Leak-down test...Analysis?

[Paul Workman]

To eliminate a possible leaking injector for my intermittent slightly rough idle, I took y'alls suggestion and bought a decent pressure gauge and did a leak-down test.

At idle the pressure was steady at 43#. I shut off the ignition, and after 20 minutes, it had leaked down by only 2#. I concluded the injectors are at least not leaking.

I'll "ohm" them out, soon as I can find my danged VOM...It's brand new too, and it seems to be avoiding my search, apparently. If they check out, then I will assume the injectors are prolly fine...for now.

If it isn't an injector, I'll prolly just conclude I have a lifter that wants to stick a bit after quite a bit of down time. When I fired it up to do the leak-down test, the car had been sitting for a couple weeks - some of that time in single digit temps. But, there was NO ticking sound as sometimes is the case - coupled with rough idle, rather, the idle was smooth as silk.

Mebby I'll fire it up in total darkness and see if that ticking I sometimes hear is actually a leaking sparkplug wire arching...??

In any case, I still have butterflies in my stomach over having a Z in the garage! It is my wildest dream come true and I can't get over it! Woo Hoo!!

P.

[bradslt5]

Yeah paul they are very cool cars. i had to buy another so i hopefully allways have one running . i get depressed if i cant drive my baby (named betty boop- black car or deb-redcar ) . just a thought the plug wires in these car fail due to heat alot .you might have an intermitten short there .i replace the wires in both cars . i used the ,msd wires. hope this helps and enjoy your baby

[Aurora40]

Your car is a '90, right? You can probably assume you have bad injectors or soon will, if they are original. Mine didn't leak down significantly either, nor did they ohm out too badly. But you could smell fuel in the intake even hours after it had run, and when I pulled them some of the injectors would form drops on the ends. The car ran a lot smoother at low-throttle after they were replaced.

[bradslt5]

which ones did you use accells a rc s

[tccrab]

Paul:

Check your MAP sensor hose and the PCV hoses too.
Vacuum leaks can cause all kinds of weird startup issues, especially if they leak when the engine is cold and then seal up when the engine gets hot.
The easiest way to check for leaks is to take a portable propane bottle (the kind the plumber uses), crack the valve just until you hear it hissing and then "spray" around all the vacuum hoses while the engine is idling. If there is a leak, the vacuum will suck the propane into the engine and the engine will momentarily idle higher. Viola! Leak found.

Good luck!

[Paul Workman]

Originally Posted by tccrab

Paul:

Check your MAP sensor hose and the PCV hoses too.
Vacuum leaks can cause all kinds of weird startup issues, especially if they leak when the engine is cold and then seal up when the engine gets hot.
The easiest way to check for leaks is to take a portable propane bottle (the kind the plumber uses), crack the valve just until you hear it hissing and then "spray" around all the vacuum hoses while the engine is idling. If there is a leak, the vacuum will suck the propane into the engine and the engine will momentarily idle higher. Viola! Leak found.

Good luck!

I'll check that. I was told the vehicle had been "gone over" and new injectors (some or all???) were installed...(by now my tongue is pushing fimly in the corner of my cheek) Well, I appreciate the tip. I will eliminate everything I can before going "plenum diving".

Though I'm not new to electronics (by profession) or mechanics (built lotsa SBCs, car restos, rebuilt some Muncie transmissions, etc), the LT5 is 'a different beast', to be sure! So! I appreciate any foot holds y'all can give me as I attempt to ascend the LT5 learning curve!

P.

[bldavis11]

It isn't that difficult of a motor to learn the basics. Now tearing the whole thing down to the crankshaft and rebuilding will be more involved than the standard SBC. Glad to hear you've got it running.

[Paul Workman]

Originally Posted by tccrab

Paul:

The easiest way to check for leaks is to take a portable propane bottle (the kind the plumber uses), crack the valve just until you hear it hissing and then "spray" around all the vacuum hoses while the engine is idling. If there is a leak, the vacuum will suck the propane into the engine and the engine will momentarily idle higher. Viola! Leak found.

Good luck!

After changing plugs (oh...that #8 plug was "interesting"), I tightened all the "zerbra" hose clamps on all the various lines coming/going to the plenum. a couple were faily loose too.

Then I re-torqued the intake plenum screws (posted farther down the thread), finding a couple that required 1/2 turn to come to spec.

After that, I fired her up and did the test you discribed. No telling what might have happened had I done the test before tightening stuff, but in any case, the idle was smooth and there was no reaction to the propane anywhere around the plenum gasket or any of the hose connections.

For what it's worth, it seemed to idle as smoothly as it ever does...By that I mean a nickle would not stand on edge as there seems to be at least one cylinder that is not in tune with the others. It isn't missing. But, it gives the idle a slight throbbing noticed when placing my hand on the plenum when it is idling.

Freezin my bunz off, so I called it good for today. Oh, I did find out why the primary fan always runs...The control wire running to the ECM has been intecepted and intentionally run to ground. So, now the question is "why"? .

Thanks for the propane tip. I put that one in my "special tips file"!

P.

[tomtom72]

I'm wondering that would be the place to tap in for a manual fan switch, if one wanted to install such a switch?????

I'd say you dunn a good day's work, Paul!

Me, well I f'ed off with my vette buds....man them blown C6's sure are a hand full.....I think I run outta nerve too quickly! ....but at least my carbon is in the atmosphere now.

I would venture a guess that all those "small" items you found were affecting the car adversely....I did the same on mine awhile ago & it added up to something when all was said & dunn.


Tom

Ya know that's interesting about the plug technology....the plain jane GM ones seem to work for my car. Maybe some day I'll play around with some different ones. Thanks for the info on the plugs!

[Aurora40]

Originally Posted by bradslt5

which ones did you use accells a rc s

I used Delphi '93-95 injectors.