Hi Guys

The boost gauge on my car has started behaving erratically. When I give the car some beans, the needle just jumps about and doesn’t give any meaningful reading. I recently rebuilt the wastegate and the gauge worked fine for a while but has now gone haywire. 
 

Any idea what can cause this, other than a faulty gauge?

An earthing issue with the boost gauge transducer can be a problem in the M100 Elan so maybe, just maybe........ 

Mine has stopped returning to zero. 

I am putting it down to the cold weather!

24 minutes ago, Dave Eds said:

An earthing issue with the boost gauge transducer can be a problem in the M100 Elan so maybe, just maybe........ 

Ah ha! You might have something there! I am currently experiencing weird goings on with the radio and the indicators at the moment and other people have advised that I check the electrics in the cabin to make sure the correct voltage is coming through from the battery. A bad earth was suggested and is something I have yet to look into. It didn’t even occur to me that the boost gauge may also be affected by that problem. Sounds very likely. 

Don't think so - your boost gauge is entirely pneumatic/mechanical unless its non-standard!

 

1 hour ago, 910Esprit said:

Don't think so - your boost gauge is entirely pneumatic/mechanical unless its non-standard!

 

Oh, my mistake. Sorry.

Do you have any idea what might be causing the problem?

Can you still hear the whoosh & flutter as you accelerate / lift off ?

Yes.  But the whoosh and flutter does seem to be quieter than it used to be. 

The boost "signal" comes from a port on the intake plenum.  It is a plastic line that runs through the bulkhead above the backbone frame, and then to a joint on the frame of the binnacle, at least on US spec cars (I'm away from my offline parts manuals, so I am not certain this is carried over to the UK cars - I know the mount for the binnacle is completely different).  This joint has another plastic line that runs from the joint to the gauge.  Each connection along the way has a brass olive that provides the seal to the joint.  I'd start with a visual inspection of the joint at the plenum and see if you can find any places where the plastic line may have been rubbing on something abrasive, got kinked, or maybe got a bit melty from something.  Problems with connections elsewhere would usually cause a low boost reading vs an erratic one, with the exception of the gauge itself.  Sometimes if you have a turbo with a leaky bearing, some oil mist can get in the charge, which would eventually find it's way to the gauge.

Thank you Mike. I will try to find the plastic line and check the connections I can see. 

 

Easy way to test is to remove the line at the plenum connection and apply some air pressure ( e.g. via a football or tyre air pump and see if the gauge responds to pressure applied.  If it does and does not drop when when the line is pressurised but no more air being added then no leaks and any problem is real variation or  the connection to the plenum itself

 

cheers

Rohan

3 hours ago, rgh0 said:

Easy way to test is to remove the line at the plenum connection and apply some air pressure ( e.g. via a football or tyre air pump and see if the gauge responds to pressure applied.  If it does and does not drop when when the line is pressurised but no more air being added then no leaks and any problem is real variation or  the connection to the plenum itself

 

cheers

Rohan

That's great advice, thank you! I will definitely do that.

I had a look for the pipe that goes from the plenum to the boost gauge this evening. Does this look like the one? (The thin black plastic pipe with the brass connector)

Maybe - It connects on a right angle union on the front side of the Plenum.  EG just to the left of the alternator.  Wouldn't normally be a connector there, but that's clearly fairly recent.  Perhaps it got damaged & spliced. 

NB What's that strange thread dropping vertically down to the alternator?? i

Edited January 30, 20232 yr by 910Esprit

That looks like the right pipe.  There is another tube just like it for oil pressure, so you might want to poke around a bit more to be sure you have the right one.  The oil pressure pipe taps off the block below the distributor.  As Steve noted, the brass connection is not standard.

On 30/01/2023 at 21:51, 910Esprit said:

NB What's that strange thread dropping vertically down to the alternator?? i

Ha ha, I think it's a cotton thread hanging down from somewhere.

Well, unbelievably the boost gauge has been performing properly for the last week or so. So perhaps I disturbed something when I was fiddling around.

Now that it's working, I don't really want to start taking things to bits. I will keep an eye on it and if it starts playing up again I will follow your advice.