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MG MGF Technical - Sticky throttle problem...

Has anyone had a ‘sticky throttle’ problem? Our dealer’s mechanic is stumped and needs some suggestions.

The throttle seems to ‘stick’ after the car has been used for a while as has warmed up. When you take your foot off the accelerator, the revs stay high and gradually fall. If I’m stuck in traffic, the MG frequently idles at 1500 rpm instead of 900prm. Turning the ignition off and on again sometimes clears it. I recently a day where the car was no longer idling at 900rpm and was trying to idle at 500 and stalling!

The car has been to the dealers on numerous occasions with this fault, and has had the throttle body replaced twice and the cat replaced. The last time it was at the dealers (Monday), the mechanic reset the IAC valve steps from 4 to 22 using the testbook, but the result was limited, and the throttle is starting to stick at 1500rpm during idling again.

I’ve got the car booked in again for two days on Monday, so any suggestions before then would be greatly appreciated!

Many thanks,

Jason
W681 JCB
Jason Ratcliffe

Have you tried resetting the stepper motor.

Turn on the ignition, to the second setting, do not start the engine.

Push down the accelerator 5 times slowly, wait ten seconds then turn off the ignition.

Then start the engine and take for test run to see if problem is cured.
Chris

Hi Chris,

Thanks for your reply. I've tried that when I've been stuck in traffic. Saying that I pressed the accelerator quickly 5 times and didn't wait 10 seconds though, so I'll try it again tonight. I suspect the MGR mechanic would have reset the idle speed when he was using the testbook on Monday.

I initially thought it was a kinked accelerator cable, but was able to open the boot with the other set of spare keys while the engine was running and stuck at 1500 rpm and checked the throttle body. No joy...

Jason
Jason Ratcliffe

Jason, how old is your F?

The reason I'm asking is I was just about to post a new thread on this EXACT subject because I have been suffering from EXACTLY the same problem, and it is now turning into the proverbial nightmare.

Mine is a March '01 VVC VIN 522'9xx. My dealer has been absolutely crap so far in solving it, and as you will see has made things much worse.

First I had the battery changed because it was weak and that can apparently cause idle problems. It didn't solve the problem.

Then the dealer changed the TB from the original plastic item to an alloy one (48mm, not 52). Didn't cure the problem although as with Jason the idle was then sticking to 1500 rpm instead of nearly stalling.

This week I was on business travel and I left the car with the dealer all week, with firm instructions not to give it back to me until the problem was SOLVED. I picked it up this evening. First thing I noticed was that the radiator cooling fan came on almost immediately after starting, although the car had been sitting in the dealer's car park for some time because they close at 6 and I arrived at a quarter to seven. The engine bay was relativey warm though. As I drove off the idle was going back down to normal as I let go of the accelerator, so it seemed the problem had been solved. I drove off, motorway for 5 miles, then dual carrigeway, sticking to the speed limits, and not exceeding 4500 revs. All seemed to be going normally.

When I got to my last stretch of open road through a forest , I dropped down to second and accelerated hard. The engined picked up as usual up to 5900 and then suddenly at 6000 revs a horrible rattle noise from the engine seemed to cut in and increase together with engine revs to 6,800 rps, at which point I let go of the acclerator, horrified. The usual "kick" of the VVC wasn't the either. The engine seemed to be unable to exceed 6900 revs.

At slow engine speeds the noise was not there but when I went past the 6000 mark there it came in again, as if some moving part was terribly loose. I immediatley thought of the VVC mechanism in the camshafts - O horror.

Anyway I drove the last two miles home at lower revs and guess what, as I stopped in the driveway the coolant fan was still running... which it never does after a such a drive because it gets a lot of cool air. Was it running all the way, or is it compensating for the engine overheating as a result of some fault in the ignition or some other mechanical fault in the engine? the bay did seem unusually warm for a sedate drive when I opened the boot. Oil and coolant levels are normal. No leaks under the car.

NOW.
while I wait to hear from the dealer how exactly he thinks he "solved" the problem, as a clue to what's actually going wrong now, I really need some help here as to the possible causes to the more general problem of which I don't seem to be the only victim.

The archives point to sticky throttle, low battery, faulty plugs or HT leads, faulty ECU.

I personally believe that in our case (Jason's and mine) it is the latter. The way the accelerator pedal reacts is definitely NOT characteristic of a sticky throtttle, and in both cases anyway it should have been solved by changing the TB. Same reasoning for the stepper motor.

