1.6k - strange intermittent 'sucking' noise

[revilla]

OK, possibly some progress ...

Didn't want to cut into the fuel line to install a pressure gauge unless I had to and didn't want to have to rob mine, so I swapped the FPR out for a known-good one, started it up and it was sneezing right from the start, no different. Then I disconnected the return line, put it into a glass jar and let the pump prime. It sounded normal, you could hear the pump coming under load as the pressure increased then settling as the pressure stabilised. It pushed a good amount of fuel past the the regulator into the jar too, so I think we can assume the fuel pressure is not far off.

Pulled the vaccum lines off the TBs, allowing an air leak and leaner mixture, not really much difference.

Pulled the vacuum line off the regulator and plugged the pipe, in theory that should raise the fuel pressure and enrich it a bit, but again no real noticeable difference.

Lambda sensor unplugged, no difference.

Scope on the lambda sensor, not cycling, low voltage showing lean condition all of the time, no closed loop.

So (probably should say at this point "don't try this at home when it's popping and banging with flames in the inlet tract!") I tried spraying a light haze of Easy Start over the mouths of the trumpets while it was idling. Idle speed immediately jumped up, the idle evened out and the sneezing stopped! In fact, if I judged the dose just right for a few seconds I could even induce the lambda sensor to cycle as the ECU went closed-loop,

So it is a lean mixture problem.

Next thing I tried was to put a big mat over the trumpets to make sure they were breathing only fresh air from below, then I sprayed Easy Start around the inlet manifold gasket area. Result, same - increase in idle speed, a much smoother happier idle and the chuffing and sneezing stopped. Repeated the experiment several times and it was pretty convincing and consistent.

So it's drawing in air somewhere where it shouldn't be. I guess probably the inlet manifold gasket, but also worth a good look over the two manifolds for cracks etc. It was hard to precisely isolate the area. Maybe something less volatile like WD40 will be a bit easier to localise.

Looks like I need to pull the inlets off for a thorough inspection.

At least I feel like I'm on its tail now.

Thanks for all the pointers.

[SM25T]

Great work Detective Revill

[revilla]

Got it, I think. Inlet manifold gasket. A quick squirt of WD40 along the top edge of the gasket line and it settles down a lot. And when it does start to pop again, you get little bursts of oil spray off the top of the gasket. I've got a spare. Will let it cool down, drop the coolant and whip it all off.

[SLR No.77]

Andrew, is the support strut in place underneath the throttle bodies?

Stu.

[Wrightpayne]

Great work again - I'm glad I live close by ;-)

[7 wonders of the world]

Carb cleaner / spray never fails  

, possibly previously overtightened manifold - may need a little flatting off

 

[revilla]

Stu, no I can't see one. I know with the heavy VVC plenum it eats the gasket if there's no strut. I'm not familiar with what is usually used with these TBs. I believe they're the KV6 ones. I did run my hands underneath to feel if there was anything and wondered about it. There might be something I've missed, I'll know when it take it off. What strut to do they usually use?

[revilla]

Err...

No strut. One bolt missing. Other bolts not properly tight. Vibration has utterly destroyed the gasket!

 

 

[SLR No.77]

There were two setups using KV6 TBs, same bodies but different manifold. I think the one you're working on is the PTP version, I only have experience with the other one although I would expect as far as struts are concerned they're very similar.  

There's various drillings in the block directly below the throttle bodies, the strut should be anchored to one of these and then bolt to the underside of the inlet manifold, likely in the centre, but not to the TBs themselves. When I fitted KV6s to my old car, the same strut that had supported my old 1400SS aluminium plenum fitted perfectly with the TBs, I can only assume the same part was used.

From memory, it was a steel tube, probably 12-18mm diameter, with the ends flattened and bent over to an angle at each end to fit flat against the block and the end reside of the manifold, it sounds a bit crude but it was a properly manufactured tube and very strong. I've seen a KV6 manifold with the inlet flange broken off due to no support bracket being fitted, I think Neil has got away lightly.

Stu.

[SLR No.77]

This is the bracket listed on Caterham's website for VHPD TBs, I recall buying one of these but it didn't seem to fit anywhere, whereas my 1400 one fitted perfectly! The 1400 version was as I described, a reworked tube and also cadmium plated rather than powder coated, it might have therefore been an original Rover part.

Stu.

[oilyhands]

Lean fuelling due to air ingress.. what a pain. Make sure you get a decent gasket too, I dont think the KV6 type support brackets are available anymore so ti will need something fabricating.

Oily

[revilla]

 

Thanks for all the helpful info and suggestions.

[revilla]

 

Thanks for all the helpful info and suggestions.

[Tazio]

A great detective story, well done Andrew.

[TomB]

Very interesting bit of detective work! I’m not sure I’ve got an inlet manifold gasket for my SLR TB, I can’t remember if I fitted one or not. Hen I last had it all apart, I made up this drawing of the support bar for my KV6 TB for someone on here. It’s not a perfect engineering drawing but the best I could do.

[21jigsaw]

Andrew,

Was there a support strut fitted to both throttle bodies?

Dave

[revilla]

No there wasn't. I think that was half the problem. The other half of the problem is that the nuts and bolts were a mixture of missing and loose! I've told Neil it really needs a support strut or it will just happen again. I'm fairly sure when I looked that there was only provision for a strut under one of the throttle body pairs (it was a single piece solid manifold) (can't check now though, the car has gone home!).

[oilyhands]

It's a solid one piece manifold and it can be quite tricky to access some of the bolts because of the IACV rail. ISTR a single plug at the front underneath for a stay.

Oily

[pgm]

Having read through all the posts it stirred a memory of a similar problem from many years ago. The fault wasn't quite as pronounced as this one but it did produce a strange squeaking sound at idle. Initially thought it was a cam tensioner but after much investigation it turned out to be the inlet gasket. I had personally never heard an inlet gasket create a noise like it. Can't remember if the manifold support was missing though. Well done on sorting the issue though!