Jerk/jolt when sitting in gear

[tocruise]

The strange issues continue with this car.

So, the car starts up and sounds great. It's still in my driveway and I'm trying to finally get it back on the road. If I shift from Park to Reverse or Neutral it's fine. The car doesn't clunk or do anything it shouldn't, but the second I get it into Drive it starts pulsing and jerking forward, roughly every 2 seconds. This doesn't seem normal, and it doesn't seem to matter if I have my foot on the brake or not, it still pulses forward. Fortunately, the front driver's side wheel is a little buried at the moment from sinking into the dirt, otherwise I think the car would just take off.

I have a video of it for anyone wondering:

It almost looks like I'm doing it intentionally, like I'm putting my foot on the throttle, or changing gear, but it does the pulsing itself. I'm touching absolutely nothing inside the cabin. What could this possibly be? It's so strange to me that it's doing it every couple of seconds, rather than something happening every engine rotation or something.

Any theories?

 

[Sheriff41]

The idle speed sounds really fast. Also, what's the transmission fluid situation?

 

[will e]

To make sure I understand. The pulsing symptom happens without you touching the gas and only in drive, it does not happen when in reverse.

 

[tocruise]

The idle speed sounds really fast. Also, what's the transmission fluid situation?

I thought the same thing. I have Holley 600CFM (M2008-series: SUM-M08600VS) carb and it says it comes pre-tuned but it does seem to run quite fast at idle.

Because I have the most idiotic gauge cluster ever made by ford, I have no idea what RPMs I’m even getting.

Transmission fluid I think might be low. I’m going to top it up today.

 

[tocruise]

To make sure I understand. The pulsing symptom happens without you touching the gas and only in drive, it does not happen when in reverse.

It does seem that way, I think the first test I did, it did it in both reverse and drive, but then when I tested it again it only does it in drive. I’ll test again and confirm.

But yes, the clip I attached above I’m literally touching nothing. Once I get it into drive, I don’t touch it again and it starts doing that itself.

 

[giantpune]

I thought the same thing. I have Holley 600CFM (M2008-series: SUM-M08600VS) carb and it says it comes pre-tuned but it does seem to run quite fast at idle.

Because I have the most idiotic gauge cluster ever made by ford, I have no idea what RPMs I’m even getting.

Transmission fluid I think might be low. I’m going to top it up today.

They come pre-tuned in a manner that will start and run for most people. They are not dialed in. Most come out of the box running rich. Rich is safer than lean.

For your idle, is it sitting there on the fast idle cam? If you dont have that electric choke hooked up to power, it'll never open. You'll always be on the fast idle cam. Even if you do have it hooked up, it will still take a while to open up. Then you blip the throttle and get off the fast idle down to your normal idle speed.

That video sounds like its idling at 1500+. You could just be trying to idle it close to the stall speed of the converter. Or if the transmission fluid is low, there's not enough to do what it needs to do in the converter and in the transmission.

 

[tocruise]

They come pre-tuned in a manner that will start and run for most people. They are not dialed in. Most come out of the box running rich. Rich is safer than lean.

For your idle, is it sitting there on the fast idle cam? If you dont have that electric choke hooked up to power, it'll never open. You'll always be on the fast idle cam. Even if you do have it hooked up, it will still take a while to open up. Then you blip the throttle and get off the fast idle down to your normal idle speed.

That video sounds like its idling at 1500+. You could just be trying to idle it close to the stall speed of the converter. Or if the transmission fluid is low, there's not enough to do what it needs to do in the converter and in the transmission.

Ah I see what you mean. Okay, in that case I’ll mess with it and try get it tuned better. I can actually smell it’s running a bit rich I think. Do you think it’ll actually help with this issue though?

As for the idle, electric choke is hooked up. I’ll dial the idle speed screw to bring it down.

 

[will e]

I watched the video a few times. It certainly sounds like it is going into/out of gear. The engine speeds up when the car STOPs pulling forward and it 'sounds' about the same RPM as when you are in park when it does this. I would suspect low transmission fluid level, at least to start.

 

[tocruise]

I watched the video a few times. It certainly sounds like it is going into/out of gear. The engine speeds up when the car STOPs pulling forward and it 'sounds' about the same RPM as when you are in park when it does this. I would suspect low transmission fluid level, at least to start.

It does, doesn't it? It makes the "jolt" that most cars make when you shift them into gear. It's so strange with the interval that it does it at though. It's also weird because it jolts forward/up an inch, waits about a second, then drops back/down an inch, then 2 seconds letter repeats and so on.

I've inspected the transmission fluid with the engine cold, and it does measure low, but I've not really let it get hot enough to measure any other time. I did add a bit more today, but haven't run it since.

Tomorrow, I'll add more transmission fluid, try get it running up to temp, buy an RPM tach so I can measure what I'm getting, and then try tuning the carb and seeing if I can make it run a bit slower. I also thought I might try lifting the backend while I have the engine running and seeing if the tires spin at all. I know they're free to move (if I spin them by hand after jacking up they move freely) but I don't know if they're actually trying to move under the power of the engine.

