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PostPosted: Tue May 29, 2018 5:03 pm 
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Joined: Wed Jun 25, 2008 6:45 pm
Posts: 151

For background, car is on standard LH2.2, non cat, with a Lucas 5AM AMM. Tuned redbox APC. It runs a Forge twin piston dump valve and has done for several years without any issue. all vacuum lines in engine bay have been replaced within the last year with silicone ones.

My 1991 C900 Carlsson recently developed an intermittent issue where the boost gauge would get "stt ambient pressure and not show a vacuum or boost reading. Despite this the car drove fine and wasn't limited to base boost etc and on first application of what / full boost would "unstick it" and send the boost gauge wanging just into the red as normal and give a very brief stutter. After this the car would drive fine for a few weeks, rinse and repeat and unfortunately I ignored it and drove around the issue when it occurred.

This then led to last week when trying to "unstick" the boost gauge, I gave it some welly and hit the boost overpressure fuel cut off switch (which happen occasionally on a cold morning as its a red box APC that's been turned up slightly. Unfortunately the fuel stayed "cut off" and left me coasting to a stop just outside Bristol on the M4. Confirmed fuel pump wasn't running at road side and got recovered home. Did some digging and suspected a dodgy boost over pressure switch.

Last night I "bypassed" the boost overpressure switch as several others have done on here to prevent the issue occurring again and the car idled and drove as normal under a brief test drive..

Today, when driving under moderate load, the CEL came on (first time ever in 8 years!), kangaroo throttle and the car died. Managed to limp back the few hundred yards home but cannot work out the issue. I have now reconnected the (unstuck) boost over pressure switch so the car is back in the exact configuration it was before the issue occurred.

Car will idle fine (with CEL) if AMM is disconnected, and will idle without CEL with AMM connected, as soon as you apply moderate load / rev from idle, the CEL will come on and after 5 seconds, the engine will die.

My thoughts:

Has all the roadside cranking at the motorway damaged the ECU or the AMM?
Is a low battery voltage (car still starts fine on the key but battery is a little low with all the cranking) confusing the LH system?

I'd appreciate where to go from here with diagnostics as the Bentley manual coveres main the the LH2.4 systems....

1990 900 Carlsson
1973 Sonett III

PostPosted: Tue May 29, 2018 7:38 pm 
Light Pressure Turbo

Joined: Fri Jun 16, 2006 9:26 pm
Posts: 538
Firstly it won't be LH 2.2
Should be Lucas CU14

Check engine light only triggered by fuel system and not boost control (APC) system. So nothing to do with over pressure switch.

Does this car have a Lambda sensor?
If it's a 1990 model it shouldn't, if it's 1991 it should and would also have had a cat although this may have been removed.

My initial thinking is that you've partially blown off a turbo hose causing a massive air leak and causing the Lambda to trigger the CEL (weak mixture)
However if no sensor then it won't be this!
Apart from poor connections about the only other things that will bring on the CEL are faulty AMM or temp sensor.

Sounds like the problem with the boost gauge could be a crushed pipe which opens up with a bit of boost. This could also affect the APC pressure transducer and give inconsistent boost control (this is plumbed into the same line as the over boost switch and boost gauge).

Hope that helps,
Check that it is in fact a Lucas injection system and if it has a Lambda.
Let us know how you get on.

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