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Idle rises to 5k - possible cause?

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#1 iani

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Posted 10 June 2018 - 08:19 AM

All the PI components on my car have been reconditioned and the linkage replaced by a new, still underslung, type. After being quite poorly installed by a local Triumph specialist, the linkage was disassembled yesterday by a TR6 owning mechanic and reassembled correctly. I have a good spray pattern from all injectors and the inlet manifolds are reasonably balanced (less adjustment on this linkage). We took the car for a run and after a couple of miles the idle revs started to build, eventually hitting 5k by the time I found a place to pull over. The throttle rods had elongated with the heat, but only causing a couple of mm movement, not enough to raise the idle more than a few hundred RPM. The throttle wasn't sticking and the both chokes were off. We let the engine cool for half an hour and fired it up again, idled correctly for a couple of seconds then up to 3k RPM at which point I shut it off.

 

I'd had a similar issue last week, however that time I stopped the car when it was idling at 3k RPM and noticed a leaking injector, had the car recovered and replaced the injector, I hadn't used it again until yesterday. We have sprayed WD40 all over looking to see if we have an air leak but nothing found, we have retightened inlet & exhaust manifolds too. The timing appears to be correct and, before the revs went crazy, the car was running well, better than it has since I bought it. Our thoughts are that the Metering Unit must be the cause, the mechanic did comment that the exhaust looked to be rich when I started the car up. Does it sound like the MU could be the cause?

 

NB: The car has a Phoenix manifold and single exhaust system fitted so there is a lot of under bonnet heat


Edited by iani, 10 June 2018 - 08:19 AM.

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#2 Waldi

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Posted 10 June 2018 - 08:36 AM

I think:
If the MU is running/set rich, you would also need the equivalent volume of air to get it up to 5000 rpm.
No air= no 5000 rpm.

But on the other hand
If the MU is working correct, a (larger) air leak will rev up the engine, as vacuum drops.

Good luck,
Waldi
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#3 iani

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Posted 10 June 2018 - 08:48 AM

I think:
If the MU is running/set rich, you would also need the equivalent volume of air to get it up to 5000 rpm.
No air= no 5000 rpm.

But on the other hand
If the MU is working correct, a (larger) air leak will rev up the engine, as vacuum drops.

Good luck,
Waldi

Where should I be looking for air leaks? we've already checked all hoses, manifold nuts, the hoses between the inlet manifolds are new and clipped, vacuum pipes are new & clipped and spraying WD40 over everything doesn't result in any change. I do believe there is an air leak as the air valve can be screwed right in without the engine stalling.


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#4 Nick Jones

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Posted 10 June 2018 - 09:08 AM

Surprising how little air is needed for quite high rpms unloaded.

 

Given the size of the butterflies (6 x 46mm!) they really don't need to be held open by much to give this effect.  My money is on the linkage and it's adjustment.

 

Nick


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#5 Waldi

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Posted 10 June 2018 - 09:25 AM

I would run it without the air inlet header, so you can see and measure the gap between the discs and throttle body.
The gap should be zero or close to zero (there are different opinions on this), but certainly be very narrow as a maximum.
Waldi

Edited by Waldi, 10 June 2018 - 09:26 AM.

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#6 iani

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Posted 10 June 2018 - 09:27 AM

Surprising how little air is needed for quite high rpms unloaded.

 

Given the size of the butterflies (6 x 46mm!) they really don't need to be held open by much to give this effect.  My money is on the linkage and it's adjustment.

 

Nick

The linkage has been set 6 times now, twice by the installing garage, 3 times by myself and once yesterday. The last few times it has been set with a warm engine and hot linkage, the rods certainly expand once the exhaust manifold heats up. The butterflies were also slackened and reset, they aren't open at all.


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#7 iani

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Posted 10 June 2018 - 09:28 AM

I would run it without the air inlet header, so you can see and measure the gap between the discs and throttle body.
The gap should be zero or close to zero (there are different opinions on this), but certainly be very narrow as a maximum.
Waldi

The gap is zero Waldi, I can't get a 2 thou feeler through

 

WRONG...this was the position before yesterday's tuning exercise. On checking my car now, I can get any feeler gauge I want through there, but no need really as I can see a gap of c. 1mm!

 

The rods seem to be incapable of being set correctly, there is so much heat coming off the phoenix manifold that I think it's expanding the rods and the linkage assembly throwing all the settings out. I've just had the car sat idling at 1100 RPM with the rods disconnected so the MU would appear to be innocent here.


Edited by iani, 10 June 2018 - 10:20 AM.

