Jittering engine when it is warm

This is about my 1986 Lincoln Continental 5L with 93800 miles.

Yes it is long to read and that is because I am hoping to avoid too many back and forth need to clarify my issues, so please take the time to acquaint yourself with my problem. Thank you kindly.

On Th Oct 8, I go in for my smog test. It failed. The timing has to be adjusted. I had to reschedule the test and the timing adjustment for the following Mon Oct 12.
On Sat Oct 10, work needed to be done on my doors by a freelance experienced mechanic and at the end it killed my battery which I recharged.
Mon Oct 12, I drive the car on the HWY 16 miles West bound then 19 miles East bound before I go for the timing adjustment and the smog re-test.
While I was driving back East bound I noticed the engine sounding louder than usual yet I also thought it might a car driving by that was loud. I gave it little attention.
At the exit ramp 19 miles later, when I accelerated after stopping, the car shook so much that it scared me. It was hesitating and jittering at the same time. It continued to do so while I reached the next stop over the bridge and while I drove up a hill all the way to the Smog Shop 4 miles farther. After getting off the Freeway where I went up to 70mph I stayed between 35mph and 45mph to keep up with traffic on surface street until I reach the smog shop.
There I revealed what the car just did and the mechanic advised that I get that fixed before I get it re-smogged. He was not interested in resolving the problem because it’s an old car.
Remembering my dad’s best friend telling me several times that my dad never tuned the car since he had it in Dec 1995, I checked the car file and found that that was true and that only the spark plugs were changed in 1998.
That following Sat Oct 17, the freelance mechanic came and tuned the car with new plugs, wires, distributor cap and rotors, no timing adjustment though. Some electrical tape was wrapped over the hole on the vacuum hose nearest the distributor since he did not plan to bring a replacement in case there would have been an issue. He claims the PCV valve to have passed his shaking check (Valve did pass the 1st smog test). He did a visual check of the fuel filter underneath the car in the back. The car ran smoothly after the job.
On Mon Oct 19, before going back to the Smog Shop, I drive about 10 miles between 55mph and 65mph on the freeway and then 35mph to 45mph on surface street for about a half hour. All went well. I’m thinking the tune up fixed the hesitation/jittering/shaking problem. Once at the shop, upon adjusting the timing, the control module came apart; the plastic housing was broken in half (I have the broken part). The mechanic replaced it as well as the vacuum hose that was wrapped with tape. The car passed the Smog Test.
Upon leaving the parking lot at the test station, I noticed the car hesitated/jittered just as it did last Monday, although somewhat subtle. I drove about 2 miles at 35mph, stopped and called the mechanic at the Smog station to tell him about it and he said that he had noticed the hesitation while it was running in the bay and he does not know what it could be since the car was tuned up and diplomatically indicated again that he was not interested in working on old cars.
I drove to the grocery store 2 miles away, spent about a half hour there and went home 6 miles away. The car had I guess cooled down some and therefor the jitter was subtle all the way home.
In my view, and based on my experience on Mon Oct 12, it seems that that reaction (hesitation/jittering) occurs when the car reaches a certain temperature.
I have not checked if the RPM jumps up or down whenever I accelerate when it jitters. One thing for sure is that when the jittering starts, it does it while the car is running and stationary, a bit more when you stop and go and more when you accelerate to a higher speed.
On Mon Oct 12, I had the air on and turned it off shortly after the jittering was felt; that did not affect the intensity. On Mon Oct 19, I did not turn the air on at all.
It seems that since the tune up, I can feel the car shift better and have never had issues with shifting anyways. Checked for slippage and that was negative.
Fluids are always where they are to be. Motor oil and filter were changed on Oct 7.

Today, I’ve decided to seek a possible answer to my problem on the internet and remembered CarTalk.
So far I’m finding out that it could be the Mass Air Flow Meter Sensor that needs cleaning, or an issue with the Temperature Sensor, the Balance Shaft, the Fuel Pressure Regulator, that a compression check may be needed.
Keep in mind that I don’t know if my car is even equipped with all that stuff since these findings are based on problem that people shared about their own cars.

I must not forget to reveal that 2 of the old plugs smelled like gas and 2 were oily and very dirty (I know that the gasket on the left hand side needs to be replaced again).

I sincerely hope that someone could shine a brighter light on my problem based on the info I’ve given.

Do you have a temp gauge or an idiot light for engine temp?

Since it changed with ignition tuneup, I’d start with another spark plug inspection.

Not so insightful. The jittering happened before the tune up and after the smog test I passed and in both instances the car was warm but not overheating that is no steam from under the hood and no warning from idiot light for coolant temp.

