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!

[Aurora40]

Originally Posted by bradslt5

which ones did you use accells a rc s

I used Delphi '93-95 injectors.

[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.

[Paul Workman]

Originally Posted by 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.

Thanks for that info, Bob, and yest it is a RED 90!. I need to eliminate all the other stuff first - pick those plums I can reach from the ground first, as it were. But, indications from the very knowledgable Z folks like you here and elsewhere seem to be inditing the injectors. I'll keep y'all posted!

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.

[bradslt5]

Originally Posted by Aurora40

I used Delphi '93-95 injectors.

Were did you get them and do they need anything special to go into @ 1990. thank you for any help