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Discussion Starter #1
I have owned my 620ti for 10 years with no serious problems. I now have two problems which I am posting together in case the two problems are connected.

Problem 1.
If the car sits in idle traffic for too long it starts to idle too high. It will start at 750rpm and when the car gets hot under the bonnet the idle will slowly and steadily increase until the car gets on the move again. If I am stuck in a traffic jam the idle easily gets as high 2,000 - 2,500 rpm and in one traffic jam the idle even got up to 4,000rpm. The engine temperature always stays normal, it seems to happen when the "under bonnet" temperature gets high. If I lift the bonnet and let the engine run with the bonnet open the idle will return to 750rpm within about 1 minute.

Problem 2.
The engine has recently developed a misfire under acceleration. It drives fine normally and when you put your foot down it misfires. Initially I thought it was fuel starvation so I replaced the fuel pump but this made no difference. Then I tried reducing the boost (for the last 5 years I have been happily running 12-13psi with no problems). When I reduced the boost down to 8psi it helped. It still misfires intermittently but not as bad as when running 12-13psi. In the last few days the misfire has started to happen occasionally under normal driving conditions, very mild acceleration, hardly putting my foot down at all.

I don't know if the two problems are connected or not but Problem 2 seems like a sensor or something failing and I wondered if anyone has any ideas.

Many thanks
 

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coolant temp sensor can cause a lot of issues on a t series , about £15 delivered for a new one ,

change the fuel filter if not already done so ,

change the plug leads ( I changed just about every sensor on a stuttering vectra that just wouldnt pull under load only to discover it was the leads in the end ) , andy
 

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I have owned my 620ti for 10 years with no serious problems. I now have two problems which I am posting together in case the two problems are connected.

Problem 1.
If the car sits in idle traffic for too long it starts to idle too high. It will start at 750rpm and when the car gets hot under the bonnet the idle will slowly and steadily increase until the car gets on the move again. If I am stuck in a traffic jam the idle easily gets as high 2,000 - 2,500 rpm and in one traffic jam the idle even got up to 4,000rpm. The engine temperature always stays normal, it seems to happen when the "under bonnet" temperature gets high. If I lift the bonnet and let the engine run with the bonnet open the idle will return to 750rpm within about 1 minute.
I have got EXACTLY the same problem and it's driving me nuts! I want to sell the car but can't pass it on to any new owner with this problem, so it needs fixed (unless anyone wants a 620ti and thinks they can fix it). I think it's to do with the stepper motor. I changed the CTS for a brand new one from Rimmer Bros but it didn't make any difference. Also, disconnecting the plug from the CTS when the problem is occurring doesn't seem to make a difference, just turns the fan on, but as you say, lifting the bonnet and letting things cool down a bit, brings the idle speed back down, and if you stand there while it's cooling, you can hear the stepper motor gradually clicking back to normal. But what is causing the stepper motor to up the idle, is the question.
 

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I have got EXACTLY the same problem and it's driving me nuts! I want to sell the car but can't pass it on to any new owner with this problem, so it needs fixed (unless anyone wants a 620ti and thinks they can fix it). I think it's to do with the stepper motor. I changed the CTS for a brand new one from Rimmer Bros but it didn't make any difference. Also, disconnecting the plug from the CTS when the problem is occurring doesn't seem to make a difference, just turns the fan on, but as you say, lifting the bonnet and letting things cool down a bit, brings the idle speed back down, and if you stand there while it's cooling, you can hear the stepper motor gradually clicking back to normal. But what is causing the stepper motor to up the idle, is the question.
Have you reset the stepper motor ? andy
 

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Have you reset the stepper motor ? andy
Just been out for a test run, got the engine nice and hot, fans on severals times, and got the idle speed thing to happen. I did the pressing the throttle pedal fives times thing but didn't hear anything happen with the stepper motor....

However I discovered that when the idle speed is stuck high, the stepper motor plunger isn't even touching the idel control screw - it appears that this problem might simply be down to the thottle flap getting stuck open when it gets hot, and the hotter it gets, the more stuck open it gets. I'm hoping that's all it is, just means it needs a good clean out.
 

