Rover SD1 Forum Snippet #14 - Efi Engine Runs Without Fuel Pump

With the feed line (or even the return fuel line, if dangling in fuel) still connected to the petrol supply the manifold depression at idle is at its highest, so the normal atmospheric pressure inside the tank pushes petrol through the pump, uphill to the filter and uphill again into the fuel rail.

Thus, without hesitation, the difference between manifold depression and atmospheric pressure allows the injectors to function, admittedly at fairly low overall pressure, but sufficient to sustain a very low torque combustion process at idle rpm

Even while these (and other) owners were describing/confirming the phenomenon, I could not get my head around it, but the more I thought it through, the more a tangible explanation emerged.

Expressed another way! With fuel pump disabled the engine runs on because it is SUCKING up fuel with the normal depression seen at the intake manifold at idle speed in sufficient volume to fill the fuel rail and keep the engine ticking over due to the normal function of the injectors.

In reality, of course, it is a mirage, because the system was never designed to run as described and it is merely a function of someone perversely pulling out a relay to see what happens.

But then again - maybe not so?

Imagine, if you will, a situation whereby the pump relay and/or associated diode steering unit, or the pump itself, or the associated wiring had an intermittent fault, its entirely possible for such a strange thing to occur, in reality, and completely fool anyone trying to diagnose a fault thereabouts!

So how did this revelation come about? Well, a misguided Forumee started to experiment with various ways of stopping his engine and found that under the circumstances described, the damn thing kept running. Lets face it Campers, the normal way to stop an engine is by switching it OFF.

So there you have it! Another piece of useless information regarding the Rover Efi System that reminds me of other strange situations when folks report that an engine continues to run/function with (say) the AFM or some other key component like the Throttle Potentiometer disconnected, then agonise themselves by asking Why?. And the answer, of course, is:

So What! It's irrelevant because it was never designed that way!

But the story cant end there, can it? What about the alternative views and explanations expressed by various forum contributors?

Mobydick says - "How long did the engine run for in this condition? Are you sure it wasn't residual pressure in the fuel rail? Would it have stopped eventually? Very curious."

Ramon replies - "Curiouser and Curiouser, Alice. According to the two people this happened to their engines ran and ran (at idle rpm) and could only be stopped by switching off the ignition. I got involved in the aftermath discussions trying understand the science involved and explaining it as best as I can. Thinking about it some more, I would imagine a full fuel tank would help as it raises the head of fuel making engine suction more effective. ie, having to lift the fuel over less height."

Tommo says - "Mine will certainly run with TPS disconnected!"

Mobydick jokes: - "Poor old CarlosV8 can't get his Vitesse to run at all and you lot are talking about people who experiencing engines that won't stop, no matter what - there's just no justice!

CarlosV8 replies: - "Yeah, talk about rubbing my nose in it! On my old XR4x4 I switched the engine off by killing the fuel pump with a crude immobilisor, but as some injectors leaked, by completely de-pressursing the fuel system it meant none would leak into the cylinders causing it too flood. Failure to do that and it would be a pain to start if still hot. PS The car was a total piece of junk!"

sd1john says: - "I do not think such a car would run at any speed much higher than tick over."

Ramon replies: - "You are absolutely right, but let's be clear, its not "such a car", apparantly its "Any Rover Efi Flapper System" with the fuel pump disabled whilst ticking over, will continue to tick over in a sustained manner, for the reasons explained. Also, This is not a theoretical proposition, merely an attempt to explain a reported phenomenon. Seemingly, it should be really easy to emulate by simply pulling the fuel pump relay whilst the engine is ticking over."

Punx0r says: - "I have some direct experience of this due to various fuel pump troubles on my Single Plenum Vitesse. I have never found the engine to keep running when the fuel pump wasn't. Due to the high volume of the fuel filter, there is a decent reserve of pressurised fuel and the engine will continue to run at idle speed for around a minute. The engine revs gradually drop like a clock winding down, until it finally stalls. If someone can keep an engine running for a substantial period of time then it's certainly an interesting situation."

