I've been skeptical if there is some vacuum leak since the warm idle still goes up and down about ±50～100.
So I covered the air inlet on the air cleaner housing completely but the engine was still running as if nothing happened.
Does this mean there is a huge air leak or is it designed to breath the air from somewhere else????

Thanks in advance

Huge air leak from somewhere. All the air getting into the engine "should" be coming from the air filter housing. If you blocked that off, then it's coming from somewhere else...

--DD

Thanks DD!

But I don't see any other holes other than air cleaner inlet. That must be huge.

I've recently changed inner injector seals, intake runner gasket & hoses and other vacuum hoses.

What could be the problems?

1.) Expansion hoses on the plenum to runners

2.) Crack in the plenum

3.) Runner to head spacer/gasket

4.) Throttle body gasket

5.) Any number of defective hoses that connect directly to the plenum

Hello Ron, Thanks for the advise. I'll follow your guidance but...some questions came up.

1.) Expansion hoses on the plenum to runners
>Mines are new but I did not put clamps on them. I can feel by touching the air outlets on the plenum and intake runners are not perfectly aligned so the hoses may be stressed and leaked. Better to use the the clamps?

2.) Crack in the plenum
>What's the easiest way to inspect the dusty plenum? Just carb spray on them?

3.) Runner to head spacer/gasket
>I installed new one while ago but I did not use gasket sealer or anything on either side. Is this causing the air leak?

4.) Throttle body gasket
>It looked tired so I cut a gasket paper and replaced today but no change.

5.) Any number of defective hoses that connect directly to the plenum
>They are all new but I will triple check and get back here.

Thanks

How about removing the air cleaner and closing off the air intake with your hand. If the engine dies, great. That means you are looking in the right place (cleaner/hoses). If not, you've got a serious plenum leak or throttle body leak.

Eric

Yeah, you're right, Eric.

The engine died in a split second as I blocked the TB.

So I looked around the engine bay and found 3 suspicious points.

1) The rubber sleeve that goes into the bottom of the air cleaner outlet is not complete in the slit. There is about 1/8" room to go but it's so tight and does wanna go in.

2) I recently replaced the air filter(Mahle) and checked today. The rounded rubber edge of the top of filter does not look like being pressed at once. It is too clean. And there is no gasket between the air cleaner housing halves. Is this normal???

3)One hose comes from canister in the front may be disconnected and vacuum cleaning in the rocker panel. The rocker's never been opened in its life.

Which one do you think the most suspect?
Any input would be appreciated.

Thanks!

All of the items you mentioned are pre-TB / plenum and would not have an effect on the idle.

You never stated if the idle oscillates or just sometimes it's a little higher than others.

If it is a steady oscillation at idle (hunts @ idle), I would suspect:

1.) Point cam bushing in distributor is worn allowing side to side slop and the timing/dwell is changing (pretty obscure)

2.) To low of fuel pressure (lean mixture) or to high of fuel pressure (rich mixture)

3.) Injectors not flowing a same rate

4.) Trigger points could could be fouled

If it is when you come off the throttle while driving (ie, coming to a stop sign or light) ... a transient condition

1.) Throttle cable is sticking slightly

2.) Gunk around butterfly shaft in TB is not allowing it to return to same positiion each time.

Hey, just guessing at things I would check ........... +/- 50 to 100 rpm is not a major problem ... irritating, but not a great problem.

These things are not electronically controlled sophisicated engine management systems and they are old.

Anyway, read the article in the link that follows:

PBanders D-Jet Site

And bookmark it.

It doesn't seem to be a TB/Plenum issue. Work the issues above and let us know what you find...

Eric

Does your motor stop running if the air inlet on the air cleaner housing is blocked off?

I'm assuming you have a same model with D-jet set up like mine. So you are being asked as my role model.

Thanks

Find vacuum leaks fast with carb cleaner. Spray on hoses, connections and listen for a change in engine note.

I have been ignoring this issue since the car runs just fine. But the injector flow rate result from the injector cleaning service shows that one injector sprays more than other 3. So that is the reason for this hunting.

And today, I accidentally located 2 hair cracks on the top of the plenum where the 2 holes are seen. So I plugged JB weld in it. I will be happy to open the idle speed screw since it is closed almost to the limit.

Thanks for the all advises!

Did you look at the bottom of the plenum? These things rust out and can have pin holes in them that can barely be seen.

Plenums get tweeked when you don't have the accelerator stop on the floorboard, and an aggressive foot overtorques the throttle repeatedly. Ask me how I know.