Thread: 3.9v8 with motec stalls at low revs.

IMHO it is probably the MOTEC tune.

I have been told that MOTEC are primarily designed as a racing ECU, and do not have the multitude of strategies that a 14CUX (3.9 injection ) has.

I know I saw a mate trying to start his 4.6 MOTEC for 5 minutes or so on a hot day at Stocton, when my 14CUX started first turn.

Its hard to say whether it has an idle valve problem as it may not even have one.Have you checked to see whether it has one and it is connected?

The MOTEC is run by the TPS , a MAP sensor and temp sensor at the intake. Any change in tune of the engine from wear etc is not automatically compensated for like with a MAF sensor run injection, so it may simply need a tune to compensate for engine changes /wear from when it was last done.
It may just need an electric tune up of new plugs, leads, rotor and dizzy cap. Seeing this is the cheapest, it is probably the place to start.

If you are in Sydney, Ward at Graeme Cooper has fitted quite a few AFAIK , so could redo a dyno tune.

Now I see you are in ZA, proably a bit hard to get to Sydney.
Regards Philip A

It is unlikely that the Motec would be out of tune. I am assuming that the system was installed when the EFI for the 3.9 was installed. Was it a Graeme Cooper Automotive installation?

The symptions sound like the vacuum issue. The thing to check is a small tube at the back of the plenum which goes to a sensor usually on the LHS rear of the plenum chamber, you have to reach around the back of the chamber to feel it. This tube has a reputation of splitting, or at least on my 4.6 Motec it does, and needs to be replaced every couple of years.

There are benefits and problems with both Motec and the OEM 14CUX systems. The Motec does not have the repeated failures of the MAS so common on the 14CUX. On the M48 Motec you can have dual mapping which is not possible on the OEM 14CUX system. My Motec M48 has a second map connected to the LPG change over so that I can run on one map for petrol and a second map for LPG rather than having a compromise setting for both. I can even re-programme the engine on the move and if I wanted could store different profiles for different conditions and load them up for the activity I'm about to embark on, like a performance profile and a towing profile. To do the same on the 14CUX system you have to burn a new chip.

The hard starting issue on the beach is likely that the new fuel line position being too close to the rear of the RHS manifold and getting vapour lock, or as happened to my Motec 4.6 recently, some unknowing mechanic has installed coolant temp sensor plug onto an adjacent but redundant coolant temp/timer sensor.

Diana

Thanks, really appreciate the replies as I don't know much about Motec. Idle control valve is still there so will replace it just to see if there's a difference.

Wiring is all over the place, very difficult to work out what's what, and as mentioned before, noticed that the coil is Toyota, as is the fuel pump and igniter.

Wasn't very clear before, it idles ok after a bit, but the revs drop as soon as 'R' or 'D' is selected on the 'box.

Are maps readily available for the 3.9? Not sure if the guy who did this one really knows what he was doing.

Cheers!

Motec M48 is computer programable, the best way to map your vehicle is to find someone with the programming cable and the software. If you can on a dyno. (I have both and can modify the ECU parameters on the road if I want.)

If you look where the harness enters the Motec ECU box, you should find an RS232 plug. This is where the programming cable hooks up.

Graeme Cooper Automotive in St Peters (Sydney) can do it, but it's a bit far from South Africa to drive there!

The type of coil is not an issue for the Motec ECU.

Hi!
Spoke to mapper, he says the Motec can't 'talk' to the LR idle control valve, so the cold start problem will remain unless I fit a different valve that can be controlled by the ecu.

His solution - let the car warm up before driving!

A simple solution to idle-up an engine is to find a suitable solenoid valve, connect it across the throttle, manually switch it on for fast idle. Various vehicles from the 80's Including EFI RRC's) had idle-up solenoids connected to the air conditioner clutch. Shouldn't be hard to find one.

Originally Posted by Leo

Hi!
Spoke to mapper, he says the Motec can't 'talk' to the LR idle control valve, so the cold start problem will remain unless I fit a different valve that can be controlled by the ecu.

His solution - let the car warm up before driving!

Do you seriously believe that a Motec over a Rover V8 will never idle.

My advice, find a different mapper! Someone who has installed Motec on Rover V8s.

BTW: If the Idle control valve is the one on the RHS of the plenum, it is redunant on the Motec.