Low vacuum

Bob_M · Aug 27, 2015

2003 3.0 V6 RWD ranger, 197,000 miles.

I know there are a lot of "low vacuum" threads but I didn't find what I was looking for so:

My idle vacuum is only 7 in Hg (3.5 psi), very hard brake pedal, rough idle, CEL will flash when idling for more than 10 seconds when engine is warm and getting misfire codes. If I raise engine rpm above 1200, flashing CEL goes solid and it just holds misfire code.

Replaced a few old vacuum hoses, pulled booster hose and plugged it, no change. Booster holds force with a shop vac. Tried spraying hoses, intake manifold, throttle body, numerous vacuum connections, pulled line from the vacuum tank & plugged it (this is where I got the 7 in Hg reading from), replaced coil pack & plugs, no change.

Strange thing is is that the truck will accelerate very strong.

I'm going to try compression test next.

Anyone know what idle vacuum pressure should be?

Thanks,

Bob

RonD · Aug 27, 2015

Vacuum is not engine specific, for the most part.

Gasoline engine should be 17-21" at idle, assuming sea level to 2,000ft, drop 1" per 1,000ft above 2,000ft

Good read here on vacuum tests: http://www.gregsengine.com/using-a-vacuum-gauge.html

And test your vacuum gauge on another vehicle, just to be sure gauge is reasonably accurate.

Worn timing chain and tensioner can produce low vacuum at idle but compression test would often show no problem.
3.0l with all spark plugs removed should show above 160psi on each cylinder for compression test.

You can test for timing chain wear using the CPS(cam position sensor) on the 3.0l
Remove the CPS from the synchronizer.
Use socket on the crank bolt to rotate crank in it's normal direction of travel, usually clockwise, Tab in synchronizer will move while you rotate crank.
Mark where the tab is then rotate crank in the opposite direction, rotate crank 180deg(1/2 turn), then see how far tab has moved, should be full 90deg(1/4 turn) if less timing chain is the issue.
Synchronizer runs off the cam which turns at half the speed of the crank, so 1 full turn of crank(360deg) = 1 half turn(180deg) of cam(synchronizer).

Repeat test a few time to be sure.

You should also hear a metallic sound while engine is running if timing chain is loose.

Bob_M · Aug 27, 2015

Thanks for all the info Ron.

I did a quick check of the vacuum gage before using it with my Mity Vac bleeder gage and it was right on as I pumped the bleeder up.

Thanks for the link too. Good info on vacuum diagnosis.

No timing chain noises but I'll follow your advice after I check compression.

197,000 miles but still more life in my truck, I'm not done with it yet.

Thanks,

Bob

RonD · Aug 27, 2015

With good power it won't be clogged exhaust
With no rattling sound timing chain is a long shot

I would think you could hear a 10" Hg vacuum leak

After engine warms up what is the idle rpm, should be 650-750
After warm up unplug IAC Valve, it will close and idle should drop to 500rpm or engine may even stall, either is OK it means no vacuum leak.

If RPMs stay high start testing for leaks with IAC Valve unplugged.

On cold engine you can loosen and remove fan belt from crank pulley.
Then start engine, it can run a few minutes without water pump, but only 2 or 3 minutes max.
This takes away fan and pulley noises, possibly making a vacuum leak hear-able.

Bob_M · Aug 28, 2015

Thanks ron,

Idle is still normal speed but the engine misses and shakes a little.

I can't hear any leak but I will follow your good advice and disconnect the belt and then try to disconnect the IAC too. I'll use my listening stethiscope to try to hear a leak.

My last experience was with an IAC valve on my Buick Grand National and it used a stepper motor to open and close a pintle on an air bypass port around the closed throttle body to maintain idle. Idle was hunting up and down due to carbon build up and it would never close so IAC had no control over idle speed.

If the IAC is cycling and open slightly when I disconnect it, won't it just stay in the position where it was when it lost power or does it have a spring return to close it?

Thanks again for the help.

Bob

Low vacuum

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