Stumped

Mr C

Mr C

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Recently my 84 Diplomat daily driver has started acting up and I am stumped as to the problem.

Details

1984 Diplomat 318/ 904 original drivetrain (stock intake/ exhaust EGR etc)
Lean Burn conversion done in Oct 14:
ECU, ballast, I had lying around (4 pin).
Swapped LB BBD with Rebuilt BBD with ported vacuum port on it.
New vacuum advance distributor.

Car has run great all winter, but has recently developed a problem (has done this twice to me now).

After driving about an hour, the car will lose power and only be able to do 60 kph. Try and give it gas and it bucks and dies. Throw it in Neutral, fires right up, put it in gear and you can gingerly accelerate to 60 kph again. It feels very "weak" and boggy. In Neutral you can floor it and it will scream- slam it into gear and it will immediately die.
Once I limped home in the driveway in park/ neutral the engine will scream if you want it to, so the problem is when it's under load.
When cold, and up until the hour point the car runs fine and gets up to speed without issues...it will bury the needle.

Engine is not overheating during this issue- all's well in that department.

I checked all vacuum lines and they're all in place- no leaks...it was getting dark, so I was unable to check the trans fluid level...I'll do that today.

I've had ECUs die, and in my experience, they just stop working. I was wondering if it was my parts stash ECU that develops some issue once hot, but I've never heard of one that "kinda works".

Any help diagnosing this "new" problem is appreciated, I need this thing operational ASAP as it's my daily wheels and I don't want to take it to a shop that probably doesn't know how to work on one of these cars anyway.
 
Could be a plugged fuel filter, ecu going bad usually dumps goo out of the bottom. . got a spare, throw in place and test? My fuel filters were filling up with crud from the lines and plugging up plus was catching air from pinholes in the metal fuel line and would "run out of gas" while driving. . .best wishes...Bert.
 
So last night, limped home from work at 60 kph (it was fine in rush hour traffic and began to exhibit problems on the hiway portion of my commute).

This morning -25, started right up and drove to town and back 100+kph no problems- short trip though.

It only seems to give me grief when warmed up and @ an hour of driving...wierd.
 
I'm asking a buddy to bounce this one.
Waiting to hear back....I remember reading about the return fuel line exhibiting a sort of vapour lock scenario in the same situation. I'd have to find it but I think the experiment involved disconnecting that line and it solved the problem.....?
I'll let you know what my resident expert says when I hear back.
 
79410aspenrt said:
check your coil. if you see fluid once you pull the wire off, the coil is toast.
Click to expand...

Well, in 30+ years of owning Mopars, I've never had a coil go bad, so maybe there's a first for everything.

I have some spare coils in the shop, so I'll swap one out and see how it goes.

Why can't this happen when it's not totally freezing out?
 
Mr C said:
Well, in 30+ years of owning Mopars, I've never had a coil go bad, so maybe there's a first for everything.

I have some spare coils in the shop, so I'll swap one out and see how it goes.

Why can't this happen when it's not totally freezing out?
Click to expand...

Yep....nothing like troubleshooting in arctic weather....
 
I've had 3 coils go bad over my 20 years of playing with mopars. 1st time was on my 69 Charger and the last 2 on my 77 woody wagon.
 
New coil is in, topped off trans fluid that was a bit low too. Fired up and runs...but it ran in the driveway before too.

The old coil showed no fluid anywhere.

We'll see.
 
Last edited:
I had similar symptoms a few years back. My car would run until it warmed up and then die. Once it cooled off it would start right up, run until warm and then die. After searching for the problem it turned out to be the pick-up in the distributor. Hope you get it figured out soon.
 
Ran to town and back- no issues, but the trip was less than the "hour threshold" that I had before. Seems to run more smoothly now...stronger spark due to the new coil?
 
Quite possible.
I blew through my FSM and weak coil was one of the things they listed as the cause for lack of power.
 
Drove into the city and back yesterday (the highway portion of my commute) 120kph no problem. Today is a full commute to work and back...we'll see.
 
Captain Caravelle said:
....I remember reading about the return fuel line exhibiting a sort of vapour lock scenario in the same situation. .
Click to expand...
It shouldn't have vapor lock in this cold. Does the motor overheat?

Mr C said:
Well, in 30+ years of owning Mopars, I've never had a coil go bad, so maybe there's a first for everything.
Click to expand...
I have had more than one coil go bad on me.

compubert said:
Could be a plugged fuel filter, ecu going bad usually dumps goo out of the bottom. . got a spare, throw in place and test? My fuel filters were filling up with crud from the lines and plugging up plus was catching air from pinholes in the metal fuel line and would "run out of gas" while driving. . .best wishes...Bert.
Click to expand...

ALLOT of people now have fuel filter & fuel pump problems with the lack of filtering that seems to occur with the ethanol blended fuel now days (not just dirty+it also mixes well with water-try HEET).

The best detergent for your fuel is SEAFOAM.

Tell us how it goes.
 
Do you have available in your area the ethanol free 87 octane fuel? We've got a few stations with it. Costs 20 cents more/gallon but worth it. Same as ethanol free premium is 20 cents higher than "up to 10% ethanol premium. Wonder why it costs more????

It does kind of sound like a weak coil, especially since it takes a while to act up. A weak spark cause buck/jerk and lack of power under load. May be sufficient in park with no load so the engine will rev up seemingly fine. We run into it on coil-on-plug vehicles (which is pretty much everything made in the past 20 years) at work all the time. Idle and light load OK, heavy load and it'll miss. of course it only affects one cylinder in that case. Heat makes it worse.
 
She failed again. During the city part of my commute home in rush hour traffic, she started to surge and lose power again. Limped home on the highway doing 60kph.

No overheating, just zero power and some surging. Occasional stalls...throw it in Neutral, fires right up- hit the throttle without load and it'll scream-drop it into gear and it's a dog.

Back to square one it seems.
 
Yup, I'm still stumped. Gonna do a fuel filter and a new ECU and roll the dice, what the hell...
 
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