Fluctuating RPM's at constant throttle (throttle hunting, surging, gear hunting)

tetsuo69

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If that is the problem, see my post #16 of 2019 - two years later, still no surging so I guess replacing the oil temperature sensor located down near the oil filter on the 4.4 V8 was the solution.

Replacing the oil temperature sensor did not cost much and was pretty easy to do - yes, that is unusual and hence worth a try if surging is the concern you have.

Well I have the same surging RPMs problem (accelerator pedal stationary, car speed constant and RPMs bounce up and down when engine is cold) and I replaced the sensor per your suggestion bbyer. Unfortunately doesn't seem to be the cause in my case.

Aside from the coolant temperature sensor in the thermostat any other suggestions for parts to through at it? 2005 SE V8 120k miles and it maddening frustrating.
 

bbyer

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Now that you mention the thermostat area coolant temperature sensor, every five years or so I replace the whole thermostat assembly, p/n LR005631.

That way I get new plastic, a new thermostat hopefully properly assembled (its tricky) and best of all, a new coolant sensor.

The one down by the oil filter I do not change often, (well never, until I had the surging problem), but the shift pattern of the ZF is in part regulated by engine rpm and the fuel mix is in part regulated by rpm and engine temperature - well you see the problem - it is a loop and as such, all the sensors best be working well.

I have about 240,000 miles (390,000 km) on my 4.4L V8 now.

I notice a mere 120,000 miles on your engine - have the spark plugs ever been changed? I replace mine about every fifty thousand miles - not that they really need it but at least they do not get frozen into the aluminum head.


Thermostat housing complete LR005631.jpg
 

Houm_WA

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I'd say that's a good practice, replacing the t-stat assy every 5. I recently had to change it on both of my LR3s and luckily it was caught before any significant coolant was lost. Just seems to be a wear item on these rigs.
 

bbyer

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I'd say that's a good practice, replacing the t-stat assy every 5. I recently had to change it on both of my LR3s and luckily it was caught before any significant coolant was lost. Just seems to be a wear item on these rigs.
That is how I see a lot of the LR parts - maintenance items that you would never change out on a Chev or Ford pickup but on the 3/4, the cost of "run to fail" it just too high.

I treat maintenance on my 3 as similar to my airplane - "time in service", and keep replacing stuff before it fails - the downside is worse than regular downtime.
 

tetsuo69

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I notice a mere 120,000 miles on your engine - have the spark plugs ever been changed? I replace mine about every fifty thousand miles - not that they really need it but at least they do not get frozen into the aluminum head.

I replaced spark plugs at about the 100k mark. I have noticed my fuel economy has dropped over the past 20-40k but was assuming its dirty injectors / old truck problems. Was going to buy a magic in a can snake oil fuel system cleaner on the oft chance it will work. Perhaps I'll try the thermostat housing as well but $120 is a lot to risk without an idea for success.
 

bbyer

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I replaced spark plugs at about the 100k mark. I have noticed my fuel economy has dropped over the past 20-40k but was assuming its dirty injectors / old truck problems. Was going to buy a magic in a can snake oil fuel system cleaner on the oft chance it will work. Perhaps I'll try the thermostat housing as well but $120 is a lot to risk without an idea for success.
I agree about the $120 and the new temperature sensor you get with all the plastic might not be the problem - but it might be. With a Land Rover, all fixing is a gamble - that is what makes them interesting; they are more a vehicle of choice for the gambler so to speak. All sensors deteriorate over time and for the most part, the engine computers compensate by defaulting to some predetermined nominal value once the sensor gets too out of wack. That means that in all older vehicles, all the sensors read less than ideal and hence the engine computer at some point in time is sending out default values rather than measured values.

The engine computer takes signals off both sensors, melds them together and then sends a signal to the transmission computer for it to do whatever it does. The combined signal also goes to the dash temperature gauge but the gauge so damped as to really only show cold, med and hot - of not much use really.

What I think is a gamble however is old plastic. How long a thermostat lasts is also a gamble and the new one might be a "cold" one or I suppose a "hot" one relative to what you now have. I have probably replaced that housing about four times now - the last one was a cold thermostat and the current is what I would call hot, relative to previous. Up here, hot is good as that means the heater puts out more heat in the winter and summer is kind of a blink in time anyway.

For me when the ZF started hunting, the solution was replacing the factory installed temperature sensor by the oil filter as the thermostat located ones are always relatively new.
I got lucky - my gamble paid off.

For me, the bigger gamble would have been not changing out the thermostat housing regularily.
 

tracyc

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I had the exact same problem. A transmission pan and filter replace & refill completely fixed it. Fixed my "California stop" issue too.

I was prepared to also replace the valve body with a good used one with new sleeves (and maybe new solenoids) but it wasn't necessary.
 

tracyc

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I had the exact same problem. A transmission pan and filter replace & refill completely fixed it. Fixed my "California stop" issue too.

I was prepared to also replace the valve body with a good used one with new sleeves (and maybe new solenoids) but it wasn't necessary.
By the way the valve body and sleeves I sourced is available is someone is looking for one
 

tetsuo69

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Tracyc, interesting. I had an indy shop do the Atlantic British transmission fluid + filter change back a few years ago and they put some random fluid in despite my specific request to use (and I pay for) ZF fluid.... perhaps this is related.

Bbyer, I was abel to source the temperature sensor itself (Ford part) from the thermostat housing finally and replaced that only, easy job with minimal coolant loss. Had thought it was better, but I think it was wishful thinking for a day or so only. If I was splitting hairs I could say it was slightly better since changing a few weeks ago, but it still surges, still annoying.
 

bbyer

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Tracyc, interesting. I had an indy shop do the Atlantic British transmission fluid + filter change back a few years ago and they put some random fluid in despite my specific request to use (and I pay for) ZF fluid.... perhaps this is related.

Bbyer, I was able to source the temperature sensor itself (Ford part) from the thermostat housing finally and replaced that only, easy job with minimal coolant loss. Had thought it was better, but I think it was wishful thinking for a day or so only. If I was splitting hairs I could say it was slightly better since changing a few weeks ago, but it still surges, still annoying.
For me, the problem was resolved by replacing the oil temperature sensor down by the oil filter.
The coolant and the oil temp sensors both feed to the engine computer. Some calculation is made with the engine computer software and that sum is sent to the transmission and the fuel controls.

What you see on the dash temperature gauge is a very damped summary of what the two gauges are reading. I say "damped" as there is so much damping that about all the gauge every shows is Cold, OK, or real bad, and then it is too late.

All sensors degrade and hence routine replacement is good maintenance. The computers know that the sensors degrade and at some point, the computers insert default valves for the sensors, the idea being to keep the vehicle running, not well, but running. Well, that is with most manufacturers but I do not think LR is one of them.

For me, I replace the whole plastic thermostat assembly every few years and that gets me a new coolant temperature sensor.
 

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