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Agitated, see if you can get the idle speed stuck high on your car and then lift the bonnet and see if you can turn the throttle cam back to its normal resting place against the throttle stop thus returning it to normal idle speed. If so then I guess your throttle body just needs a good clean, that might possibly explain your other problem too.
 

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The problems with high idle are almost certainly a sticky throttle body. Clean it out (properly) with carb cleaner or similar, making sure the butterfly action is nice and smooth when you're done.
 

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Discussion Starter #8
"Agitated, see if you can get the idle speed stuck high on your car and then lift the bonnet and see if you can turn the throttle cam back to its normal resting place against the throttle stop thus returning it to normal idle speed. If so then I guess your throttle body just needs a good clean, that might possibly explain your other problem too."

I ran the engine up to "ridiculous idle level" and looked at the Stepper Motor and Throttle cam. At first glance I thought the plunger on the stepper motor was not touching the throttle at all but on closer inspection I noticed the plunger on the stepper motor is actually really tiny and thin. Unfortunately this was pushing the throttle. I reset the stepper motor by depressing accelerator five times and then watched the stepper go through a routine where it started clicking a lot, retracted the plunger then pushed the plunger right out and finally the plunger returned to the exact same position where it started. I then switched the engine back on and it was idling ridiculously high again. I forced the throttle position backwards and the idle returned to 750rpm, obviously by forcing the throttle back to this position it pushed the plunger back inside the stepper motor. However slowly, but surely, the stepper motor got it's own way and kept poking the f***ing plunger back out to return my idle to a stupid level.

I am fairly confident the throttle body is not seized in any way because, when the plunger is forced back inside, the throttle moves freely and springs back to 750rpm.

What I don't know is what is causing the stepper to poke it's plunger out and will replacing the stepper actually fix or is something else (ECU?) telling the stepper to stick it's plunger out?
 

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mg_zt_t
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I have had TWO 620ti which did this. In both cases it was the three wires to the TPS ( Throttle Position Sensor ) had been stressed at some stage probably by someone previously pulling that plug off the throttle body by the wires ... heavy handed ...

I cut another plug from the loom of a scrap T16 and spliced it in place of the original. .... Immediate return to perfect idle both cold and warm...

I discovered this whilst looking for reasons for the high idle ~ 1500-2000 rpm ~ and whilst fiddling with the wires to the TPS, noticed that perfect idle could be obtained with the wires fiddled in a certain way. Fiddle again with those wires and idle went high. I suspect a poor or broken wire connection in the TPS plug was responsible.

Not saying this is the case here, but it was on my two examples.

Moral: ALWAYS release the retaining clip to release that little TPS plug on the throttle body. You can pull it off using only the wires to hold onto but, that is asking for trouble.
.
..
 

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Discussion Starter #10
I have had TWO 620ti which did this. In both cases it was the three wires to the TPS ( Throttle Position Sensor ) had been stressed at some stage probably by someone previously pulling that plug off the throttle body by the wires ... heavy handed ...

I cut another plug from the loom of a scrap T16 and spliced it in place of the original. .... Immediate return to perfect idle both cold and warm...

I discovered this whilst looking for reasons for the high idle ~ 1500-2000 rpm ~ and whilst fiddling with the wires to the TPS, noticed that perfect idle could be obtained with the wires fiddled in a certain way. Fiddle again with those wires and idle went high. I suspect a poor or broken wire connection in the TPS plug was responsible.

Not saying this is the case here, but it was on my two examples.

Moral: ALWAYS release the retaining clip to release that little TPS plug on the throttle body. You can pull it off using only the wires to hold onto but, that is asking for trouble.
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After seeing the plunger on the stepper move the throttle do you still think it could be a wiring problem?
 

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I am fairly confident the throttle body is not seized in any way because, when the plunger is forced back inside, the throttle moves freely and springs back to 750rpm.

