I have a 75 with 2.0 GA engine. I replaced the CB Performance FI since it ran too rich and installed D-jet FI.
Spent the last year and a half going over everything.

Replaced all the hoses and gaskets, set the valves, MPS is good, etc. Compression is about 110 in all cylinders cold.

I’m down to a low vacuum reading of 6” hg. at 1100 rpm.

AAR closes but I plugged it anyway. Sprayed starter fluid all over the place and no surges.
Plenum was removed and all ports plugged, shined a flashlight inside in a dark room and no leaks.

With air filter removed I blocked off the throttle body with a sheet of rubber and a wood block and it still runs.
Idles a little slower at about 900 but strong like a tractor.

How does my engine run with the throttle body completely blocked off?

Before I pull this engine is there any chance there’s a vacuum leak somewhere else?

Can a modified cam give a vacuum signature this low? I know overlap is same as stock but lift might not be.

Any thoughts?

If you can close off the throttle body and the engine still runs, you have a major leak somewhere.

1.) Around the mating of the manifold to the head

2.) Rubber slip joints on plenum to runners

3.) Throttle body gasket

4.) Stacked elbow is cracked

5.) Somebody drilled a hole in the plenum

6.) No seals on the injectors

7.) Somebody drilled holes in the runners

8.) Cold start valve air inlet is not hooked up (hose comes from AAR)

9.) Oh well .....................

You might not see a crack in a plenum till it either hot or under vacuum. I would also guess if the MPS leaked bad enough it might still idle.
Could be a severely worn shaft on the TB.

And..what you gonna do with the CB injection?

Thanks Ron – all good points.

1. Replaced the gaskets and torqued to spec
2. Replaced the slip joint and band clamped
3. Replaced TB gasket
4. Stacked elbow? PCV? - it is working properly
5. No holes in the plenum – checked for holes with flashlight
6. Injectors and seals are new
7. Don’t see any holes in the runners. Sprayed starter fluid all around the welds
8. Cold start valve inlet is connected and AAR intake is plugged
9. “Oh well” seems popular lately

Good suggestions, thanks.

Vacuum is low (6"hg) right from the start (cold). I'll spray starter fluid all around it some more to make sure.

MPS is rebuilt and holds vacuum

TB looks ok but will look more closely

CB went to another member here, sorry.

I'd think with a big leak you could hear it whistle - maybe not.
Since you know the MPS holds a vacuum, try removing the hose and plug it. Then try pressurizing the intake - run a hose through a rubber ball or something and jam it into the throttle body. You may be able to hear or feel air leaking out somewhere. Just a few PSI should be enough.

Some shops have a smoke machine for this purpose, shade-tree guys like us, probably not...

Stacked Elbow .... PCV valve is connected to it via large diameter hose. The deaccel valve or distributor can be connected to the small inlet.

You might also want to check and see if your PCV valve is operational. If they used the valve (oil filler assembly) from a GC engine ('75 - '76 2.0L) it will be just a hole with no restriction in it, which will be a gigantic vacuum leak. Plug it momentarily, cover the throttle body and see if the car dies.

I agree with the other member who said you have a leak and have not found it.

I have just gone thu the very same thing with my 75 2.0-ga, D-jet. - i thought i had checked everything but still could not eliminate the hunting idle problem, -
- if you drop the idle low enough with a vac leak it will start to hunt, at that point,
you spray starter at these areas and it will level off when it sucks it in-- i'll bet you are leaking at the rubber boots that connect the plenum to the runners- after i had eliminated all other possibilities- it came down to that(#2 on that list on other post)

i am just getting ready to re-install my system - ordered new intake runner gaskets - throttle body gaskets and runner boots- if you do this i'll bet you will have it.

phil

Aftermarket cams can, and usually do, cause poor vacuum at idle. D-jet HATES "performance" cams.

The Cap'n

But how does a performance cam'd engine work without any air getting through the throttle body? He says he sprayed starter fluid over the common vacuum leak areas without any rpm change.

I was addressing only the question about the cam profile. He obviously has additional issues.

The Cap'n

Ok, stacked elbow is old but doesn't look cracked. While running I unplugged it from the plenum and idle and vac went up a bit, but then calmed back down when I plugged the port.

Oil filler assembly is the GA type with the PCV valve intact. I dissasembled it and made sure it was working per PBAnders site. I will pay more attention to that area next time I run it though.

I've never really had a "hunting" problem, probably since it runs so damn rich. I replaced all of the gaskets you mentioned, and even clamped the runner boots. But sounds like a good idea to drop the idle way down and then spray around - to make it more apparent if/when it sucks it in.

Good luck with yours and let me know what your vacuum reading is when you get it running.

Bryan

Ok, so can an engine ever run with no air through the throttle body? Have you or anyone out there ever seen this?

Put a shop vac in blow mode and tape the hose to your TB. Spray soapy water on the intake. You will find the leak.

As far as rich running, I haven't seen you post anything about fuel pressure being set / tested as yet. 29.4 psi is the target. If well above that, it can run rich and if high enough it will override the injector closing spring and flow fuel.

Also, who rebuilt the MPS? While it may hold vacuum, did you check the Ohms between the terminals and/or shorts to ground?

A long overlap cam will not produce a great amount of vacuum at idle speeds.

Fuel pressure is about 28 psi. My gauge isn't that accurate.

MPS was rebuilt by Fuel Injection Corp. I checked the ohms when I bought it and it measured correctly but I don't recall the values.

I don't think the overlap is much different than stock. Put a paperboard disk on my drivers side wheel while the car was set up to do the valves. Put it in 4th gear (1 to 1 ratio). Made a mark on the disk at TDC for #1 and then started turning the wheel. As the valves opened and closed made additional marks on the disk. Could see the range of overlap graphically on the disk. Then took the paper disk to my friends 73 2.0 and did the same thing. All the points matched, so I believe the overlap is pretty much stock.

Valve lift on the otherhand might not be stock – hard to measure that. Could that also lend to low vacuum at idle?

See it many times. It'll crack on the underside right next to the tube for the through-bolt.

You won't see it from above, you have to actually remove the plenum and carefully inspect the underside.

.....Spray some carb cleaner(spray) around the connections
it might help find the leak
amd then take into account what other's have said!
(a pin hole in the plenum, is really hard to find?)

I knew i had a pic somewhere ...

Underside of the plenum, on one of the two bolt-holes, hairline crack that started at the weld.

will do.- you are right, running rich will help compensate, if i fool mine into running richer , like with changing the resistance in the CHT circuit, it will level out the idle some, but its not the cure or the long term solution- ask me how i know.

the good Capt'n s advice was such some time ago - keep looking to find that
leak(s) then get the timing set, and fuel/air mix and all will be good!

That trick works on air-flow measuring systems (like carburetors, AFC, CIS) because they operate on the principle that leaks mean unmetered air and cause leanness. When you temporarily replace the air leak with a fuel source (by spraying something combustable like carb cleaner or propane) the leanness goes away and you can hear it in the engine response.

Unfortunately, with D-Jet, a vacuum leak means - low vacuum - which the ECU interprets as 'Need More Fuel.' So D-Jet vacuum leaks can result in too-rich operation (as in this case, probably) and further enrichening the mixture at the leak source may not tell you much.

Ok thanks. That explains why the spray didn't work. Now to find the pin-hole or hairline crack. This is progress!