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P38A bad start when hot, tested crankshaft sensor

Discussion in 'Range Rover' started by mjb.thomassen, Mar 19, 2020.

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  1. mjb.thomassen

    mjb.thomassen Member

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    my p38 4.6hse 1994 lpg prins injektion starts very bad when half warm.(petrol) Cold start is good (petrol)
    When maximum temperature it stars automaticly on lpg so no problem.So it could be the crakshaft sensor.
    I tested it by removing it and then starting with cold engine .....no start.
    So the sensor works.
    I assume a sensor works or does nor work , nothing in between.
    The problem started when installing another becm, before that the half warm start on petrol was 80% ok.
    what tests can i do
    I have no entrance to nonocom
     
  2. Henry_b

    Henry_b SPARES OR REPAIR

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    The Crank sensor can/will break up when hot..

    run her upto temp when it starts to splutter shut it off. spray some freeze spray on the sensor and restart if it smooths out you're found the issue.

    rather common especially on an aged Gems unit..
     
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  3. Datatek

    Datatek Well-Known Member

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    Crank sensors often fail when hot and can be intermittent or produce a weak signal. Diagnostics are needed or an oscilloscope, if neither are available trying another sensor is the easy option.
     
  4. Grrrrrr

    Grrrrrr Technician, Bodgit & Scarper Ltd

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    @Doo had rough running with a dodgy crank sensor.

    Check the output on the o2 sensors too. When cold it ignores them.
     
  5. romanrob

    romanrob Well-Known Member

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    You really need diagnostics to check the set-up. I chased a warm-start error for a long time until I had the injectors cleaned. Once the car is turned off after a run the fuel rail temp will rise from ambient to c.60+ within 10-15 mins as it absorbs the radiated heat from the engine block (for a UK observer, in the autumn). If restarting less than 60 then it would start no problem, at 60+ it would fail. The report came back from the inject cleaners without noting any major leak/ blockage, although improved flow, since reinstalling the injectors there has been no repeat of the issue. In the background I upgraded the rad, which was partially blocked, and that lowered the running temp of the coolant (read at ECT) by 5+ degrees - which will have helped to keep the heat off the fuel rail.

    I went via O2 sensors, IAT, TPS , ECT, fuel pressure & filter - probably forgot a few... the more data you can read the faster you'll zone in on your issue. STFT is 38.75% after starting for the first minute or so, and then GEMS reads your o2 sensors - I can't remember if it does the same at a hot start, or it reads the ECT and works out it's already warm. Find a GEMS reader - there are some app-based ones that use the OBD II port. Nano better
     
    Last edited: Mar 21, 2020
  6. Doo

    Doo Well-Known Member

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    I had the same issue and I chased it for months. In the end, I noticed the cold hot issue and suspected the crank sensor.

    Bought a new one, fitted it and away she went.

    What got me was the fact it started and ran hot or cold, but the misfire when hot was as if one spark plug or lead was faulty.

    I can only imagine the fact it's a V8 that somehow affects the crank sensor and prevents an actual total hot failure...

    Like I said, my car now runs perfectly hot or cold, although because the misfire killed my cats, I had to remove them and so I now lack top end power :rolleyes:

    No doubt a new set of cats will fix that.
     
  7. Stanleysteamer

    Stanleysteamer Well-Known Member

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    This has got me fascinated. Why would removal of cats lead to a lack of top end power? To me removal of any obstruction to the passage of exhaust usually results in better breathing and therefore more powere throughout the rev range. What were the cats replaced with? Straight smoothbore tube?
     
  8. Henry_b

    Henry_b SPARES OR REPAIR

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    The V8 or indeed any engine creates back pressure which in simple terms is an opposing force acting on a gas that attempts to move through a confined space any engine will pulse the exhaust gases through the exhaust manifold and then through the exhaust pipes. These pulses occur as the individual cylinders work through the four stages of the combustion process/cycle.
    As the pulses move along, they generate an exhaust flow. If you have a restrictive exhaust system blocked cats are a good example, it can generate back pressure that works against the positive flow of the exhaust gas that's trying to exit the exhaust. a restricted exhaust flow that builds up back pressure is only degrading the power th engine can deliver because it's not working efficiently enough.

    A little back pressure is good however the rover v8 has multiple "runners" (one from each cylinder) into a collector where they all meet. This collector is effectively a rapid expansion in pipe diameter from a single pipe. When a cylinder exhaust pulse hits the collector a rarefaction (low density) pressure wave is formed, which travels upstream back to the exhaust valves, decreasing pressure at the valve.

    Depending on the length of the runners or any restriction or de-restriction, this will cause peak performance to shift to different RPM ranges. You can also use the cylinder pulses from the other cylinders to help each other out and improve your scavenging performance
    There are some "so so" reasons why but from what i gather the backpressure with the tuned length pressure wave scavenging, it helps performance in some RPM ranges, and makes it worse in others So, with the P38 fitted with a stock exhaust system, the change can shift where the volumetric efficiency peaks are on that engine at any given RPM range..

    Hence why in some cases you can lose MPG or peak HP in certain rev ranges when de catting..
     
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  9. Doo

    Doo Well-Known Member

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    Exactly!

    I left the empty cat shells in place rather than putting a straight through pipe. This allows an expansion soon after the manifolds instead of back pressure. It also sounds "hollow" and quite treble rather than bass o_O

    Still, the car runs well enough for this apocalypse...:confused:
     
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  10. oakey

    oakey Well-Known Member

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    Superb explanation Henry
    Never seen it explained so succinctly
     
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  11. Stanleysteamer

    Stanleysteamer Well-Known Member

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    I did wonder if that was what you were talking about, "tuning the pipes" as it used to be called and world beating when it was first used in 2 stroke motor bike exhaust design. When I changed my D1 300tdi exhaust for a sports stainless version, the effect was immediately noticeable, and it only had one box at the back, but it was wider bore so all this was to be expected and was what I paid out the money for.
    On my D2 TD5 I removed my centre box and replaced it with a through pipe. I expected some improvement and was a bit surprised to get none at all.
    On all my kit cars you have to design and build an exhaust, it is fun but also a bit frustrating, especially as many are so low slung that you have to put twists in, some times so complex you actually have to pay a specialist to do it. I am a great believer in getting the breathing of the engine right, inlet manifolds, ditto exhaust, air filters, cam, valves particularly exhaust, and obviously exhaust. My aim always being to improve performance without affecting economy. and usually I have succeeded. I see others throwing huge carbs on and getting frustrated when they it does little but hit them in the back pocket. I suppose this just shows that Rover actually knew what they were doing when they designed the system. Looks like a cat shaped pipe might help. Maybe this is why some take the cat off and just destroy the stuff inside it. But in the end it is all a compromise, until someone designs and exhaust that slides like a trombone slide!:)
     
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  12. Stanleysteamer

    Stanleysteamer Well-Known Member

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  13. Henry_b

    Henry_b SPARES OR REPAIR

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    Janspeed used to do a Cat delete pipe and by some accounts it worked for performance, economy is hit though..
     
  14. oakey

    oakey Well-Known Member

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    The Janspeed tuned length system worked very well in my Avenger Tiger many years ago.
    Just fitted Janspeed to my P38 V8 and I reckon it made a difference
     
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  15. Datatek

    Datatek Well-Known Member

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    Yamaha had a variable exhaust system on some performance bikes
     
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