Auto Repair Help
Reiner’s Corner – Auto Repair Help
DIAGNOSE – BURNING ODOR COMING FROM UNDER THE HOOD By Reiner B

Continued from Part 1

TIPS & TRICKS TO FIND AND FIX THE SOURCES OF BURNING ODORS

The odor of something burning under the hood is not only an inconvenience; it is also an unmistakable sign that something is seriously amiss with your car’s engine. If these types of issues are not resolved immediately, there is a high likelihood that that serious and even fatal damage to the engine can result, but worse, many engine fires are caused by oil and fluid leaks. Here is a quick rundown on the most common sites of fluid leaks, and what you can do to fix them-

OIL LEAKS

Oil leaks can occur almost anywhere on any engine at any time, but some leak sites are common to all engines. However, if oil leaks are left unresolved over long periods of time, the entire engine can eventually become covered with oil, which makes finding the site of a leak almost impossible. Therefore, it is recommended that the oil residue be cleaned off as much of the engine as possible before you attempt to trace an oil leak. However, if the engine is still reasonably free of oil deposits, things are considerably easier. Here is where to look for oil leaks-

NOTE: While evidence of oil and other fluid leaks are easy to see, finding the exact site of the leak can sometimes be very difficult, and sometimes impossible due to the design of modern engines and transmissions. One way to overcome the issue is to add a special dye to the engine, transmission, power steering system, A/C system, or cooling system that glows bright green when the dye escapes from the leak along with the fluid. The downside of these products is that they will only fluoresce under a UV light, but most major auto parts stores supply the dye and a UV light in a kit. Leak tracing dyes are an invaluable aid in tracing fluid leaks, and we strongly recommend that you treat the fluids in your vehicle with a dye product to make finding the sites of future fluid leaks easier.

Valve covers

While almost all manufacturers use rubber gaskets to seal the valve covers on their engines today, rubber gaskets harden and shrink over time. No amount of tightening bolts and or screws will resolve a leak past a degraded rubber gasket, and you will probably just damage screw threads in the cylinder head if you over tighten retaining bolts or screws.

  • Make sure the engine is cold before you start your inspection, and remove all protective shields and covers on the engine to improve your chances of spotting the leak.
  • If the valve cover gasket is leaking, oil will have collected in the gap between the cover and the cylinder head, but it will also have left a trail as it ran down the engine. If the trail stops at the gap, it is certain that the gasket is leaking.
  • Use the correct tool to check all bolts or screws holding the valve cover down for tightness, and tighten any that are not fully screwed down. Wipe off as much leaked oil as possible, and operate the vehicle normally to see if the leak returns.
  • If the leak does return, replace the gasket with an OEM replacement to ensure a proper fit, but bear the following in mind-
    • Once the valve cover is removed, clean all mating surfaces with a lint-free rag and a solvent such as acetone to remove oil residues. Doing this improves the sealing properties of the new gasket greatly
    • Do NOT use silicone sealants on rubber gaskets, because not only do these sealants do nothing to improve the sealing properties of rubber gaskets, but because uncured silicone sealants can become detached and end up clogging oil passages and galleries, with potentially disastrous results for your engine
    • Make sure the replacement gasket fits securely in the valve cover before fitting the cover in place to test the location of the new gasket. DO NOT start inserting and tightening bolts and screws before making sure that the new gasket is properly in place, and not protruding from under the valve cover at any point
    • Insert all retaining bolts or screws, and screw them down in several stages in a crosswise pattern to ensure even sealing of the gasket. DO NOT over tighten bolts or screws- refer to the manual for details on the recommended torque settings for the application.
    • Operate the vehicle normally for a few days to verify that the valve cover is no longer leaking before replacing all protective shields and covers.

