P38A bad start when hot, tested crankshaft sensor

mjb.thomassen said:

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

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Doo said:

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

No doubt a new set of cats will fix that.

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Stanleysteamer said:

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?

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Henry_b said:

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..

Click to expand...

Henry_b said:

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..

Click to expand...

Stanleysteamer said:

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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