3.5 Hotwire V8 running rich... any ideas?

Howdy,
I am running a 3.5L Disco V8 in a hybrid on Hotwire injection and the bugger won't run right It has the correct 3.5 hotwire ECU (as fitted to a handful of early Discoveries) and the electrical values seem to check out OK. So far it has had all the sensors buzzed back to the ECU plug to check that there is continuity and had their resistances / voltages measured.... and they all seem to be registering pretty much right AFM CO trim has been set as per RPi suggestion to 1.5V, throttle pot to 0.35V at rest etc. and the temp. sensors measure similar resistance at cold, which then alters as they warm - values look about tight. The system is not running Lambdas and does have the correct tune select resistor. The symptoms I am suffering are reasonable thirst (well it is a V8 ) but more so I am getting smuts out of the exhaust - the back of the vehicle actually has flecks of soot on it! The biggest symptom though is spitting/hesitating under throttle - it struggles to rev towards the limiter and spits back (sounds like it is spitting into the plenum), feeling like it is choking up under heavy throttle. Light throttle loads are OK and it runs nicely. Stepper motor is working as it should. I am pretty much out of ideas :doh:..... anyone help with any suggestions?

Have you checked or changed, spark plugs, HT leads, ignition amp?

Yes. Had a new set of leads, new coil, new Ign. module, spark plugs, swapped the dizzy cap and rotor arm for spares - no change to symptoms. Checked spark the other day - big fat spark right up the rev range, no obvious deterioration.

Ok then....try disconnecting the cold start injector. I know you have tested the temp senders but this would eliminate this completely.

It wont have a cold start injector on a hotwire,that was on the old 3.5 flappers.Check the resistance of the coolant temp sensor when the engine is up to temp.Cant remember what the value should drop down to.Probably about 40 - 60 ohms.Another common fault is for the coolant temp plug to get swopped with the fuel temp plug - they are easy to mix up.

eightinavee said:

It wont have a cold start injector on a hotwire,that was on the old 3.5 flappers.

Click to expand...

OK, my mistake

If you also have a bit of trouble with the starting with the engine spluttering, as i did on my 3.5, then u have props within the engine ECU which I had, and to get the ECU referb'd by Lucas but that was a few years a go.

No problems with it starting, starts on the button hot or cold and settles to a normal idle - MoT test didn't show any emissions concerns either. I had previously considered swapped plugs on fuel and coolant temp sensors, so checked that again recently at the ECU and they are wired correctly. Will have to check resistance on coolant temp sensor when hot later.
Curiously it does its spitting through all engine temp. conditions, hot or cold - it seems to be slightly improved if the ambient air temp is cooler or it is dewy etc. .... which is part of what has led me to think it is over rich. I am baffled, but there are some good suggestions above, thank you. Is there a reference site anywhere for the hotwire set up that lists sensor ECU and voltage inputs / outputs?? This should be such a simple system when it is running and this install runs so well other than at wide open or heavy throttle.... very odd.

Get your ECU tested by substituting it with one that is known to work.

Also, check inlet side for any possible leaks, make sure your ignition timing is spot on. have you checked/set base idle? I presume air filter in good order and engine breathers clear.

If I have any other thoughts I'll let ya know.

Had a thought, probably nothing but have you done a compression test?