rough idle holding back - MG-Rover.org Forums
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post #1 of 20 (permalink) Old 18-06-2017, 16:14 Thread Starter
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rough idle holding back

I have a problem local garages cant sort out for me so heres hoping someone out there can.My 05 1.6 115 tf, starts fine after 30 seconds or so it starts to stutter while idling then cuts out restart ok then drive off it holds back violently until its hammered then fine but finding its holding back when pulling from low revs sometimes with a small back fire, have had,plugs leads,coils,water temp sensor,idle control valve,cat,fuel filter,after cat lambda sensor,it comes up with no faults but still the same,anyone had this problem and got it sorted would be great to know how,thanks dusty
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post #2 of 20 (permalink) Old 18-06-2017, 17:17
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What condition is the air filter in?

Have they pulled and examined the plugs?

Sounds like fuel starvation if the ignition side has been thoroughly checked.

Alternatively have they checked the timing marks? It's not unheard of for the belt to have slipped...

Good luck!

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post #3 of 20 (permalink) Old 18-06-2017, 17:23
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He said 'it comes up with no faults but still the same' are you not paying attention ? lol
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post #4 of 20 (permalink) Old 18-06-2017, 17:33
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Why do garages replace good parts playing the guessing game.

Parts canon on overtime at your expense.

Start by reading fuel trims

Is she starved of fuel or getting to much?

Cam & crank signals ok? You will need to put a scope on them to see. If your garage doesn't have one, go somewhere else!

Whats the fuel pressure doing?

etc etc

Very difficult to call from symptoms which is why testing is a must.

Good luck.

I am going to test every sensor on my TF soon so that we have a list of known goods to help diagnose problems.
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post #5 of 20 (permalink) Old 18-06-2017, 17:36 Thread Starter
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1thanks Ianmc, yeah the plugs were ok and I forgot to add that the timing was checked and throttle body removed and cleaned, thanks for reply
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post #6 of 20 (permalink) Old 18-06-2017, 17:48
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get them to pull the crankshaft sensor ( gear box end ) and clean it off .if ianmcs suggest yeild no results , not unheard of a build up on it to cause erratic low end problems.
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post #7 of 20 (permalink) Old 18-06-2017, 17:51
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Quote:
Originally Posted by Whitwell.Mike View Post
He said 'it comes up with no faults but still the same' are you not paying attention ? lol
Not at first, then re-read the OP's post - so deleted mine lol
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post #8 of 20 (permalink) Old 18-06-2017, 18:20
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I just don't understand how these engine diagnosis thingies come back with no faults when there clearly is. My check light has come on three time in the seven years of ownership. Guess what, no logged or pending faults shown.
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post #9 of 20 (permalink) Old 18-06-2017, 19:21
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Barkingdog has me thinking, No codes means the computer thinks all is ok. I'm thinking un-metered air getting in via a vacuum leak. Try spraying water around the intake and vacuum lines and see if the rpm changes.

Also what is the iat reading via a scan tool, if it is faulty and reading something stupidly low the computer would over fuel to compensate.
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post #10 of 20 (permalink) Old 19-06-2017, 21:54
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Lots moving parts of the engine are without a sensor to check if is working fine, like sparks plugs or oil leaks.
There is no magic even in a coolant level sensor.

Diagnosis is done by collecting sensors input.
Very easy concept and will not tell you anything more than the sensor could detect.

SW and sensors are very simple on these engines, so diagnostic is limited too.
But mind it: is a bonus.

