Need help with diagnosis- car stutters and bucks ....

S

Scott Wilson

Active Member
Site Donor
Messages
49
Reaction score
7
Location
Carlsbad, California
Okay deQuicy was correct. Repacing the fuel pump didnt stop the stuttering. So im reaching out for help to diagnose my problem. Any help would be appreciated.

The car:
‘74 CS. 4 speed. Factory replacement engine 3.0. Two down-draft Webers. Compression is great. Car runs really smoothly and strongly.

Shortly after i obtained the car about two years ago, i had a knowlegeable shop go through it including giving it a full tune up. This included a new distributor, points, condenser, wires, plugs, etc.

The issue:
A couple months after the tune up i was driving along and the car began to stutter for 10 seconds then recover. This became more common and began getting worse. At times i would go for a drive and out of no where the car would stutter...then get worse to the point it was bucking...literally like the engine was pulling one second then dead the next then pulling again then dead, etc. this would jerk the car forward whiplashing you as you tried to pull over. The engine usually stalls as you coast to a stop. After letting the car sit for say 5 to 10 minutes it starts and drives normally, but may or may not begin stuttering and bucking again.

Important in all this is that at no time during the bucking and stuttering do the instruments bounce around as if experiencing an electric short.

What i have done so far:

Fuel Related:
1) My first thought was bad gas. Fuel was replaced and fuel drying formula (Heet) was added. Several tanks of fuel have been run through.
2) Fuel filter in engine compartment replaced.
3) Fuel pump just replaced

Ignition Related:
1) Tune up was initially performed. As mentioned new distibutor, point, plugs, condenser, wires.
2) I recently replaced the coil.

I’m sort of at a loss at this point. Thus i’m reaching out to the group . Any thoughts on next steps to diagnose and fix this issue would be appreciated!
 
I would recheck all ignition components. Check that the wires on the coil are clean and attach snugly. Check the timing. Check the points and the connection to the condensor. If that doesn't solve the problem I would replace each ignition component one by one. It's not unusual to get a bad coil, condensor, points, they are now often made in 3rd world countries and quality can be crap. Check all grounds.
 
you mention the fuel filter in the engine compartment. do you have a fuel filter in between the fuel tank and the fuel pump? if so, when was it replaced? water in that filter can do some bad things.

you have changed all of the things that popped into my head. Steve has a really good point to check all of the connections

if it was doing it at idle then you start wondering about air leaks, but since its while driving that shouldn't be the issue. you didn't mention replacing the distributor cap / rotor. check to make sure the cap doesn't have a crack in it.
 
I’d jumper a green home run wire fused to 15 amps from battery to coil.
Running the battery + backwards will rule out ignition switch forwards voltage loss.
 
sfdon said:
I’d jumper a green home run wire fused to 15 amps from battery to coil.
Running the battery + backwards will rule out ignition switch forwards voltage loss.
Click to expand...

200w.gif
 
Perhaps a photo of your distributor and wires? I had similar symptoms and found that the wires into and out of the coil were old, baked, cracked, weak. also terminals had corrosion and age related detritus. I cleaned or replaced terminals, replaced suspect wires and this solved my issues. Good luck.
 
Thanks for all the suggestions.

My gut tells me this is an electrical problem rather than fuel. I say that only because the bucking can be so violent that it is difficult for me to believe that it is simply running out of fuel or that a carb jet is plugging. But i could be wrong. At any rate i have decided to focus on electrical.

1) i dont think it is a bad grounding as i think that would show up as the dash instruments bouncing around. Tach does not move when bucking occurs.
2) i have ordered a new condenser. Ill put that on this week. I have run into bad condensers myself ...this,could be the problem.
3) Bluecoupe30, i have checked all terminals and wires.....all look to be in good shape.
4)HB Chris, the car came to me with a blue Bosch coil and no ballast resistor. As part of my trying to fix the bucking i replaced the blue with a new silver Bosch coil. I was under the impression that both of these coils do NOT require a ballast resistor. Can you confirm this? My car is a US 1974 CS. I see no extra wire.
5)SF Don, i like your idea of isolating the coil forward. I may have to get to that if the condenser doesn't do the trick.

Thanks everyone! Ill let you know how the condenser swapout works.
 
Blue is internally resisted so it’s not correct with your resistor wire, the silver coil is best, it is not internally resisted. The coil will state resistance required, hopefully 0.8 ohm I think.
 
Likely an electrical issue, but one other thing to consider would be a significant vacuum leak.

Simple to check and maybe worth a minute before the replace and test electrical regime.
 
002A5F5E-E802-4FA0-BAE9-495CFBF0E3D7.png
HB Chris said:
Blue is internally resisted so it’s not correct with your resistor wire, the silver coil is best, it is not internally resisted. The coil will state resistance required, hopefully 0.8 ohm I think.
Click to expand...

Chris, thanks for your comment on my coil. As i mentioned the blue coil came with the car before it started stuttering and bucking. So i replaced it with the silver, in vain as it stilled stuttered. So based on your comments i have it wrong and either need a ballast resistor with the silver or i should go back to the blue. You mentioned the silver is better, why?

Also here is the wiring on the coil, if you can identify any problem here id appreciate it. Not sure i know where the resistor wire is.

Thanks
 
The silver coil requires a resistor, your wire has one so you should be good unless wire was replaced. Someone has a pic of the resistor wire, I don’t. I think it is clear/yellowish and is on the + terminal and into the harness. On a 2002 it is the size of of an aspirin and not an entire wire and is hidden in the blue rubber sleeve with emission wiring.
 
to me the stuttering and bucking is not indicative of overheating coil from lack of ballast (usually total shutdown until cool) but rather a component failing so again I'd start replacing them one by one after all electrical connections etc are checked.
 
You must log in or register to reply here.
Back
Top