Weird Jerkiness after 45 min - 91

I own grandpa's 91 Town Car with several fairly recent updates, including a new egr valve, egr line, idle sensor, intake manifold gaskets, alternator, wires, plugs, and battery among other things. The tansmission was rebuilt about 8 years ago and runs smooth. After driving the car for a period of about 45 minutes, especially in warm weather, the car will begin jerking and hesitate while shifting, during acceleration, or when coming to a complete stop. The vehicle acts as though it's going to stall, but never does. The sensation reminds me of a choking carb. and you feel the whole engine shake. When given a little gas, it stops. The actual shift itself remains smooth. During all this, the damper door will also open and cycle the air through all the openings before returning to the designated selection. This jerky routine will continue for the duration of the drive once it has begun and can only be temporarily pulled out of it by punching the gas. Once the vehicle shifts again, it starts it up again. When the car sits idle for several hours, the whole cycle starts over. I have also noticed that a heavy foot and/or aggressive driving on the highway will bring on the issue without reaching the 45 min time frame. Again, the transmission shifts smoothly through all this. All the vaccum lines have been checked and suction is good. I believe the check valve is bad, thus explaining the damper door issue, but something is still pulling on that vaccum in the first place. Air conditioning on/off makes no difference to the problem. In cooler weather, especially during the winter, the car drives 100 % normal. The car is not overheating during any of this, and fuel economy averages about 18.5 mpg. I am at a total loss for what is causing this weather-related issue. I suspect there is a peak temperature being reached to trigger the cycle, and the 45 minutes just happens to be about the time that temp. is reached. It has gotten to the point where I no longer feel comfortable even driving the car much over 25 minutes. The check engine light is off right now, but comes on periodically. When diagnosed a year ago, it only read codes for the egr and brakes. The egr was replaced and the brakes operate normal. There are no leaks with the emergency brake release either. Does anyone have a clue how to resolve this rather bizzare quirk and what is causing it ? I also suspect the O2 sensors are involved, but surley they are not causing all of this.

Hi and welcome to the forum!

I'm sorry you had to replace all those parts to no avail. There is a passage under the intake tube which directs the EGR flow from the egr valve. With time this passage becomes clogged with carbon mixed with oil from the PCV.

You can clean it yourself and takes a couple of hours the first time you do it. I would advise you to remove the wiper cowl in order to have better access to the back of the engine.

Also make sure you buy a new throttle body adapter gasket first. When installing it DON'T use any sealant.

Click here (http://www.lincolnsonline.com/tech/00015.html)

Oscar.

I forgot to say that the check engine light coming on and off is one of the main symptoms when EGR channels are clogged. This usually happens during decceleration.

As an additional note you might want to disconnect the harness running to the MAF sensor, starting the engine and driving the car for a while, that way you will find out if your MAF is causing any problems.

NOTE that you need to clean those EGR passages in any case.

Thanks. I'm going to check into that. Most of the work I do myself, but last year I had the neighbor do the intake manifold gaskets for me. I'm going to check with him to see if he recalls cleaning that area out. He's pretty through, but you never know. If he can't recall, I guess I will be cleaning it out. I appreciate the help. Anyone else that has other ideas about the jerky cycle please let me know.

I was wondeirng if you tested fuel pressure, sometimes fuel pumps can make the engine to fuel starve when they start going bad, after they cool down will start working normally.

Using a fuel pressure gauge tester measure how many PSI you're getting from the schradder valve on the passenger side fuel rail.

Key on Engine off - 46 PSI
Engine running - 32 - 36 PSI