Ramon replies: - Yet, according to Peter Humphries on the TVR forum, his explanation was: "If you turn the pump off the fuel rail remains at atmospheric pressure and since the injectors have manifold depression on the bottom, fuel will still be sucked through under light load conditions. How much fuel would depend on how much depression you have and what the ECU is doing to the injectors. If you have a lot of depression or the ECU is being very generous on the pulse length then it's quite possible that the engine could run with the fuel pump powered off. Typically though you would feel it firing but not quite enough there to keep the engine running."

Which seems to makes it conditional on what else is happening in the system at he time.

Mobydick says: - "It's beginning to sound a bit more theoretical and less of an observed phenomenon. I guess the height difference between the engine and the tank is considerably less on a TVR (God bless them) than an SD1. I'm not over-familiar with modern machinery, but on my old Vixen (with Daimler Dart V8) the tank top was about level with the cylinder heads.

GForce replies: - I don't think there is any great mystery about why this can happen. The fuel pressure in the fuel rail is roughly 36psi, as soon as the fuel pump is turned off and the engine stopped this pressure can be maintained "on a fault free system" for a good hour or more, well over the quoted figures of something like 10psi drop per miniute. The fuel return to the tank is blocked by the closed fuel pressure regulator and a non return valve in the fuel pump stops the fuel line pressure bleeding back through the pump. With the engine running and only the fuel pump disabled the injectors are still being opened by the ECU so the engine will continue to run at idle on the residual pressure in the rail until the point where no more fuel is available or the pressure in the rail is equal to the absolute manifold air pressure (manifold vacuum). In theory this could be a good number of miniutes!

And from Ramon: - "I know, it gets to sound theoretical and in many ways its a struggle to explain, but people report strange goings on and then have the temerity to ask questions, so, if the answer is not already written down then that's what this chap (Peter Humphries) was also trying to do." Particularly, rationalizing that the system may not be in "normal" running mode but with extended injector open time (ie; engine not yet hot) for the wierd result to occur. The quote (above) from Adam Quantrill, was anecdotal not theoretical.!

Conclusion and Summary for All (Except for CarlosV8)

Dont ya just lurve the things that happen with our ancient Rover Efi Flapper Systems when they (mis) behave in these unusual ways and the wild theories that are projected to try and resolve them? And don't ya just lurve the ensuing debates that can very often shake out acceptable answers? I recall, over the life of the Rover SD1 Forum, and beforehand by email, having many technical jousts with my dear friend, Mr Chrisp to try and resolve the unresolvable. Sometimes with success, sometimes not. The more I hang around the more absorbing these "oddball" situations become. I mean, lets face it, it's only because of the interaction of enquiring minds that quite a few of our Rover SD1 Efi Systems are still alive. With no inquisitorial function they might be long dead by now, hence my personal mission is to encourage SD1 owners to keep the faith.

The Diagnostic Challenge Continues (TDCC) and Enquiring Minds Need To Know. (EMNTK)

The SD1 Efi Diagnostic Challenge Continues

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A mysterious phenomenon has been discussed on different occasions in parallel RV8 Forums as to how a Rover (SD1) V8 Efi engine can continue to run when the Fuel Pump is suddenly disabled.

A Range Rover owner reported, during some un-informed fiddling with engine idling, when, either the Fuel Pump Relay is pulled out or the Fuel Pump is otherwise disabled the engine continues to idle until the ignition is switched off.

I re-posted these "goings-on" across several RV8 Forums and TVR freak Adam Quantrill confirmed - quote - "Exactly, I had the fuel pressure gauge on there and the pressure did indeed drop to zero (relative to atmospheric) but the engine kept running! Could it be that the injectors are somehow being jammed on full (despite the pulsing I see from the ECU) and the manifold vacuum is sucking the fuel in! Weird Innit! At least it means there are no plenum leaks!"

So what is happening? Well,! Obviously, no amount of vacuum could jam the injectors open but the observed pulsing indicates the ECU is active and the fuel pressure created by manifold vacuum is clearly sufficient to cause this effect..

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Ramon Alban


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