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MG MGB GT V8 Factory Originals Technical - Surging FI

The heat in Phoenix has abated and I took the MGB out for a Sunday drive. It has been sitting for about six months.

(The car has a stock 1980 SD1 engine, fuel injection and transmission)

I drove about a mile to the gas station and put about 4 gallons in the tank and continued on my drive. About 7 miles later there were a couple of mild surges at about 50MPH. I turned down a somewhat bumpy asphalt road and the surging became quite severe.

As I turned to head back home, the idle speed would go from 500 to 1000 RPM continually. It would accelerate OK, but once it hit a steady speed it acted like cylinders were cutting out. It occilated in and out of this surging at a frequency of about once every one or two seconds.

I turned down the road twards the house and the symtoms disappeared. I drove the last two miles home and it ran fine. I parked it out in the street and started it about two hours later to put it in the garage and it was fine.

Any thoughts on where to start the troubleshooting? I'm tempted to put an Edlebrock Weber on it and be done with it.


I swap the FI parts from your 1980 engine with my Weber/Carter 4bbl carburetor. Had it on the car for 4 years and ran fine with not a single problem. I bought the engine with FI but was missing most parts (Harness, senders, ECU unit)so I converted to carb. Now I am restoring the body (Rust on footwels and sills) and would like to go FI.
Let me know if you are interested
Werner Van Clapdurp

I supect one of the sensors but it could be the fuel line too - Can you get to a LandRover agent and get them to run your car on their computer - check all your connections before you set out (and you fuel filters).

I would persevere with the FI despite the good offer

If it worked fine before you put it to bed a few months ago, I would suspect the fuel pump relay. These have a tendency to go bad and cause intermittent fuel starving. Ahh but that would be a stumble, not a surge. Is is surging like the additional idle you get while the engine is cold? Maybe bad sensor has engine thinking it is still cold and adding additional fuel.

If the engine runs fine, but occasionally surges, I would suspect a vaccum leak or sensor snafu. If the engine runs badly and stumbles, but occasionally clears up then check fuel delivery first - pump, filter and relay. I don't know what the symptoms of a bad Mass Air Flow sensor are, but if yours has that flap thingy instead of a hot wire sensor, would the car act this way if it was bouncing around??

Brian Corrigan

The fuel relay is part of the master relay that powers up the ECU. Opening the flapper on the air measuring device causes the relay to close and energize the fuel pump.

The thing I don't understand is the oscilation. It never misfired really, just surged back and forth about every 1-2 seconds, and it did it at idle or at cruise, but not during acceleration. Could the fuel pump relay itself be shutting on and off? Is the airflow meter turning it on and off?

I'm rigging up a hose to read fuel pressure on the fuel rail from the drivers seat abd I'll know in a day or two.

Any other suggestions to contemplate?


It could be cavitation in the fuel pump due to heat- The EFI version of 'vapour lock'. Your fuel pressure gauge will pick this up.

Here's a good link to an SD1 injection page- even if it doesnt help, its interesting reading. (copy and paste link into your browser)

Good luck!


Yes, I was referring to the fuel pump relay. I've seen them go bad and exhibit intermittent starting and running problems, but not so much a surge as a overall lack of power, misfire, stumble or complete failure to start. It doesn't sound like the kind of problem you are having, but it has the beauty of being easy to swap out with a similar relay to rule out the problem.


Brian Corrigan

Have you tried disconecting the O2 sensor? My old SD1 used to do this as well and disconecting one of the O2 sensors fixed it. Never could figure out what the problem was caused by.

In looking over the fuel injection system last night, I noticed that the wire going into the connector on the water temperature sensor had come away from the housing. I'll put it back together tonight and see if the problem goes away.

Thanks everyone for your suggestions.



Depends which temp sensor your referring too the thermotime sensor (brown) will have no bearing unless it becoimes active due to a make and break and kicks in the cold start injector. If it is the white (probably a dirty cream now) then this can cause a surge as the injector open time will be varied as a result of potentially dropping the connection and then adopting a default value.

This default value is intended to provide a start and half reasonable running of a really cold engine and so if the engine was hot and the weather was warm then this would create a fair degree of over fuelling for the conditions. This could and does cause surge.

Also if the engine does suffer the surge again use a pair of pliers to gently sqeeze and close of the hose between the extra air valve and the plenum chamber. These valves are quite crude and simple in operation but they can fail and cause too much air to pass and this mixture upset when the engine is operating with fuelling for a fully warm engine can cause surge too.

The horrible combined relay is something that never gave me trouble with my previous injection set up from the ex Lucas test system I used for nearly 10 years. (See a photo of it on the front cover of Roger William's book @how to Give your MGB V8 Power) However it was an over complicated device and had I not converted to Hot Wire then I would have used the later TR8 wiring harness I got which is in Roger William's car. It has two standard relays and a diode unit, much simpler and easier to diagnose problems. Early systems can be converted to this later style if need be.

Roger Parker

This thread was discussed between 19/11/2001 and 20/11/2001

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