KV6 engine racing

[SD1too]

Originally Posted by xsport

Are the rubber seals in good condition without compression set?

Yes Stefan, I renewed all of them (black and green) six years ago but it made no difference to the (then) hunting idle.

Originally Posted by xsport

Can i ask if you have any open screws missing at the rear mounting rubber area (the ones that snap off ,and are a pain to relocate).

I'm pleased to answer that my rear mounting brackets are intact and both screws were fitted and tightened to spec.
The loan manifold chamber currently on my engine has lost both its brackets!

Originally Posted by xsport

... suspect a crack/broken seal along its joint surfaces where the manifold was joined in manufacture.

I agree; that's what I intend to examine first but detecting a defective weld is going to be a challenge.

Originally Posted by xsport

Also check the purge valve seal , i know you disabled this but have you checked the seal on this to the manifold.?

Yes I have. I removed the purge valve which was fitted with a juicy bright green 'O' ring seal with no sign of a defect. But how reliable is a visual inspection alone?

Originally Posted by Matt_75

You could try a smoke machine fed into the inlet ...

That's an interesting idea Matt, thank you. I'll look into that further.

Simon

[StewartIngram]

Years ago, on a previous KV6, I had a manifold with a leak. I eventually found that the manifold was leaking, by hearing a faint hiss, hardly detectable above the normal engine noise. By putting my head close to the manifold, and slowly running my hand around, I found a leak around the back, probably on the seam.
Previously I had a rattle from the manifold and investigated taking it apart, but without actually doing much, abandoned that idea, so I may have caused the leak? A change of manifold was necessary.

[Fred Byrne]

My sympaties are with you Simon. It seems that you are in in some alternative universe where the unthinkable happens! Yesterday I disconnected the pipe leading from the iinlet manifold and forgot to replace it. When I started the engine it raced up to 2500 rpm before settling down at 2000 rpm. I just wondered could your problem be with the servo connection.

Fred

[xsport]

Worth considering inspection of the alloy inlet manifolds have no cracks or open thread holes . Some of these are for coil /and engine cover mounting and also there are screws for the plate along at the rear. Without having the manifolds in front of me , i cannot recall which if any have open threads into the alloy manifolds. A resin applied along the manifold seams would at least provide a tempory seal,as well as the throttle body to manifold join as you now have nothing to lose ,should a smoke test prove to be negative. Your previous posts going back with missfire and tickover history over many years , might suggest that there is another underlying issue present here. You have stated that in previous instances ,idle and performance were absolutely fine ,only for the problem to return at a later date. So you would not really know that the donor manifold that you tested would be a permanent definitive fix, unless it was used long term. I have a feeling that a smoke test will not reveal a result, but well worth a try. That result failing, would point to another underlying root cause i would think, that has worsened over a period of time. Hopefully it will , but doubts are now stacking up. Good luck simon ..

[xsport]

Originally Posted by Fred Byrne

My sympaties are with you Simon. It seems that you are in in some alternative universe where the unthinkable happens! Yesterday I disconnected the pipe leading from the iinlet manifold and forgot to replace it. When I started the engine it raced up to 2500 rpm before settling down at 2000 rpm. I just wondered could your problem be with the servo connection.

Fred

Absolutely fred. !! Any of them ... They are a pain to renew and ,if you can get them , very expensive. Difficult to remove without breakage or damage. Im sure that simon would have checked the function of the servo , as he has been using the car off and on. Simon will know im sure if these well known pipe connections were not reasonably secure or sealing. Good point though fred !!

[xsport]

Just a little further to the brake servo. I have renewed my servo to engine vacuum pipe. They are a pain to remove and refit . The inlet to the servo is very difficult to refit,and i ended up using a little resin when rejoining ,as although it entered with difficulty , i could not get the pipe to fully engage a couple of MM short. There is also a "one way" valve in the pipe which can stick or not function fully after years of service. This can be checked before removal by the use of a length of hose attatched to the pipe end. It should give 100 % one way seal only. If not then there will be air drawn back into the system. The brake operation would seam to function ok on the pedal and would not be that noticeable,as it could be intermittent. The "sinking" pedal syndrome on the automatic was the reason that i changed mine originally , but this in itself made no difference to the outcome on this,and has always been present on the autos.so i understand. ....

[SD1too]

Thank you Stewart, Fred and Stefan for your helpful suggestions. Please keep them coming because even if I've already checked, it helps to focus our attention and narrow down the possibilities.

Originally Posted by StewartIngram

A change of manifold was necessary.

They seem to be obsolete now, both new and used. Even if I found one it would be expensive, probably in poor condition and with no guarantees.

Originally Posted by Fred Byrne

... I disconnected the pipe leading from the iinlet manifold and forgot to replace it. When I started the engine it raced up to 2500 rpm before settling down at 2000 rpm. I just wondered could your problem be with the servo connection.

I've done that as well Fred! Yes, the tube push-fit connector with the red collar might be the problem. The symptoms certainly indicate that extra air is being drawn into the engine after the throttle butterfly. Unfortunately there are many possible sources of this.

Originally Posted by xsport

Worth considering inspection of the alloy inlet manifolds have no cracks or open thread holes .

My alloy manifolds remain in use with the loan chamber so I think that rules them out.

Originally Posted by xsport

... should a smoke test prove to be negative.

I've found a company specialising in smoke testing. They have equipment for hire. I intend to contact them to find out more about this process.

Originally Posted by xsport

So you would not really know that the donor manifold that you tested would be a permanent definitive fix, unless it was used long term.

Good point Stefan. It's still fitted to my engine as the owner doesn't need it back for a while so I'm going to test it on a 250 miles round trip over the next few days.

Originally Posted by xsport

There is also a "one way" valve in the pipe ... This can be checked before removal by the use of a length of hose attatched to the pipe end. It should give 100 % one way seal only. If not then there will be air drawn back into the system.

After reading this Stefan I immediately raised the bonnet, withdrew the brake servo vacuum take-off tube from the manifold chamber and sucked on the end. There was no passage of air from servo to my mouth. Perhaps I was unable to maintain the required amount of suction. I'd be interested to know if yours behaves the same way.

This afternoon I removed the balance VIS actuator. The seal was very flat and I have spares. My preferred solution though is a smoke test so that I don't waste time, money and effort replacing things which are perfectly o.k.

The diagnostic work continues! Thank you all again for your interest.

Simon

[xsport]

Quick question simon. Maybe stocktake would know .. Under the fine clips that ALWAYS break ,whilst removing the motor covers on the vis motors , Is there a gasket ? just another point to mention that this is also a point of possible air ingress. Definite a NO NO to check this as the clips WILL break unless you use the stocktake method. A tempory strip of cellotape around the join would suffice to check whilst in situ . I would bet that there are gaskets under there ,but unfortunately cannot be viewed without dismantle of the vis box itself...

[SD1too]

Originally Posted by xsport

Quick question simon. Maybe stocktake would know .. Under the fine clips that ALWAYS break ,whilst removing the motor covers on the vis motors , Is there a gasket ? just another point to mention that this is also a point of possible air ingress.

I've been discussing the VIS actuators with Stocktake today. I noticed that the "breather" on my balance actuator is a simple hole. All the others I've seen have an insert. This could indeed allow air ingress unless the lid has a perfect seal.

I'll report back in a few days.

Simon