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#8 stuart

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Posted 10 June 2018 - 02:59 PM

What are the rods made of if they are expanding that much! Whose linkage is it and can we have pictures?

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#9 Waldi

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Posted 10 June 2018 - 04:31 PM

Looks like you are one step closer:)
Keep going.
The whole linkage should have some clearance, so the slightest movement of the gas cable should not open the butterflies. Do you have some clearance still?
Waldi
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#10 iani

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Posted 10 June 2018 - 04:33 PM

Looks like you are one step closer:)
Keep going.
The whole linkage should have some clearance, so the slightest movement of the gas cable should not open the butterflies. Do you have some clearance still?
Waldi

We adjusted the throttle cable yesterday so it has slight play, I have an issue in that the pedal bushes are missing, ordered them last night to dial out any unexpected slop.


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#11 Waldi

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Posted 10 June 2018 - 04:39 PM

I recommend the blue PU bushes, they are an easy fit.
I installed them both on the pedal and on the (standard cp) linkage, I think 4 or 5 in total.
You may need two only for the pedal.
The gap of 1 mm you mentioned on the butterflies, dies it change to practically zero when the engine is cold again?

What if you release both the gas cable and the choke cable from the linkage to the TBs?
Do the butterflies close then?
Waldi

Edited by Waldi, 10 June 2018 - 04:43 PM.

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#12 iani

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Posted 10 June 2018 - 04:51 PM

I recommend the blue PU bushes, they are an easy fit.
I installed them both on the pedal and on the (standard cp) linkage, I think 4 or 5 in total.
You may need two only for the pedal.
The gap of 1 mm you mentioned on the butterflies, dies it change to practically zero when the engine is cold again?

What if you release both the gas cable and the choke cable from the linkage to the TBs?
Do the butterflies close then?
Waldi

That gap was there with the engine cold Waldi, I disconnected the rods and the gap closed, obviously I reset the rods. I started the car, it idled ok for a minute or so then, as the temperature rose, so did the RPM, I shut it down again. I adjusted the centre rod yet again and have just adjusted and connected the other two, I can't try it again today as my neighbours will have had enough by now. Note that the 1mm gap was caused by a mechanic adjusting things yesterday, there was no gap as supplied and yesterday morning I couldn't get a 2 thou feeler past the butterfly.

 

I have ordered the PU bushes, the linkage seems to already have them fitted.


Edited by iani, 10 June 2018 - 04:52 PM.

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#13 Steves_TR6

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Posted 10 June 2018 - 06:35 PM

Do you have the ‘choke’ cable that has two cores?

If so is the ‘fast idle’ part too tight?

Steve
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I thought TRs were meant to leak outwards!

#14 iani

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Posted 10 June 2018 - 06:36 PM

Do you have the ‘choke’ cable that has two cores?

If so is the ‘fast idle’ part too tight?

Steve

No Steve, the MU end seems to work fine, the inlet manifold end misses the cam completely so isn't an issue.


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#15 Mike C

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Posted 10 June 2018 - 09:54 PM

Is this from a recognized manufacturer or home made: "the linkage replaced by a new, still underslung, type"


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#16 k_raven_smith

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Posted 10 June 2018 - 10:18 PM

I think its time to get it to me
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#17 Tim D.

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Posted 11 June 2018 - 06:37 AM

This is interesting. I replaced the throttle rods with new a year or so ago and had a similar if less extreme issue. No matter how I adjusted as the rods heated the idle increased. In the end I never sorted it as I switched to the supercharger. But I suspected that the play in the old rods meant that this wasn't a problem.
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#18 iani

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Posted 11 June 2018 - 06:45 AM

Is this from a recognized manufacturer or home made: "the linkage replaced by a new, still underslung, type"


They’re from a recognised reconditioner Mike, one often recommended on here.
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#19 iani

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Posted 11 June 2018 - 06:46 AM

I think its time to get it to me


If it was driveable I would bring it over tomorrow, unfortunately that’s not an option.
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#20 Mike C

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Posted 11 June 2018 - 06:56 AM

They’re from a recognised reconditioner Mike, one often recommended on here.

OK, I guess the basic design should work. I'd put a vacuum gauge on the brake booster connection and start the car. If the idle vacuum decreases as the engine warms up then the problems probably in the inlet manifold/linkages and not the MU.

 

One thing I've done with in the past with components that I've suspected might be affected by heat is poured water on the suspect component and see if the problem goes away. If it's the linkage this might tell you which part of it  is unduly expanding. 


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