What could be the cause of this odd rev behaviour? (video included) - MG-Rover.org Forums
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post #1 of 16 (permalink) Old 18-04-2017, 20:52 Thread Starter
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What could be the cause of this odd rev behaviour? (video included)

Hi folks,

My F is showing some rather odd behaviour of late in terms of low revs at idle when stationary and worse when I'm actually driving her. During normal driving, she appears fine. When I'm shifting down and approaching a stop, clutching down into 1st seems to result in revs dipping down to almost zero on most occasions and often results in a complete stall. Same thing when reversing at really low revs - take my foot off the clutch to select 1st to move away and she drops to almost zero and again will occasionally stall.

As you can see from the video (which was shot whilst stationary) the revs drop down fine when revved hard, but when a gentle blip on the throttle is pressed, the revs go right down and it almost stalls most of the time.

Does anyone have an idea what is causing this behaviour and what I can (or should) look at first to try and remedy it?

Here's the short video - about a minute in length:


Oh, I should add that I've already done the TPS reset procedure several times when the engine was up to temp - no difference. Actually, when the car is just sat idling with no throttle action at all, she quite happily sits at 750-800rpm with no hunting up or down which leads me to think that the throttle (52mm which I upgraded recently) is maybe not the cause. I'm a bit stumped as I'm no mechanic, really. I could do with some help to try and sort it out.

Thanks in advance of your replies.

Last edited by steviejones133; 18-04-2017 at 21:05.
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post #2 of 16 (permalink) Old 18-04-2017, 21:18
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I would start with cleaning the throttle body and the bypass valve.

IIRC the reset should be done when cold.

What car did the throttle body come from?
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post #3 of 16 (permalink) Old 18-04-2017, 21:19
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Try cleaning your idle control valve.

Failing that start with the simple stuff like checking the quadrant on the throttle body, the throttle cable and so on.

Good luck.

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post #4 of 16 (permalink) Old 18-04-2017, 22:21
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Sorry you are having issues.
I usually find if I change something, it is that, that is the culprit.

idea 1, A shot in the dark:- get hooked up to a testbook. Get them to set the throttle.

idea 2, Bob the 48mm back on for a bit with the original position sensor. See if the prob continues.

Have you got the cotton filter in yet, as well?

I had a nearly stalling problem, after I changed the air filter box around from the tf 1.6 to the tf 1.8. and the o2 sensor number 2. I did not know which was the offender. The mg would lose power after start up but be ok once at temperature.

You now have more air going in with the new airbox and hoses plus the tbody change. Maybe and this is a big maybe - the ecu needs recalibrating. not just a tps reset.

Last edited by batoutofhull; 18-04-2017 at 22:43.
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post #5 of 16 (permalink) Old 18-04-2017, 22:25 Thread Starter
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The throttle looked like new to me (see below) and I think it came from a Lotus Elise but carries the same part number as for the MG's. After installing the TB and a Trophy airbox with dual intakes, everything was fine - this has only started up over the last week or so. Bat, I still have yet to replace the air filter with a new one - it should arrive in the next day or so. The additional Trophy air feed was re-rerouted to the passenger side vent and the original left in place. Can't see that being an issue. This was the TB I bought:

Quote:
Originally Posted by steviejones133 View Post
Well, the 52mm TB got collected Today from the sorting office as I missed it Yesterday. I knew I was buying used and that it looked good in the eBay listing, however, I was really surprised to see how good the condition was - practically brand new I think. I think the previous owner must have recently upgraded with a brand new one and possibly the car is no more - who knows, just that if it had been in use for a while, I'd have expected condition to be a lot worse. All in all, a good saving on new for just 43 delivered. A little clean and here she is:

Bypass valve? - what or where is that! - if I knew what to look for and clean, that'd help. Same goes for the TB quadrant......

Ian, I had thought that it could be the IACV/Stepper but I was under the impression that it was non-servicable like older F IACV's - my F is a very late (almost TF) model and I think it has the same IACV as the TF's that I thought were not cleanable.

Also, what would I be checking the throttle cable for? - looseness, slack etc?

God knows I am no mechanic but I don't mind getting stuck in if I know what to do or look for........
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post #6 of 16 (permalink) Old 18-04-2017, 22:52
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Quadrant Steve.

Try revving the engine from the throttle/quadrant. You will see if it is moving smoothly and if the spring brings it back ok.
I have quite a bit of play in my throttle cable. It is taken up when i step on the gas pedal.

I dont think the extra hoses are the issue but you are getting more air in now.

Last edited by batoutofhull; 18-04-2017 at 23:17.
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post #7 of 16 (permalink) Old 18-04-2017, 23:19 Thread Starter
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Thanks Bat - well, the quadrant part looked fine to me. Made sure the cable was seated correctly. Is there any adjustment to be made to the cable itself? - I did notice that the place that holds the cable to the body might have some nuts to adjust slack, possibly. Maybe mine is too slack, hence dropping below 750/800rpm, and could do with nipping up a bit?????
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post #8 of 16 (permalink) Old 18-04-2017, 23:57
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Quote:
Originally Posted by steviejones133 View Post
Thanks Bat - well, the quadrant part looked fine to me. Made sure the cable was seated correctly. Is there any adjustment to be made to the cable itself? - I did notice that the place that holds the cable to the body might have some nuts to adjust slack, possibly. Maybe mine is too slack, hence dropping below 750/800rpm, and could do with nipping up a bit?????
Ive never adjusted the cable. I think you unclip the square black plastic part that the cable goes through, then twist it around and put it back.
It doesn't want to be drum tight.
Throttle Cable Change - How To
This post could help.