But I also remember that someone from Belgium reported similar problems on his TF remained unsolved depite all the parts listed above having been changed. I don't recall seing any recent posts from him on this so
*please, whoever it was*, let us know how it all ended and what the cure was (if any!).

I am of course terrified of giving the car back to the dealer. He seems to have absolutely no clue as to what he is doing. Does any of the experienced members of this board have an inkling as to what may be going wrong here, based on our descriptions?
Anthony Braham

I assume you call for Erik.
As far as I know him he will report.

Strange problem though :(
Dieter K.

Hi Anthony,

It must be great taking your MGF around the Swiss countryside! My MGF is an MPi and I bought it new in March 2000. I've had the fault since around October, but due to it's intermittent nature, the mechanic wasn't able to do very much until December, when he faxed Rover asking for help and got no response. I decided to leave it a while and let the fault 'develop' a bit more. I noticed later on that the car was struggling to get to 6000 rpm and that it's performance was suffering.

I took the car to a dealer that was closer to home at the end of March for it's service, and to see if a second opinion would help. That dealer told me I needed new front discs and pads, costing another £200, and that they weren't able to look into the fault. 30 miles later the newly fitted brakes locked on the Mancunion Way, and the car had to be towed back to the garage by the AA.

A week later, this new dealer had the car for the day for the accelerator/performance problem. They changed the TB again, then informed me that they thought the performance problem was due to the cat starting to pack in, and it would cost £303 + fitting + vat. I popped in to pick up the car with the intention of bringing it back at the end of the month to get the cat changed. Once I got the car onto the road and started to accelerate above 25mph, I heard a sound which can only be described as gravel being dropped onto a frying pan! The engine at that point had no power and kept stalling. I tried to get the car home and only managed 1/2 a mile before I could no longer move it. Needless to say, at that point I'd had enough of that dealer, as every time I took the car in, it came out worse! I’m no mechanic, but felt that in both cases when I took the car there, that they weren’t being honest with me.
The AA confirmed the cat was blown, and I had the MGF towed to an independent mechanic who fitted a replacement cat for £200. On one side of the cat the ceramic honeycomb was totally destroyed, and must have been due to petrol getting into the cat when the car was being tuned. I can’t pin anything on that dealer, but needless to say I won’t be using them again.

Once it became apparent that changing the cat did not get rid of the fault, I took the car to the original dealer again. From going through the archives on this site and from the fact that the mechanic had alter the steps again on Monday, I’m inclined to think that the idle air control (IAC) valve itself might be faulty. However, I’m an IT manager, not a mechanic! The mechanic suspects the ECU as well, but the fault ‘feels’ like there’s some dirt or something causing the problem.

If Erik was able to solve this problem, I’d love to hear from him!

Jason
Jason Ratcliffe

Hi Jason !
Yes driving in Switzerland is great. You should come down some time, I'd show you some twisty mountain roads that are a real hoot in an F top down. That is, if it's working.

From your description of dealer problems it seems there's nothing new under the sun. FYI the brakes seized on my former MY 2000 F ONE DAY after I had bought it new. There again the dealer had fitted them badly at delivery in Switzerland.

It is SO infuriating to have to "guess" and do personal research on a problem when the MGR dealers are incompetent. I am a jurist, not an MGR mechanic, and the fact that I see faults the dealer has no clue about or has not even seen makes my blood boil.

This morning when I started up, icy cold engine, and hey presto, on comes the cooling fan. Got tothe dealership OK and then spent 30 mins talking to the manager who served me a pile of bullsh*t about how experienced the dealership was. He completely missed the point that the car should have been fully tested, including past 6000 rpm (it's a VVC, Duh!) before it was delivered to me.

anyway, this is what I gathered from our meeting, at which the mechanic was also present:

1) one of the electrical wires (blue colour: he showed me the part) leading to the stepper motor and/or idle air control valve (or something similar in that area) was severed, and they didn't notice it when they changed the TB. They put that right. Now to me that seems like it's all it needed.

BUT

2) They said they also changed the engine ECU. Now, after listening to the problem (rattle in the engine starting at 6000 rpm) the mechanic suspects ..... guess what .... that they fitted an MPI ECU instead of a VVC ECU :-((.

