MFI troubleshooting - high idle, pops and bucks at part throttle

[edmayo]

I tend to agree that you getting ignition failure, are you using the correct Bosch black short coil for C.D. ignition? no newer silver Bosch coils or generic Pep Boys specials. As to the C.D. box,,,we cheat, we plug in a known good one, less time pontificating means less charge to customer. There are however two fuel system faults that can come on quickly. One is the return circuit check valve in the outlet of the fuel filter housing (hopefully that wasn't lost when all that stuff was removed and replaced) or a partial clogged fuel tank strainer (filter).

[Arne]

Definitely seems temperature related. I let the car cool down a couple hours, went out and started it and it ran reasonably well again. Didn't take it for a drive that time, I'll give that a try probably tomorrow. If it drives OK until something (CDI box?) gets warm, then I'll go the next step. I have a friend who has a '73.5 T, I'm assuming that car's CDI is the same 3-pin as mine. She will gladly let me borrow the box, I'm sure.

Yes, Ed, the coil is a Bosch black with brown top. Still has the German 'high voltage' sticker on it. Which means it is probably also 48 years old, and therefore suspect as well.

If I can confirm an ignition problem, no further tuning on the MFI will happen until I get the ignition issue addressed.

[MAH]

When the CDI is good it should hum/buzz with the ignition on, but the engine not running. So the first time you go out when the engine is cold check for the buzz and then once it warms up and runs poorly check for the buzz.

[Scott72]

Had the same issue with our 69. Was trying to tune the injection and the CDI box started to go. Definitely put a strain on the backyard mechanics skills as it caused a few days of head scratching.

[beh911]

With this newly described behavior, Ed's suggestion of checking for fuel flow issues is a simple task.

Remove the fuel level sender and shine a light down on the strainer sock area around the outlet bung. If it is all silted up, or looks like it could get that way after some sloshing around and pump running, you should consider draining the tank and cleaning it.

[Arne]

Definitely not CDI box or coil. No change when those parts were swapped. Putting both cars back to original, then looking at fuel delivery.

Will check inside tank, also wondering about the fuel pump. Do they tend to have problems with keeping up when they get older?

[Arne]

Tank has virtually no sludge or grit or silt in it at all, and the strainer sock looks new. Pretty confident there's nothing blocking the tank outlet.

Would the return check valve start misbehaving only when things get good and warmed up? That doesn't seem likely, to me. The issue still seems temperature related. It doesn't start cutting out right away, has to get warmed up. If I start with it cold, 2-3 miles driving before things start cutting out and popping.

Wondering about the fuel pump itself. I've had other vehicles where failing pumps were first noticed when they get warm.

Inside the tank:

[edmayo]

Easy enough to look at the check valve, unscrew the banjo from the filter console (has the return arrow stamped on housing) and look in the of the banjo and you will see a spring loaded, phenolic washer. Should easily spring back when pushed in. To check the pump pressure remove the fuel feed line at the fuel filter housing (has cast in arrow pointing in towards filter) hook up a (min.) 60 psi pressure gauge and turn on pump. Should make about 30 psi dead head pressure. To test for check valve remove inlet line into fuel filter housing (the fitting aimed towards rear of car) and install your pressure gauge using a tee fitting so you can run car with gauge installed. With car running you should be seeing 10 to 12 psi. Try to read gauge when car acts up. An AFR gauge would really help here. Need to know mixture strength when hot and acting up. A volume test of the pump can be done from that fuel feed line you tested pump pressure at, aim hose into a small coffee can, should easily fill that up in less that 60 seconds.

[Arne]

So today I traced and checked the fuel delivery path from the tank to the MFI pump. As noted with the picture above, the tank is very clean inside with a new strainer screen in place. I found that all fuel hoses for the entire path (both supply and return) are new, done when the engine was out for rebuild. All clamps are proper "not-worm-gear" type, and are tight - so the pump is not sucking air from a loose connection anywhere.

One is the return circuit check valve in the outlet of the fuel filter housing (hopefully that wasn't lost when all that stuff was removed and replaced)...

Easy enough to look at the check valve, unscrew the banjo from the filter console (has the return arrow stamped on housing) and look in the of the banjo and you will see a spring loaded, phenolic washer. Should easily spring back when pushed in.

I ended up pulling both banjo fittings and the plain barb fitting for the return-to-tank line from the top of the filter console. First the banjo for the return from the MFI pump. No check valve present. So I pulled the other banjo (inlet from fuel pump) and the plain barb fitting (return to tank) just in case it got into the wrong port on reassembly. Nada. So your fear about it being lost during the rebuild, rust repair and engine compartment painting was spot on. I'm assuming that this could conceivably be the root cause of my fuel supply problem.

I'm not seeing any indication that the check valve is available as a separate part anywhere. Am I going to need to source a new filter console to get one?

[Arne]

Late addition. Is the check valve supposed to be inside the plain barb for the final return? Part number 901-110-915-01? Not inside the banjo? Gonna have to go give that another look.