up pulling the head to see how carboned up it is.

 

Unfortunately, diagnosing a sound over a forum is pure guesswork. Educated guesswork at best, but still guesswork.

Who knows, maybe it's a cracked valve spring / piston that only really makes noise when it has enough load on it? Not sure. =/