We'll see how it all ends but I can't help thinking that there is now probably some permanent internal damage to my VVC camshafts. The mechanism is already Very Very Complicated so with the wrong ECU heaven only knows what could happen.

More news later.

Anthony
PS. Dieter thanks for your quick response last night. It's comforting to know someone who knows his stuff is listening when you're in trouble :-)
Anthony Braham


Exact problems suffered - and cured!!!

After you've been down the throttle body and cable route. ie. it's not physically sticking, look at two things:

1) TPS (Throttle Position Sensor)
This is fixed to the throttle body and has three wires.
It can...
i) Break internally. replace it
ii) Have broken/severed wires where it leaves the loom
iii) Have broken wires inside the connector
If this is faulty it can cause high revs, no engine braking etc.

2) Temperature sensor(s)
Look at the water temp sensor and the oil temp sensor on a VVC. Note that there are different sensors for the insruments and for the MEMS unit.
If the Oil temp is wrong the VVC will not kick in -> poor performance.
If the water temp is wrong then it *can* affect the idle and performance... but not as much.

For all sensors they can be tested by both the testbook and by a good old fashioned volt meter!
Note that some sensors will "work" but read incorrectly. Watch this.

Good luck with your search for a cure.

Paul.
Paul Nothard

>>problem was due to the cat starting to pack in

Sounds like baloney to me. However, I have a spare so if you have to get one, it's yours for a reasonable offer.
Ddraig

I was going to give the exact same answer as Paul! :o)

Sticky throttle can be:
1. Sticking throttle cable (rare through)
2. Sticking butterfly valve: dirt in the throttle body (can be cleaned with proprietary carb cleaner) or by distortion of the plastic throttle body (can be related to heat, but more often to the use of a cheap jubilee clip to retain the airfilter hose).
3. Problem with the IACV (can need cleaning, or the stepper motor attending to).
4. Electronic: be that a problem with the MEMS engine temperature sensor or a problem with the throttle position potentiometer or its wiring.

Let us know what the solution is in your case Jason (and Anthony!)
Rob Bell

Paul many thanks, I think you've put your finger on it: the cause was a severed cable on the throttle position sensor, which as a result also caused some damage in the ECU.

In my case however the garage said the replacement ECU was also faulty (!) They said they replaced it a second time today. The car's going fine now. They even delivered it to the office. (I must have spooked them this morning - I was pretty harsh. But it worked). I just really hope the engine hasn't suffered any mechanical damage. The future will tell.

Thanks to all those who helped.

Jason I hope your problems get solved soon too and at reasonable cost. From your description it looks like you will probably need a new ECU. Try a different dealer and get it done under warranty - after all the problem appeared last year and if the dealer made the wrong diagnosis and didn't solve it first time you shouldn't have to bear the cost.

Anthony
Anthony Braham

Don't see how they could have fitted an MPI ECU instead of a VVC one. The VVC one has an extra socket so the spare plug would have been left hanging loose!

Bruce
Bruce Caldwell

Hi Folks,

Many thanks for your feedback on this! I'm printing this thread out for the mechanic tomorrow, and hopefully he'll be able to resolve it now. I think if the problem isn't the IAC valve or its wiring, the best thing to do is push for a replacement ECU. I'll let you know how I get on when I get the car back on Tuesday. Fingers crossed!

Jason
Jason Ratcliffe

Hi Folks,

Got an update from the garage. They've tried changing the IAC valve and that hasn't worked so they've got an ECU on order and are keeping the car overnight. Anthony, it looks like your prediction may be right...

Fingers crossed for tomorrow then!

Jason
Jason Ratcliffe

Jason, make sure they check for severed cables in the TPS. That's how my ECU got damaged so before they replace it they should check. That severed cable (they showed it to me) was very small and - I suspect - not easy to spot.
Anthony Braham

Sticky throttle?
hmmmm.
nup, too thick to solve thissun
Tony Porritt

Folks,

Just to bring you up to date, the dealers STILL has my MGF! Apparently MGR only exchanges ECUs on an exchange basis and they seem to think they should get one on Tuesday.

So it looks like I'm stuck in a battleship grey Fiat Punto courtesy car for the bank holiday :(

Jason
Jason Ratcliffe

Sorry to hear that Jason. I'm realzing I was not that unlucky with my dealer... . Try and bully them into getting you a better car for the w/e. ZR?
Anthony Braham

This thread was discussed between 15/05/2003 and 24/05/2003

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