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Possible hidden vacuum leak on Z51 after rebuild - where could it be?

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Possible hidden vacuum leak on Z51 after rebuild - where could it be?

Old 09-17-2018, 01:01 PM
  #1  
jimxms
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Default Possible hidden vacuum leak on Z51 after rebuild - where could it be?

Short background: Had my engine rebuilt with a stage 3 cam, new pistons, lt4 fuelling. Since it come back it idles like crap and often stalls. If I hold the throttle down a bit and get the revs up over 1000 it smooths out a bit, but anything below that and it just wants to stall. No CEL's. Tuner is Ben Charles - so well respected. He thinks there may be a vac leak as the tunes he has sent me have not remedied the stalling.

What I've tried: Sprayed carb cleaner around everything after the MAF. All visible vacuum hoses, the vac hose that plugs in behind the intake manifold, around the intake manifold port gaskets..etc. I've also tried removing my intake manifold (an MSD Airforce) and put the OEM manifold back on. If I create an intentional vacuum leak it doesn't make the engine run any worse than it does already. I am waiting on some smoke pellets to see if i can rig up a home made smoke tester. I've also tried disconnecting the MAF to see if the car runs any better in SD - it doesn't, it runs much worse.

Am I on the right track with a vac leak? Could it be anything else? Are there any VAC hoses that are not easily accessible/visible that could have been left unplugged when the engine was reinstalled?

Thanks guys.
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Old 09-17-2018, 03:21 PM
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Double check the intake bellows connection onto the throttle body. Had a car once where the lip folded under so it didnít seal properly and let air into the intake that bypassed the MAF. Behaved exactly as you describe.
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Old 09-17-2018, 03:50 PM
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Originally Posted by raylo View Post
Double check the intake bellows connection onto the throttle body. Had a car once where the lip folded under so it didnít seal properly and let air into the intake that bypassed the MAF. Behaved exactly as you describe.
Thanks for the suggestion, but yeah I've looked there. If there is a vacuum leak I'm inclined to think that it is somewhere out of reach. Maybe a fuel injector gasket missing (although on a direct injection engine I'd imagine that would spray fuel everywhere), some kind of vacuum hose at the rear of the engine, or something much worse like something not bolted down properly inside the engine.
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Old 09-17-2018, 04:12 PM
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The only vac line that's hard to reach/see is the brake booster and I'm sure you'd know if that thing was loose. What about all of your pcv lines are they properly routed?

I tuned a 6th gen camaro that had an msd and a tight lsa cam with high lift and I was only ever to get it to around 65kpa at idle, which it wasn't very happy about. I had to make some pretty drastic changes to get the thing to run right. No vac leaks that I could find unless the manifold was cracked either...we never did get it on a smoke machine though.

Last edited by sevinn; 09-17-2018 at 04:12 PM.
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Old 09-17-2018, 04:33 PM
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Originally Posted by sevinn View Post
The only vac line that's hard to reach/see is the brake booster and I'm sure you'd know if that thing was loose. What about all of your pcv lines are they properly routed?

I tuned a 6th gen camaro that had an msd and a tight lsa cam with high lift and I was only ever to get it to around 65kpa at idle, which it wasn't very happy about. I had to make some pretty drastic changes to get the thing to run right. No vac leaks that I could find unless the manifold was cracked either...we never did get it on a smoke machine though.
Yeah, I did check that vac line as I remember it from when I installed my MSD intake. I also tried using the OEM intake manifold too, which didn't help

I'm running an Mighty Mouse catch can with their recommended PCV routing:
Drivers side valve cover outlet > MM Can
2x Passenger side valve cover outlets > Dry sump oil tank
Intake manifold vacuum nipple > MM Can
Valley Port nipple (below the throttle body, on the engine block) > capped off

The only thing I have different to MM's instructions is this:



The nipple this breather is on was originally connected to the OEM air intake. So I don't think there should be an issue with it having access to open air?


Is there any chance the engine might not be firing on all cylinders? Would it be wanting to stall if that were the case? I suppose I can easily check that with a laser temp probe on the headers?
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Old 09-17-2018, 06:20 PM
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Double check the connections from the harness to the coil packs then make sure all the plug wires are secured both into the coil and on the plugs.
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Old 09-17-2018, 07:01 PM
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Originally Posted by jimxms View Post
Is there any chance the engine might not be firing on all cylinders? Would it be wanting to stall if that were the case? I suppose I can easily check that with a laser temp probe on the headers?
Yes, you can use a laser temp gauge to see if all the cylinders are indeed firing.

Originally Posted by stab1991 View Post
Double check the connections from the harness to the coil packs then make sure all the plug wires are secured both into the coil and on the plugs.
I agree, check this, as well as the spark plug wires on both ends. On the end that the plug wire connects to the coil, it actually has to “click” twice when pushing it on or else its not seated and could be causing issues.

there are no other vacuum lines other than the brake booster and PCV system that I can remember.

Last edited by FYREANT; 09-17-2018 at 07:02 PM.
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Old 09-22-2018, 06:42 PM
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I'm unclear, did you actually install the engine yourself? Doesn't sound like it, so what about your installer?. What does he think? Can he go over everything he did, step by step? Has he done an identical engine before? How does he work? I ask the latter because I've seen people who do things by halts and starts, constantly interrupted and returning to various projects which makes it easy to miss things.