Turbo chargers

The most common sites of external oil leaks on turbo chargers is where the oil return line connects to the underside of the turbo charger body. On most applications, this line is a rubber, oil resistant hose that connect to the oil pan. However, the high temperatures associated with turbo chargers cause these hoses to harden, split, or crack, which causes major oil leaks that give off intense clouds of smoke when the oil comes into direct contact with the searingly hot turbo charger casing. The only remedy for this type of leak is replacement of the hose with an OEM replacement part. Note however, that while many aftermarket parts suppliers claim that their hoses are and temperature resistant, aftermarket hoses seldom provide the level of durability and reliability that OEM hoses do.

NOTE: It should be noted that accessing the oil return hose on many applications is far easier said than done. On many applications, working space is severely limited, and to reach the hose it may be necessary to remove or disassemble unrelated parts and components. If you have any doubts about your ability to perform the replacement yourself, the better option is to refer the vehicle to a competent repair facility for professional assistance.

Oil seals

Oil seals that commonly leak are the crankshaft seals both on the front and back of the engine. In the case of the front seal, this seal is always located out of reach behind the harmonic balancer (crankshaft pulley), and replacing these seals involve removal of drive belts, A/C pipes and hoses in some cases, the harmonic balancer itself, and the timing cover along with the timing belt if the engine uses an external timing belt.

If the rear seal (commonly known as the main bearing seal) is leaking, engine oil can often be seen dripping out from between the engine and the transmission. The only way to replace this seal is to remove either the engine or the transmission form the vehicle- which is removed depends on the application.

Other seals that often develop leaks are those that seal the camshaft(s) on engines that use external timing belts. Replacing these seals requires removal of drive belts, A/C pipes and hoses in some cases, the timing cover along with the timing belt, and in many cases the valve cover, and camshaft(s) as well.

WARNING: Attempting to replace the oil seals described here is NOT recommended for non-professional mechanics that do not have the required skills, tools, equipment, and technical abilities. Oil seal replacement is a highly technical procedure that should ideally ONLY be performed by trained technicians that have access to the special tools and technical knowledge that are often required to perform seal replacements successfully.

Transmission leaks

Both manual and automatic transmissions most commonly leak through the output shaft or selector shaft seals, though one or more sensors that are screwed into the transmission, and in the case of automatic transmissions, at the point where fluid cooling lines attach to either the transmission and the radiator or fluid cooler, or sometimes, at both attachment points.

Other possible leak sites include the mating surface between the transmission and the oil (fluid) pan, the point where the dipstick tube enters the transmission casing, or the drain/filler plug(s).

The repair depends largely on the site of the leak, but non-professional mechanics are strongly urged to refer transmission leaks to a competent repair facility for professional diagnosis and repair, since some repairs could include removal and disassembly of the transmission.

Power steering fluid leaks

On older vehicles, the most common sites of fluid leaks is the high pressure hose that hardens, which allows fluid to escape from between the hose and the crimped metal fittings, but also through the braids of the hose itself. Replacement of the hose with an OEM replacement hose is the only reliable, long-term remedy.

Other leak sites could include joints and attachments point in the low-pressure hose, or the power steering rack itself. While replacing the low-pressure hose should not present most non-professional mechanics with undue difficulties, all other repairs should be referred to the dealer or other competent repair facility for professional diagnosis and repair.

Engine cooling system leaks

While oil leaks usually give off an acrid, chocking odor that is often accompanied by thick clouds of white smoke, leaking engine coolant usually gives off a sweetish, almost pleasant smell when the antifreeze comes into contact with hot engine parts.

Since coolant leaks can occur (often unexpectedly) anywhere in the cooling system, the best way to prevent leaks is to perform regular inspections of the radiator, hoses, and expansion tank, and to make repairs or replace parts as soon as possible after a problem is discovered.

If however, a leak develops suddenly, DO NOT attempt to stop it by covering the site of the leak with your hands, or objects such as jackets, sweaters, or other items of clothing. Trying to cover or stop a leak will almost certainly cause you to sustain serious injuries, since the escaping mix of coolant and steam is super heated and highly pressurized.