I ve worked for so many years for defense & NATO projects, especially HW diagnostic for complex systems, and is a bloody hell having all working without lots of false pasitive.
And relay too much on these diagnostics... Well, good luck.
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post #11 of 20 (permalink) Old 20-06-2017, 14:14
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too much or too little fuel. in relation to the air intake... i.e the mixture is wrong, and stops igniting... the reason for that might be a faulty injector... or faulty injector feed line... maybe an injector is not closing properly... until the revs change.. maybe its not opening properly until the revs change too much or too little fuel in relation to the air intake.. means the ecu will try and compensate to set the mix based on the oxy sensor input.. and if the injector input fluctuates the oxy sensor starts telling the ecu all kinds of false positives.. and so two things start to happen at once... if the mixture is lean .. the ecu richens it... if its rich it leans it... and all the time the faulty injector is allowing something different to happen,,, so either the ignition leans out and falters or gets rich and falters. the plugs might tell you that.. if you let it stop when it falters, and look carefully at them... since.. if the mix is rich it will foul the plug and it wont fire... if its leaning out.. the plugs will be very white and dry...

if it not a faulty injector... then either its an air leak or the ecu is going awol because the iacv isnt working properly.

because the fault is semi permanent, and only goes away under particular circumstances.. then its got be fuel and air mix error., or the ignition circuit

it could be the iacv not closing or opening properly... since the ecu might be telling it to open or close more.. but it doesnt.. its sticking.


if the mix is right and the ignition circuit is good.. the engine will run... if its not... it wont..

The worst scenario would be your getting conductive vapour in the cylinders ( coolant ) and that happens at the time of engine warm up... and goes away when you rev hard... the indicator of that would be loss of coolant, oxy error, and bad idling... spark plugs will fire to the nearest earth.. and that might no longer be the plug tip, if its wet.

i think you need to look at the plugs carefully.. when the fault occurs... ie let it falter and die, then take out the plugs and look at them. Also, bear in mind an electrical wiring fault, might only happen at a particular moment... based on temperature, proximity, and function... ie a bad connection only happens when the circuit is at a particular moment..

Last edited by Incony; 20-06-2017 at 14:23.
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post #12 of 20 (permalink) Old 20-06-2017, 14:53
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You would think if it was fuel or ignition fault the OBD system would flag a fault code, wouldn't you? Hummmm.
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post #13 of 20 (permalink) Old 21-06-2017, 21:24
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why Barkingdog.. the ecu compensates... everything it can see is within range... until the moment the engine dies... and then everything the ecu can see restarts from that moment... and the moment is lost... its not registering... its not seen by the ecu.. right up to the point of failure.. the ecu will see out of range faults.. but not those it cant compensate for.. AND the worst thing about that, is that its trying to compensate for an error it cant see... that influences its logic... so in effect the ecu makes things worse... it compensates and increases the fault.

so.. what happens when you suddenly change engine revs...? everything that was.. is now different... the ecu gets its sensor input.. and changes the air fuel mix based on that.. the original fault isnt happening now.. things have changed... so the new situation works... the fault the ecu could not see, isnt there... so now the ecu sensing logic does what its supposed to.. everything works... and the fault? well the rev changing affects that too... so it changes... the circumstances that caused it are not there now.... so when you drop back to idle.. the fault isnt there.. the ecu just deals with the things it can see.. not those it cant... and it could not see the original fault... else it would flag it.

you think that the ecu should flag a fault when things it cant see are wrong? think again.

the ecu cant see a valve that dont close properly.. or open properly... it will try to compensate if the logic parameter are in scale... but lets suppose you increase revs.. and now the valve closes and opens correctly at the right time... yet, before that happened.. it was still within the ecu operating logic... ? that maybe idled like a sick pig until revved... the oxy sensor was getting near to telling the ecu it was wrong... even though the ecu was modifying the mix... and it was still within range.. and then suddenly, by revving... everything comes back inside the " window" and all is happy.. until the naff valve seat decides to put a spanner in the works.. again...

and at the moment the engine stops? well there at that moment is the truth... if one can see it.

the ecu doest tell you - you turned off the ignition and killed the spark and therefore flags an ignition fault... ie it could not compensate for you turning off the ignition... it cant see that as a fault... only you know why the engine stops... and you would be concerned if it did not... but the ecu? well unless the not stopping affected another parameter.. it would happily keep running the engine.. even though you had turned the key off.. and it didnt turn off.

