95 discovery bone stock 115k miles has always been reliable. month ago would not start, cranked no fuel pressure. couple of hours later it started and drove fine. did it again the following day, i turned the key off and on while listening for the relay four or five times and it started and also ran fine. i tried to replicate the problem but it always started right up, i let it idle for a while no problems. left it parked for a month but started it once a week no problems. today i drove it about four miles started and ran fine in city traffic then it would not idle would slow down and stall. started it up and kept the rpms up with my foot and made it home. stalled in the driveway and would not start, had some fuel pressure. what is the problem?? any ideas, i suspect fuel pump but my experience with other cars is once they go they dont come back on

 

Put a psi gauge on the schraeder (tire) valve on the passenger side fuel rail. Expect 32-34 PSI. Can be old fuel filter, weak pump, fuel pressure regulator, or rotten vac hose to regulator. One test tells you if all of those OK.

Assume no codes since you mentioned none. May be time to clean throttle body (with cleaner made for it), and IACV (highly suspect).

Resist the temptation to clean MAF until you have exhausted everything else. It should only be cleaned with material made for the purpose, and put the can of spray under the hood and park truck over night. Clean when cold (spray will be same temp as engine). The hot wire would not like being sprayed when hot. MAF sensor is like $1,000 at Rock Auto (more for a 97).

 

The fuel pump relay is cheap, start there.
Next time it happens check the fuel pressure, and then check for spark.
Also starting it and just letting it idle once a week is not good for any car.

 

fuel pressure rose to 35psi as soon as i turned the key to start position and stayed there after i started it. what problems does that eliminate as i fear driving it as i know it will stall.

 

That eliminates the fuel pump and fuel filter.

 

Time to clean the IACV. It controls idle when you let off the gas. When you have it out to spray, don't turn, twist, or pull on it. It is a 180 - 200 position stepper motor that is basically a computer controlled vacuum leak.

 

Yes, that 35 psi at key on indicates that the fuel pump is working as it should and that the fuel filter and fuel lines aren't plugged up in any way.

However, I think the fuel pressure should drop a bit after the engine is started, as it revs a little. Yours is a 95 and mine is a 97 with a little bit different setup, but I'm pretty sure there is a fuel pressure regulator on your truck. On mine it's vacuum operated and located behind the plenum, near the throttle cable linkage... with a couple larger hoses, one connected to the fuel rail, the other a return line to the fuel tank, through a vacuum actuator and with a smaller vacuum line coming from that. Don't disconnect the fuel lines, but check that the regulator is working.

When there is a high vacuum condition in the engine, the regulator allows more fuel to return to the tank, reducing pressure in the fuel rails. If yours is the same or similar to mine, check the small vacuum line between the fuel regulator and the intake manifold or plenum. Be sure that hose is not cracked and leaking, damaged or missing. I had to replace it on my truck recently. I get 35-37 psi at "key on". That drops to about 30 psi once the engine is started and further to about 25 psi when revved to around 2000, when the fuel pressure regulator kicks in.

If your's is maintaining max fuel pressure all the time, that can be a problem. Looking in the manual, I think your truck is supposed to operate exactly the same way as mine, so too high pressure can result in overfueling, a too rich condition, particularly at idle. A vacuum leak, the most likely reason the fuel pressure regulator wouldn't be working, also would mess up idle by allowing uncontrolled air into the intake manifold.

Other things.... If you are seeing poor power at various rpms, yes I agree with previous suggestion that you should remove and clean the IACV. That's fairly easy to get to and do. Carefully clean inside the air bypass too. Throttle body cleaner in a spray will do. The IACV allows bypass air into the engine so it will idle, when the throttle plate is fully closed. It draws air from the crankcase via a hose to one of the valve covers, so tends to get oily, gunky and sooty over time. That can really effect the air flow if it's not cleaned occasionally.

However, I'd also suggest removing and cleaning the mass air flow meter or MAF. This has to be done carefully... Only use MAF spray cleaner that's sold for that purpose. Be sure the part is cold (one of the wires inside the MAF heats up when the engine is running). Don't disassemble, but spray into the pitot tube you'll see inside, behind the screen. Don't remove the screen or poke anything in the pitot tube. The rest of the MAF body can be wiped out, it tends to get coked up with soot. Let it dry for a while before reassembling and starting... The MAF cleaning fluid evaporates pretty rapidly, but give it time to completely dry.

