Danny Rogers

Hey guys! I've learned a lot from this forum over the years, but I have a problem that I haven't yet found a solution to. Once I crank the engine and it's warm, it idles nicely at ~1k RPM. Then, when I put it into reverse or drive, the vehicle will lurch in whichever direction and the idle will drop super low and it'll almost immediately stall out unless I hit the gas. Even when I do hit the gas, the stalling will resume once I let off the gas. If I were to drive it in this condition, I will experience poor acceleration and usually have to pop it into neutral when at a stop sign or a red light. In this condition, when in neutral or park, it does idle higher, but I also notice that it's just not quite right. Like it's not the usual strong idle (if that makes sense). I experience this issue about 95% of the time. The other 5% it works normally, accelerates well, and has a beautiful idle sound. Sometimes, it starts out normally, but then these issues pop up after driving it for a while.

Here are a few more details that you might ask about:

- This issue happens whether the engine is hot or cold.
- 351 Cleveland engine
- Automatic tranny (idk which specific kind)
- Edlebrok carb - 4 barrel (idk which specific model)


In trying to research this problem, I keep finding solutions that only apply to modern vehicles. I don't think O2 sensors, MAP sensors, and computers were a thing in '72 lol. I've tried a bunch of different things that I thought may have been the issue (based on the zero knowledge I have, since I'm not a mechanic), but I can't figure it out and I'm at my wit's end. The only thing I can think of is that it's a vacuum issue or the torque converter.

Help me, Mustang Forums. You're my only hope.

Gun Jam

It could be a vacuum leak I would check all vac lines using a can of carb cleaner with the engine running. Spray it around the carb base and suspected ports and listen for changes in rpm.

The Torque Converter is also suspect. The idea is below the Stall RPM they slip and allow the engine to rotate independent of the driveshaft. At stall rpm the converter locks ups completely and the engine will stall if the vehicle isnt moving. So if you have a 1000 RPM TC and mash on the brake then rev the engine up at 1000 RPM the engine should die as the TC locks. I dont know a bunch about Auto trans but yeah there could be an issue with the TC. Start with Vac first as it easiest to track down...Usually Vac leaks aren't intermittent.

When did the issue start occurring? Did the car run well before this?

danube7

Howdy Danny (or Dan?),

Don't rule out an ignition issue! I just came across your post randomly just now as I am looking to post a question about spark plugs, but I just recently worked through a problem similar to yours. I have a '67 mustang C-code car but with a later model 1978 Mercury 302 in it. 2bbl Carburetor. HiPo centrifugal advance distributor. But my car sat idle for 5 or more years and I had bad gas AND rust in the fuel lines. Because of this, I have rebuilt my carburetor 5 times in the past year, trying to work out a stalling issue. But by now, the bad gas is flushed, the rust is mostly gone, and because I can rebuild my carburetor with my eyes closed in 15 minutes now, I know for sure there are no plugs in it. What happens is the car runs beautifully at idle, smooth and fast. The car warms up, the fast idle cam unloads, the choke opens up, everything works fine. I can take the car for a 15-20 minute drive. And then at some point, I put it into gear and it just stalls. I can't even get it home at that point.
But having rebuilt my carb 5 times, I know there is nothing wrong with it. I measure the vacuum, and sometimes its bad, but then it's good again. Well, it turned out to be a bad ignition coil. I needed a lot of convincing that was the problem because I seemed to have a good spark and the car ran fine for a while. But as the *coil* gets warm, its primary resistance rose from the nominal 1.4 Ohms to about 1.6 Ohms, sometimes even 1.8 Ohms. That's enough for a marginal spark, and under load, the car fails. Some posts on various forums also suggested he warm idle stalling could be a bad coil.
So I replaced the coil and found out why my vacuum was bad. It seems that a missing cylinder can result in bad vacuum. With the new coil, I had arcing from cylinder #7 to the fuel line. I cleaned up the distributor cap and plug wire, stopped the arc and the vacuum went from 15" to a solid 20". Now the car idles beautifully at 800 RPM, and drops smoothly to 600 RPM in gear, and does NOT stall. But it misfires at higher RPM under load so I am going to replace the plugs.

So, check your ignition coil when it's cold and again hot and see if there is a change. It is *difficult* to get a reliable measurement of something with only a couple Ohms of resistance due to contact resistance, so keep that in mind as you try to diagnose your coil. On the other hand a new one is only $30 or so.

Good luck,

Dan