930 boost cuts out

[SportChief]

Hello, I figured I would post this on the ROW market as I an in the US and there are a few differences in cars made for ROW.

I purchased an 88 930 from Europe and delivered it to the US. Car was running great and the previous owner installed a few upgrades, rarely8 headers, rebuilt k27, high flow exhaust. I installed a fabspeed intercooler and the first time I drove it boost was great till 5200 rpm. Came home found a few air leaks and fixed the hole in the turbo pipe, replaced the poly head gaskets, grounded the overboost wire, rear fuel pump replaced as it was not strong enough to keep the car running, waste gate diaphram was pinched with a small hole so I replaced it with a Tial 1 bar spring.

I am at a loss of what could still be causing the car to loose boost above 5200 rpm.

Thanks,

[Type911]

Car still runs just no boost?

Edited October 29, 2025 by Type911

[SportChief]

It will run on boost from 3500-5200 on full throttle. I can modulate the throttle and maintain some boost till 6000 rpm

[Type911]

I would do a smoke test on it

[SportChief]

Thanks Type911, 

smoke test was done and fixed them such as small leaks. I was expecting to find a big leak.

I grounded the overboost wire and it didn’t change anything. Weird…. 

[flatsix777]

That seems quite an odd issue so I don’t really know. You seem across air leaks, the normal issue, but a smoke test is only going to show an issue off boost and not if it only occurs when pressurised. Still odd as it runs OK on full boost below 5,200. Maybe change every O Ring if not done, including the one under the recirc assembly if it’s still stock. May be worth  checking the seals in the recirc assembly too and that the piston moves OK. 

Getting enough fuel could be the issue as it only happens on boost and high rpm when you need the most but I presume you’re happy with flow from the pumps if you’ve replaces the rear one. Good to look at fuel pressures but  you’ve probably done this too.

I’d check the ignition system if it’s more rpm related. The standard rotor arm includes a mechanical limiter but it should be set well above 5.200 but I I think you can get them for lower limits too. I’d change with the cap and leads to rule out a high load/rpm ignition issue.

I wouldn’t think it’s an overboost issue as it’s be a problem well below 5,200 with a wide open throttle. Anyway, grounding the send I should rule it out. 

Not much help I’m afraid, but that’s what I’d look at. I think it’s one of those where you have to systematically test/replace induction, fuel and ignition to verify all is OK and in spec. It’s had a lot of recent work and mods so maybe something isn’t quite right.

David

[Type911]

Yes, sorry, it needs some pressure when smoke testing, somehow get it up to 15 psi, as David says, static all seals might be fine but leak under pressure

[SportChief]

Thank you Flatsix777 and Type911. 

All of your insight is helpful allowing me to think through the possible options. Once I find the issue I will post back what I find. 

Cheers!

Dwight

[SportChief]

No solution yet. I had a boost gauge and it would climb to 1.3 bar on a 1 bar Tial wastegate. 

Dwight

[carrpet]

Hi there, I have an '89 930.66 with a K27, Fabspeed intercooler and a Tial wastegate. I also experienced cut out on overboost. The key learning was that a Tial wastegate with a 1 bar spring delivers more than 1 bar boost pressure. I had a lot of dialogue with the Tial technical department about this ..spring specs etc. They recommended Tru Boost see below

I fixed the issue by doing the following;

Grounded out the overboost switch

Replaced the spring in the Tial from a 1 bar spring to a 0.7bar spring

Installed an AEM Tru boost controller set to 1 bar boost pressure

I also removed the high rpm cut off switch from the rotor arm to rule this out.

Now I have perfect boost control. 

I also have microsquirt so that I can analyse the log for high boost events to confirm all parameters are in-control

I hope that helps

[carrpet]

On 01/11/2025 at 01:23, flatsix777 said:

That seems quite an odd issue so I don’t really know. You seem across air leaks, the normal issue, but a smoke test is only going to show an issue off boost and not if it only occurs when pressurised. Still odd as it runs OK on full boost below 5,200. Maybe change every O Ring if not done, including the one under the recirc assembly if it’s still stock. May be worth  checking the seals in the recirc assembly too and that the piston moves OK. 

Getting enough fuel could be the issue as it only happens on boost and high rpm when you need the most but I presume you’re happy with flow from the pumps if you’ve replaces the rear one. Good to look at fuel pressures but  you’ve probably done this too.

I’d check the ignition system if it’s more rpm related. The standard rotor arm includes a mechanical limiter but it should be set well above 5.200 but I I think you can get them for lower limits too. I’d change with the cap and leads to rule out a high load/rpm ignition issue.

I wouldn’t think it’s an overboost issue as it’s be a problem well below 5,200 with a wide open throttle. Anyway, grounding the send I should rule it out. 

Not much help I’m afraid, but that’s what I’d look at. I think it’s one of those where you have to systematically test/replace induction, fuel and ignition to verify all is OK and in spec. It’s had a lot of recent work and mods so maybe something isn’t quite right.

David

Hi David

The installation of the Fabspeed Intercooler replaces all of the standard recirculation hardware so no piston seals etc. Also the cast aluminium boost pipe from the turbocharger outlet is replaced with a moulded silicone hose so no seals involved. There is also a short silicone hose connecting the IC with the throttle body. All of the silicone hoses have hose-clips so very easy to check for tightness. The Fabspeed IC installation includes the installation of a turbo boost dump / bypass valve . The orginal Fabspeed item is made out of black plastic. Forge make a beautifully engineered alternative. This could be a point of failure.

Having said that I have carried out a smoke test on my intake system and it did identify a leak in an aluminium weld to incorporate a threaded bung to accommodate an air intake temperature sensor for microsquirt.

 

Edited December 4, 2025 by carrpet

[carrpet]

One of the risks you face is that your issue may be fuel starvation under full boost at high revs (WOT) in which case you are going to be getting very high AFRs which may cause piston damage.

Data collection in these situations is critical. Microsquirt offers you the ability to log rpm, AFR, manifold pressure (Boost) and fuel pressure which is critical to the Bosch K-Jetronic fuel injection system. If you had all of this data we probably wouldn't be having this conversation because the answer would be quite obvious.The AEM gauges are easy to integrate into MS.

If you have removed the rev limiter on the rotor arm and earthed out the overboost switch my best guess is fuel starvation under load which means high AFRs which is not good news.

If you want any help with MS please let me know

 

 

 

Edited December 12, 2025 by carrpet