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MG MGB GT V8 Factory Originals Technical - flooding 3.9 with FI

I've put about 5K on my motor scince new plugs, cap, rotor, wires and ignition module. Car ran great started first time in about 3 seconds of cranking.

Over the last few weeks it began to be hard to start (would sometimes backfire through the plenum) and developed a stumble of idle. So I pulled a plug, it looked good kind of light brown in color.

The car has got progressivly worse and now won't start at all. Plug are flouled with fuel after a few seconds (10 or so) of cranking. Kicks every now and then but won't start.

This is the first time I've seen it flood so bad, I mean smelly.

I have spark, tested with grounded spark plug. Blueish yellow spark.
It will cause inductive timing light to fire. My Ignition module (only 6 months old) is still on the distributor. My engine bay runs a little hot. Do the modules "weaken" or do they just fail.
I tested the fuel pressure at the rail. about 48psi with engine off.
I tested the input sensors, TPS, Stepper, temp, MAF. All good.
Even swapped ECU for known good one.

So I'm thinking it's ignition related as the FI all checked good.

Any ideas, I'm stumped...


M Mallaby

Mark. you fuel pressure seems very high, TVR recommended pressure was 36/40 psi at idle, I run 35psi on my V8 , have you got an adjustable regulator ?
M Barnfather

I think at idle mine is about 34 psi. RR says 34 - 37 at idle. With the engine not running it reads higher as the vacuum lowers FP at idle.
M Mallaby

If I may, the engine is not flooding (maybe) this is very hard to happen on a FI engine.

Fuel pressure should always be tested warm an at idle, fuel is at a deadlock so it will read high. There two ways to test fuel pressure, one while cranking, to test pump, the other while is running. Your problem sounds like electrical and yes the first thing to check would be the module. Also the coil or if you have MAP, a bad MAP will register as rich mixture and sometimes, but rare will appear as flooding.
Bill Guzman

Also, the water temp sensor (top of engine, behind top hose) may be giving duff readings , a meter should show a resistance of 2500 ohms or so cold, dropping to 200/400 ohms when the engine has reached running temp.
M Barnfather

Mr. Barnfather makes a good point. The thermistors tends to degrade over time. The resistance over temperature range should be as follows:

Temperature Resistance
(deg F) (ohms)
14 7,000-12,000
68 2,000-3,000
122 700-1,000
176 200-400
185+ Less than 200

Another possibility is a leaky injector or cold start injector. I've seen cases where they continuously dribble fuel. Makes for hard starting because there is not only too much fuel, but it is also poorly atomized.
Steve Mc

Minor correction the FP is at 38psi not 48, my bad.

Bill, I am running the 14CUX stock RR with all connections but no MAP sensor. Lots of fuel in the cyliders, I suspect your right about the electrical being the culprit.

Barn & Steve
Measured 2310 ohms cold. So it seems OK.

Dose anyone know how to test the Amplifier module short of parts swaping?
M Mallaby

Mark, It's the hot Ohms you need to check....the higher the ohms, the longer the injectors run....You could try dis-connecting the temp sensor and see if it leans out.
M Barnfather

Hi Barn,

I would give my left nut to be able to test the hot ohms. Car won't even start never mind run. My research to this point is telling me that these ignition systems are pretty frail at the best of times and component quality really counts.
M Mallaby

Testing the sensor through it's temperature range isn't too difficult. Try this: Dangle the sensor by a string or wire in a pot of water on the stove with the ohmmeter connected to the sender. Put a candy thermometer in the pot and turn the stove on. Monitor the ohmmeter and the candy thermometer up to the point the water boils. You've got the full opreating resistance range of the sensor.

On the igition side, I ditched the OPUS system in favor of a rebuilt Buick distributor with a pertronix system. The only minor issue is that you'll have to replace the oil pump drive to a Buick unit. The Rover pump drive is female and the distributor male. The Buick is the other way around, go figure. D&D provided the distributor with the Pertronix kit installed for $200. They even tossed in an oil pump drive as a freebie. One of the nice things is that the cap and rotor are Chevy small block. Available anywhere and cheap. If the Pertronix ever gives me trouble, I can drop in a Chevy small block set of points and get myself home, too.
Steve Mc

Steve, the pump drive on the Rover engine was switched around when the SD1 engine came out, the previous P5 and P6 Rover saloons had a male drive, subsequent cars (Sd1, Range Rover etc) had this new arrangement.

