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!! :thumbsup:

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]

which ones did you use accells a rc s
I used Delphi '93-95 injectors.

[Paul Workman]

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]

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]

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

[Jeffvette]

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.

Paul, don't worry about a leak down over a 20 minute period. What's the leak down after 30 seconds?

Also injectors can fail on several issues. One being stuck open or closed, but also in the nozzle with puddling or not atomizing correctly.

[Paul Workman]

Paul, don't worry about a leak down over a 20 minute period. What's the leak down after 30 seconds?

Also injectors can fail on several issues. One being stuck open or closed, but also in the nozzle with puddling or not atomizing correctly.
Thanks for that info, Jeff. After 30 seconds, there was no appreciable drop in pressure that I noticed. Yes, I understand about "other issues" injectors might have; e.g. sticking, puddling, etc. I was doing the test looking for an "ah-HA!" kind of reason to go plenum diving. I'll check for vacume "leaks" and run a dynamic scan when it is acting up and see what that turns up before diving in. (I prefer to troubleshoot by isolating the problem, as you do, rather than "shotgunning" i.e. changing parts until the problem goes away as they love to do at most repair shops.) So, any other clues you might have, and I'm all ears.

I might mention too, that after it has been redlined a couple times on an outing, the problems seem to go away. The stumbiling only returned after 5-6 periods of idling for about 20 minutes at a time until normal operating temp was reached. I guess that would be 6x20 or about 2 hours of idling:rolleyes: and most of that time the temp was significantly below operating temp...Methinks that is a pretty tall order for any vehicle, now that I think about it...[sigh].

I've about concluded that injectors might be necessary, but I think I'll change the routine and avoid idling - waiting for those occational dry days for a few winter joyrides and see what effect that has on the stumbling characteristic.

My only point of reference was my previous '95 LT1, and it didn't seem to get "loaded up" as result of idling in the same fasion. But, this LT5 is new to me, and so I'm not sure what is or is not normal. I just may have to pull the plenum and "do" the injectors and see what happens (hence my quest for info on LT5 characteristics...I'm in "learn" mode in that regard!:wink:).

Jeff, your comments (and those of the others too) are always welcome - and I'm grateful to have 'em. I certainly would like to fast-track my schooling on this classic Corvette Z as is possible! Now that I have the FSM (thanks, Santa), I kin get me some book learnin' an be paper shmardt at least!:mrgreen: Now I can really get into trouble!!

P.

[HIZNHRZ]

Haven't done a leakdown test on an LT5 but I have wrestled with a rough idle problem. I agree that eliminating prossible causes one at a time is the way to go. Jeff probably knows better than anyone but it seems to me that a likely root cause for rough idle once injectors are ruled out are vacuum leaks. My injectors turned out to be a significant cause of my problems but under-the-plenum (poor) seals on some very old vacuum lines was also a major issue.

[Aurora40]

One quick way to tell if air leaking in is a problem is to hook up a scan tool/laptop and look at the IAC counts at idle. If there are counts, then the ECM is in control of the idle. If there are zero counts, you have a reasonable leak somewhere (especially if the idle is above the desired idle rpm).

[Jeffvette]

Paul, let's start with something simple. Change the plugs. Try the NGK 6341 plugs.

[Paul Workman]

Paul, let's start with something simple. Change the plugs. Try the NGK 6341 plugs.
Check! I've got em coming and will be in hand tomorrow morning.

While I'm at it, I'll try the propane trick and check for air leaks as well - at least in the accessible parts of the plenum and hose connections. Stay tuned:wink:! (Ahl B back!!)

P.

[Jeffvette]

Paul, I think what you will find is the plugs will clean up your miss. It doesn't sound like you are having any other issues. I'm guessing your plugs are probably originals.

[Tyler Townsley]

Also it can be old/bad gas. Run at least 1 tankful using a good injector cleaner after changing plugs.

Tyler

[bradslt5]

what is the best inj cleaner to use?

[tomtom72]

I was told the good stuff is Techron by Chevron. Black bottle any autozone or Costco has a 4 pack for about $20.00.

Paul, not to be forward.....I notice with mine that around town driving is the worst on my car. I go out with my vette buddies & try to keep up and the car never runs better...for a few days untill my around town driving "loads" it up again...if I drive it like a wuss = car don't seem to like it. If I beat it with an ugly stick = car seems to laught at me & wants more & she runs sweet. Oh, I'm using the std GM replacement plug for the FR2LS's that were OEM in my 90.

:cheers:
Tom

[Paul Workman]

I was told the good stuff is Techron by Chevron. Black bottle any autozone or Costco has a 4 pack for about $20.00.

Paul, not to be forward.....I notice with mine that around town driving is the worst on my car. I go out with my vette buddies & try to keep up and the car never runs better...for a few days untill my around town driving "loads" it up again...if I drive it like a wuss = car don't seem to like it. If I beat it with an ugly stick = car seems to laught at me & wants more & she runs sweet. Oh, I'm using the std GM replacement plug for the FR2LS's that were OEM in my 90.

