Engine Stuttering Options

[GulfRacer]

I'm hoping some collective wisdom can help me through a frustrating problem. I have 1974 Bumblebee that I've owned for about 6 months. The 2.0 was rebuilt to a 2056 shortly before I purchased it. It has a Raby 9590 camshaft, original D-Jet and 123 distributor. When I first got the car, it was running great. It has since developed an intermittent stutter/bucking. It occurs most commonly around 3000 RPM when cruising, but once it happens, it continues when applying throttle. It doesn't usually occur until the engine has warmed up - the first several minutes of the drive are usually good.

I replaced the throttle sensor board which did not solve the problem. I disconnected the throttle sensor, and the problem is still present. I replaced the fuel injection harnesss which did not help. The fuel injectors are all new with the engine rebuild, but cylinder 3 injector stopped firing a few months ago, and I replaced it with a new one.

I'm at a loss as to where to go next. Any advice would be greatly appreciated.

Jason

[Dave_Darling]

Wiggle all of the wiring in the engine bay while the engine is running to see if the idle changes.

Install a fuel pressure gauge and look for fuel pressure to change when the engine is stuttering.

Chase the fuel line routing and see if there's anywhere that might wind up kinking a line.

--DD

[emerygt350]

Interesting. That is a spot where a lean miss is most likely. If one of your injectors is an underperformer that is where you might feel it the most. Since you already replaced one injector I wonder if another is having issues? Maybe there was some crap in the lines that is working it's way through. Is it a full engine stop or is it more like a single cylinder dropping out?

[Olympic 914]

I was having a similar problem

On mine it was an intermittent CHT failure.

It tested good cold and hot. And usually started fine cold. But after a mile or two it would start bucking badly.

I fought with it for 2 months before finally changing the CHT.

I am also running a 2056 diet 9590 cam and 123 dizzy.

[Lockwodo]

Would a faulty MPS cause the lean condition and missing?

[emerygt350]

Would a faulty MPS cause the lean condition and missing?

Normally no, a faulty mps is normally a cracked diaphragm and that leads to a rich condition.

[emerygt350]

Would a faulty MPS cause the lean condition and missing?

Normally no, a faulty mps is normally a cracked diaphragm and that leads to a rich condition.

Have you checked your plug wires? I have had situations where everything was fine with a loose wire until it found a better place than the plug terminal to jump to ground, and once it found the new route it wouldn't go back to the plug.

[GulfRacer]

Would a faulty MPS cause the lean condition and missing?

Normally no, a faulty mps is normally a cracked diaphragm and that leads to a rich condition.

Have you checked your plug wires? I have had situations where everything was fine with a loose wire until it found a better place than the plug terminal to jump to ground, and once it found the new route it wouldn't go back to the plug.

Thanks everyone for the suggestions. I did the "wiggle wire test" early on and found the engine stalled when playing with the CHT sensor connection and some of the injectors. I replaced the FI harness and resolved those issues hoping that would solve the problem. It didn't.

Plug wires are new and look perfect. I don't think it's that.

I'm going to replace the CHT sensor (although it's pretty new) as that may explain why it happens primarily when warmed up and it's a cheap and easy thing to replace. If that doesn't fix it, I'll find a way to run a fuel pressure gauge to the interior and see if I can document a drop when it's acting up.

I'll report back when I learn more.

[cgnj]

I'm thinking ignition coil. Sits on the tin above the cylinders, gets hot, gets intermittent.

[Lockwodo]

Or perhaps a dodgy ground connection?

[JP101]

Thanks for additional suggestions. Coil may make sense and easy enough to replace. I’ll look at it and see if it was replaced when engine was rebuilt. I’m pretty sure grounds are good but also a good thought.

[GulfRacer]

I still haven't sorted this problem out, and since the original post, the problem has become more consistent which at least makes it a little easier to troubleshoot. The problem no longer seems to be heat related or at a specific RPM, and the car is pretty much undrivable now.

Since the last post, I replaced the ignition wiring harness and redid the spade connectors on the 123 distributor to the coil. I replaced the distributor cap (carbon center piece worn away) and rotor. I replaced the CHT sensor. I think as a separate issue last week, the fuel pump wouldn't come on, and I traced that to a bad relay board. I was hoping maybe that was the problem for the poor running. A borrowed relay board allowed me to get the car started, but the rough running (and backfiring) is still there. Fuel pressure remains steady at 28-29. MPS holds vacuum. I've disconnected each fuel injector, one at a time, and idle drops like you'd expect. Rough running persists when firing on 3 cylinders. Plug wires all look fine and firmly attached. All ground connections are good

Any other suggestions. Remaining possibilities that I can think of are a bad ECU or bad 123 distributor. What am I missing?

[Ishley]

Do you have a Bluetooth 123+? If you do you can connect your phone/tablet and while driving around it creates a log file in the files area on your device. That might give you some hints.

[emerygt350]

I would try replacing the coil and verify it is wired correctly. I had an original Bosch that read perfect on the ohm meter but in the car it resulted in some real badness, much like you describe.

[GulfRacer]

I have some additional information. Thank you for the suggestion to check the 123 data. I wasn't aware that was saved. I didn't see anything out of the ordinary there when the engine was stumbling (steady vacuum and voltage).

I hooked up a noid light to each injector plug, one at a time, and when the engine stumbles, the noid light dims and becomes erratic for each injector. A also hooked up an inline spark plug tester to each plug, one at a time, and the light pulses didn't significantly change when running rough.

The noid light data, I believe, would suggest that the engine is stumbling because the computer is triggering the injectors erratically. If this is true, then I think it is either because the computer is bad or one of the inputs into the computer is providing erroneous information. With all of the other results I have, the only input I haven't eliminated is the trigger points from the 123 distributor. I don't know of any way to test these. Otherwise, I think I'm narrowing it down to the computer.

Thoughts? Anyone know of someone who can test a computer or have a known good one to try out?

Jason

[Lockwodo]

It does seem like the trigger point pulse from the 123 to the injectors is being disrupted somehow. If memory serves there are spade lug connectors between the wires from the 123 and the fuel injection harness, could these connections be corroded or shorting to ground somewhere?

[GulfRacer]

It does seem like the trigger point pulse from the 123 to the injectors is being disrupted somehow. If memory serves there are spade lug connectors between the wires from the 123 and the fuel injection harness, could these connections be corroded or shorting to ground somewhere?

FI harness was replaced and those connections are perfect. Good suggestion though. Thanks.

[emerygt350]

I wonder if this could be one of those rare times the ECU is going? Do you have an extra?

[GulfRacer]

I wonder if this could be one of those rare times the ECU is going? Do you have an extra?

That's my thought, either the ECU or a fault in the 123 distributor trigger mechanism. Unfortunately, I don't have an extra to test.

[Ishley]

I’m wondering if you should check the spray pattern on the fuel injectors. It’s a
No cost item. I do 2 at a time on the same side. Disconnect the coil (no sparks during this operation) take out the 10mm nut and pull the injectors out. I use 2 small dishes to place the injectors in… have a helper crank the motor and watch how the injector sprays. It should be a nice fine pattern.

I had one that would dribble somewhat and that was the cause of my intermittent problem. It’s an easy fix and it’s free to check.

Just a thought.