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Old May 26th, 2011, 02:20 PM
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HawaiianF150 HawaiianF150 is offline

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The answer is probably three-fold;
1. replace the mod valve and the connecting vacuum tubing.

Once the rubber hose is contaminated it will leach out the absorbed the trans oil will break down the rubber hose and turn them hard and they will crack in time and continue to plaque you, your trans, the engine sensors, the computer timing electronics for a long time to come. Not too sure how transmission oil effects valve seats and seals.

2. Verify the return porting from the trans oil pump back to the trans sump is clear (internal valving - theres a small ball or 'check valve' that is either damaged, loose, stuck open or not installed - look in the trans valve block).

Youre getting back pressure from there and it is good chance it's the cause of the damaged the mod valve diaphragm. The diaphram is noramally under vacumm not pressure. One inch of mecury (vacumm) is equal to .5 psi. Thats a half of a pound of pressure. Normal engine vacumm is roughly 15-25 inches of mecury. That is roughly equal to 7 to 12.5 psi. The transmission oil pump is quite a bit higher. The engine vacumm is sucking up the oil. You should have seen red exhaust smoke prior.

3. The Transmission oil pump filter is plugged. Looks like a round tube covered by a fine mesh screen.

I recommend flushing the transmission and replace all vacumm hoses and filters connected to it.

Other than replacing the intake manifold gasket. use some intake cleaner through the carb with the engine above 2500 rpm for about 15 minutes. theres a good chance that other vacumm lines are contaminated too. trans oil is very thin - viscous- and it gets into everything.

The extent of conatmination can be checked by pulling off the vac hoses one at a time and checking with a white paper towel to see if you notice traces of reddish/black oil. if youre in doubt use a Q-Tip in the hose. Start at the intake and work backwards to determine . Spray cleaner will clean each hose but theres a chance they will dry out sooner than normally and need replacement. Rubber hoses usually has a five year shelf life. Of course nobody listens to that because we all have vehicles with rubber hoses that are older than dirt.

To put it in real world, check the cure date on your tires and add five years to it. Under normal situations such as weather and wear the tire rubber will start to show cracking around the sidewalls.

The pressure your experiencing is most likely the result of pressurization coming backward from the trans pump through the vent on the trans and into the vent coming from the gas tank to the intake manifold. the solution is to ensure the check valve in the gas tank vent or return line is installed and operating correctly.

thing to remember for all combustion engines is -
all vacumm goes to the engine in some form or fashion. there can be straight runs or canisters or some other do-hickeys. Bottom line is the intake is the ultimate source for all vacumm no matter if its the start or the finish point.

What we leave behind are our words and our deeds. Nothing else really matters.

Last edited by HawaiianF150; May 26th, 2011 at 03:04 PM.