Stupid mechanics thread

it is the 3.0 BTW, it's pretty damn obvious since it says it right on the valve cover

But TBH, the only thing wrong with the car right now is a slightly rough idle that goes away as soon as you let go of the brake pedal. I've tested MAF, IAT, IAC, throttle pos sensor, even my fuel trim numbers are close to zero, which is amazing considering how old it is. Everything checks out as far as what would affect idle. (BTW this car does not have a MAP sensor). Edit: I also checked for vacuum leaks with propane and I got nothin. I suppose there was no need to check for vacuum leaks, since the fuel trim was damn near perfect, but there wasn't much left to test at that point.

What about a bad torque converter? It seems to make sense because the rough idle is NOT RPM related, since letting off the brake and letting it coast (not changing the rpms from an idle) makes everything smooth again.

And SOMETIMES, but very rarely, when it's idling, it stumbles as if it's going to die, then immediately springs back to life. The sensation is like when your driving a manual and you drop the clutch at a stop and stall the engine.

All things considered, this car has been THE most reliable car I've ever driven. You wouldn't believe how well this engine runs, ESP for one with 223,000 miles on it.

Just in case you were wondering, the engine I got was the ONLY one they made that year that still uses pushrods. And it also has an electronic distributer. So yeah, the engine is kinda old school technology.

I don't know shit about automatic trany's yet, but I do know that torque converters sort of take the place of a clutch in a manual. I can imagine that if it's not allowed to slip very well then it could cause vibration (which is mostly what I mean by rough idle).

Oh and I forgot to mention that if I put it into park or neutral, the rough idle goes away. It's ONLY when it's in drive and ONLY when the brakes are applied at a stop.

I WILL say however, that my GF's family had a 98 taurus with a 3.8 engine, and a 2000 windstar with the 3.8 engine, and BOTH of them crapped out before 200,000 miles. This 3.0 I got is still running FAR FAR better than those engines were at 150,000 miles.

Here's my guess as to why. When ford made the switch to OBD2 in 1996 (as everyone did), they hadn't worked out all the bugs and kinks of the new system. Also, right around that time ford starting outsourcing their manufacturing to places like mexico. So it's quite possible that a few years after the OBD2 switch, a lot of fords were crap.

my 94 uses old school technology that was well established and was tried and true at that point.

I could be wrong, but that's my theory.
 
I think the compression is supposed to be somewhere between 150 and 200 PSI?

And when you disable the ignition and crank the engine, do make 3 complete engine turns for compression testing or more? less?

I need to write that shit down. I forget alot
 
I would be suspect of the brake booster. The symptoms you describe would lead me to believe that. Of course the booster works off vacuum. And would make sense if you feel a stumble when you got your foot of the brake and then trying to take off. Take off the vacuum hose to the booster. Then remove the vacuum switch. Make sure its not plugged.

The issue you describe can be a difficult one to figure out since its a problem that doesnt happen consistantly. Could be lots of things really but the booster would be the first thing I would check.

Yes 3 revolutions is what you want to test compression. Compression isnt controlled by anything electronic. Its a combination of cylinder head chamber size (measured in CC's) and in the dome of the piston. Compression is lost one of 2 ways. Either down past the piston rings (in which case you would have blue smoke out the tail pipe) or up past the valves (this would warrant a valve job type thing). IIRC the 3.0 should have 180lbs of compression or so. A difference of only about 10lbs between all the cylinders is what you would want to find. Anything more than that indicates a problem.

The old ford IAC motors are an arrow in the knee for old fords. Im suprised it checked out ok. Typically those things will make your engine rev really high at idle. You can typically take those off, clean out the inside with some carb cleaner and put it back on.

There is indeed a MAP sensor for those. But they call it a DPFE sensor. Looks like a MAP sensor cause it also runs off vacuum but works a little different.
 
the thing is, I looked it up on alldata. Doesn't show a MAP sensor.

The MAF sensor on the other hand, on the description it says something like barometric pressure, which I would guess indicated that it measures pressure somehow. I'm thinking they worked some kind of pressure sensor/MAF assembly possibly? Or am I just interpreting it wrong?

Also, I can identify everything attached to the upper intake manifold, and I don't see a MAP anywhere.

Next time I get access to alldata I will type in DPFE and see what I get, have to wait till monday though.

The brake booster thing makes total sense, and I'm surprised I didn't think of it. Cause yeah that's how boosters work, they pull from intake vacuum. Plus, it only goes rough when I push the brakes at an idle. Thanks for the tip mate :)

Oh and BTW, I recently cleaned out the main EGR port by taking off the manifold, cause that shit was clogged as hell! Didn't seem make any difference, But I suppose I wouldn't cause EGR only opens under cruising and coasting.
 
To test out the booster i suggest disconnecting the big vacuum hose from the booster and plug it off with a bolt. Try to replicate the instance that you notice the stumble, and see if anything changes. Dont think i would suggest actually driving it without the bosster hooked up, instead chalk up the wheels, put it in drive and with YOUR FOOT FIRMLY ON THE BRAKES, give it some gas and see how the engine behaves.
 
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