Weak Running

[griffsmith]

During a recent trip to the Surrey Rolling road. I discovered that my car a 4.0L 92 precat was running 'leaner' than I believed it should do.. I'm not sure if the AFR could cause damage to my car? or if it is just reducing peak power

I have an AFR & Power vs rpm graph which I have added to my profile. cos I don't know how to add it to this thread.

Is this normal (the AFR graph that is)?? I was surprised to see a sudden change in AFR at 4800rpm, which I believe is restricting power at the top end which I think should occur with an AFR of around 12.5-:1 (ish).?? I have also got low speed, 'traffic surging' which recent threads on here have suggested could be caused by lean mixtures and a possible very occasional lean misfire on accel..

Has anyone got experience of this type of AFR graph? What could be causing it?:confused:

My first thought would be fuel supply (sounds obvious I know!).. The injectors were checked and cleaned by TVR Power only a few weeks before although I did the removal and refitting myself.. Could it be fuel pump / regulator?

I have just bought an in line fuel pressure gauge to see what it tells me. The bible says I should have 33-40 psi (presumably at idle).. but it doesn't say a lot about what should happen when the throttle is opened to the 'rising rate' what fuel pressure should I have at WOT or when the plenum vacuum hose is removed?

Are there any other tests I could carry out?:confused:

Thanks all

[dnb]

The idle fuel pressure is normally quoted with the vac hose to the regulator disconnected so that you read the pressure relative to atmosphere - it's more consistant than the depression in the manifold.

I've never seen an AFR graph quite like that one! It's almost a step change at 4800 RPM.

No idea why it's happening that way unless the pump suddenly decides it can't meet the required pressure above a certain level... But I'd have expected a bit more of a gradual reduction in AFR in this case.

[mongoose]

griffsmith said:

During a recent trip to the Surrey Rolling road. I discovered that my car a 4.0L 92 precat was running 'leaner' than I believed it should do.. I'm not sure if the AFR could cause damage to my car? or if it is just reducing peak power

Your afr is never too lean so as to cause damage,it doesnt even reach stoich(14.7:1),so your not in trouble there.It isnt ideal for max power past the 4.8k mark though.Optimum afr for max power is 12.6:1

>> Edited by mongoose on Saturday 29th October 13:01

[griffsmith]

Cheers guys

I fitted the in line pressure gauge on saturday.. I carried out a few pump priming tests first to ensure it wasn't leaking, and to see what the pressure was this should be max pressure as before the starter turns there is no manifold depression..

The gauge spun up consistently to 38psi.. ok then I went to the engine running tests, both vac tube on and off... the engine wouldn't start not even turn over.. Charged the battery still nothing... now I know its not connected to fitting the gauge.. so another thread may be required need to do some more checks first...

Still on the little info I have on the pressure testing so far.. the pressure rises quickly to 38psi then when the pump stops it drops back to 0 in 2-3 seconds... Now from the Range Rover workshop manual it says the pressure should drop back slowly 10psi/min.. this is a RR spec on a RR 'in tank' pump. The question is, should the TVR be the same should the bleed back be slower is this an indication of a pump issue??