I should add that I don't know when this car last driven. I bought it about a year ago and it was last registered in 1999. This is the first time it's probably been put into drive in 26 years.

 

[Hemikiller]

It's dropping in and out of gear, likely that your fluid level is low.

 

[will e]

I wouldn't mess with the carb too much at this point. Slowing down fast idle isn't hard to do so that might be worth it. Another trick is just turn the keyed ignition on but don't start and use a volt meter to confirm the electric choke is getting power. If you leave the ignition on for 20 seconds or so before you start it the choke will have already started to 'open' and will run at a slower speed.

 

[basstrix]

Have you checked the engine mounts? Maybe you have a broken left engine mount and the engine is torqing over and pulling on the throttle cable when in drive? Try disconnecting the throttle cable from the carb and see if it still does the surging.

 

[tocruise]

I wouldn't mess with the carb too much at this point. Slowing down fast idle isn't hard to do so that might be worth it. Another trick is just turn the keyed ignition on but don't start and use a volt meter to confirm the electric choke is getting power. If you leave the ignition on for 20 seconds or so before you start it the choke will have already started to 'open' and will run at a slower speed.

That's good to know. I will probably lower the fast idle because, as you say, it's quite easy to do and it's definitely too high. I'm going to have to do it at some point. From memory, I think the normal idle speed was actually okay. I'm going off hearing at the moment though, but I should have tach today to make sure it's correct.

 

[tocruise]

Have you checked the engine mounts? Maybe you have a broken left engine mount and the engine is torqing over and pulling on the throttle cable when in drive? Try disconnecting the throttle cable from the carb and see if it still does the surging.

Good shout. I'll check that today.

 

[tocruise]

It's dropping in and out of gear, likely that your fluid level is low.

It really looks that way. Let me get it up to temp and then check it again today.

 

[will e]

Have you checked the engine mounts? Maybe you have a broken left engine mount and the engine is torqing over and pulling on the throttle cable when in drive? Try disconnecting the throttle cable from the carb and see if it still does the surging.

It's a bit of a long shot but a good idea to check. If he has stock style mounts they are 'captive' and shouldn't allow this. My 73 had the wrong 'non-captive' mounts and the driver side was broken. It would do some weird things but it wouldn't torque the engine at idle. "Your results may vary" so your idea (and quick way to verify) is a good plan.

 

[tocruise]

Okay so I’m out working on the car now and I thought I’d give you all an update:

The first test I’ve just done is taken the throttle cable off (to test basstrix theory about the engine mounts), installed an RPM gauge to collect some data, and then made sure to see if the choke is opening.

The conclusion was that, even with the throttle cable off, it still jolts. The RPMs measure 2500 at fast idle which is just ridiculous so I’m going to lower that down to 1500 (which I think is the recommended RPMS at fast idle for a 2 valve 351C). I suspect the car is struggling to get into gear because the RPMS are simply too high. It would be like sitting with your foot on the throttle and shifting out of park. I’m surprised it’s not stalling.

With the engine running, the choke is partially open but doesn’t continue to open, and closes straight away if I turn the engine off. If I put the key to start without turning the engine over, I don’t see it opening at all, not even after a few minutes. Because it’s an aftermarket carb, I believe people usually hook this up to the red-yellow-hash wire that goes to the EGR solenoid and throttle position sensor, or the window wiper motor wire. I use the EGR solonoid wire. It measure 0V at key off, and 0.4V at key to start (engine off) which obviously isn’t correct at all.

I also found during this test that with my foot on the brake at ignition and then shifting all the way to drive without taking my foot off, it only starts jolting once I take my foot off. I dont know if this adds much context but I thought it was interesting.

Also checked the transmission fluid after letting it warm up for a minute or so and it’s even lower than what it is when it’s cold. I’ve added an extra quart but I may add more because it’s really low. I suspect if I can get it just above high with the engine cold then it’ll drop to about the right level once it’s warm (obviously this is something I’ll keep an eye on).

Keep the theories coming, but I’m going tackle this a bit more in the meantime.

 

[tocruise]

Found the culprit as far as the choke not opening…

 

[Don C]

When the transmission is cold, with engine at low idle and transmission in Park, the transmission fluid level should be between the Add line and the end of the dipstick. When the transmission fluid is warm and the engine is at low idle and the transmission in Park, the fluid level should be between the Add and Full lines.

 

[midlife]

I wouldn't mess with the carb too much at this point. Slowing down fast idle isn't hard to do so that might be worth it. Another trick is just turn the keyed ignition on but don't start and use a volt meter to confirm the electric choke is getting power. If you leave the ignition on for 20 seconds or so before you start it the choke will have already started to 'open' and will run at a slower speed.

If you do this, please remove the lead to the + side of the coil, particularly if you have points in the distributor. You can burn up the points this way.