Gas Engine

Capacities
Vehicles with J7T-239 Fuel Injected Gas Engine


 * Without Motoraud Water Heater Or Rear Heater

Valves
Valve adjustment is so very important to these engines. Since the valves on the two Renault power plants have no system (hydraulic valve lifters) to automatically keep valve clearance within limits, they must be manually adjusted.

When this (or any engine) is designed, the designer, after thoroughly testing the engine, determines the various operating clearances. It is a clearance that allows for normal expansion of the metal parts of the engine, valves, engine block, in fact everything that changes temperature. He must find a clearance, for the valves, for instance that allow for this expansion, which if too close, would allow the valves to be held open the tiniest bit and then, because they could not cool from seat contact, become very hot, white hot I am told. The hot gasses flowing thru the space between the valve and seat will then start valve erosion. Resulting in gaps developing on the valve to seat face. This results in burned valves, loss of compression, and in some cases, VALVE STRETCH and/or VALVE STEM-VALVE HEAD SEPERATION where the valve head drops into the cylinder, COMPLETE DISTRUCTION OF THE PISTON, HEAD, CYLINDER SLEEVE can occur.

Another consideration is that he must be sure the clearance is close enough to insure that the camshaft lobe cam profile is followed as close as possible--without being wide enough to cause excessive valve rattle or click, (the noise coming from contact between the rocker arm and the valve stem.)

It might be noticed that the clearance is usually different for the intake and exhaust valves, because the exhaust valve gets more heat and expands more. On some engines, like the old Chevy "stove bolt 6 engines and others, the valve clearance was set with the engine at normal temperature and idling. Luckily we don't have to put up with the mess of oil flying all over because our engines are "cold set" engines, that is, the valves are set with the engine cold, i.e., at normal room temperature.

The manual will give the following settings for the exhaust and the intake valves. They are:

Diesel:

Gas:

Replacement of the rocker shaft oil filter and a valve adjust is REQUIRED at 40,000 mile intervals.

Valve cooling is most important. Engine intake and exhaust valves are cooled:


 * 1) By the intake air or fuel mix during the Intake stroke. A very lean fuel mixture can accelerate valve wear, result in burned or warped valves.   Mix too rich results in unwanted deposits on the valve face and seats.
 * 2) They also cool by transferring valve heat to the valve seat during the short time the valve is on seat. This is why valve clearance is critical. Valves with less than specific clearance can grow from the engine heat and be held off seat (not fully seated), allowing for valve face and seat erosion which results in burned valves. Overheated valves can also fracture just under the head, which allows the valve head to fall into the cylinder and is then driven into the head and piston on the next piston stroke, resulting normally in catastrophic damage.
 * 3) Heat is also dissipated through heat transfer through the valve guides into the cylinder head. Worn valve guides are not in full contact with the valve stem due to wear, thus cannot conduct heat away from the vale into the head.

An engine valve is a covering of one port in the cylinder head, either an intake port or an exhaust port. The valve is made up of a valve head, which is precision ground to fit the port it sits on, a valve stem attached to the valve head and is positioned in a critical position by valve guides...which function to keep the valve in a position that allows it to return to the correct seating position each time the valve actuates. The valve stem goes through the "roof" of the cylinder head port as dictated by the valve guide. The top end of the valve stem will then be found outside protruding from the valve guide inside the valve cover which is sitting on the top of the cylinder head.

In the valve chamber on the cylinder head, surrounding the valve stem and upper end of the valve guides is a strong spring attached to the valve stem by keepers. The spring is there to close the valve. On the stem/guide is a seal to prevent excess oil from reaching the combustion chamber through the valve guides, and also a base and cup washer and finally the keeper.

So the valve is constantly being pulled against the valve seat by the valve spring. This spring tension plus overheating the valves can lead to valve stretch, that is, the valve can grow in length and eventually reach the point that it will hold itself off seat. That is one reason that the valve clearance is often time found to be less than specified during checks with a feeler gage.

And finally, with the valve sitting there, spring loaded to keep closed, it comes time for valve actuation. These engines are belt driven single overhead cam engines. Meaning, that the valve is operated directly by the overhead cam through an adjustable rocker arm. The clearance between the valve and the rocker arm is what it is all about. To accomplish a proper valve adjust, follow one of the two procedures in the engine section of the book.

(Larry S. - Yahoo Group)

Timing
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Rocker Arm Clearance / Valve Adjustment
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Belt Tensioning
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Vacuum Lines
Cracks in the tubing in the emissions system is one of the main causes of poor engine performance.

Ignition
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Fuel System
Bosch or Bendix


 * Testing


 * Adjustments


 * Wiring

Troubleshooting
The most common and easiest to fix causes of poor engine performance (poor idle, hard cold starting) is misaligned valve rocker arms and cracked vacuum lines. These are the first things to check and repair upon purchasing a LeSharo and/or experiencing engine problems.