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Are the boost gauges accurate?


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When I bought my 84 Turbo (in 2013), it would overboost. You will tell that I am no mechanic if I tell you that I used to just free off the rod thingy under the turbo unit to cure it. 

With use, it was fine for several years. If I did not use it for a week or two it would need a clout again. Eventually, I had it professionally rebuilt.

It now performs perfectly and the boost indicator goes just past the vertical. Not sure what number that is. I am indoors in the warm and the car isn't!

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Oh and to answer the original question, my (Smiths?) boost gauge seems to agree pretty well with the MAP sensor for my EFI, at least close enough I've never noticed a difference. But these cars are old, I'd tee in a cheap gauge to check against the one in the dash.

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Thanks again guys. Before I dive in and start taking the wastegate to pieces, can the spring and diaphragm be changed without having to remove the whole wastegate unit from the exhaust? Likewise, can the shaft of the valve be cleaned/lubricated without removing the whole thing from the exhaust?

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26 minutes ago, GreenGoddess said:

can the spring and diaphragm be changed without having to remove the whole wastegate unit from the exhaust?

Yes.  We have replaced our diaphragm...retained the original spring.

https://www.sjsportscars.com/parts-and-accessories/A910E6571F.htm

 

Regards the shaft, this was one shop's solution

 

https://www.lotustalk.com/threads/88-turbo-esprit-runability-issue.121943/

 

When we refreshed our engine, I took the turbo to a couple of truck repair shops and no one could point me towards a place that would turn the shaft and install a matching bushing...so I left it...

 

Should be easy enough to feel any binding, with the diaphragm out.

 

 

 

 

 

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Atwell Haines

'88 Esprit

Succasunna, NJ USA

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  • 2 weeks later...

I finally got around to removing the wastegate this weekend. It was a total nightmare to get off (took me around 6 or 7 hours just to remove the thing from the exhaust using nut splitters and every other tool in my garage).

It was absolutely seized solid with exhaust soot. It must have been like that for a very long time. In fact, I wouldn't mind betting that was the reason why my car had an engine rebuild before I bought it. The previous owner had probably been driving with 11.5psi of boost for ages.

Anyway, I got it all cleaned up and put it back on the car today. The fibre gasket that sits between it and the exhaust is in terrible shape so I plan on removing it and fitting a new gasket next weekend, along with stainless nuts and washers so that I can easily remove the thing next time.

I took it out for a test drive and the overboost problem has definitely gone. However, now I'm only getting around 6psi max boost. Do you think the ruined fibre gasket could be to blame for that? I personally can't see how a bad gasket would cause low boost because it's the opening of the wastegate valve itself that regulates the pressure doesn't it? The only other thing I can think of that would reduce the boost is a weakened spring in the wastegate. 

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As you assume, the gasket is irrelevant to boost as the only gas going through that part of the manifold is the gas that has already been released by the wastegate valve.  As we know your turbo can produce 11.5, so I'd agree that the spring is the most likely culprit. 

Edited by 910Esprit
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1 hour ago, 910Esprit said:

As you assume, the gasket is irrelevant to boost as the only gas going through that part of the manifold is the gas that has already been released by the wastegate valve.  As we know your turbo can produce 11.5, so I'd agree that the spring is the most likely culprit. 

Thank you. The only other thing I can think of is the way the valve is seated in the exhaust. If it’s not seated quite right and is leaking around the edge, would that limit the boost? My limited mechanical brainpower thinks not but I’d be interested in hearing another opinion. 

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It's very common (I would almost say standard) for the seat in the adapter manifold to be cracked and degraded, but any leakage, at the seat, also reduces the overall pressure in the exhaust turbine and therefore the boost detection pipe (located in the turbo compressor housing), which in turn will slow down the opening of the valve (cos there is less pressure acting on the diaphragm), thereby counteracting the leakage.   

My assumption is that boost will be moved higher up the rev range, but the overall max boost will be unaffected.     

 

Edited by 910Esprit
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Thank you Steve, that's excellent info. I did notice some cracking had started around the valve seat in the adapter manifold but it wasn't terribly bad.

Do you mind me asking what the small metal pipe that exits the elbow at the upper part of the wastegate is for? I am wondering what would happen to boost if it was leaking (I don't think it is, but I'd be interested to know).

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The metal pipe is connected directly to the turbo compressor, so that the wastegate 'sees' what the boost pressure is.  i.e., if the air passed through this pipe is greater than atmospheric pressure, it tries to open the wastegate by deflecting the diaphragm, which in turn is counteracted by the wastegate spring to determine how much air pressure is required to overcome the spring tension - eg the boost pressure you see on the gauge.   

So...  A strong spring gives more boost by holding the valve shut longer and a lighter/worn spring would give less boost by opening sooner 

 

Edited by 910Esprit
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Sorry to keep reviving this thread. I have another 3 questions:

1. I noticed when taking the wastegate apart that there was a paper gasket between the diaphragm and the cast iron upper part of the wastegate. Should there also be a second paper gasket on the other side of the diaphragm?

2. If the area around where the diaphragm is clamped is leaking air, would that reduce boost?

3. When reassembling the wastegate, would you recommend using Hylomar either side of the diaphragm seal area? 

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2. If the diaphragm edge is leaking air, (lots of it) you would end up with more boost not less. Pressure from the compressor scroll/manifold is what opens the valve and allows exhaust to bypass the turbine, taking away speed from the compressor that would have built excess boost.

Not sure on the other Q's; given the heat on the exhaust side of the wastegate, the paper might have been there to protect the diaphragm or prevent it from fusing to the hot cast iron? My memory of the diaphragm is that it's soft and rubbery, and should seal just fine when compressed without sealant, also not sure how Hylomar takes to hot conditions.

 

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Thanks guys. That’s great to know. I can stop worrying about a leak around the diaphragm in that case as I now have low boost, not excess boost. 
The paper gasket is definitely supposed to be there (SJ Sportscars sell them) and I have one on order. I was just wondering if there was supposed to be two paper gaskets. I will cross Hylomar off the list. As you say, it would probably make it more difficult to separate next time. 

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Hi Guys,

I rebuilt the wastegate today with new gaskets and a new spring. I also used stainless steel flange nuts to attach it to the exhaust so that it will be easier to remove next time. I took it for a drive and it is absolutely perfect! The boost comes on strong and steady and tops out at exactly 7.5psi as it should. This is the best the car has ever driven. 

Photo of old spring versus new spring attached. You can see that the old spring is shorter than the new spring as it has obviously lost some strength over the years.

Thank you all so much for your help. 

IMG_4735.JPG

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