WARNING: Required repairs depend on where the leak occurred, but NEVER work on the cooling system when the engine is hot. Also, note that refilling the cooling system after repairs often require that air be purged from the system in a prescribed procedure, so always refer to the manual for the application for details on how to purge the cooling system successfully. Getting this step wrong can cause serious and often fatal engine overheating.

BURNING RUBBER ODORS

The odor of burning rubber on a modern vehicle is a somewhat rare event, since most engines are designed in such a way that rubber hoses and components are routed and secured away from hot exhaust parts.

However, if you do smell rubber burning under the hood, keep a fire extinguisher (if you have one) ready at hand when you open the hood, since opening the hood can cause smouldering rubber to ignite into actual flames when additional oxygen is added when the hood is lifted.

Typical sites of heated rubber include unsecured or badly routed radiator, A/C, or power steering hoses that rub against drive belts or other rotating parts, or drive belts that are slipping on frozen/seized pulleys and/or tensioners. Repairs or remedial action depends on the actual problem but bear in mind that rubber that is hot enough to give of a bad odor is extremely hot, so DO NOT touch the problem area/object with your bare hands- doing this can cause serious scalds and burns. Wait for the engine to cool down before you attempt repairs.

BURNING INSULATION ODORS

Burning electrical insulation always indicates a serious problem, since many, if not most engine and vehicle fires start as the result of short circuits that melt protective insulation around wires and cables.

Electrical short circuits have many possible causes, including badly and secured wiring that rubs against body or engine parts, wiring that runs to close to hot exhaust or turbo charger components causing insulation to melt, or often, poorly executed repairs that leave live wires exposed.

Whatever the cause of the short circuit, the best thing to do when insulation starts to burn is to switch off all electrical consumers, and to exit the vehicle as soon as it is safe to do so; i.e., don’t stop the vehicle in the middle of traffic and jump out just to get driven over by a passing bus.

If it is possible to open the hood and you see an actual fire, get as far away from the vehicle as you can if you have no means of fighting the fire. Bear in mind though that if there is no actual fire, but you see the site of the burnt/burning insulation, DO NOT touch the damaged wiring with your bare hands- if you do, you will almost certainly sustain serious burns.

BURNING PLASTIC ODORS

Many modern vehicles have a lot of plastic in the engine compartment these days- from protective shields and covers, to plastic inlet ducting, and other components. Under normal driving conditions, these components rarely melt or catch fire, but lost and/or broken retaining clips often cause shields, covers, and other components to move into close proximity with hot exhaust parts, where they can melt and give off an acrid, choking odor and sometimes, clouds of black smoke.

If you encounter such a situation, switch off the engine to reduce the heat acting on the plastic part or component, but do NOT touch the object with your bare hands, since the object is almost certainly hotter than you might have thought. Use an object of clothing or a rag, newspaper, or other insulating material to remove the melting/melted object from the engine compartment to avoid burning or scalding yourself. Let the engine cool down and remove all, or as much as possible of the melted plastic material from where it had fused onto the hot surface before continuing your journey to avoid the possibility of the remaining material starting a fire.

BURNING FRICTION MATERIAL ODORS

On front-wheel drive vehicles that have manual transmissions, the clutch is located under the hood, since the engine and transmission combination set is located transversely. Therefore, if the clutch starts to slip for whatever reason, the acrid odor of burning friction material will emanate from under the hood, as opposed to coming from under the floor pan on rear wheel drive manual applications.

Note that on some applications, the same, or almost the same odor as that of a burning clutch might appear to come from under the hood if the front brakes are overheating. While a burning clutch can be confirmed if the engine speeds up but the vehicle does not accelerate, overheated brakes can be identified by either a mushy feel to the brake pedal, or a serious reduction in braking performance when the brakes are applied.

In both cases, repairs are best left to the dealer or other competent repair facility. In the case of overheated brake parts, other components such as sensors and/or wheel bearings may have been damaged by the excessive heat as well, while removal of the engine or transmission is required to replace a damaged clutch.

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