"Ballpark" inspection of MFI throttle bodies

[kenikh]

I just went through "Check, Measure, Adjust" and didn't find what I was looking for.

I would like to know a seat of the pants method for testing throttle bodies to know if they are probably in plug-and-play shape or not. Possible methods that come to mind are:

• Jiggling the throttle arms for play
  Holding the stacks up to the sun for light leakage
  (Gross) wrapping your lips around one end and blowing to feel how much air passes through the plates when closed.

Do any of you have any insights?

[chan1408]

"Ballpark" inspection of MFI throttle bodies

Hi Kenik - I'm giving you the long version in the hopes that some guys out with actual experience can give some more detailed, FACTUAL info on this arcane subject. IMHO, the TB's in good shape is one of the most overlooked contributors to poor running.

What to look for: (1) wear of the hard coat around the throttle plates - when worn, there will be a shiny line around the bore where the plates flutter around. This line can be felt with the finger when dis-assembled, but can make the "light tight" method of checking difficult; (2) clamp the TB's to a plate and measure the slop of the shaft with a dial indicator. If your car has more than ~ 50k orig miles, they'll probably be worn.

Too loose, and they'll clatter as much as a loose valve. I had idle speed ~ 1100 rpm with hot engine, throttle plate adjustment screws in to ~ 1/4 turn before throttle plate completely closed in bore, bleed screws damn near shut, and idle not smooth.

This is what I've done to date, because I think I can have a good rebuild jig completed for the amount (a grand) I'd spend at Eurometrix, or Supertec, and just because I like piddlin with this stuff:

First set, rebushed only, because did not have time nor spare set of TB's to do otherwise. Set throttle shaft w/ ~.001in/.002 to bushing id clearance.

Did not touch throttle bores tho they had distinct wear lines, that I could feel with finger. This improved idle quality and ability to get air flows equalized somewhat, but wasn't very stable - in other words, I'd have to re-synch every couple weeks to keep idle halfway smooth. Still had hard time getting idle speed where I wanted it, because of flow around throttle shafts from grooves in bores.

Bought spare set of TB's. Had surface plate made to mount TB's to, had TB's surfaced top and bottom, (you'd be surprised at the out of flat and parallel) rebushed the shaft bores, opened throttle bores .025in (.63mm) over, locating off throttle shafts (not the best way to do it) bought throttle plates from Kinsler, made a jig to turn them to a 7 degree closing angle (Kinsler is only set up to make at 14 degree) and re-assembled. Results: best idle I've had on car so far, able to get idle speed down, able to synch and remain stable (~ 2 months now).

Next steps: add to surface plate to create jig that allows boring of throttle bores and then perp and centered boring of throttle shafts, and finalize jig to turn throttle plates, and find out what kind of hardcoat the bores have. Also, I'm working on a simple flow bench to allow calibration and synching before mounting on car.

FWIW, the shafts should measure ~ .3125 inch (or ~7.94mm) od. On two sets I've re-worked, none have been more than .0005 away from this dimension, even tho the bores were very worn w/ deep grooves from the throttle plates. It's the throttle shaft bushings that wear. The bore in the TB's for the shaft bushings are about ~.400in (10mm) and I've bought some bronze bushings from Grainger which have .4 od and .312 id :-))

I've talked w Supertec, sounds like they are set up to do a complete rebuild, with the ability to get all surfaces re-established, and you're getting other work done there also as I recall (?). WTH, it's just another grand..... ;-)

If you've got oodles of spare time, and an inexpensive local machine shop, you can do it yourself like I've done, but you've gotta be a bit warped... I've got about ~ $400.00 in my jig so far. Didn't dare count the hours.

Input from those with more actual experience than me, please chime in. I'd really like a delta P vs mass flow curve data from the factory for small throttle openings.