Mechanics

VACUUM PRESSURE WHILE CRUISING

2008 Ford F-150 • 74,000 miles

I have 22 in. Hg at idle out of gear. 20 in gear at idle. (Very good from my experience) while accelerating it drops significantly (as it should) however while driving at a constant speed with or without cruise control on or off it remains very low. It varies between 5-8 in. Hg as if the ac compressor cycling on and off is dropping the vac. However turning ac off does not matter (even unplugged the ac clutch from the harness to make sure. It will stay at 8 in. For about 45 seconds then drop to 5 in and my gauge twitches fast (speed of a reed valve on an air compressor open and closing). Disableing Overdrive doesn't effect it. Check all vac. Lines I could find and only problem is a blown check valve. But 8 inches down the hose there is a 2nd identical check valve and nothing is branched off the hose inbetween them. So question is that 5 in. Hg while cruising a problem? First Ford I've had and seems very low too me. I don't hot rod her but did just for diagnose purposes and I get positive pressure! Vac. Gauge is teed right after the brake booster. Actually between the two check valves discribed earlier.
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Bilskie87
September 17, 2011.




That seems low. This may sound off the wall, but could the IAC be opening and causing a drop? Also, have you checked all vacuum to the EVAP system?
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It doesn't have a traditional IAC. It has electronic pedals so the throttle plate (single butterfly in my case) is controlled by an electric motor/IAC unit. It mimics an IAC by openening the throttle plate at idle. I checked all hoses for the evap but didn't see the need for any component tests because of numerous system check sensors for the evap system. Some self check sensors are: evap valve flow monitoring (think MAF), leakdown detection (think map to check for bad vac. Hoses). Evap valve position monitoring (comparison between evap valve return voltage and engine map, temp, TPS, and previous two check sensors) so seemed to be capable of detecting any problems and displays a MIL. I do have access to refernce volts for set conditions (alldata pinpoint testing). But no MIL on the dash so didn't bother.
FYI the pvc valve is "hi-tech" also. Rh valve cover has a spring loaded screen/reed type valve that's made into the valve cover. Spring loaded to offer resistance to raise crank case pressure slightly to prevent natural blowby from healthy engines (from what I understand). It connects with a hose to a heated tube in the intake manifold (Ford calls heated pvc valve). Its shaped to control amount of flow from the engine and only heats when below 30 degrees. (Says alldata). Other valve cover is open to intake after MAF but before the tb to allow open flow into crank  case.
At idle When I press the brake in the vac. Drops <1 in. Hg but when releasing the pedal it drops ~7 in. Hg. I've never paid attention in past but seems a bit much.
The 5in. Hg is at any speed. 30mph is same in. Hg as 75 mph


Tiny
Bilskie87
Sep 17, 2011.
You've done your homework. Don't you love technology? Have you checked to see if the brake booster is leaking? It shouldn't drop when you release the brake.


Ha thanks. Ya I wanted to be a mechanic when I was younger. Even had a starting position in a NASCAR drivers main garage held for me. (Cleaning parts Prolly) but affraid making my hobby a job would make it unenjoyable. I did a "test" of the boosters vac inlet check valve. I removed the inlet fitting from the booster after engine was off for 2 hours and it had plenty of vac. Inside it. Kinda scared myself cuz I was adding a T for my vac. Gauge to run in the cab to see how it acts while driving and forgot it was a check valve. Possible I ruptured the diaphram by pressurizing it so fast? I was diagnosing vibration but only had idle vac. Reads at that point. I have iwe (integrated wheel ends) that unlock the hubs from cv axles when vac. Is applied. And locked with no vac. This vac. Line comes from a T at booster (the check vakve hose I mentioned) it has its own vac. Reserve another check valve before the selonoid that applies vac. In 2wd (cv-t for vac gauge-cv-t to vac box and to 3rd cv-selonoid- to iwes) so my vibration my be from such low vac to slight positive pressures in vac. Lines allowing check valves to open slighty? Might be reason for 2 cv right by eachother? And eninge has nasty shakes. Almost makes harm. Balancer look loose or it is loose lol.

I'm about to unplug sensors one at a time to see which if maybe bad map is allowing such lean mix that vac. Isn't being mad or fuel temp/press. Sensor is dictating too low fuel press.

Ps. Fuel rail has a temp/pressure sensor that used vac. As refernce for optimal rail pressure at current engine load. Is think it would be very rich and obviously smell as such if it was allowing fuel into vac line.


Tiny
Bilskie87
Sep 17, 2011.
Sorry for the novels its just so complex and different from the average vehicle. Seems to be something done, used, pcm looks at, etc after the tp is opened past idle.


Tiny
Bilskie87
Sep 17, 2011.
I watched the booster and master and the "flex" away from the fire wall when the brake is applied. I haven't dug into it yet. (Working 75+ hours a week make for little free time). It goes into dealer Wednesday for warranty evaluation. Just don't trust a dealer doing warranty to to correct the cause of a failure just to replace the obvious.

Booster makes sense cuz rotors are. Orange, pads r glazed but thick, wheel bearings shot, both diffs oil doesn't look promising, thumps like the spider gears are bad, wheels cracking at lugs, leaf springs appear axle wrapped occasionally, iwe rusted and blued from heat, rust between where the wheels meet with the rotors and look. Can't describe just look at the pics, and sounds like hubs r trying to lock in but not sure. All can be traced to brake and vac. Problems.


Tiny
Bilskie87
Sep 18, 2011.
Looks like a 4x4? If the front seals leak then it will have low vacuum the 4x4 is vacuum driven is it not? Also check the vacuum reserve tank for leaks.


Racefan966
Sep 27, 2011.
Yes it is 4x4 and yes vacuum driven. Vac. Applied to disengage the hubs and atmosphere pressure engages them. Failsafe design to ensure 4x4 says Ford cuz u can rip off a vac line to engage the hubs if trouble ensues. Id rather be stuck in mud than have gears grind on the highway if something fails. Suck as a leak in a rubber hose. Ha. But anyways it is at a Ford dealer now and was told everything is fine except TRANNY mount is broke. The blued metal in those pics says otherwise. I believe the hubs are loosing vac and trying to engage while cruising. Rust in a vac. Area is not right. Wheel bearings are catching and stiff. (Effort to get them to turn then they break free and are smooth. Dealer said they r fine cuz they aren't loose) those two combined are causing massive heat generation causing the blueing and vibration. Also explains the "rings" on the hubs and grooves in my rims. Aluminum doesn't just softening up and marking hubs and leaving grooves in rims. U know how aluminum turns your hands silver. Few other problems mostly unrelated the dealer says are fine but obviously not. (1/2" of play in bearings where cv axle meets front diff, 100rpm drop when selecting manual 2nd at parked idle for split second then back up to normal, TSB 09-20-14 uncommanded tcc apply. Gradual Idle rpm rise. Ex. 200 after 15 min. Of idleing) Sry for rambling just aggervated and first dealer never reported any visit to Ford warranty and refuses to show me their personal records which he admitted to having 8+ dates that r not on Ford service report.


Tiny
Bilskie87
Sep 27, 2011.
Sounds like you are on the right track to me. I would be aggravated too.


Racefan966
Sep 28, 2011.
So I guess since I kinda answered my own question. The real question is. What do I do about dealer #1 not reporting to ford and what do I do about 2 dealers saying nothing is wrong when obviously there is? It's not a blown speaker its a drivability and possibly a safety concern.


Tiny
Bilskie87
Sep 28, 2011.

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