Burning smell at high RPM

[brnco_luvr]

Everytime I rev the engine a bit, for example sustaining speed at 65mph for about 10 min, or go up a steep hill, there's a slight burning smell that then completely goes away a couple of min after I slow down or stop. I've tried to stop right away and see if I can trace the smell in the engine bay to no avail. Any ideas? There's no overheating. My emergency brake is off too ;D !!!!

 

[langester]

Are you running headers or stock exhaust manifolds?

 

[SHX669]

Hot exhaust manifolds / header pipes is my first thought also

 

[brnco_luvr]

I believe stock exhaust manifold.

 

[brnco_luvr]

Is it a problem? Should I do anything about it?

 

[spap]

Could oil,be leaking from the valve covers on the exhaust
When you said going up a hill maybe thought small leak from the back of the valve cover

 

[Broncobowsher]

What is your air filter and crankcase breather setup?

I'm guessing an open element air filter and breather oil fil cap. High load = low or no vacuum. The PCV can't pull fresh air through the engine. High loads are also high cylinder pressures which is the most blowby. The blowby just vents out the breather cap. In a stock setup the breather is routed up to the air cleaner to be burned in the engine.

How much oil does it leak/burn? High RPM taxes gaskets and seals a lot more. The oil inside is being thrown around a lot harder.

 

[DirtDonk]

When was the last time you rummaged through the entire engine compartment and underside to make sure there were not some things touching other things that should not be?
When was the last time you gave it a tune-up including new PCV?
What is the ammeter saying when you're driving? Is it way over into the positive side, or down near zero?
What do all the wires look like?
What year is the Bronco and what all has been done to it?

And the next time this happens pull over when it's safe to do so, and wander around the truck to see where the smell is strongest. It might be under the dash, might be under the hood, might be under the vehicle.
Is it undercoated? Dual exhaust?

Hard for us to tell where the smell is coming from way out here, because it's not normal behavior. So yes, it just might be something to worry about.

Paul

 

[jckkys]

A picture of the engine compartment would help a lot. The fact that you don't know what the stock manifolds look like, makes me wonder what has been done to your engine that you're not aware of. The majority of EBs sold recently are filled with shiny crap that uninformed buyers see as improvements, when the reality is that cheesy billet and chrome plated bling is likely causing problems in many of these trucks. The crank case ventilation/PCV system isn't sexy looking, so it gets ignored, tho it's a necessity. Most of the open element air cleaners have no place to connect a breather to supply clean air to the crank case to replace the dirty air removed by the PCV valve. Under high load a warn engine's blow by will overwhelm the PCV and has to go somewhere. The stock air cleaner had a breather hose to one valve cover so pressure won't build up pushing oil leaks. Broncobowsher is correct.

 

[Rustytruck]

if you have an automatic it may be tranny fluid from the vent. the c-4 doesnt tolerate an overfull condition. it burps out the vent. Sometimes you will see drops of fluid on the tailgate. Mine needs fluid extremely low on the dipstick when cold. when I drive up the Cajon pass out of LA is when it would rear its ugly head. C-4 can be a picky beast.

 

[brnco_luvr]

Wow, thank you for all your questions. I'm a novice, so will do my best to answer questions.

'74 C4 stock 302 4B carb

1. The valve covers were changed in June by the PO before my eyes. No leak around them. The only leak it has is a very small leak from the oil pan. Oil levels are maintaining and haven't had to add oil over the past couple of months of ownership. I have taken a goose neck camera and looked all around and no other leaks are found. I don't see a tranny leak.

2. Paul, I've taken a look at the bay with the engine on several times right after the smell occurs. Can't trace it any where. There are probably some hoses or wires that are touching hot surfaces, but again, I don't see any signs of burn or melting any where. The smell dissappears literally seconds later once I've slowed down the drive, and have looked at the engine bay right after the burning smell comes, and can't smell anything.

3. Rustytruck, the tranny fluid level on the dipstick seems to be higher than where it should be. I will send pictures of the engine bay, exhaust manifolds, air filter, etc. tonight.

4. Will take some pix of the engine bay tonight.

 

[Rustytruck]

another smell I use to get was from the transfer case with 50 weight oil in it. the first trip I made each year over 200 miles the transfer case would burp an oil smell no leaks or anything. Probably the time the oil got hot and vaporized any built up moisture in the oil.
Went away when I went to Amsoil synthetic 90 weight in the transfer case.

you can get a burning smell if you grease your driveline and drive off slinging grease from the lube job on your exhaust.

any power steering leak or oil pressure sending unit leak goes straight to near by exhaust manifold.

 

[brnco_luvr]

another smell I use to get was from the transfer case with 50 weight oil in it. the first trip I made each year over 200 miles the transfer case would burp an oil smell no leaks or anything. Probably the time the oil got hot and vaporized any built up moisture in the oil.
Went away when I went to Amsoil synthetic 90 weight in the transfer case.

you can get a burning smell if you grease your driveline and drive off slinging grease from the lube job on your exhaust.

any power steering leak or oil pressure sending unit leak goes straight to near by exhaust manifold.

What's interesting is that the smell disappears immediately; one would think that a leak would cause the smell to be there for a little while at least as long as the engine is hot

 

[brnco_luvr]

And it only happens after few min of high speed or pushing it a bit up a hill. I could be driving at 50-55 for 10-20 miles and no smell

 

[Broncobowsher]

And it only happens after few min of high speed or pushing it a bit up a hill. I could be driving at 50-55 for 10-20 miles and no smell

That really sounds like a blowby issue

 

[MarsChariot]

I have experienced the same. High speed mostly. Every time that I have tracked it down, what Rustytruck said has been my main suspect. Usually you can find a driveshaft-slung grease blob somewhere that was not there before.

 

[JB Fab]

I have experienced the same. High speed mostly. Every time that I have tracked it down, what Rustytruck said has been my main suspect. Usually you can find a driveshaft-slung grease blob somewhere that was not there before.

^^^^ Been there.....

 

[brnco_luvr]

here are a few photos of the engine bay.

 

[DirtDonk]

Very sharp! Not much to burn down under there unless there's a hidden wire somewhere that ends up touching something. But that looks doubtful.
Time to go underneath and follow the exhaust along to see if there is any burned on oil buildup along the tubing adjacent to the u-joints.

And make sure that the front hubs are unlocked too, just to be sure that nothing in the front is turning.

Paul

 

[DirtDonk]

Oh, and feel around on the spark plug wires to make sure that the high-speed fan air is not pushing any of them closer to the header tubes. That one in the back on the passenger/right side (#4) is angled fairly close to one of the header tubes. If the timing is not advanced enough then a header tube can get glowing red hot at sustained higher speeds.

Look too, between the alternator and the cylinder head. See if any of the wires are bunched up in such a way as to be touching the metal of the head.
And speaking of which, what is that blue thing in there? Maybe it's just some painted part of the engine, but I did not blow it up enough to see and it looks a little different. Like it might be something wedged up in between.

Paul