A little history;
After thermostat stuck shut and overheated on the interstate, engine ran badly. Compression check found #'2 and 3 low. Rebuilt head installed, great compression, smooth running, no fast idle, noticed HVAC vents cycling under long periods of foot to the floorboard. checked vac with gauge and have steady 12 HG at idle, and near zero 5th gear in cruise. checked all vacuum tubes, sprayed brake cleaner all over everything,no results. disconnected and plugged all vac accessories from plenium(except vac guage), to include PCV and brake booster, no results. Replaced intake manifold with brand new one, and gaskets after finding tang broken off rear end of old manifold. same result. Removed front oxygen sensor and preformed exhaust backpressure test, no backpressure at idle and 2 PSI at about 2500 rpm. also removed EGR tube and plugged valve, still 12 HG after all that.

Some one tell me what I am missing. Is it possible that my engine is only capable of making that little vac ? It is my understanding that 21 to 23 is normal. My elevation here in Grenada Ms. is an astounding 213 MSL. I have devoted many many hours under the hood, and hundreds of hours online and thinking about this. Somemone please give me some relief.

 

Have you checked your ignition and cam timing? Any OBD2 codes?

Normal idle vacuum is 14 to 22.

 

No codes, and the Ignition timing is controlled by ECM. Valve timing in 4.9 is by solid gears.

 

Are you sure the distributor is not adjustable? Fords with distributors historically are set to 10 BTDC with the SPOUT connector off. I know it's solid gears, but is it possible it may be off?

 

I'll have to get the droplight out and check the distributor for adjustability and try to get timing light on it.. the engine runs really smooth and quiet, no vibration, no rattles, no blowby. when I put on new head, the valve cover and rockers had no sludge at all. Just had the pan off for new gasket and it was in the same condition. I'm 70 years old and have never saw an engine with 181k on it with so little wear. thats why I can't fathom whats causing the vac probs.

 

Eyeballed the distributor and looks to be adjustable, the only problem i have with it is finding the timing reference marks. on 95 and subsequent models they had notches in a plate near where my knock sensor is located, and reference marks on the dampner itself . that area on the 96 is almost totally obscured by various pulleys and belts. Having researched this a lot some say use the raised marks on the timing gear cover. and others sat they're the wrong ones. Any sage advice?

 

Read this thread:

https://www.f150online.com/forums/pr...cid-6-cyl.html

Post #2. 10 BTDC with the SPOUT connector disconnected. You may have to crawl underneath to see the marks on the pulley, when you find them mark them with white paint.

 

Thanks, I have no problem finding the marks on the dampener , It is only a few months old, had to replace after complete failure, so at least one problem shouldn't be present, the marks being in the wrong place because of slippage of the dampener on the hub. The 96 F150/ 96-97 F250 4.9 is one of a kind, that being said, I need to figure out where/what is the correct marking/pointer/reference on the engine. I suppose I can take off distributor cap and rotate to the #1 spark plug wire position and see if i can find the reference somewhere near the 0 deg dampener mark, only thing I can think of.

 

From that linked post:

 

ah ha, I checked out the link, and tomorrow I will check it out if these feeble old peepers can pick it out of the maze . Thanks.

 

Had to jack up and crawl underneath to spot pointer. scraped it bright with knife blade and marked 10 deg BTDC with a thin strip of HVAC aluminum tape. I will have to remove serpentine belt to get light on it but should be do-able. you cant even see the pointer from above with belt on. My timing light should arrive in a day or two. Hopefully good news follows soon.

 

The strangeness continues. I put the timing light on it and it was showing the marks on the pulley at about 30 deg ATDC. I could turn dist enough to make timing mark at 10 deg BTDC line up, but had no throttle response. I placed dist back to original position, and it is still idling smoothly and decent throttle response. I just installed new harmonic balancer maybe 5K miles back, so I don't think it's possible the pulley has slipped on the hub, maybe the aftermarket pulley was mismarked or put together wromg. I think I am going to remove the #1 sparkplug and determine when the piston is coming up on the compression stroke and use a plastic drinking straw to determine when it teaches the top where movement stops, mark the pulley with a dot under the timing pointer, and rotate further until the straw starts a downward movement, mark that, and the middle of those marks should be a close TDC position. Are there any major flaws in that plan? P.S. spout was removed.

 

You could time it by ear - advance it till you get spark knock, then retard it a bit.

 

Thats where I am now. I did have presence of mind to slip a 1/8 inch metal rod between the spark plug wire towers on the dist cap and mark the location of the dist on the head. I'm just a degree or two advance of that right now and running OK still with the 12 HG vacuum at idle. This may be one of the few mechanical/electrical/etc problems that I was never able to solve. Hater for it to end this way.

 

I went to the trouble of making a paint can/resistance wire/wick and oil smoke generator and associated air pressure line . Plugged the throttle body and checked for any major vacuum leaks. I found a small leak at the TPS and also at the other end of the butterfly rod. Another at the back side of the EGR valve. nothing significant. So at this point, I am giving up. If I ever do find the cause, I'll post it here, If not, Good day all and thanks for the attempted help. Merry Christmas.
Bob Mullen