Barkydog, there is an idiot light. It did not come on. There was no sign of the car overheating.

That following Sat Oct 17, the freelance mechanic came and tuned the car with new plugs, wires, distributor cap and rotors, no timing adjustment though. Some electrical tape was wrapped over the hole on the vacuum hose nearest the distributor since he did not plan to bring a replacement in case there would have been an issue. He claims the PCV valve to have passed his shaking check (Valve did pass the 1st smog test). He did a visual check of the fuel filter underneath the car in the back. The car ran smoothly after the job.

So the last sentence is not correct?

I recall that the 86 model had throttle body injection and it can be difficult to diagnose. An analog DVM can be used to pull codes and that should be the next step. If the codes are posted here it is likely that someone will give some insight into solving the problem. If I had to throw something at it the intake air temperature sensor would be my first toss. And regardless of this problem if the mileage is mostly on city streets the timing chain may be on its last legs.

A faulty thermostat causing the engine to run too cool, is one plausible explanation for poor engine performance when supposedly warm. That is why I asked if you had a a gauge or an idiot light. An idiot light will only indicate overheating, but if you had a gauge to see if operating temp was coming up to normal that would have been useful. @dellat

Barkydog that’s an interesting thought. On Oct 12 when I drove the car West and East bound, I had the air (presumably the fan) on but not the AC. It was at 70F I think and I thought it felt too cold. The fan was also set on high so I set it back to low. It still felt too cold for 70 so I increased the temp to 80 and I actually noticed that the temp did not drop. I went up to 85 and still no noticeable drop. Shortly after that I exited after driving nearly 40 miles and because the car jittered quite a bit at acceleration at that point, I thought I better shut the air off (fan) so I can listen for unusual sound from the engine while driving to my actual destination.
At the moment 3 mechanics I’ve talk too are focusing on the fuel system because it had never been tuned.
One suggested a fuel pressure test and a leak down test.
Another said that those tests would be worthwhile only if I change the fuel filter which is likely to be the original before conducting those tests. He then suggested to pour some Berryman fuel injector cleaner in, which I did yesterday. I chose the Total Berryman, filled the car up and drove it for the next 30mns or so, stopping and going. It still hesitated of and on and a couple times it jerked hard but never killed the engine. I understand it would take some time for the product to really be beneficial. In fact I was advised to put another can in at my next fill up.
While driving around after the fill up yesterday, I stopped at my regular mechanic’s and he drove the car with me since the car was in its acting mode. I gave him the run down I posted and he suggested to change the fuel filter to begin with because it’s overdue and also suspected 1 or more injectors to be messed up or other stuff. Unfortunately for me most injectors are under a large cover. He knows I’ve put a can of Berryman in and said to see if that would help however a noticeable change will take a bit of time for it to be felt. This mechanic works along side his boss who is a cooling system specialist. I shall call him about your thought and tell him about my experience with the fan air being cold and not dropping even at 80.
Any more thoughts Barkydog?

Insightful, I get your confusion.
On Mon Oct12, the car jittered a lot after I drove it about 40 miles and specifically from the time I stopped and accelerated at the exit ramp.
On Sat Oct 17 after the tune up it ran smoothly when I took it for a short spin of 4 to 5 miles round trip on flat surface and up a 40 degrees climbing road. I drove maybe 15mns only.
On Mon Oct 19, I drove the car 30 mns or so before the smog test. It passed the test. Upon leaving the parking lot, it jittered again although not as rough as on Oct 12. When I called the mechanic at the shop he reported feeling a slight jitter too when it was running in the bay.
I hope that helps.

Rod Knox you’re going for the intake air temperature sensor first, Barkydog is going for the thermostat.
I have 2 converging thoughts here.
Let me share this as well about my drive yesterday.
I drove about 6.5 miles to Riebes from my house going up a low grade hill on the FWY for about 15mns. This time, sooner than it’s been, the car jittered slightly upon pulling in the parking lot and it continued to do so until I got home. The jitter was absent between 65mph and 70mph on the way home.
Prior to going home, however, after a short stop at the grocery store, while leaving the parking lot, the car still jittering not so roughly, stalls.
I quickly remember being told that if the fuel filter is an issue, your car can or will stall while driving (I know the car’s main computer can do that). Nonetheless, I start the car back up and it does and I drive home 6.5 miles away on the freeway, worried silly that it would stall while I’m driving.
So now, I’m wondering what else might have damaged in the car or what clue to the problem might it be?

