All second opinions solicited here. Here are the details: 1995 F150 XL, 4.9L inline 6. The problem is at highway speeds, there is not enough vacum to operate the dash vent valves or the cruise control. Backing off the throttle and everything works well (cruise is iffy at times). I have checked and rechecked the entire sytem for leaks to no avail. At idle, vacum holds steady at 19 inches, upon acceleration there is a 7-8 inch drop in vacum before recovering back to 19 inches. From scouring the post here, I am on the beleif that it may be plugged cats. Before I place the green out for a new system, cats on back, any other areas I should test out to ferret out a simpler cause? Thanks, Karl

 

I have the same problem with my 86 4.9l i6. I have found a cracked intake, various small vac leaks and ended with the same readings as you are getting. My mph is between 12-13 there are still problems with engine.....I am beginning to believe ford designed it this way. The fix is to acellerate 10mph above speed limit then slow until vents work when going to slow repeat. Also the cruise control is disengages with wind gust or hills so only use when going downhill.

 

Gckk, any idea what the vacuum is while at stable cruise speed, and just how fast is your cruise speed?
Vac line from intake to dash controls has a check valve in it, and a vac reservoir on the cab end of the check valve. All to keep vac there to hold the controls still, when engine vac decreases due to load of hills or acceleration. Vac reservoir may be on engine facing side of the A/C evaporator, under the hood. An iffy check valve, or leak in res. can cause problems.
I believe vac for cruise control was always direct, for those that used vac to create the pull. They usually worked down to a couple inches of vacuum, before vac was so low that they didn't have enough muscle to pull throttle open against the return spring pull.

 

>Gckk, any idea what the vacuum
>is while at stable cruise
>speed, and just how fast
>is your cruise speed?
>Vac line from intake to dash
>controls has a check valve
>in it, and a vac
>reservoir on the cab end
>of the check valve. All
>to keep vac there to
>hold the controls still, when
>engine vac decreases due to
>load of hills or acceleration.
>Vac reservoir may be on
>engine facing side of the
>A/C evaporator, under the hood.
>An iffy check valve, or
>leak in res. can cause
>problems.
>I believe vac for cruise control
>was always direct, for those
>that used vac to create
>the pull. They usually worked
>down to a couple inches
>of vacuum, before vac was
>so low that they didn't
>have enough muscle to pull
>throttle open against the return
>spring pull.
One more thing, a 95 would have an electric motor for the cruise, not vacuum operated like older ones.

 

Hey greener, I'll put your notes right there on top. Appreciate your humor. I havn't checked the check valves nor the canister for leaks but will tonight. I havn't ran a hose from the manifold to the dash yet, but that seems like a logical next thing to do so that I can see the vacum through all ranges. As far as the cruise is concerned, it has only worked very sporadically and I'm usually doing around 65-70 when I try to engage. Sometimes it lopes in a 5 mile range and sometimes it is right on the money. I'll post the leak test later tonight.

 

As for the cat, I checked it by loosening the bolts to the exhaust manifold and retested on the highway. There was no difference in vac during accells or at different speeds and no noticable power increase but it was much louder so I assume that the cat is not clogged causing any problems.

 

Heavy storms wiped out any testing last night. Will reschedule tonight.

As for the cruise, there is a large vacum canister with direct manifold pressure hooked up with what appears to be the cruise control linkage. Maybe I am just assuming the cruise is vac controlled.

I like your style Greener. I have not found a way to test the cats, I even went to midas and they didn't seem to know how. Sounds like a test I need to do before I shell out the $250 for new cat system. Take care all.