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Old 6th December 2006, 13:53   #1
Rover Roy
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Question 75 1.8 manual; Surging deceleration and wavering idle!

Has anyone experienced anything like surging deceleration and wavering idle between 600 & 1200 RPM. ????
When starting from cold the idle goes straight up to 2000 - 2500. Symptoms just started to show a few weeks ago so not related to the recent wet weather. Drain under EMU clear. Swapped the Idle air control valve off another 1.8 without fixing the prob. Dont know any Rover service in Hampshire. Any ideas?
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Old 6th December 2006, 18:41   #2
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A couple of snippets in reply to a similar question, I have never touched a 1.8 so don't know


"Part of the problem on no turbo 75s was the throttle body, If you have a plastic one, you have the problem part. My 75 non turbo had a plastic one and suffered from the same, it was changed for a "alloy" one and the problem was 90% gone. I think the problem was traced to the throttle body deforming when the air intake from the air cleaner was fitted to it, hence the throttle butterfly doesnt quite close, leaving the revs "hanging". The other 10% of the problem is engine tune related but far too complicated to explain. It is mostly to do with the fuelling during gearchanges which makes the revs stay a little higher to aid smooth gear changes."

And

"Once my TF was having the same symptoms but a bit worse, strange revs, up and down, and stalling, and doing that solved it!

On the F/TF it's turn ignition one click, fully press the loud pedal 5 times, turn off, then start.

Or something like that anyway.."

hth
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Old 7th December 2006, 08:38   #3
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Are alloy throttle bodies expensive?

Is it easy to fit? Do i need to swap over all the butterfly, springs etc?

Is it worth trying for a used one?
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Old 7th December 2006, 10:47   #4
Christopher
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When I experienced that it turned out to be a dodgy HT lead, once changed it was sorted straight away.
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Old 7th December 2006, 14:34   #5
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Quote:
Originally Posted by Rover Roy View Post
Has anyone experienced anything like surging deceleration and wavering idle between 600 & 1200 RPM. ????
When starting from cold the idle goes straight up to 2000 - 2500. Symptoms just started to show a few weeks ago so not related to the recent wet weather. Drain under EMU clear. Swapped the Idle air control valve off another 1.8 without fixing the prob. Dont know any Rover service in Hampshire. Any ideas?
Not sure if this applies to the 1.8 K series, but I remember on the 1.4 K series R8, there was a problem with the idle switch, which I believe was located on the throttle pedal assembly, and it caused exactly the problems you are experiencing. It is possible I guess that the switch is now on the throttle body, and so it may or may not be related to the plastic throttle body issue that has already been mentioned in a previous reply.
Andy
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Old 7th December 2006, 14:57   #6
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ordered a Haynes Rover 75 manual today. Hope it has a few hints about this prob. I dont quite understand what the ECU does with the throttle position sensor info. I thought the fuel injection process was NON variable?? Can replacing this be the fix?
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Old 7th December 2006, 22:39   #7
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Quote:
Originally Posted by Rover Roy View Post
ordered a Haynes Rover 75 manual today. Hope it has a few hints about this prob. I dont quite understand what the ECU does with the throttle position sensor info. I thought the fuel injection process was NON variable?? Can replacing this be the fix?
The ECU in your car is very clever. It continually adjusts the fuel/air ratio according to numerous variables to give the best output. The throttle position is just one of these many variables it monitors. For example, when you lightly press the pedal it knows you are after more economical output whereas if you push the pedal to the floor it knows your after the best power output.
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Old 8th December 2006, 08:21   #8
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Thanks Simon D for the explanation.
If the ECU is making decisions based on the input from several different sensors then the ECU output could be influenced by any mal-functioning sensor. Question is, which one? (As we asay in the IT dept, GI=GO, garbage in = garbage out) Is there any literture avaiable on the ECU control program?
Is the ECU made specifically for Rover?
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Old 8th December 2006, 11:34   #9
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Quote:
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Is the ECU made specifically for Rover?
Yep and the firmware is calibrated to the engine.

My old 1.8T did this (had about 10K on it) and never saw it as a problem - took it as being what it needed to do to keep it going!
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Old 8th December 2006, 14:01   #10
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Quote:
Originally Posted by Rover Roy View Post
ordered a Haynes Rover 75 manual today. Hope it has a few hints about this prob. I dont quite understand what the ECU does with the throttle position sensor info. I thought the fuel injection process was NON variable?? Can replacing this be the fix?
The idle switch tells the EC that it should be running the engine on the idle map (tune), or on the part/full load map. If the switch is faulty and is not working at all , or is cycling between on and off, the ECU will be cycling from map to map, which will tend to give different levels of fueling, and cause surging, and or high idle speed.
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