Another thing you might want to consider is a intake air leak.

Cougar, could be.
I was advised it could be many things and I’ve learned that surfing too.
I’m adding your suggestion to my list.
Thank you.

Well people, this afternoon I decided to drive the car to help the fuel cleaner do its work.
I back out of the car port and as I began up the short drive way which is at a 40% grade, the car stalls.
It starts back up and I decide to pull it back in the carport.
There I let it idle and after 2mns, it stalled.
Waited a couple seconds and cranked it again, let it run and 3mns go by and it stalled.
I did that one more time, revved it up just a tiny bit for 5scds or so, let it idle and it stalled at about 3mns again.
Clogged fuel filter maybe?
Keep in mind it’s the original one from 1986; there are no records in my dad’s car file that it’s ever been changed.

If the fuel filter is clogged the engine would lose power when accelerating. The problem seems to occur when the computer goes into closed loop operation. Getting the codes could speed the diagnosis considerably.

That’s a good looking car, assuming it is the one in the photo. If it has some kind of built-in engine diagnostic system that outputs diagnostic codes, that’s the place to start first. Nothing like that, or the codes say everything is working? …

hmm … well, it sounds like the car was running fine even though it didn’t pass the first smog test, then engine started acting up, and the only thing that occurred prior to that was the first smog test and some work on the doors, which accidentally drained the battery.

If this were a newer car I’d say the drained battery caused the computer to forget the parameters needed to run the car in its present condition. And just driving it for a couple weeks would cause it to re-learn. But that’s unlikely in a car of this vintage.

Since it seems to occur when the engine is warmed up, first thing I’d do is to replace the thermostat. It might not help, but it shouldn’t hurt. You likely need to replace it anyway, if it is original to the car. And it is usually an inexpensive repair. Plus it could in fact solve the problem, if you are lucky. So cross your fingers. I had a thermostat go bad on my Corolla, caused the engine to run cooler that it was designed for, and it caused some difficult to diagnose symptoms.

After that, if I had this problem on my own car, my next step would be to bring all the routine maintenance up to date. And replace all the parts that are on the verge of falling apart. Hoses, electronic, connectors, etc. Doing that is probably the most economical and fastest way to get the car purring smoothly again. But it might be fairly expensive due to all the deferred maintenance in the past.

I’m assuming you want to take the risk of a low-cost fix … ok … I think what you call “jittering” is the engine misfiring. The gasoline smell on the spark plugs is an indication. Best guess is one or more of the plugs isn’t getting the correct spark. I think you’ve replaced the spark plugs, distributor cap, wires, right? And the electronic ignition module? The next-up, what I’d do is use a timing light to see if a spark pulse was going to each plug. If ther’s a pulse, the timing light will light in a regular sequence. I’d check the ignition timing at the same time. If that all looked ok, I’d put the car someplace dark, and see if I noticed any light flashes occurring in the engine compartment, indicating a spark was jumping somewhere it shouldn’t.

GeorgeSan Jose, yes I’d like to not have to spend a fortune but I have to be ready for it and realistic. Freelens mechanics do not come with guarantees.
Now 3 of you are pointing the finger at a temperature related part.
Confirming that plugs, wires, distributor cap, rotor, control module, 1 vacuum hose were replaced with new parts.

Now the car jitters some witin 6mns at start up and when left idling and if I keep it revved up some. Could thermostat or temp sensor still be the issue?
Am having it towed at the shop Mon where they will scan diagnose it. Am not driving to possibly stall in the middle of the road.
By the way before it was re-smogged, the timing that was off was adjusted from 4 to 10 where it’s expected to be, that is different from what you are suggesting, right? I will share that with the shop too.

Oh, I was aware of the car needing to be driven up to 100+ miles for the computer to resume complete communication with all parts of the car if your battery dies. That was why, I had driven that 40 miles and to warm the car up before the resmog, instead it jittered badly then and I had to fix the problem before resmoging hence the tune up.
And yes that is the car.

Both ignition timing and engine temperature problems could result in this symptom.

Hey all of you, once a tune up is done like I had done, would the timing adjust itself on its own?
And just in case, how proper is it to use a regular flash light to check the timing after a tune up like mine?
Finally, is it possible to do a visual check of the fuel filter without detaching it and say it’s good?

The timing can change on its own, as parts wear. Or if something wasn’t tightened up properly after the adjustment. Regular flash lights are not able to measure ignition timing. That requires a special kind of light called a timing light; i.e. a strobe light. A plugged fuel filter could show up on a fuel pressure test, but not always. It would depend on how plugged up it was.