Last edited by batoutofhull; 19-04-2017 at 00:04.
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post #9 of 16 (permalink) Old 19-04-2017, 08:57
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If it just happens on idle then I would suspect the IACV, or Idle Air Control Valve. It allows air to pass the closed throttle body and compensates for fluctuations in load on tickover, to keep a steady rpm. They can get blocked up. Old ones you can take apart and clean, but newer ones are sealed. You can try flushing it through with carb cleaner or replace with a new one.





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post #10 of 16 (permalink) Old 19-04-2017, 09:08
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Quote:
Originally Posted by steviejones133 View Post
Thanks Bat - well, the quadrant part looked fine to me. Made sure the cable was seated correctly. Is there any adjustment to be made to the cable itself? - I did notice that the place that holds the cable to the body might have some nuts to adjust slack, possibly. Maybe mine is too slack, hence dropping below 750/800rpm, and could do with nipping up a bit?????
A slack cable would not influence the idle speed, only if it was tight would it raise the idle to high, as said, sounds like IACV needs cleaning.
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post #11 of 16 (permalink) Old 19-04-2017, 18:21
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two things happen with the iacv...

1. the rubber hose is old and doesnt seal properly sometimes but not always, so the air going in at idle isnt settled... its constantly changing..

2, that pipe sits on an aluminium pipe and it gets coated in the dreaded ally white dust.. that dust gets into the iacv mechanism... and then reacts with anything already in it... its an electrochemical reaction, you cant stop it.. rain water.. puddles splashes, react with the ally pipe.. and then that crap reacts with anything in the iacv.. eventually that crap dust stops the iacv moving properly... one can hear that sometimes with the engine cover off... clicking noises as it jumps instead of resetting smoothly..

the result is the iacv mechanism gets covered in dust and dont work properly... that and age, mean it dont do what its supposed to do..

one can try cleaning it.. as suggested.. brake pad degreaser... pipe freezer spray... something with a totally evaporating action without water in it.. not anything oil based like wd40... dust will quickly stick to any oil residue left inside...


ptfe based cleaners might help... the ptfe aids slippery surfaces... and that is what one wants in nearly all the iacv... to help it seat properly... and ptfe is pretty non reactive as far as the engine is concerned..


the only other solution is buy a new one...
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post #12 of 16 (permalink) Old 19-04-2017, 20:15 Thread Starter
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Thanks for all the replies, fellas. Much appreciated.

Oddly enough, the car has behaved Today and no odd symptoms of falling rpm's. I did have a brief look at the TB quadrant earlier and when moved with a finger, it seemed to go back to the stop easily without any signs of sticking. There is a bit of slack in the cable between the TB and the retaining clip - not sure if removing that slack would help or not, I suppose I could always adjust it and see if it makes any difference. Point taken that it should not be too taught, though.

Other than the above, I will be looking at trying to clean the IACV before replacing with a new one. I will also have a look at the breather hoses to see how well they are seated to the TB - might even take them off and clean them up if required.

Would there be anything else to consider? - I'm going to be dropping in a Green Cotton filter soon and whilst I am at it, is there anything else worth doing in terms of cleaning or changing such as fuel filter, plugs, leads etc? - I don't often gain access to the inspection area as it's not my forte to fiddle, generally speaking.

Still, odd that she drove fine Today - weird!
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post #13 of 16 (permalink) Old 19-04-2017, 20:51
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Quote:
Originally Posted by steviejones133 View Post
Bypass valve? - what or where is that! - if I knew what to look for and clean, that'd help.........
It is another name for the idle control (from someone who was extremely tired).

When I was having problems I took the valve out and cleaned it all with white spirit, I also did the throttle body. It was a while ago so I can't remember all the details.
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post #14 of 16 (permalink) Old 19-04-2017, 21:18 Thread Starter
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Originally Posted by Talkingcars View Post
It is another name for the idle control (from someone who was extremely tired).

When I was having problems I took the valve out and cleaned it all with white spirit, I also did the throttle body. It was a while ago so I can't remember all the details.
Ahhh, I see. Thanks for the clarification - that clears that up.
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post #15 of 16 (permalink) Old 21-04-2017, 10:37
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I have a 2000My F with 52mm trottle body fitted with similar problem as when I go to pull off at the traffic lights after reving up while leaving the clutch out she appears to loose the revs and almost stalls. Could this be the IAVC
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post #16 of 16 (permalink) Old 21-04-2017, 20:14 Thread Starter
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Originally Posted by Hobby View Post
I have a 2000My F with 52mm trottle body fitted with similar problem as when I go to pull off at the traffic lights after reving up while leaving the clutch out she appears to loose the revs and almost stalls. Could this be the IAVC
Well, my issue is not quite the same. She's fine pulling away, it's just when coming to a slow stop and dipping the clutch to go down into 1st - the rpm's will then drop to a close-to-stall level.

Saying that, I've not done anything to her over the last day or so and she is behaving just fine.....maybe a bit of crud somewhere has been dislodged. I'm going to keep a close eye on things for a while, drop in my Green Cotton filter and see how she behaves then. Fingers crossed, normal service will continue.....

Watch this space, as they say!
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