What I don't know is what is causing the stepper to poke it's plunger out and will replacing the stepper actually fix or is something else (ECU?) telling the stepper to stick it's plunger out?
It seems there could be several things that have the same effect here. In your case what I would try next is to see, when you have the idle stuck high again, what happens when you unplug the Coolant Temperature Sensor (cooling fans should run and idle should go to 1200rpm)? But I don't understand how that could be adversely affected by under bonnet temperature. Nor can I see how faulty wiring can make the idle speed prgressively increase as the temperature increases. I know exactly what you mean as my car has been doing this very thing, but as I say I've changed the CTS and now it looks like in my case the throttle is also sticking which is giving me the same symptoms.
 

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Discussion Starter #12
It seems there could be several things that have the same effect here. In your case what I would try next is to see, when you have the idle stuck high again, what happens when you unplug the Coolant Temperature Sensor (cooling fans should run and idle should go to 1200rpm)? But I don't understand how that could be adversely affected by under bonnet temperature. Nor can I see how faulty wiring can make the idle speed prgressively increase as the temperature increases. I know exactly what you mean as my car has been doing this very thing, but as I say I've changed the CTS and now it looks like in my case the throttle is also sticking which is giving me the same symptoms.
King Arthur, have you cleaned your throttle body yet? Did it make any difference?
 

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King Arthur, have you cleaned your throttle body yet? Did it make any difference?
I have cleaned it but won't know if it's fixed until I drive into London at the weekend. There was a fair bit of gunk on the manifold side of the throttle though which I'm sure would have got sticky as it got hot.
 

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Update: Cleaning the inside of the throttle body hasn't stopped it from sticking open when hot. I can get the engine hot and quite clearly see that it is sticking by blipping it by hand and watching it settle back. Something is still sticking somewhere, and it's becoming a bit of a mystery. I suppose I'll just have to take it apart completely and clean all the parts.

Tell you what though, it didn't half improve the throttle response of the engine. Much smoother, and I might have been imagining it but it seemed to have more low down torque. That inspired me to treat it to a new set of spark plugs and now it's like a different car, it's completely come alive. So just this last niggle to sort out and then someone else can enjoy it.
 

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Discussion Starter #16
Okay, I have finally fixed my "high idle" problem. Firstly I must apologise to people who said "clean your throttle body" and I dismissed the suggestion and ignored it.

I removed the throttle body and examined it carefully. in the very middle of the throttle body where the "butterfly" is, the body has been very precisely machined to match the exact size of the "butterfly" just leaving a slight gap, just enough to see a slither of daylight through. When I inspected my throttle body, the precisely machined area had a build up of carbon / dried oil / black crap (?). This built up "stuff" was making the fit between the "butterfly" and the throttle body an exact fit with no gap around it at all. It seems that, when the throttle body got hot in traffic jams (and the current hot weather), the metal must have been expanding ever so slightly but significantly enough to stop the "butterfly" from closing properly causing the idle to increase.

I cleaned the throttle body until it was literally spotless and now problem has completely gone! I sat in a traffic jam in London yesterday for 30 minutes and the idle stayed perfectly at 750RPM.

Now all I need to do is find a solution for my second problem, the misfire. I have a suspicion that a sensor has failed. My reasoning for this is because I read the MEMS ECU has a "fixed map" which it uses when a sensor fails, like a "plan B" fixed MAP which gives a lower output. Now my car runs fine until you put your foot down, then it feels like fuel starvation. I suspect that the MEMS thinks it is running in a "safe mode" with lower boost and lower fuel etc but because I have fitted a bleed valve to run higher boost it must reach a point where there is not enough fuel to match the amount of boost I am running. The car appears to run okay up to around 4psi and anything over this invokes a misfire.

I now just need to find a "friendly diagnostic tuning place" who won't charge me too much to plug my car in and tell me which sensor has failed.
 

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mg_zt_t
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Okay, I have finally fixed my "high idle" problem. Firstly I must apologise to people who said "clean your throttle body" and I dismissed the suggestion and ignored it.

