I've got a 94 4.0L Automatic 180K miles. It’s my second Aerostar. I have a couple of minor problems that lead me to the conclusion that I have a vacuum leak.

1. Loaping (oscillation in idle) while at idle. After sitting at a light for about 20-30 seconds the engine starts loaping. Sometimes the whole van will rock forward and back.
2. Knocking while accelerating. This happens sometimes but not all the time. I use a fuel octane that is recommended by the owners manual. Haynes manual suggests a vacuum leak could cause both of these.

There is also another oddity with regards to the vacuum lines. Occasionally when starting, the engine will die and you will hear a sound like a sudden puff of air being released. A vacuum line will have come (blown?) loose from the connector at the rear driver’s side of the engine. A small line would come loose until I replaced the rubber boot on it. Now a larger hose comes loose.

My question is: Where can I get information on the vacuum system for this van. I really don’t have a good basic understanding of the vacuum system and the Haynes and the owner’s manual are vastly lacking in this information. How does the vacuum get generated and where is it routed to. Etc. Any help would be greatly appreciated.

 

It is unusual for 2 vacuum lines to come off. One possible problem is the vacuum modulator on the transmission starts to leak and the engine vacuum sucks up the fluid. This fluid softens the vacuum hoses and they fall off. Follow the vacuum line to the modulator on the side of the transmission. The vacuum modulator is only about $12 though it may be inconvient to get at.

 

Try to get a copy of the Ford E&VTM(Electrical & Vacum Trouble Shooting Manual) for your year van off ebay. I have one & it helps with all the vacum lines & wireing.

 

Thanks for the info. It's interesting that you mention the vacuum modulator. I had mine replaced about 4 months ago. So it is possible that the hoses have been softened by transmission fluid. Do you think I should just change the vacuum hoses?

I will look for the Ford E&VTM on Ebay. That's where I got the owner's manual for this van. Thanks again.

 

I have obtained the E&VTM from ebay. I also got it on CD. There is a little vacuum troubleshooting in the manual. But not much. It has only two diagrams of the vacuum system. One being only the heat/AC portion.

I guess the first thing I need to know and can't find in the manual is where is the vacuum canister and check valve? And is there more than one of each.

Second, from the vacuum tree on the rear driver's side of the engine, how many vacuum lines should there be and where do they go?

Also, does anyone have a copy of the vacuum decal that is normally under the hood for a 94 4.0 automatic?

With this information and the manual I may be able to fix it. Thanks.

 

After pondering this post for a good while, the oddity, to me, seems to be that the vacuum hoses appear to become pressurized and thus 'blown' off their mounting - just the opposite of a vacuum.

How then, would a vacuum system become pressurized? A loss of vacuum could easily induce the symptoms you mentioned in your first post -loaping at idle and knock under acceleration. However, at zero vacuum (steady state), there would be no pressure in the vacuum lines - and nothing to cause the line to pop loose.

The fact that you can pinpoint the occurence of a popped line during start-up is really perplexing. I can only think of three ways this could happen. All which seem at this point, wild guesses.

1. One or more bad valve seats pressurizing the crankcase and a faulty PCV valve venting pressure into the vacuum system.

2. The fuel system uses vacuum to passively operate the fuel vapor recovery system. A failing vapor cannister may be allowing pressure from a warmed gas tank to suddenly be released into the vacuum system.

3. (Wildest of all) On 4.0l engines, the coil pack ignites two cylinders simultaneously. The one under compression as well as a companion cylinder on its exhaust stroke to further clean up residual combustion byproducts. Concievably, a dirty or failing coil pack could be inducing pre-ignition in nearby cylinders on an intermittent basis. Along the same line, broken or weak plug wires could cause spark jump where the wires cross.

OK, I'm out on the proverbial limb here --- someone go ahead and saw it off!

There is a oblong, cylindrical, vacuum reservoir for the HVAC system located along the top, left side of the engine compartment, near the blower. There is also a vacuum line going to the Manifold Absolute Pressure (MAP) sensor located at the top, middle section of the engine compartment, just below the bottom edge of the cowl, encased in a black, paper box, looking affair. Check these for loose/missing/broken lines.

Very interesting problem, keep us advised.

 

In addition to what's already been said by the others, here are a few more things to think about.

Regarding the loping, I'm wondering if perhaps the torque converter clutch is trying to come on for some reason (either electrical or mechanical). Does it feel like a car with a manual transmission with the clutch still in or do you have to press harder on the brakes to remain stopped? If the rocking were side to side (rather than forward and back), I would suspect the rocking were due to the engine running roughly, unless there are big swings in the engine rpm. I can't remember if the enigne of the guy with the timing chain problem had any symptoms like you describe, but you might want to check out that discussion to see.

As aerocolorado pointed out, it takes pressure to blow the hoses off. In addition to a bad valve/seat causing pressurization of the manifold, backfiring could cause this and is an indication of a lean condition. To see if the vacuum leak is through any of the various hoses coming from the octopus on the manifold, remove all the hoses, plug the octopus and reconnect the hoses one by one with the engine running and see if you notice a change, or no change, in the idle. Or, you can use a vacuum pump to test each line separately. If all of the vacuum lines and the components are tight, then it suggests the problem is elsewhere (like a bad gasket). A somewhat common cause of a lean condition on the 4.0 is a bad lower manifold gasket. This is usually indicated by the presence of oil on one or more of the plugs in addition to pinging. If you havent' done a tune up in a while, it would be wise to do plugs and wires before you go too much farther in your troubleshooting.

Since air/fuel ratio is affected by the MAF, try cleaning it. When dirty, they have been known to cause pinging.

Regarding the generation of vacuum, it comes from the engine trying to draw more air in than can get through the throttle plate. Diesels have no throttle plate and that's why you'll find vacuum pumps on some of them (like Puegeots and Mercedes).

Regarding where the vacuum is routed, it goes to the fuel pressure regulator, the AC system, power brakes, cruise control, the tranny vacuum modulator (not sure that I got them all, but the list should be pretty complete). There should be a sticker under the hood with a crude diagram.

 

Greetings all,
For historical purposes or for anyone searching for solutions to their problems, I decided to update this thread.

Before I got any further in diagnosing the problem I had a catastrophic failure of the engine. A warped head. Water leaking into a cylinder and oil. One cylinder not firing. Overheating….

When you read the suggestions above by Aerocolorado, Mikeman, and Opera House Works, you will see some good suggestions. Some turned out to be very astute guesses. My hat is off to you guys.

Anyway here is what I’m talking about. I took the engine apart (in the van) to get the heads out and replace the head gaskets. See my other thread to read about it.
https://www.ford-trucks.com/forums/s...B%25252Bgasket
When taking it apart I found that there was no vacuum line from the vacuum tree to the Fuel Pressure Regulator. I guess there was an open port on the vacuum tree that I didn’t see. The machine shop who processed my heads said there was a bad valve seat that needed changed. After the new gaskets and reworked head were installed, everything works now without the loaping and pinging.

Now as to the advise/guesses mentioned above:

First was the guess about the vacuum modulator. Opera House Works was right on. It had already been replaced but I didn’t see the connection to the hoses coming loose. They now have clamps on them.

Second was Aerocolorado’s idea of a bad valve seat causing or allowing pressure to enter the vacuum system. I don’t know if that is what happened but there was a bad valve seat.

Mikeman answered my question as to where the vacuum came from and gave some good ideas on how to diagnose the problem.

In the end I have my van working again (better than before) and with the help of the Forum I fixed it myself.

 

Great info. Wish more posters would respond like this once their problem has been fixed.

Happy Thanksgiving to you and yours.

Ron