rich misfire?

[350Matt]

Chaps

I thought I'd ask this little poser, my 4.3 precat griff has developed a misfire which comes and goes, now as I recently fitted my wide band lambda kit I know that its going rich by about 20-25% before it starts missing, its worse at low throttle positions and it you give it full throttle it does clear it for a little while then creeps back in . then all of a sudden the mix will return to normal and all is well again?
The car apparently had a Mark adams chip at some point but other my wideband kit its currently standard

My first thought is the coolant temp sensor has gone awol but upon measuring it while it is a bit out its not hugely out. I have swapped it for a spare but not had a chance to drive the car since

any other ideas?

Edited by 350Matt on Friday 27th July 09:01

[trackcar]

Have you seen the thread about emerald group buys? there's yor answer

[GreenV8S]

If the problem is intermittant you would need to measure it (ideally at the ECU plug) while it was occuring in order to pick up the problem. Open circuit corresponds to very cold so intermittent wiring faults could cause this sort of symptom. From your description I guess that the problem occurs outside the lambda control region (i.e. over 2500 rpm or whatever the threshold is set to) iin which case you could rule out narrowband sensor faults, but it might be worth checking that the narrowband sensors are cycling sensibly. If the ECU thinks it has detected a major fault it might be dropping into 'limp home' mode which is likely to be rich. Is the fuel pressure consistent and correct? Vac hose connected and not leaking etc? Monitor the injector duty cycle to make sure the problem is the ECU adding too much fuel rather than some other fundamental fault such as a blocked intake, trumpet falling out of the base etc.

[spend]

GreenV8S said:

...Vac hose connected and not leaking etc? ..

That would weaken the mix wouldn't it?

Collapsing intake hoses might momentarily richen but unlikely to have the effects stated.

Dave

[GreenV8S]

spend said:

GreenV8S said:

...Vac hose connected and not leaking etc? ..

That would weaken the mix wouldn't it?

Collapsing intake hoses might momentarily richen but unlikely to have the effects stated.

Dave

Might, might not. If the vac line to the fuel pressure regulator is playing up the pressure might be going up or down - or failing to go up and down correctly as the manifold pressure changes. A collapsed air intake should act a lot like a partially closed throttle but might cause the air flow meter to misread.

[350Matt]

Chaps I should point out that its a precat so no dodgy lambda sensors are on it......

Also all intake hoses are shiny silicone or new rubber

And thanks Jools I know emerald is the way..... ( helped develop it, mutters), just a bit short of readies at the mo'

Matt

Edited by 350Matt on Thursday 26th July 18:53

[spend]

As a shot in the dark, is it worth cleaning / greasing all the associated connectors first, and a check for wiring continuity across the major components (whilst giving them a good wiggle )?

Dave

[griffter]

Sounds a bit like my problem. One bank of injectors opens full duration due to the ecu corrupting. What causes the ecu to corrupt seems to be a momentary break in the ignition switched 12v supply. So momentary the engine doesn't cut out, but enough to corrupt the ecu.
To diagnose you need to observe injector pulse duration on the move - easiest with a 12v LED across each bank of injectors. You can do this on the ecu plug. You'll see one LED brighter than the other at part throttle during misfire episodes. At full throttle (and over 4k ish revs) they'll both look the same as both banks should be open full duration.
To solve is a bit harder. Go over every inch of the loom, checking and cleaning connections, then start replacing relays and ignition switch if necessary.
PM me or check my previous posts on the subject for more info if this sounds possible.

[rev-erend]

Matt - maybe the fuel pump is on it's way out ..

Check & monitor the pressure

Also could be a sensor giving a bad signal to the ecu which reverts to limp home mode a fault code reader should show if this has happened.