Starting when engine warm
#1
04-30-2016, 01:01 AM
#2
04-30-2016, 07:12 PM
Join Date: Feb 2014
Location: Crossroads of America
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Hi Ian,
There are multiple possibilities that can lead to hard starting when the engine is warm. The first question to ask is how long has it been since the car had a basic tuneup: new spark plugs of the correct type and properly gapped, new air filter (you already replaced the fuel filter), new distributor cap and rotor, and new spark plug wires if they are several years old?
If all those bases have been covered, the first likely suspects that come to mind are the Fuel Pressure Regulator (FPR), Fuel Check Valve or Non-Return Valve, and the Coolant Temperature Sensor (CTS).
The FPR is mounted at the forward end of the fuel rail on top of the intake manifold on the engine. It works in conjunction with the check valve to maintain fuel pressure in the rail so the injectors have a continuous supply of pressurized fuel to atomize into the combustion chambers. On your '88, the check valve threads into the outlet side of the fuel pump, which is mounted above the rear suspension on the left side of the car. If either the FPR or check valve loses its ability to hold pressure, then as soon as the engine is shut off the hot fuel in the rail boils to vapor, creating classic vapor lock until the fuel condenses to liquid again. Unfortunately there is no valve on the fuel rail of the XJ40 for testing fuel pressure.
Some owners have assembled groups of pipe fittings in order to adapt a fuel pressure gauge so they can read the pressure while the engine is running and the rate at which the pressure dissipates. Since it's a hassle to test the pressure, my approach was to simply replace the FPR to see if the situation improved, and then install a new check valve on cars with external fuel pumps, or add an inline check valve on cars with in-tank pumps.
Another possible problem with the FPR is failure of its diaphragm, which allows raw, unmetered fuel to be inhaled into the intake manifold, which leads to rich running the ECM cannot eliminate by adjusting fuel injector pulse duration.
Regarding the CTS, the Engine Control Module (ECM) uses this sensor's signal as part of its calculations for determining the Air-Fuel Ratio (AFR). If the CTS has failed or has become lazy and sends the ECM a signal that indicates the engine is cold when in fact it is hot, the ECM will incorrectly apply cold-start fuel enrichment which can choke a hot engine and make starting difficult. Especially if other components are out of spec (spark plug gaps, reduced voltages due to corroded grounds, etc.) The CTS can be check by testing the resistance across its terminals when the engine is cold and then again when it has reached full operating temperature. When the engine is cold, the resistance should be a few kilohms, and when the engine is hot the resistance should be near zero ohms.
Another possible contributing component is the carbon canister purge valve. The carbon or charcoal canister is mounted behind the wheel arch liner ahead of the front left wheel. Fuel vapors from the fuel tank are stored in activated charcoal and then released into the engine intake by way of the purge valve, which is controlled by the ECM. If the purge valve siezes in its open position, the fuel vapors are inhaled by the engine at times when they can cause over-fueling. The easiest way to check for this problem is to pinch the hose from the purge valve that connects to the forward side of the EGR port on the underside of the intake manifold behind the throttle body. Follow the rubber hose from that general vicinity under the intake manifold down to the front left wheel arch, and pinch it somewhere far enough from the hot engine to avoid being burned.
It's also worth checking for corrosion on your battery power connections starting at the battery, at all three fuse boxes, at the jump-start terminal on the firewall, and at the alternator and starter. Also check for corrosion on the engine ground strap terminals and the grounds used by the ECM (two on the intake manifold and one on the firewall behind the cylinder head).
Please keep us informed.
Cheers,
Don
There are multiple possibilities that can lead to hard starting when the engine is warm. The first question to ask is how long has it been since the car had a basic tuneup: new spark plugs of the correct type and properly gapped, new air filter (you already replaced the fuel filter), new distributor cap and rotor, and new spark plug wires if they are several years old?
If all those bases have been covered, the first likely suspects that come to mind are the Fuel Pressure Regulator (FPR), Fuel Check Valve or Non-Return Valve, and the Coolant Temperature Sensor (CTS).
The FPR is mounted at the forward end of the fuel rail on top of the intake manifold on the engine. It works in conjunction with the check valve to maintain fuel pressure in the rail so the injectors have a continuous supply of pressurized fuel to atomize into the combustion chambers. On your '88, the check valve threads into the outlet side of the fuel pump, which is mounted above the rear suspension on the left side of the car. If either the FPR or check valve loses its ability to hold pressure, then as soon as the engine is shut off the hot fuel in the rail boils to vapor, creating classic vapor lock until the fuel condenses to liquid again. Unfortunately there is no valve on the fuel rail of the XJ40 for testing fuel pressure.
Some owners have assembled groups of pipe fittings in order to adapt a fuel pressure gauge so they can read the pressure while the engine is running and the rate at which the pressure dissipates. Since it's a hassle to test the pressure, my approach was to simply replace the FPR to see if the situation improved, and then install a new check valve on cars with external fuel pumps, or add an inline check valve on cars with in-tank pumps.
Another possible problem with the FPR is failure of its diaphragm, which allows raw, unmetered fuel to be inhaled into the intake manifold, which leads to rich running the ECM cannot eliminate by adjusting fuel injector pulse duration.
Regarding the CTS, the Engine Control Module (ECM) uses this sensor's signal as part of its calculations for determining the Air-Fuel Ratio (AFR). If the CTS has failed or has become lazy and sends the ECM a signal that indicates the engine is cold when in fact it is hot, the ECM will incorrectly apply cold-start fuel enrichment which can choke a hot engine and make starting difficult. Especially if other components are out of spec (spark plug gaps, reduced voltages due to corroded grounds, etc.) The CTS can be check by testing the resistance across its terminals when the engine is cold and then again when it has reached full operating temperature. When the engine is cold, the resistance should be a few kilohms, and when the engine is hot the resistance should be near zero ohms.
Another possible contributing component is the carbon canister purge valve. The carbon or charcoal canister is mounted behind the wheel arch liner ahead of the front left wheel. Fuel vapors from the fuel tank are stored in activated charcoal and then released into the engine intake by way of the purge valve, which is controlled by the ECM. If the purge valve siezes in its open position, the fuel vapors are inhaled by the engine at times when they can cause over-fueling. The easiest way to check for this problem is to pinch the hose from the purge valve that connects to the forward side of the EGR port on the underside of the intake manifold behind the throttle body. Follow the rubber hose from that general vicinity under the intake manifold down to the front left wheel arch, and pinch it somewhere far enough from the hot engine to avoid being burned.
It's also worth checking for corrosion on your battery power connections starting at the battery, at all three fuse boxes, at the jump-start terminal on the firewall, and at the alternator and starter. Also check for corrosion on the engine ground strap terminals and the grounds used by the ECM (two on the intake manifold and one on the firewall behind the cylinder head).
Please keep us informed.
Cheers,
Don
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