Any further thoughts on Mark's problem.? certainly seems more likely to be ignition rather than fuel, and the Lucas system with the amplifier on the distributor was supposed to be more prone to failure than the one with the amplifier on the coil.

I think I would risk replacing amplifier & coil, if I was retaining the Lucas system.

I have a nearly new Lucas 35DM8 distributor, and coil with attached amplifier to sell, as I've just switched to a 123Ignition from Holland.

Shipping costs would probably make a deal prohibitive, but I think that's what you may well need.
M Barnfather

Are you are getting nice big fat sparks at the plugs?
Unplug each temperature sensor in turn. This will look like a cold engine (infinite resistance) If the car starts from cold then there's your problem.
If those are OK and since you are getting back fires then next most likly is the timing.
Peter Sherman


Cleaned the MAF, assured it is not grounded, new wires. regapped the reluctor. Still no start. I do get spark just not enough. Ordered a nre ignition amp module. We'll see... I hate parts swaping.
M Mallaby

Just to throw this out there: Does the coil have the capasitor wired up? It should be grounded next to the coil and hooked up to the positive side of the coil. This is to prevent spikes to the distributor.

I also burnt out my ingition module a few years back for no apparent reason, and I did not have the coil capasitor wired in.
Just a thought...
Wally Jonker

Go buy a new coil.
Even if it is not that, the money isn't wasted. When you get is going again, even if that was not the cause, it is an upgrade and will make the engine go better.


Still not running with new Ignition module. I did install a capacitor at the coil. I checked the coil with an ohm meter and it checked out OK. I hvae spark, Still pouring fuel into the cylinders. It maybe time to send her of to the Rangie guys in Encenitas to diagnose and fix the problem. Have any FI gurus out there?
M Mallaby

A long shot,
Unplug the computer for a while and reconnect. A kind of "reboot"

Is there any possible way that (over the last few weeks especially) a ground connection somewhere could have been getting progressively worse?

Also you mentioned that you are using the 14 CUX... Is the on board diagnostic display reading any codes?

I would agree with Peter, get a new coil. Not much money, and you'd be sure you're not getting any false readings.
Wally Jonker

Another long shot here I don't know what year your MGB is but my 78 had a problem with the diode in the emergency brake circuit doing something similar. I believe the diode is in all late model (74 1/2 & up RB models) The diode would short the ignition to ground, maybe yours was having increasing leakage before it failed completely.

I never could locate the diode to replace it (somewhere under the dash board) but instead cut the wire (one of two white wires from my coil) that went to the diode from the coil.

My car would start if I didn't have the emergency brake on but because I always parked with it on it was a long time before I made the connection.
Michael S. Domanowski

OK, this weekend. compression test, one cylinder slightly down all the rest OK. I did reset the computer several times, no luck with that and no codes. that's why I think it is electrical still.

BTW, Michael the diode is behind the glove box on a 77, two of them. charging system seems OK.

The saga continues.
M Mallaby

The throttle sensor?

anyway try these links, more useful than my guesses I think.


Gosh Peter....the first link is almost identical to my TVR EFi service manual, just re-badged by them by the looks of things.
M Barnfather

Amerikanische Firma „ACG Logistics“ sucht nach Mitarbeiter in Europa fuer die Arbeit im Logistikbereich. Interessanter Job mit guter Verdienstmoeglichkeit.

Schicken Sie bitte Ihre Bewerbung an

OK the 3.9 is up and running! The culprit was BAD FUEL! After exhausting my 14CUX knowledge and ignition tests. I finnaly drained that entire fuel system, then put in 5 gallons of fresh gas, dried the plugs (for the umpeenth time) and cranked her over. She sputtered and spat then fired up! Drove to work today with no issues.

Thanks to all for the suggestion or atleast allowing me to rant.

All the best,

M Mallaby

What brand of fuel ?? I don't want any !!

Edd Weninger

Crikey often do you fill it up ....even with a gentle right foot I doubt a tank will take you more than 300 miles or so, you need to get out more !!!

Seriously, I'm glad it was something simple !!
M Barnfather

Thanks for the update, Mark. Glad it wasn't serious, but too bad it was such a difficult thing to pin down.
Steve Mc

Amerikanische Firma „ACG Logistics“ sucht nach Mitarbeiter in Europa fuer die Arbeit im Logistikbereich. Interessanter Job mit guter Verdienstmoeglichkeit.

[b]Schicken Sie bitte Ihre Bewerbung an[/b]

This thread was discussed between 06/09/2007 and 28/09/2007

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