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MG MGB GT V8 Factory Originals Technical - idling problems with hotwire injection.

Ok, I've been toying with this car for WAY too long now and I could really use some help here! I have a 3.5L V8 with hotwire injection system on top and I cannot for the life of me get the car to idle proplerly. Timing is in good order, throttle potentiometer is set properly, as is the air flow meter, and I replaced the stepper motor this morning and still no progress. (By the way, the stepper motor is the EXACT same as that of a 1986 buick grand national fuel injected turbo model and only costs $34.00 at Auto zone.) There are times when it'll start up, rev to 3,000 rpm's then drop back down to 1000 and sit there very hapily. Other times when it starts it revs right to 5,000 rpm's and won't come down! There is no difference in what I'm doing when this happens, and it's really baffling me! I followed the rover test procedures for testing the injector banks, and the two temperature senders, and all appeared to be working properly. *but* that won't tell me if I have a leaking or clogged injector. I'm really thinking this is a fuel related problem because of how haphazard it is. The pertronix II electronic ignition under the cap probably isn't likely to cause those kinds of fluctuations, from start to start, but I'll be damned if I knew what was.

Another thing that's bugging me is there are times when I can depress the gas pedal and the rpm's will slowly increase until I let off, and then other times I'll barely touch the gas pedal and the rpm's skyrocket and stay there! I tested the throttle potentiometer while doing this and everytime the gas pedal is at idle the voltage from it reads .32 (withing the acceptable idle range of .31 to .35v) yet the rpm's are staying through the roof. I don't know what to do from here. I'm about 3 days from buying an edelbrock 500cfm carb and manifold and pitching this injection system.

Please help! I'm really at the end of my rope here!


A friend of mine and I have converted several MGB-V8's to the Rover hotwire injection system. One thing you might want to check, does the stepper moter seat tightly when closed? You somehow have air entering the intake plenum after it has passed thru the airflow meter. Check to make sure that your throttle plate is seating in the closed position as well although our description of the problem with revving the engine points to the stepper motor.

Another thing to check is the electrical connection to the stepper motor. The Rover test procedure is pretty clear on how to do that. If you have any other questions, e-mail me direct.

Hope this helps,

You have got problem with airflow, not fuel injection, IMHO. The fuel injection is merely reacting to some source of excess air that is probably leaking in from the throttle not closing all the way consistently or some other source of 'false' air.

It is easy to isolate the stepper motor - just take a clamp and put it on the hose that feeds it and you will take it out of the system.

If you want to get really fancy, there is a way to fool the stepper motor all the way closed then disconnect the electrical connection. If you are curious, I'll find the note on the process. However just blocking the hose will do the same thing.

If I'm not mistaken, the first thing you have to do is establish a "base" idle of about 800 RPM with the stepper motor fully closed and the connector removed or the hose clamped off. The stepper motor is designed to control speed around this base idle value and has limited authority to contorl speed. There is an aluminum cover on the plenum that houses a bleed bypass screw that will control airflow across the butterfly. It does, however, have only a limited authority over the base idle speed. There is also a allen screw on the bottom of the plenum that is the stop for the butterfly. Take the allen screw out from the bottom and put it in the top so it can be adjusted without taking the plenum off its base by opening the throttle and uncovering the grub screw.

Use the butterfly stop setscrew to get the idle close to 800 and then fine tune it with the bleedscrew under the aluminum cover, reconnect the stepper motor and see if it will hold idle. (I must warn you that even after you do this you will likely see RPM at idle vary by a couple of hundred RPM)

If everything is working properly, you should be able to start the car with the RPMs at about 2000 or so then slowly decrease to the correct idle speed with the stepper motor. The rapidity of the decrease in speed after starts seems to be a function of how warn the engine is and the ambient temp.

It seems from your description that you either have a big airleak somewhere or, more likely, the throttle isn't returning to the fully closed posistion. Make sure your linkage is correct and you have a sufficient spring to return it to its fully closed position. You can add another revolution of wind to the factory spring if you want to get it to close well.

If this fails to remedy the situation, then start looking for leaks with a bottle of propane. Do this outside, please. I take the mixing tip off and let just a little propane escape then use it as a sniffer. Put the escaping gas down by the intake filter and you'll get an idea of the magnitude of the effect you're looking for.

After you get the idle to settle down, make sure your timing is spot on and that your mixture is correct with the screw on the side of the air flow meter.


You might find this link interesting-


Just a Thought......
Did you remove the intake manifold heater?
if so, did you plug the four threaded holes in the bottom of the intake tract? Seems obvious,but has been overlooked by many!
Good luck, let us know what you find.
DTR Ross

I connect an air hose from my aircompressor to the hose feeding into the stepper motor port and set it to about 30 psi and started listening and feeling around the injection system for leaks. There was an airleak the size of the grand canyon coming from where the trumpet stack met the intake manifold, and from the four holes from the manifold heater.... DOH! After a little machine of the trumpet stack, and filling those four holes, the idle settled back down to about 1200-1400 rpm's. I've been told I can get it lower then that but I didn't have a chance to fiddle with anything yet. It appears my problem is getting resolved, but I'm not about say it's fixed. I'm not that naive.

I hooked up my drive shaft and while in neutral my car lurched forward while I tried to start it. I'm hoping to God the transmission is just really tight after it's rebuild and I didn't miss something there. That's tomorrow afternoons project.

Thanks everyone for the help!

I spent quite a while working on the car today, and even got my first test drive out of it! I worked on the idle and I have it just below 800 right now, but I think I'm gonna raise it a little bit more just to be on the safe side. It seems to "burp" from time to time. The clutch pedal is INCREDIBLY stiff, and I'm hoping I either A) get used to it or B) it softens up as the clutch wears a little. The lurching problem worked itself out, but the car is HARD getting into first gear, and I can't get it to downshift from third into second, yet it'll go from first to second no sweat. Syncromesh? I'll do a little double clutching tomorrow and see what I get. From here I think I just have to work out some minor kinks in the system, but I think it's pretty much operational. There's a lot more power here then I think I was bargaining for, and I'm not starting to ask myself "what the heck did you build here!" Anyone else ask themselves this question? What was your answer?

I already know that if I could do it again I would use the borg warner T5 box, and I just may do that this coming winter. They're a dime a dozen here in Detroit, and rebuild, mods, installation and clutch will be easier then what I have on my hands now.

What an incredible project, and thank you everyone for the help!


If it 'burps' it is probably a little lean. Give it a couple of turns on the airflow meter pot. Idealy, you should have it set with a CO meter.


This thread was discussed between 04/07/2002 and 09/07/2002

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