merchauser

lets say all four of the mentioned bearings are leaking thru the seals: how significant would that register/effect
engine idle and overall performance, and exactly what symptoms would occur?

Speedtoys

Hunting idle runs rich..same as any moderately large vac leak.

merchauser

^^^^ wow; same as moderately large vacuum leak? I have had a VERY minor (50 or so rpm) so I hoping the new bearings
will completely eliminate that. was assuming that those seals would present little to no effect since they are so small and when
I did my smoke test, only the most faint trace of smoke was emanating from the passenger side TB bearing. on removal. all four
bearing seals were toast.

Speedtoys

Id say the would all together(4) leak more than a minor gasket issue, but less than a vacuum line totally off. If you only saw smoke from 1...

merchauser

^^^^^ thanks for input.

worf928

SwayBar

LOL!

worf928

First, let's keep in mind that the original factory-installed bearings leaked a little bit. If this super-tiny bit of unmeasured air had a deleterious effect then every 928 S4 would have had it from the factory.

If everything else about the engine and the engine management system is functioning correctly and is in-spec, the false air from the bearings would have no obviously noticeable effect once the oxygen sensor control loop activates. There would be an effect but it would be a very slightly lower duty cycle on the ISV pulse wave. During 02-loop warm-up you might have a very-slightly higher idle. But, during warm-up you do have a higher-idle so determining the effect of only false air from these leaky bearings is an academic exercise.

If everything else about the engine system is NOT ok then this additional false air might - might - result in a slightly-higher idle once the 02-loop is active.

False air that affects the entire engine causes issues when the amount is so large that there's no way for the engine management system to compensate. In some cases false air is only one manifestation of a failure mode and the additional manifestations can confound the function of the engine management system. (For example, pulling all the vacuum lines from the fuel pressure dampeners and regulator not only manifests as false air, but also as too-high fuel pressure.)

False air that affects a specific cylinder can cause issues.

All of the above is based upon an understanding of what does what and why it does it with respect to the engine management system along with very recent practical application. I've just finished a week-plus long odyssey diagnosing and repairing a GT with a surging unsteady cold and warm idle. This diagnosis included smoke testing the intake system and the only leaks were minuscule leaks from the throttle plate spindle bearings. This GT now idles exactly were it should at 775 and it is quite steady and those bearings are still leaky.

worf928

merchauser, if your idle is hunting up and down:
- warm only?
- cold only?
- both?
- what is your measurement device for the RPM? Your ears? Timing light? Sharktuner? Hammer?

merchauser

dave, thanks for the detailed information. I would not say for certain that I have the classic usual hunting idle. I got into the intake
because the WOT portion of my TPS has failed. in addition, the smoke test reveal leakage from one of the TB bearings, so I thought I would
replace all 4. here is photo of what all 4 bearings looked like. seals are crumbling.



even if they are only a minor contributor to vacuum issues, figured they just should be replaced.

I am at the point where I am fine tuning my car, and perhaps splitting hairs, or getting nitpicky. my car starts perfectly, and
gets to proper idle after about 30 seconds; by sound, it is rock steady. using the hammer, the reading will bounce between 743 and 795,
but I don't hear this difference in the car, or see my tach deviate at all.

after the car is warm, and at a stop light, I get three numbers on the hammer, bouncing from 743 to 848 to 795, in various combinations. I can
hear a very slight idle change by ear. tach does not seem to move, but the oil pressure needle will float, very slightly, in sync with the idle change.
(IMO: I don't think the hammer is a good tool for rpm since it only has certain fixed values displayed, as opposed to actual real time numbers)

and finally, after car is warm, driving for maybe 45 minutes, if I stop for a few minutes, and restart, car idles as previously described, but upon
take off, idle will dip down and attempt to stall. depressing the clutch allows idle to quickly recover. this does not always happen. could this be the
the idle contact portion of my TPS acting up?

in addition to replacing the TPS, roger suggested to replace the 28 year old ISV, even though I don't think I have an ISV issue? will order today.

many have told me that it is unreasonable to expect a dead rock steady idle of 775, and that all 928's will exhibit a slight "float" of idle

worf928

worf928

merchauser

dave: . CPS, O2, MAS, injectors, all done within 60 days. LH not touched. TPS to be replaced this week, along with the new bearings.

I don't think I have an ISV issue. agreed??
not exactly sure how to test, but I don't hear you recommending to replace? 28 years and 88k, I would think you would tell me to toss it?
roger has the lowe unit for 70. , wouldn't that be a no brainer and just take it off the table?

to be clear, I would not call this a hunting or surging idle that I have experience with other cars I have worked on; perhaps a better term
is a "floating" idle?? I cant stress how minor and innocuous this is, but wondering if my car should be spot on?

there are times, infrequent, and again minor, where I get a hanging idle for maybe 15 seconds; perhaps at 900 rpm

do any of these symptoms or the combination jump out at you and point to something specific?

worf928

I don't put anything except genuine Bosch-boxed parts under the intake since I have to warranty my work. Cost to diagnose and R&R a bad POS part is too high.

I might try the Lowe part on my SC'd '91 since it takes about 2 minutes to R&R the ISV unlike a stock 87+ where it's hours and hours of work.

I have never seen a dead S4/GT ISV. I have seen them dirty and sticky. A dirty/sticky ISV will cause actuation delay that could cause idle control loop to perform poorly or go unstable (engine dies, idle sticks high.)

They are easy to test and not too hard to clean. Plenty of threads on cleaning. Testing instructions in my inspection guide. Bench testing is obvious. Don't use a cleaner that leaves a residue (like WD-40) or will promote rust (like water.) I use 50/50 ATF/Acetone.

So far everything is consistent with a sticky idle switch and/or dirty ISV. If you want to try the Lowe ISV I can't stop you. But, you should still clean and test your old one and make sure both it the Lowe behave the same (unless your old one can't be made to operate as well as the new one.)

merchauser

^^^^^^^ thank you
would love to know what you did to remedy the idle issue on that GT? surprised to hear that you fixed
the issue without addressing the leaking shaft bearing(s)?....

should I have ROCK STEADY 775 idle on this 28 year old, or is a little float more the norm?
I may be searching for perfection, when perfection was not present in 1990? just trying to make this
the best I can.