I removed the throttle body and examined it carefully. in the very middle of the throttle body where the "butterfly" is, the body has been very precisely machined to match the exact size of the "butterfly" just leaving a slight gap, just enough to see a slither of daylight through. When I inspected my throttle body, the precisely machined area had a build up of carbon / dried oil / black crap (?). This built up "stuff" was making the fit between the "butterfly" and the throttle body an exact fit with no gap around it at all. It seems that, when the throttle body got hot in traffic jams (and the current hot weather), the metal must have been expanding ever so slightly but significantly enough to stop the "butterfly" from closing properly causing the idle to increase.

I cleaned the throttle body until it was literally spotless and now problem has completely gone! I sat in a traffic jam in London yesterday for 30 minutes and the idle stayed perfectly at 750RPM.

Now all I need to do is find a solution for my second problem, the misfire. I have a suspicion that a sensor has failed. My reasoning for this is because I read the MEMS ECU has a "fixed map" which it uses when a sensor fails, like a "plan B" fixed MAP which gives a lower output. Now my car runs fine until you put your foot down, then it feels like fuel starvation. I suspect that the MEMS thinks it is running in a "safe mode" with lower boost and lower fuel etc but because I have fitted a bleed valve to run higher boost it must reach a point where there is not enough fuel to match the amount of boost I am running. The car appears to run okay up to around 4psi and anything over this invokes a misfire.

I now just need to find a "friendly diagnostic tuning place" who won't charge me too much to plug my car in and tell me which sensor has failed.
I have fitted a bleed valve to run higher boost
Is it a misfire or a boost cut!

Have you checked simple things like the state of spark plugs, HT leads and the various vacuum hoses for splits, perishing or loose fitting?

Here's a check worth doing. Disconnect the vacuum pipe from the actuator and place a clean tube in its place long enough so that you can suck on it and create a vacuum. You should be able to sense that vacuum by putting your tongue on the end of the pipe when sucking to feel the negative pressure. If there is not any negative pressure, chances are the diaphragm in the Actuator is no longer able to hold pressure. That will mean there will be 'misfire' like effects as you describe..

There again, it maybe a genuine misfire.

============================================

Glad you cleaned the Throttle Body [ TB ]. All our 600 cars are at least ten years old now and all those "out of sight, out of mind" things like TBs and Brake calipers sometimes do not get the servicing they should... that's when the problems start and will not go away without attention.

As me old Granny used to tell me...

A stitch in time....

.... saves nine!

So very true.
..
 

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duff ht leads will cause the problem you suggest , I had the same situation with a vectra moons ago , changed nearly every sensor , damn thing ran sweetly , could cruise along at 70 on the motorway with no problem but as soon as you put it under any load it was spluttering , missing and seriously lacking power , turned out to be the ht leads ....doh , andy -
 

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I have owned my 620ti for 10 years with no serious problems. I now have two problems which I am posting together in case the two problems are connected.

Problem 1.
If the car sits in idle traffic for too long it starts to idle too high. It will start at 750rpm and when the car gets hot under the bonnet the idle will slowly and steadily increase until the car gets on the move again. If I am stuck in a traffic jam the idle easily gets as high 2,000 - 2,500 rpm and in one traffic jam the idle even got up to 4,000rpm. The engine temperature always stays normal, it seems to happen when the "under bonnet" temperature gets high. If I lift the bonnet and let the engine run with the bonnet open the idle will return to 750rpm within about 1 minute.

Problem 2.
The engine has recently developed a misfire under acceleration. It drives fine normally and when you put your foot down it misfires. Initially I thought it was fuel starvation so I replaced the fuel pump but this made no difference. Then I tried reducing the boost (for the last 5 years I have been happily running 12-13psi with no problems). When I reduced the boost down to 8psi it helped. It still misfires intermittently but not as bad as when running 12-13psi. In the last few days the misfire has started to happen occasionally under normal driving conditions, very mild acceleration, hardly putting my foot down at all.

I don't know if the two problems are connected or not but Problem 2 seems like a sensor or something failing and I wondered if anyone has any ideas.

Many thanks
Yea I'm facing the same problem but someone told me about an too air mixture can make it got misfired but I haven't check the idea so I think you can try then if it's work then let me know. Thanks
 
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