A dirty MAF will cause all sorts of power loss at speed, starving the engine of fuel particularly at higher speeds. Under load, the engine will miss and stumble and struggle, particularly when fully warmed up. It might make the engine difficult to start, too. A completely failed MAF the engine might start, but usually will die immediately, may not even be able to idle. One that's working but dirty can give intermittent problems.

The MAF might be the single most important sensor in the entire fuel injection system. It measures the flow of air into the engine, so that the ECU (or ECM) can control the fuel ratio, advance, and various other factors. The way the MAF works is that it has two wires inside... one heated and one not heated. As air passes over it, the heated wire cools. The ECU compares the resistance of these two wires to measure air flow and make its various adjustment to the system.

In general I'd also suggest to check the electrical harness connectors to as many of the sensors as possible. Clean any corrosion or dirt out of them, to be sure all connections are good. (Radio Shack sells a contact cleaner and lubricant spray that works pretty well.) I'm not sure if it's a good idea or not, but I often put a little dielectric grease on the connecting pins, too, to keep them bright and shiny and clean. Check the plug wires at both ends, the harness connector to the coils, harness connectors to the injectors, etc., etc. Most of the harness connectors are a type that simply unclips, a lot use a wire clip, others use a plastic tab.

Some other important sensors to check are the cooland temp sensor (near the front of the engine, sort of underneath the top radiator hose), the fuel temp sensor (on the driver's side of the fuel rail, toward the front), and the intake air temp sensor (on the air filter container).

The coolant temp sensor is the one with multiple wires connected to it (I think the one right next to it with a single wire only serves the temp gauge in the dash). This is an important sensor for the ECU to know the engine operating temp and make adjustments accordingly.

The latter two sensors mostly are related to hotter conditions, hot restart and hot running. The fuel temp sensor is not immersed in the fuel, actually only senses the temp of the rail, but if temps are high it causes more fuel flow through the system to reduce bubbles that might cause problems at the injectors. The intake air temp sensor simply retards timing above a certain temperature, there can be some slight loss of power if it's not working.

Another critical sensor is the crankshaft position sensor. However, if you are getting a good spark it's probably fine. It's underneath, on the front flange where the bell housing bolts up and is a pain to get to! Might be worth checking that it's connector is in good shape and clean... It's covered by a heat shield because it sits pretty close above the exhaust... but it is usually working if the engine starts and runs.

If you continue to have trouble, and you have a dealer or independent LR service shop nearby, it might be worth taking the truck to them to have a diagnostics run on it. The independent shop I use has a software that they tell me is better than the dealership's. It can be set up to just run a diagnostic on the engine systems, for example, while the dealership's software has to look through all on-board systems and takes longer to run. I think the shop charges me for an hour or 3/4 hour of labor, to run a diagnostic on the engine. It can quickly pinpoint problems and save chasing things and buying parts unnecessarily.

 

Thanks for the post, i cleaned the IAC and the MAF. engine started but idle was low. drove it for several miles no stalling. After it cooled down i restarted it and would not idle. Runs fine if i keep the idle up with my foot. Am i supposed to reset the IAC and if so, how?

 

IACV may be defective, it is a stepper motor that has 180 - 200 steps of moving in/out. Don't try to force it to move. When reattached the bolts need to be firm to depress washer under the unit. The unit may re-learn your driving in a few days, and if not a dealer can reset your adaptive values. Needs a special code reader.

 

Last week I tried to start it and it started but idle dropped off then it died. Today it would not start no fuel pressure. I checked the relays and they would click when the key was turned to run. I disconnected the wire to the fuel pump and checked the voltage at the pump 14 volts when turned to run then drops to zero and goes back to 14 volts. While cranking. Reconnected the wiring plug to the fuel pump and it started right up. It is about to drive me crazy it must be some intermittent electrical glitch but how do I find it without swapping out all the components