Last edited by Incony; 21-06-2017 at 22:02.
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post #14 of 20 (permalink) Old 22-06-2017, 14:15
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Had this similar problem with my 2002 115 TF. I changed the coil packs, the spark plugs, the HT leads, the cam position sensor, fuel filter, injectors and fitted induction air free 52mm throttle body. Didn't make the blind bit of difference. Finally replaced the down pipe with a Toyo stainless steel unit and O2 sensor. All the bad idle, hesitation and backfire gone. Ok it was probably the O2 sensor. No real ODB faults except misfire and fuel codes but these were not all the time, only occasionally did they show up.

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post #15 of 20 (permalink) Old 22-06-2017, 15:41
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i am often thought of as a radical - CyprusMGTF... who says things that upset folks... because i question reason. so to see your reply gives me faith... ty for that..

if the O2 sensor gives the ecu odd readings... the ecu may not see a fault, if those readings are within its spec range..

it will try and compensate by changing both air and fuel settings to match the inputs it can see.. and in fact what happens is that things get worse.. because the ecu is compounding the error... but for the ecu all is fine... everything is in range,,

and then the engine stops... or the inputs change..

if the ecu cant see a fault.. its not going to tell you there is one. if it does.. it may have nothing to do with what caused it... ie the ecu itself compounding a fault it cant see.

in my teens, when i learned about what kept an engine running, and revving freely... there were no computers assisting that... the engine worked because i understood what it needed... and now? we have machines controlling that... but they dont learn like i did... they just keep doing the same thing... all the time... at least they are predictable... unlike me...
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Last edited by Incony; 22-06-2017 at 15:54.
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post #16 of 20 (permalink) Old 22-06-2017, 17:44
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Quote:
Originally Posted by Incony View Post

if the O2 sensor gives the ecu odd readings... the ecu may not see a fault, if those readings are within its spec range..
If that's the case, and the readings are within the spec range then there is no fault.
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post #17 of 20 (permalink) Old 22-06-2017, 18:11
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If that's the case, and the readings are within the spec range then there is no fault.
think he means that the sensor is saying everything is ok but isnt reading what is actualy happening because is it faulty ie should say 9 but is saying 3 all the time ?
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post #18 of 20 (permalink) Old 22-06-2017, 18:13
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no because the ecu is compounding a fault it cannot see... there is a fault... the ecu cant see it.. tells you nothing, but makes the fault worse.
a machine tells no lies? ok.. i think it does.
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post #19 of 20 (permalink) Old 22-06-2017, 19:29
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no because the ecu is compounding a fault it cannot see... there is a fault... the ecu cant see it.. tells you nothing, but makes the fault worse.
a machine tells no lies? ok.. i think it does.
Well, it's not lying. It's truthfully answering the questions asked of it. Just that the questions don't ask what we want to know.

You've given excellent advice IMO. If the O2 sensor reads too much O2 in the exhaust, the ECU will increase fuelling to match. First thing to check is that there are no leaks letting air into the exhaust stream. Then check the exhaust sensor.

(I don't know much about MGFs, but this is good general troubleshooting advice.)
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post #20 of 20 (permalink) Old Yesterday, 09:43
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For me the symptoms were that the engine would rev freely with no load,as you might expect, under load and below 2000 rpm the engine would simply bog down and stop pulling. Quite scary when pulling out onto a roundabout. The plug in number 4 cylinder would soot up and the others were fine. While the engine was bogging it would also back fire, just POPs.
I suspect that the O2 sensor was providing incorrect signal and the ecu compensated by over/under fuelling causing the backfire and bogging.
I Might be wrong but changing the sensor has liberated the engine and she now pulls well from standing start all through the revs as well and roundabouts are less of a scary place, and my fuel consumption has improved.
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