Turbo engine cutting out at high load/boost

[sublimatica]

Mate of mine has recently installed a C20LET engine (turbo 2 litre Vauxhall 4-cyl, fuel injection) in his Caterham and now that it's run in and he's turned up the boost, he's got a major problem at high load. The engine's power suddenly cuts on/off in half-second bursts. If he backs off the throttle he can accelerate through the problem. It's not a mis-fire; the engine goes silent for the moments when there's no power.

It doesn't happen at the same revs every time, nor at the same boost level. (Boost should be up to 20psi but he's only getting max 18psi.) Problem sometimes happens at 15psi and sometimes higher.

I'm thinking it's either a fuel problem or a sensor/ECU problem because the 'kangarooing' is similar to a problem I had on my carburetted Corvette under load, which proved to be caused by fuel starvation. Can anybody suggest anything specific with the limited information I can provide in a post like this?

He's tried a different ECU with no change.
Isn't rev-dependent, so I'm guessing it's not electrical. (?) Engine pulls well right up to the red line if not under heavy load.
Could it be the ECU reacting to a sensor somehow? If a sensor detected knock then would the ECU cut the power completely, or would it just change the fuel/ignition timing?
Fuel pump is a regular one, not uprated. Should it be uprated for the higher output of this engine compared with the 200bhp engine previously installed?

Would appreciate any help/advice anyone can offer.
Thanks,
Ben

[900T-R]

Does the engine management feature an airflow limit-based fuel cut-off, perchance?

[sublimatica]

I'll check. Thanks for the reply.

Why do you ask? Does this sound like a familiar problem?

[MR2Mike]

sublimatica said:

I'll check. Thanks for the reply.

Why do you ask? Does this sound like a familiar problem?

Most OEM ECUs in turbocharged cars will cut the engine if a preset amount of boost pressure is exceeded as a safety feature.

[siztenboots]

Make the plug gaps smaller, 0.8 - 1.1 mm

[900T-R]

MR2Mike said:

sublimatica said:

I'll check. Thanks for the reply.

Why do you ask? Does this sound like a familiar problem?

Most OEM ECUs in turbocharged cars will cut the engine if a preset amount of boost pressure is exceeded as a safety feature.

Or - one step more 'advanced' but a pain in the lower back if you're modifying - limits of 'expected airflow' (in terms of CFM) are set in the engine ECU, rather than just boost pressure. If the ECU detects a level of intake air flow that's outside the expected range for that specific engine, it assumes that something's awry and will shut off fuel supply until that value drops into that range again...

Edited by 900T-R on Tuesday 27th March 10:11

[cnhss1]

the jap scene guys use a Fuel cut defender which fits between the air flow meter and ecu and cheats the signal. essentially the AFM may be flowing more air than its range is capable of and even though the ecu map may be able to cope the AFM cant. When the AFM 'sees' more flow than it thinks it should, it sends a signal to the ecu to cut the spark or fuel or both and often for a failasfe period. on the nissan engines im used to it seems like an age, but is probably only 3-4secs. The fuel cut defender interprets teh signal and keeps the AFM signal to the ecu at its max threshold, rather than going over it. The ecu map must be capable for this to work though.
get it on the rollers and soon, turbo engines dont run badly for long at above stock boost. You may get to see the engine internals sooner than youd like!
As has been said, plug gaps too. My engine specifies 1mm (39thou) but over about 10psi boost, the extra boost blows the candle out! 25 thou cures the problem :-)

[sublimatica]

On Matt's behalf, thank you all for your replies. With your suggestions he's going to investigate the AFM signal and find out whether his needs tweaking or intercepting.

Plugs are already gapped to 0.7mm so guess it can't be that?

Cheers,
Ben

[splatspeed]

knock sensor???

[900T-R]

splatspeed said:

knock sensor???

Will make boost and/or ignition settings revert to 'worst case scenario' if and when it fails, not cut fuel delivery.

[sublimatica]

900T-R said:

splatspeed said:

knock sensor???

Will make boost and/or ignition settings revert to 'worst case scenario' if and when it fails, not cut fuel delivery.

That's good to know - Matt and I discussed this yesterday when he took me for a spin. I wondered whether the knock sensor could trip the ECU into emergency mode and shut down the power. Thanks for clarifying it.

[andygtt]

As mentioned could be as simple a plug gap... you need a smaller gap on a high boost engine than std, my high boost cossie used to do exactly the same if I had a std gap.

There is a balance as wider gap means smoother idle so by reducing the gap it may pop a little on idle.

[iaint]

sublimatica said:

Plugs are already gapped to 0.7mm so guess it can't be that?

You could also look at different temp plugs - standard ones will work at a set range of temps. On my RX7 upping boost has meaant moving to colder plugs to maintain a strong spark.

[stevieturbo]

It sounds exactly like an overboost safety cut....

Why do you think this shouldnt happen ? especially when you say it happened right after you turn the boost up ?

Does the car have some sort of aftermarket chip or fuel/ignition control that might allow it to use more boost "safely" ??

[sublimatica]

stevieturbo said:

It sounds exactly like an overboost safety cut....

Why do you think this shouldnt happen ? especially when you say it happened right after you turn the boost up ?

Does the car have some sort of aftermarket chip or fuel/ignition control that might allow it to use more boost "safely" ??

I'll suggest that Matt checks this. I don't know whether there's an overboost cut-out, but I'd imagine that any problems that could be caused by this would have been anticipated. There are only two other Caterhams that Matt's aware of with this set-up, but it's not a particularly unusual engine or outrageous level of tune in other cars. (It's essentially a Calibra Turbo engine IIRC.)

The problem started happening when Matt had finished running the engine in, and started exploiting all 300 bhp and maximum boost. Before this he hadn't reached such levels of output.

[ultra violent]

I had real issues with irridium plugs (correctly gapped). At £20 a pop and needing 12 of them i was not happy, until I found one of the best non-irridium plugs only costs $2.3 and works perfectly. Happy days...

[sublimatica]

Update:

Matt took the car back to the chap who did the conversion work for him, and they tested a range of things to find the problem.

It turned out to be the ECU. The one originally fitted was a Phase 2 unit and couldn't handle the boost settings. Changing it out for a Phase 3 ECU solved the problem.

They'd ruled out the ECU since they'd tested it in a Calibra with (I think) the same turbo and spec and it had worked, but it seems as though Matt's Caterham performs differently enough to catch the ECU out. Perhaps it's accelerating more quickly and the ECU and sensors aren't reacting quickly enough or something. I don't know.

Important thing now is that the engine works perfectly, so Matt is a happy chappy. If he got off his arse and registered at Pistonheads I'm sure he'd tell you himself how grateful he is for all the helpful replies and emails you posted. Thank you to everyone.