QuestionQUESTION: Back again with another problem that just started. Reguarding my 1977 MGB Roadster.
Driving down the road ( each time a down hill) in forth gear, the engine cut out. once at night - a cool evening, and once during the day - a warm day. My headlights stayed on, and all dash lights were ok. I just replaced the fuel pump a few months ago with an electronic SU type. I forgot to check the fuel filter when this happened to see if there was gas in it. After letting it sit for a few minuts, it started, and I made it back home. I never dropped the fuel tank to see if there was any junk in the tank but my fuel filter looks clean. I have electronic ignition not points, a new ignition coil, wires, dist. cap & plugs. It has been running fine for the past year. I checked the wiring, and ground on pump. I replaced the ground plug on connector when I replaced the pump. Should I replace it with a new ground? I had no problem with the old pump, just wanted piece of mind that it was new. When the car cut out and was coasting down the hill, I wiggled the ignition key wiring harness under steering colum, turned the key on and off , still did not start up. it tried a couple of times to start with the clutch in and coasting, but just cut out.I have a webber carb on it which is fairly new.I don't think it is vaporlock. Does this car have a fuel cut off switch which may be bad? Sorry for the lenghty letter. Tried to give you all the info I could. Thanks
ANSWER: Hi Bob,
Even though you have checked and done several things there are a thousand possible things that can cut an engine off. So trying to check each and every item that it could have been, you need to see what is missing of the three items that an engine needs to run.
The three are compression, fire and fuel. Since it comes back in a short time it is no use checking after it returns to running. That means you must see which is missing at the time of failure.
Compression can go away but when it does, it will not return without major repairs, so you can rule out compression as the fault.
This leaves you with only fire and fuel. Both can go away and return so you need a method to see both when driving. All cars today have an ECU that has a memory that stores codes for fire and fuel and many other more detailed info. You don't have an ECU so you will have to supply a monitor for each of the two items.
Most mechanics will tell you that the easiest car in the world to fix is a car that will not run and the hardest car in the world to fix is one that will not run on occasions.
Cars like yours came to me at times in the dealership and here is what we had to do to "Catch" that intermittent failure.
First we had to isolate it to fire or fuel and the way to do that is to be able to see both. For fire all we did was to connect a timing light to a plug wire and tape the trigger down and run the light out from under the hood and put the timing light under a wiper arm. Because we were working in the day time we had to tape a piece of cardboard over the light on the windshield so we could see the flash of the timing light in the bright sun light. Then we connected a "T" in the fuel line close to the carburetor/s and run a long hose out from under the hood and place a fuel pressure gauge under the other wiper arm so we could monitor fuel pressure at the time of failure. Then we just drove the car until it failed and noted if either the fire or fuel pressure went away as the engine coasted down when it failed.
This divided the failure into a smaller area. Thus made it easy to locate without throwing a lot of money and parts at it to fix.
I later designed a box with several LED lights in it and long test leads to go to various points on the ignition and on the powers and grounds of electric fuel pumps. With this I was able to just let a customer drive the car (if I couldn't get it to quit) until he had a failure and he would look at the box to see which LED light had went out. It worked so well that several local dealers on other brands of car use to borrow the box for troubled cars that would only die at odd times.
If you don't have a timing light nor a fuel pressure gauge you will find that they are not expensive tools and can be used on any engine for testing.
Unless you work on a lot of cars you don't need to go to the trouble of making an LED test box but if you do let me know and I will tell you how to make one from parts you can get from Radio Shack or any electronics store.
Howard
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QUESTION: Instructions for a tast box would be helpful. Thanks
AnswerThe box was a plastic box about 6 in x 4 x 2 and the LEDs were 12v and I used 3 Red LEDs and 4 green LEDS. One green LED was power for that green LED and power for the 3 Red LEDs
One Red LED was for the ground of it's red LED and all of the green LEDs. This made these two a "must be connected" to a power and a ground for all the others to operate.
I installed 7 sockets in the box. Each socket was next to a LED. Connect the one green LED (+)to all of the (+) red LEDs and to the one socket next to that green LED. This test lead will give you power to one green LED and all of the red LEDs.
Now connect the closest Red LED(-) to the (-)test lead of that Red LED and to all the Green LEDs (-)
Now connect the 3 remaining Green (+)LEDs to each (+) test lead.
Then connect the 2 remaining Red (-) LEDs to each (-) test lead.
This allows you to monitor 3 grounds and/or grounding signals and 4 powered circuits at the same time.
I would place my box on the dash in front of the driver so he could see the LEDs while driving and when a failure popped up a LED would go out. The two main test leads must be connected to a steady power and ground for the others to work but in practice I found that they could be placed at key powers and grounds that I wanted to test anyway. If either the power or that ground was lost all of the LEDs would go out when that circuit failed. On the other LEDs if they had a failure only their LED would go out.
I found this box handy on electronic fuel injection before they started putting in memory and codes. I have used it on everything from wipers to gangs of relays. It worked great on electronic ignition systems to isolate down to which part of an ignition system was failing.
All of the components came from Radio Shack except the long test leads. At first I probed each wire to test with a straight pin and had a small alligator clip on each test lead.
I later had a local electronics company make the box and I purchased long test leads with self probing connectors from a test lead company. I sold the units through a tech rep who traveled the US giving seminars on electronics and electronic fuel injection.
The sales dropped off when each car manufacture started putting memory codes in their ECUs as that was the main use for the "Fault Finder" as it was named by my wife.
If my description is not clear enough let me know and I will put a diagram up on my web site that you can copy.
Howard
