My 1997 Discovery XD is hard to maintain idle when warm. It starts normally when cold, however, after 10-15 minutes driving when engine gets hot, it sometimes stall and became very hard to maintain idle. At the stop, have to put one foot on the brake and another foot on the gas pedal to give it a bit gas so the engine won't stop running. Thr truck occationally throw out codes P0300, P1316 and P1496.

So far, I have put in a bunch of new parts including:

1. a new 120A alternator;
2, a new IACV, got one from eBay and another one from Amazon, no difference played
3, a new MAF, got one from eBay and another one from Amazon, no difference played
4, changed all spark wires to a 8mm premium set;
5. a new TPS
6. a new air filter

All these replacement parts seemed no help. The idle problem still comes back on an off. I also found when the truck lost idle if I turn the car off and restart after a while, the idle will go back to normal till next time it comes back after some distance of driving.

Any advise on how I should continue to check and diagnose this problem?

 

Did you ever have the adaptions reset after all the new parts?

 

Well it's a process of elimination.

Here's what I would do to start. 1. make sure my spark plugs are all good (the misfire codes would make me check them all) 2. temporarily remove the vacuum line going to the evap canister from the upper manifold and cap the port (P1496 is bad juju). Maybe I'd do this first since it's easy. I'd also make sure the passages to and from the IACV are clean and clear, and carefully check all vacuum hoses for leaks or blockage. You might need an ECU reset because of the new TPS.

A few questions to help clarify the situation: Can you read and trend upstream O2 sensor voltages real time? Long and short term fuel trims? What is the idle speed when first started, then what does it drop to as the engine warms up, idling in park? Do you know your miles per gallon?

 

1) If you haven't done so, remove the air intake hose and thoroughly inspect/clean the throttle body with like a nylon brush and throttle-body cleaner (yeah you should use that). Clean behind the throttle plate too. Let the vehicle sit a while for the solution to evaporate before starting. One wouldn't think a dirty throttle plate would make much of a difference but I've seen terrible running vehicles suddenly run great afterwards!
2) PCV. Make sure it's working properly and all the vacuum hoses and fittings are good and not cracked.
3) Quadruple-check the vacuum hoses. This is kind of a weak area on these Disco's in large part bc the vacuum hoses are metric sizes, and usually parts store hoses don't fit very well on some disco hoses. Someone may have put new part's store hoses in place of the original and you're not getting good sealing. A cheap vacuum gauge could come in real handy for checking vacuum before and after it starts stumbling. Your engine needs to maintain good vacuum to run properly.
4) EGR. Real common when a vehicle warms up the EGR may be carboned up and need cleaning in the area below.
5) Temp Sensor. I had a pick-up that had a temperature sensor which had 'drifted'. When it warmed up, the computer thought the temp was much colder than the engine temperature actually was. The computer was attempting to deliver the wrong amount of fuel. This is easy to check: Ohm out the temperature sensor before starting the vehicle, and compare the ambient temperature to a L/R temp/Ohm chart with the readings you got on your multi-meter. I can't remember if there are two temp sensors on the D1 or just one. Sometimes there's one sensor for the gauge, another for the computer. But look it up. There's probably an ohm chart in the D1 RAVE service manual. Or if you have a scanner, you can also attempt to read it. But beware which sensor the scanner is reading. Ohming is always the more sure way to go.
6) If none of the above bear fruit. You could try the ole' vacuum leak test: What you do is put a piece of hose about 13-14" long on the end of a propane torch. Start the vehicle and let it get to the point where it's stumbling. Then turn on the torch a little and move the hose around the intake and all hoses ...and see if the engine idles regularly at any point where you are waving the torch. The idea is a vacuum leak will suck in propane and it'll suddenly run better. You can also spray around with Ether or carb cleaner. But Propane is less messy. This works great for hard-to-find vacuum leaks and intake manifold leaks. You could also check the 'check-valve' on the brake booster too in case that had become faulty.
7) You said you replaced the IAC, but did you thoroughly clean the area behind it with carb cleaner and maybe a brush or soft instrument (wood stick)? If not, do that too.
8) Sounds dumb, but remove the air filter and make sure there isn't a mice nest or something below it choking up the air intake. Probably is clean, but it's good troubleshooting.
9) Make sure all your grounds and battery connections are good. Sensors, injectors and electrical paths require a good grounds to the engine/body/battery. Easy to overlook, but just sayin'

Hope these help out. If I think of something else I'll post it. Let us know what it ends up being. Best of luck!