Rover SD1 Forum Snippet #13 - Efi System "Hunting" Solutions

What this does is reduce the cold starting resistance of the CTS, depending upon temperature, fooling the Electronic Control Unit (ECU) into reducing the fuel enrichments requested by the CTS at its higher resistance values when cold, but has insignificant effect when hot!

Expressed another way, the mixture was deemed, by Rover, to be too rich at start up when cold and the added 10,000 ohm resistor weakens the mixture for the existing climatic situation.

Take for example the CTS temperature at or below the - 10°C region from the following chart, being approximately 10,000 ohms, for purpose of illustration,

Coolant Temperature Sensor Chart


- 10°C......9100-9300.........20°C..........2400-2600.......60°C.......500-700...........100°C.......150-200


A by-product of Ohms Law says, with a 10,000 resistor in parallel with the CTS also measuring 10,000 Ohms, its effective resistance will change significantly to 5,000 ohms, emulating the value required nearer to, or just above 0°C. This in turn is sufficient, by Rover's (then) calculations, to make a relatively significant Fuel/Air mixture difference in these typically cold UK winter conditions, to eliminate the Hunting characteristic,

On summer mornings, at say + 10°C.with the CTS resistance being approximately 2500 ohms, the 10,000 ohm resistance in parallel reduces the overall value to 2,000 ohms, A relatively small change, yet still sufficient to result in mixture adjustment by the ECU to affect Hunting, if required.

However at normal engine operating temperature of say +90°C the resulting change in resistance from say 200 ohms to 196 ohms is totally insignificant. in terms of mixture adjustment by the ECU.

Ah! I hear you say! If it's fixed at 0°C why is it still there at + 10°C? Well, of course, there are also other temperature sensitive components at play, such as the Air Temperature Sensor (ATS), the Extra Air Valve (EAV) and the Cold Start Injector (CSI,) so they will all variously effect the Efi system as temperature changes. Suffice it to say, this was Rover's original calculated (or was it more simply empirical?) solution to effectively address the overall issue in the cheapest way possible.

Thus, what the above explanations show is how the addition of a 10,000 ohm resistor solved Rover's "Hunting when Cold" problem without affecting normal engine running performance. What it does not explain, however, is what causes the problem in the first place and why Rover did not address those causes at Dealership level.

Remember, as mentioned earlier, its a characteristic of the system that emerged as it got older, many cars were affected, yet, clearly, these cars did not hunt when they left the factory. Rovers chosen solution was to weaken the idle mixture when the engine is cold, so, to find the causes one must seek out age-related changes that would cause some degree of unwanted enrichment. and, by identifying the causes, thereby suggest what might have been done, or indeed still can be done, to stop it happening.

Probable contributing age-related causes for Efi Hunting when cold.

# AFM Carbon Monoxide (CO) adjustment is maladjusted or the bypass gallery is cruddy reducing the available air at idle, inducing a degree of enrichment.

# Crankcase ventilatiion system partially blocked thereby reducing the fixed (not metered and not rogue) amount of foul air coming into the plenum from the crankcase, again inducing partial enrichment..

# The little hole (0.040" dia) in the left hand rocker cover breather blocked, having similar effect.

# An age-weakened flap spring in the Air Flow Meter (AFM) inducing unwanted enrichment at idle.

# A stiff and aging Fuel Pressure Regulator (PFR) will raise the fuel pressure beyond 36 psi, enough to enrichen the mixture across the whole range, including idle, again when not needed.

# Its probably not air leaks into the plenum, but taken together with other things, who knows?

# Any combination of the above!

Many possibilities then, logical but unproven, so why am I convinced the answer lies hereabouts?

Well, reviewing the above, Rover undisputedly decided the solution to the problem was to weaken the Air/Fuel mixture ratio at idle by placing a 10,000k ohm resistor in parallel with the CTS.

Ohms Law says, with a 10,000 resistor in parallel, that the change in CTS resistance would be sufficient to make a difference by weakening the mixture in typically cold conditions, to a lesser degree on summer mornings. but virtually no difference when the coolant is hot.

So reducing enrichment when cold at idle speed, was the preferred Rover solution, of that there can be no doubt. Yet why did they not simply tell the Dealerships to correctly adjust the Efi systems under their care?

My contention is, Rover realised that the problem would simply happen again and again unless THEY changed the characteristics of the system to cope with the age related changes, which is exactly what the parallel resistor modification does. It says to the ECU, "Hey, not so rich Buddy, else this engine goes Hunting 'cos some of the other players in this team are working against us!"

Another solution would have been to change the CTS specification but the same unit had aleady been comitted to many thousands of engines and many more early Range Rover systems too.

Anything in the future could be easily designed away inside the control system, but to have TWO CTS's in the spares bins would be "cat amongst pidgeons" time so they chose the easiest and cheapest possible solution and only for cars where the owner complained to the Dealership during the warranty period. Anything after that and the owner pays, so no worries there, then?

So what does all this dross mean? Well it don't amount to a hill o' beans if the Efi owner says "How do I fix Efi Hunting?" On the other hand if the question is "Why does my Efi Engine Hunt?", then this analysis may satisfy the enquiring mind!

The SD1 Efi Diagnostic Challenge Continues

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Rover SD1 Coolant Temperature Sensor

Here's is an anti-hunting protest aimed at our Rover SD1 Efi community who cheerfully tolerate hunting, even when it happens right under our noses.

LeeEFI said. "I wasn't impressed with the running of my Rover SD1 EFi when I bought it. But now it's had Ignition, Timing and Mixture issues resolved, I'm very impressed. The only little gripe is a slight hunting for about 30 secs when starting from cold, Needs to be sorted at some stage, but as it performs remarkably well at all other times I tend not to put it on the top of the 'to do' list".

A case of tolerated hunting, methinks!

Rover SD1 Efi System Hunting or Surging is when the engine idle speed rises and falls usually immediately after starting and more often when the engine is quite cold. This annoying condition is generally fixed by putting a 10,000 ohm resistor across the Coolant Temperature Sensor (CTS) terminals, soldered neatly and safely to the wires inside its connector shroud.

This resistor trick first emerged in a post production Technical Bulletin, around 1986-8 to correct a characteristic that manifested itself during the warranty periods, requiring dealership intervention when the customer reported the phenomenom.

A Small 10,000 Ohm Resistor

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Ramon Alban


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