In any event, all the best.
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Old 09-23-2018, 05:09 AM
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Originally Posted by Avanti View Post
I'm unclear, did you actually install the engine yourself? Doesn't sound like it, so what about your installer?. What does he think? Can he go over everything he did, step by step? Has he done an identical engine before? How does he work? I ask the latter because I've seen people who do things by halts and starts, constantly interrupted and returning to various projects which makes it easy to miss things.

In any event, all the best.
It was built and installed by the engine builder who is quite a distance from me. It would cost me $700 to get it trailered there and back, so I'm trying to eliminate any simple problems that it could be before I go down that route.

However, I've just dug myself into a WHOLE tonne of shi*t that makes me want to throw the whole project away.....

So I was checking the spark plug wires, and noticed that one of the wires to the plugs on the passenger side wasn't seating properly. I could slide the wire boot over the plug tip, feel it go into the metal connector inside the plug boot, but there was no 'click'. Tried it with a different spark wire and it was the same. It appeared to be an issue with the tip of the spark plug itself.

So I set about removing the spark plug, which was no easy task because the eBay exhaust headers I'd purchased got in the way of the spark plug hole! Eventually got it out, and set about installing the new one.

Threaded it in the first few threads by hand and then got a wrench on it. After a few turns it seized.....the plug was going in cross threaded!!!

This was enough to ruin the first few threads of the hole, meaning that I just couldn't get the plug to go in correctly no matter how many times I tried.

So I purchased a reverse thread tap. You put the tap in the hole, expand it with a screw and then tap the hole out in reverse so that it uses the good threads.

This happened:



Using only a small spanner, the tool broke apart inside the engine! I managed to retrieve 3 of the 4 pieces, but the fourth was inaccessible. I tried to vacuum it out, use magnets on sticks, but nothing could get close enough.

Eventually I got fed up and tied a magnet to a bit of string to try and fish it out of the cylinder - not realising that the cylinder walls are steel lined (i assumed aluminium). The magnet instantly stuck to the cylinder wall, and now I have two foreign objects in the cylinder that I cannot get out.

So I guess its time to pull the head....

Last edited by jimxms; 09-23-2018 at 05:34 AM.
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Old 09-23-2018, 10:11 AM
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Post #9 brings tears to my old eyes! That is a major case of suck...

It would appear that you had found an issue with the plug and were on your way to recovery and then the

Good luck!

Elmer
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Old 09-23-2018, 11:28 AM
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Originally Posted by eboggs_jkvl View Post
Post #9 brings tears to my old eyes! That is a major case of suck...

It would appear that you had found an issue with the plug and were on your way to recovery and then the

Good luck!

Elmer
Yep. My neighbours learned some new words the day that happened. I think I shouted several at the top of my voice.

It seems like part of pulling the head also involves taking the water pump off and draining fluids. I am so bummed and I donít even have a mechanically minded friend to help me
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Old 09-23-2018, 12:07 PM
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Oh, man, I hate to read stories like this. I learned a good cursing technique from a member of our old f-body group.... when you have a really crappy thing like this happen just just string every curse word you know into one long "sentence". Repeat as necessary. Good luck.

Last edited by raylo; 09-23-2018 at 12:08 PM.
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Old 09-23-2018, 05:27 PM
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Originally Posted by raylo View Post
Oh, man, I hate to read stories like this. I learned a good cursing technique from a member of our old f-body group.... when you have a really crappy thing like this happen just just string every curse word you know into one long "sentence". Repeat as necessary. Good luck.
Hahah I'll have to remember that one. I normally go the same curse word and a variety of suffixes.........f**kety, f**king, f**ksticks, f**k, fuuu**, fuuuuuuuuu**!!!
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Old 09-23-2018, 05:46 PM
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Originally Posted by jimxms View Post
Hahah I'll have to remember that one. I normally go the same curse word and a variety of suffixes.........f**kety, f**king, f**ksticks, f**k, fuuu**, fuuuuuuuuu**!!!
It brings a tear to my eye looking at the beauty of your curse word sting!

Elmer
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Old 09-23-2018, 07:32 PM
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You definitely have a good attitude bro. Hang in there,it could be worse. Sometimes I look at things like they were meant to happen. All will be good in short time.
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Old 09-24-2018, 07:49 AM
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Nothing quite like the smell of success in the morning:
J


The silver lining here is that even if the tool hadnít broken in the cylinder, the amount of metal shards from re-tapping the spark plug hole was enormous. And despite me vacuuming and using a magnet, there would have still been way too much shrapnel in there for my liking.

Now one I just need to wait for my head gasket to arrive and I can put this baby back together again

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Old 09-24-2018, 08:36 AM
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Excellent! Progress with my coffee this morning!

ELmer
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Old 09-24-2018, 10:03 AM
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Factory head gaskets are reusable
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Old 09-24-2018, 10:23 AM
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Originally Posted by stab1991 View Post
Factory head gaskets are reusable
For real? I thought that once they had been compressed once then they were junk.
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Old 09-24-2018, 11:35 AM
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Can reuse them. In fact the gaskets that come with headers are usually not used and most guys reuse the factory ones.
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