:cheers:
Tom

Yeah...I'm beginning to see that same performance pattern among some other Z drivers posting here and at that "other place". Mine too. After idling (especially from sub-freezing to just operating temp), mine went from running great to stumbling, balking, missing - primarily when giving it some gas. After it got warmed up good and ran the snot out of it a few times, all was well...Until next time.

Well, I bought some new plugs yesterday, and I'll get them in today and see what difference that makes - assuming the ones in there are original. My usual parts supplier didn't have the NGKs Jeff recommended, but he told me the Platinum/Irridium plugs are all made by Champion, as they're the only ones that have the tooling to make those plugs. According to his sources, Champion makes the plugs for NGK, AC/Delco, Autolite, and others...Interesting:dontknow:.

Jeff, if you're reading this too, I couldn't find the NGKs locally, so I cross-referenced them to AC/Delco #41-800 (listed for the 90 ZR-1). We'll see how they work later today, perhaps.

P.

[Aurora40]

What??? I'm pretty certain that NGK makes their own spark plugs.

[A26B]

Paul,
What you describe sounds more like it's just loading up from the idling. I'm sure that when you change plugs, you'll be looking at the old ones. Assuming the engine has been through a number of idling cycles, the plugs may be black. If it's loading up, the problem will return eventually after repeating the idling cycles.

Also, the LT5 is prone to oil collection in the plenum as a result of the crankcase ventilation system required on the engine. As such, oil may collect and add to the miss/load plug fouling problem until you "blow it out."

Another possibility is oil collection in the MAP sensor hose from the plenum to the underside of the MAP sensor. Will mess up the sensor signal. Just remove it and clean it out with air.

I like your style, go for the simple & no cost stuff first.

Keep us posted.

[Paul Workman]

What??? I'm pretty certain that NGK makes their own spark plugs.
To clarify, specifically the info indicated Champion had a lock on the Platinum/Irridium plug technolgy(??). It was the plugs with that particular electrode combination which was reported to be made by Champion for NGK, etc. T'was a auto parts dealer that told me about the Champion making the plugs. Is it true? I dunno. I never heard that before either!

P.

[Jeffvette]

Jeff, if you're reading this too, I couldn't find the NGKs locally, so I cross-referenced them to AC/Delco #41-800 (listed for the 90 ZR-1). We'll see how they work later today, perhaps.

P.

41-800 or the 41-808?

The 800 is not listed for a ZR-1, the 808 is.

[Paul Workman]

41-800 or the 41-808?

The 800 is not listed for a ZR-1, the 808 is.

I went back to the AC/Delco site and double checked. The 41-800 was listed for the 1990 CHEVROLET CORVETTE ZR-1 V8 5.7L 350cid GAS FI N Engine VIN = J

http://198.208.187.182/internet/VehiclePartDescription.jsp?selectedfld=&application=Car%2FLight+Duty+Truck&year=1990&make=CHEVROLET&model=CORVETTE+ZR-1&engine=V8+5.7L+350cid+GAS+FI+N+Engine+VIN+%3D+J+++ +++&productline=25&part=7212

In any case, the plugs I pulled were the standard AC/Delco 41-602s They looked new, and were not the least bit carboned up. Insulators were just off-white. Hmmmmm......:icon_scra So, for grins, I ran over all the "zebra" clamps on all the vacume hoses. a couple were a bit loose.

Then I re-torqued the intake plenum. There I found about 1/2 of the screws were OK, and the other half took between 1/8 to 1/2 turn before they "clicked" at 20 ft# (FSM recommendation). I cranked her up and it ran smooth - not bad considering the ambient temp was 20º and the engine was cold soaked to be sure.

I then opened the valve on a bottle of propane and directed the gas around the edges of the plenum and the various vacume hose fittings/ends. There was not a whisper of affect and it continued to idle "sweetly" for several minutes before I cut it of (not yet at operating temp - maybe close to 100º at best.).

No chance for a roady today. The temp crept up past freezing and the sun is out. So, the road is a slimy mess. Further road tests will have to wait, but I do very much appreciate the support you and others have offered here. I'll do another "roady" as soon as possible.

Well, armed with the FSM and a VOM, I'll go and see what's up w/ my cooling fan(s?) coming on with the ignition, while I wait for a road test opportunity.:wink:

P.

[Paul Workman]

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":rolleyes:), 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"? :icon_scra.

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

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!:thumbsup: =D>

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!:o ....but at least my carbon is in the atmosphere now.:mrgreen:

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.

:cheers:
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!:thumbsup:

[Paul Workman]

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 don't see why not. Intercept the blue-black wire on the (left hand) fan relay plug wire and run it thru a plain ol SPST switch to ground. Throw the switch and the fan comes on instantly. (If you need a photo of where those relays are, lemmeno and I'll snap a pic for ya.):thumbsup:

P.